Dynamic Performance Requirements for Permanent Grandstands Subject to Crowd Action

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The Institution of Structural Engineers

The Department for Communities and Local Government

The Department for Culture Media and Sport

December 2008

Dynamic performancerequirements for permanentgrandstands subject tocrowd action

Recommendations for management,design and assessment

Published by the Institution of Structural Engineers



The Institution of Structural Engineers

The Department for Communities and Local Government

The Department for Culture Media and Sport

December 2008

Dynamic performancerequirements for permanentgrandstands subject to

crowd action

Recommendations for management,

design and assessment




ii Dynamic performance requirements for permanent grandstands subject to crowd action

Membership of the Joint Working Group

Dr J W Dougill – Chairman

Professor A Blakeborough – Oxford University

Mr P Cooper – KW Ltd to July 2007, then INTEC

Dr S M Doran – IStructE, Secretary

Dr B Ellis – BRE to 2006 now Consultant

Mr P F Everall – DCLG (to 2005)Dr T Ji – UMIST/ The University of Manchester

Mr J Levison – Football Licensing Authority, d. 12th Dec. 2007

Dr J Maguire – Lloyd’s Register. (received papers from 2002)

Mr S Morley – Bianchi Morley

Mr M Otlet – W S Atkins

Professor G A R Parke – Surrey University (to 2002)

Mr J. G. Parkhouse – Parkhouse Consultants

Professor A Pavic – The University of Sheffield

Mr L Railton – Health and Safety Executive (to 2003)

Mr W Reid – Consultant, URS

Mr R Shipman, DCLG, (from 2005)

Mr P Westbury – Buro Happold. (received papers from 2002)Mr M Willford – Ove Arup

Professor J Wright –The University of Manchester/J2W Consulting Ltd

Corresponding MembersMr D Allen, National Research Council of Canada

Dr M Kasperski, Bochum University

Dr P Reynolds, The University of Sheffield

Mr P Wright, Health and Safety Executive, from 2003

ContributorsThe Joint Working Group wishes to acknowledge the contribution of the following individuals who, though

not attending as members of the Group, made presentations on different aspects relating to grandstand

design, operation and behaviour.

Mr D Allen, National Research Council of Canada

Mr J Cutlack, J. Bobrowski and Partners

Dr J Dickie, Crowdsafe Ltd

Mr C Gleeson, Chelsea Football Club

Professor M J Griffin, Institute of Sound and Vibration, Southampton University

Dr M Kasperski, Bochum University, Germany

Dr J Littler, BRE

Dr A J Soane, Bingham Cotterell


International HQ, 11 Upper Belgrave Street, London SW1X 8BH, UK

ISBN: 978-1-906335-12-0

© 2008: The Institution of Structural Engineers

The Institution of Structural Engineers, DCLG, DCMS and the members who served on the Joint Working Group

which produced this report have endeavoured to ensure the accuracy of its contents. However, the guidance and

recommendations given in the report should always be reviewed by those using it in the light of the facts of their

particular case and specialist advice obtained as necessary. No liability for negligence or otherwise in relation to this

report and its contents is accepted by the Institution, the members of the Joint Working Group their servants or agents.

Any person using this report should pay particular attention to the provisions of this Condition.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any meanswithout prior permission of the Institution of Structural Engineers, who may be contacted at 11 Upper Belgrave Street,

London SW1X 8BH.



iiiDynamic performance requirements for permanent grandstands subject to crowd action

Contents

Foreword v

1 Scope of the Recommendations 1

2 Design event scenarios 2

3 Listed Engineers 43.1 Requirement for spec ialist engineering expertise 43.2 Technical support 43.3 Discretion on relevance of the Recommendations to specific structures 43.4 J udgment on relevance of the Recommendations to specific structures 43.5 Monitoring 5

4 Natural frequencies and other dynamic properties 64.1 Background 64.2 Structural modelling to determine modal properties of the

empty grandstand 64.3 Values for initial design 74.4 ‘Relevant’ natural frequency 7

5 Testing 85.1 Need for testing 85.2 Aims of testing 85.3 Circumstances requiring testing 8

6 Management responsibilities 106.1 Overall responsibility 106.2 Design of new stands 106.3 Change of use and assessment for spec ific events 106.4 Operational strategies to reduce dynamic response and crowd alarm 116.5 Handover of new and structurally modified grandstands 12

6.6 Operations Manual 126.7 Operation 13

7 Route 1: Compliance with natural frequency requirements 157.1 Vertical excitation 157.2 Side-to-side horizontal excitation 157.3 Nodding modes due to front-to-back excitation 15

8 Route 2: Design for managed events 178.1 Outline 178.2 Idealised description of crowd activity 178.3 Serviceability: Tolerance of motion 188.4 Serviceability: Displac ement limits 18

8.5 Ultimate load capacity 188.6 Fatigue 18

9 Analysis of dynamic performance 199.1 Impulse loads 199.2 Horizontal loads due to periodic excitation 199.3 Analysis for vertical periodic excitation 199.4 Human structure interaction 19

10 Use of the Recommendations 21



iv Dynamic performance requirements for permanent grandstands subject to crowd action

Appendix 1 Background to human structure interaction 22A1.1 Introduction 22A1.2 Basic principles 22

A1.2.1 Modelling human structure interaction 22A1.2.2 Active and passive behaviour 23

A1.3 Application of the theory 23A1.3.1 Direc t application of the theory 23A1.3.2 Approximate analysis using an assumed mode shape for

the crowd’s motion 24A1.4 Body Unit properties and loadings 24

A1.5 Analysis and results 24A1.5.1 Modal analysis 24A1.5.2 Root mean square (RMS) accelerations and acceleration limits 24A1.5.3 Analysis with a dominant mode. 25A1.5.4 Multi-mode analysis 25

A1.6 References 26

Appendix 2 Body unit properties and recommended loading 27A2.1 Body unit and structure 27A2.2 Crowd body elements 27A2.3 Representation of periodic loading 28A2.4 Internal ‘drivers’- G i – producing dynamic crowd loading 29

A2.5 The crowd effectiveness fac tor ‘t’ 30

A2.5.1 Scenario 4 30A2.5.2 Scenarios 2 and 3 30A2.6 Monitoring and back analysis 31A2.7 References 32

Appendix 3 Calculation of modal properties 33A3.1 Introduction 33A3.2 Modal analysis and natural frequenc ies 33A3.3 Basic errors: two prime suspects 35

A3.3.1 Distinction between force and mass 35A3.3.2 Stiffness 35

A.3.4 Methods for calculation of natural frequenc ies 36A3.4.1 General comment 36

A3.4.2 Approximate analysis 36A3.4.3 Computer based analysis 36A3.5 Consequences of mistaken idealisations 36A3.6 Comment 37A3.7 Further information 38

Appendix 4 Dynamic testing of grandstands and seating decks 39A4.1 Introduction 39A4.2 What should be tested and what results are needed? 39A4.3 Analysis and testing 40A4.4 Principles of dynamic testing 41A4.5 Excitation sources and testing techniques 43

A.4.5.1 Ambient vibration survey (AVS) 43

A.4.5.2 Heel-drop testing 43A.4.5.3 Measured impact testing 43A4.5.4 Shaker testing of different types and complexity 44A.4.5.5 Future developments 45

A4.6 Specification and procurement 45A4.7 Reporting 47A4.8 Further Information 47

Appendix 5 Bibliography 48



vDynamic performance requirements for permanent grandstands subject to crowd action

FOREWORD

The Joint Working Group met first in January 2000. Its Interim Guidance was published inNovember 2002 in response to concerns over crowd action on structures generally and on therelevance of available recommendations to dense crowd loading on permanent grandstands. TheInterim Guidance used the vertical natural frequency, for the mode of vibration that could beexcited and felt by people on the seating deck, as the currency to determine different categories

of permissible use. No attempt was made to recommend a method of estimating performance bycalculation as it was considered that existing procedures, though widely used, could not be reliedon. The Interim Guidance was designed to be safe and straightforward to apply. It provided asignificant relaxation of the ‘trigger value’ frequency limits of BS6399 (1996) and the 1997 GreenGuide. However, because of the simplification of using natural frequency as the single factordetermining a category of use, it was a broad brush treatment. What was needed was a methodof design and operation that was based on an estimate of performance that was reasonable whencompared with the effects observed with active crowds in real structures. This was the task of theJoint Working Group from 2002 onwards.

In addressing the technical issues relating to the analysis of the structure, the Joint WorkingGroup has been closely involved with a number of UK research projects (almost all supportedby the Engineering and Physical Sciences Research Council, EPSRC) that have been undertaken

since 2000 and which have contributed to an improved understanding of the physical problem ofhow human beings interact with moderately flexible structures. This work has provided the basisfor the technical content in the new Recommendations. However, this could not be seen as an endin itself. A key issue in design is how to deal with uncertainty. With grandstands, there can be noabsolute certainty on the way any random group of people will behave. Accordingly, the technicalprovisions have been set in a framework of procurement, management and operation aimed atminimising risk by managing uncertainty.

The Recommendations propose that specially ‘Listed Engineers’, having particularexperience and capability, be used in the design and assessment of grandstands for dynamiccrowd loading. Design should be based on the concept of managed events described by standardDesign Event Scenarios that form part of the specification for a stand. Within these scenarios, theManagement of the facility takes responsibility for specific agreed measures to mitigate the effectsof motion. Hand-over procedures are outlined with the aim of ensuring that the design calculationsrelate to the as-built structure. The aim of each of these recommendations is to reduce uncertaintywhere it is possible to do so.

The Recommendations are written for everyone who has responsibility for grandstands. Thisincludes the owners, operators, managers, architects, insurers and engineering designers as well asLocal Authority staff dealing with building control and safety issues. The Recommendations areaccompanied by Appendices directed particularly at the engineering analyst and designer.

The Recommendations relating to specification, management and operation should beconsidered to be of equal importance to providing comfort and safety as the technical guidanceaddressed principally to the engineering designer.

Dr John W DougillDecember 2007



vi Dynamic performance requirements for permanent grandstands subject to crowd action



1Dynamic performance requirements for permanent grandstands subject to crowd action

1 Scope of the Recommendations

The Recommendations given here are for use in the design or assessment of permanent

grandstands and relate solely to dynamic action due to crowd activity. Reference should

be made to other guidance and relevant Standards for other load cases and design

requirements.

The Recommendations also include guidance relating to Management’s role indesign and in implementing operational requirements for stands being used for events

at which dynamic crowd loading can be expected. This guidance is concerned only with

those aspects of crowd management that directly influence the structural response of a

grandstand and so supplement, but do not replace legal and Standards’ based requirements

for safe operation. The operational arrangements adopted should be taken into account by

those undertaking an overall risk assessment at the scheme design stage or for a specific

event.

The Recommendations apply to grandstands with seating decks constructed in

structural steel, reinforced or prestressed concrete and combinations of these forms of

construction. No recommendations are made for the use of subsidiary systems to provideadditional damping or active control.

The Recommendations are considered relevant to grandstands with seating decks

having a supported span greater than 6m or cantilever spans of more than 2.5m. However,

it is recognised that, even within the declared scope of these Recommendations, there may

be particular grandstands for which the layout, form of construction or limited use might

render it unnecessary to undertake a full check on dynamic performance. The manner in

which this can be dealt with is treated in Section 3 on Listed Engineers.

The Recommendations revise and extend the recommendations given in the November

2001 report ‘Dynamic performance requirements for permanent grandstands subject tocrowd action: interim guidance on assessment and design’, published by the Institution of

Structural Engineers and adopted by DCLG and DCMS. This Interim Guidance used the

vertical natural frequency of the empty grandstand as the sole criterion for assessing the

acceptability of grandstands for use with crowds likely to generate dynamic loading. This

convenient, but coarse grained, approach is retained as an option in the present guidance

which now provides an alternative approach that depends on engineering estimates of the

likely performance of a grandstand for events at which the event organiser is responsible

for specific agreed measures relating to crowd management. This alternative approach

provides further options for the designer and management as well as addressing a need to

account for influences on behaviour additional to natural frequency.

As a consequence of this approach, there are two Routes available for design and

assessment of grandstand structures subject to dynamic crowd loading,

• Route 1: Based on limiting values of natural frequency for the grandstand empty of

people.

• Route 2: Based on estimates of performance of grandstands calculated for specified

managed events.

A grandstand may be considered to meet the Recommendations for dynamic crowd loading

if the requirements of either one or other of these routes are met together with the separate

conditions for horizontal strength and stability. (See Sections 7.2, 7.3 and Table 2).



2 Dynamic performance requirements for permanent grandstands subject to crowd action

2 Design Event Scenarios

The Design Event Scenario forms the basis for a design specification for dynamic

performance of a grandstand under crowd loading. Table 1 gives standard Design

Event Scenarios for use in communicating design objectives in terms of anticipated

performance.

The Table shows a range of different events together with the expected activity ofthe crowd and an indication of crowd control measures to be provided by the management

of the grandstand.

The four performance based scenarios (numbered 1 to 4) correspond to increasing

crowd involvement and activity together with increased loading. Scenarios 1 and 2,

appropriate for viewing sporting events and classical concerts, would normally be satisfied

by Route 1 requirements. Scenario 3 refers to lively concerts and high profile sporting

events whilst Scenario 4 is for high energy events such as pop/rock concerts. The Scenarios

are provided in order to assist event specific assessment and to provide a yardstick for

authorities concerned with safety certification.

It will be evident that the Design Event Scenario is the statement of what should

be covered in design and what needs to be managed. The Scenario comprises a reference

to the category of event in Table 1 with a statement of any additional specific crowd

control measures that have been agreed as being required. (See Section 6.4 on Operational

Strategies).

It should be noted that the descriptions of exemplar events are indicative rather

than prescriptive. For example, an event may be described as a pop-concert for publicity

purposes but the crowd’s reaction may be only moderate and so more consistent with a

concert with medium tempo music as envisaged for Scenario 3. Accordingly, in using

records of past events to assist an assessment for a future event, care should be taken to dothis on the basis of observed performance and not solely on a record that a ‘pop-concert’

had been run satisfactorily in the past. (See Section 6.7 on Operation, re. record keeping).

The Scenarios are based on experience of events in the United Kingdom. In assessing

any specific event, judgment will be needed to decide the appropriate category particularly

between Scenarios 3 and 4 and, on occasion, whether the crowd at a particular sporting

event is likely to be more than usually active with coordinated rhythmic activity. This

has become common at football matches in mainland Europe where groups of fans have

rehearsed bobbing, treading or stamping in time to a beat provided by their leader. The

improved coordination accompanying behaviour of this sort can lead to motion that is more

severe than that anticipated for Scenario 4. This could lead to possible discomfort for seated

or standing fans not participating in the activity. Such situations need to be recognised and

appropriate operational measures adopted by Management. (See Section 6.4 on Operational

strategies).




T a b l e 1 E v e n t S c e n a r i o s t o b e s u p p

l e m e n t e d w i t h a s t a t e m e n t o n

a g r e e d o p e r a t i o n a l a r r a n g e m

e n t s

S c e n a r i o

E x e m p l a r e v e n t

C r o w d b e h a v i o u r

M a n a g e

m e n t

A c c e p t a n c e c r i t e r i a

E x p e c t e

d c r o w d m a k e u p

S t e w a r d i n g

C r o w d e x p e c t a t i o n

N a t u r a l f r e q u e n c y o r

d e s i g n a c c e l e r a t i o n

l i m i t

1

S t a n d u s e d f o r v i e w i n g o f

s p o r t i n g a n d s i m i l a r e v e n t s

w i t h l e s s t h a n m a x i m u m

a t t e n d a n c e

N o r m a l l y r e l a x e d

v i e w i n g p u b l i c w i t h

s p o n t a n e o u s r e s p o n s e

t o s i n g l e e v e n t s

P r e d o m i n a n t l y s e a t e d

S t e w a r d s r e q

u i r e t o

b e i n s t r u c t e d

a s t o

t h e p o s s i b l e e f f e c t s

o f m o t i o n o f

t h e

s t r u c t u r e r e s u

l t i n g

f r o m c o o r d i n

a t e d

c r o w d a c t i v i t y a n d ,

p a r t i c u l a r l y f o r

S c e n a r i o s 3 a

n d 4 ,

n e e d t o b e p r o v i d e d

w i t h t h e m e a

n s f o r

i n s t a n t c o m m

u n i c a t i o n

w i t h c e n t r a l c o n t r o l .

C o m f o r t

R o u t e 1 w i t h 3 . 5 H z

m i n . o r a c c e p t e d a t

t h e d i s c r e t i o n o f a

L i s t e d E n g i n e e r

2

C l a s s i c a l c o n c e r t a n d

t y p i c a l w e l l a t t e n d e d

s p o r t i n g e v e n t

A u d i e n c e s e a t e d w i t h

o n l y f e w e x c e p t i o n s –

m i n o r e x c i t a t i o n

P r e d o m i n a n t l y s e a t e d

C o m f o r t

R o u t e 1 w i t h

3 . 5 H z m i n . I s

n o r m a l l y a d e q u a t e

b u t o t h e r w i s e

R o u t e 2 a n d 3 % g m a x .

R M S a c c e l e r a t i o n

3

C o m m o n l y o c c u r r i n g

e v e n t s i n c l u d i n g , i n t e r a l i a ,

h i g h p r o f i l e s p o r t i n g e v e n t s

a n d c o n c e r t s w i t h m e d i u m

t e m p o m u s i c a n d r e v i v a l

p o p - c o n c e r t s w i t h c r o s s

g e n e r a t i o n a p p e a l

P o t e n t i a l l y e x c i t a b l e

c r o w d w i t h c r o w d

p a r t i c i p a t i o n

A l l s t a n d

i n g a n d

p a r t i c i p a t i n g d u r i n g s o m e

p a r t o f t

h e p r o g r a m m e

A f e w i n d i v i d u a l s m a y

c o m p l a i n a t l a c k o f

c o m f o r t b u t m o s t w i l l

t o l e r a t e t h e m o t i o n

R o u t e 1 w i t h 6 H z m i n .

O t h e r w i s e R o u t e 2 ,

a n d 7 ½ % g m a x . R M S

a c c e l e r a t i o n

4

M o r e e x t r e m e e v e n t s

i n c l u d i n g h i g h e n e r g y

c o n c e r t s w i t h p e r i o d s o f

h i g h i n t e n s i t y m u s i c

E x c i t e d c r o w d , m o s t l y

s t a n d i n g a n d b o b b i n g

w i t h s o m e j u m p i n g

M a i n l y y

o u n g

a n d a c t

i v e w i t h

v i g o r o u s p a r t i c i p a t i o n

E x c i t e m e n t a n d

m o t i o n b u t w i t h

e x p e c t a t i o n o f

p e r s o n a l s a f e t y

R o u t e 1 w i t h 6 H z m i n .

O t h e r w i s e R o u t e 2 ,

a n d 2 0 % g m a x . R M S

a c c e l e r a t i o n

N o t e A p p e n d i x 1 , S e c t i o n A 1 . 5 d e a l s w i t h R o o

t M e a n S q u a r e ( R M S ) a c c e l e r a t i o n s

a n d h o w t h e s e a r e d e f i n e d a n d u s e d i n a n a l y s i s .




3 Listed Engineers

3.1 Requirement for specialist engineering expertiseGrandstands, and particularly the combination of a crowd of people and a grandstand,

provide technical and managerial problems that can be addressed satisfactorily only

by engineers with appropriate specialist expertise in addition to all round competenceunderpinned by professional qualifications.

Whichever Route is chosen for design or assessment, it is important that these functions,

and the provision of advice on safe operation, are undertaken by an engineer having relevant

expertise in the structural design and safe operation of grandstands. At the request of

Government, the Institution of Structural Engineers is putting in place a scheme to provide

a list of engineers with specific experience in the design, assessment and safe operation of

grandstands for dynamic crowd loading. In these Recommendations, such engineers will be

referred to as Listed Engineers.

It is proposed that any Design Team dealing with new grandstands or significant

alteration to existing grandstands should include a Listed Engineer with particularresponsibility for overseeing those aspects of design concerned with dynamic crowd

behaviour. Similarly, a Listed Engineer should be employed in assessing existing stadia

and in overseeing the hand-over procedures of new and altered grandstands.

3.2 Technical supportListed Engineers will be expected to have sufficient personal specialised knowledge and

expertise relating to the design and safe operation of grandstands to advise management

on all aspects connected with dynamic crowd loading. However, it would be expected that

Listed Engineers themselves may need to employ specialist support in respect of physical

testing and the conduct of dynamic structural analysis undertaken under their direction oron their instruction.

3.3 Discretion on relevance of the Recommendations to specific structuresGrandstands vary in type, size and manner of use. Depending on the circumstances, detailed

consideration of crowd action may not be appropriate for a particular grandstand and,

based on their experience and technical background, Listed Engineers will be expected to

use their discretion in deciding the relevance of the Recommendations to such cases.

The Listed Engineer’s report to the grandstand’s Management should make clear

whether discretion has been exercised and the reason for doing so. It will be appreciated

that the management of a grandstand has responsibility for the safety of the facility

and so needs to understand and take responsibility for accepting the Listed Engineer’s

recommendations. The Listed Engineer’s report should also be made available to the Local

Authority concerned with Building Control and Safety Certification.

3.4 Judgment on relevance of the Recommendations to specificstructuresListed Engineers will be expected to have the necessary background to use their own

judgment in interpreting and applying the Recommendations to particular grandstands.

The main areas for exercising judgment will normally be in advising the client

when choosing the appropriate Design Event Scenario, in detailed consideration of the

approximations involved in structural modelling and in assessing the need for structural

testing.




In making these judgments, the Listed Engineer should bear in mind that, in respect

of dynamic behaviour, the aim should be to remove the need to consider structural

behaviour from the wider risk assessment necessary for an event as a whole. If followed,

the Route 1 approach provides for this based on measured natural frequencies and the

frequency limits in Figure 1. (See Section 6.6 ). The Route 2 approach provides more

flexibility in decision making but depends on estimates of performance using calculations

based on recommended values for crowd loading that are considered typical for different

types of events. Sufficiently accurate values of natural frequency are required for both

approaches whilst a Route 2 analysis needs a more complete knowledge of dynamicbehaviour including a full set of relevant modal properties.

Because of the importance of accurate knowledge of dynamic properties, either

in characterising the admissible use of a grandstand using Route 1, or, as the basis of

design or assessment by Route 2, it will normally be considered necessary for the values of

dynamic properties to be established, or checked, using physical testing. (See Section 5 on

Testing). There will be circumstances where testing is not necessary, so placing a particular

responsibility on the Listed Engineer to exercise personal judgment on the need for physical

testing to check or establish values of dynamic properties. Such a judgment should not be

influenced by considerations of cost, convenience or time pressure but, solely, on a decision

that, in the particular circumstances being considered, the wider information availablefrom testing would not materially affect decisions on either design or assessment.

As in the use of discretion on the relevance of Recommendations to a particular

grandstand, the Listed Engineer’s report to the grandstand’s management should make

clear whether personal judgment has been exercised on any particular Recommendation,

including the need for testing, and the reason for doing so. The Listed Engineer’s report

should also be made available to the Local Authority concerned with Building Control and

Safety Certification.

3.5 Monitoring

The Listed Engineer should be involved with and advise on any programme to monitor thebehaviour of a stand whilst in use under crowd loading. Monitoring could include visual

recording of crowd behaviour, acceleration and load measurements, stewards’ reports etc.




4 Natural Frequencies and other dynamic properties

4.1 BackgroundPartly as a result of testing undertaken since the adoption of the Interim Guidance, the

concern on possible differences between calculated values of natural frequency and values

determined by competent testing has been reinforced rather than allayed. Differences of upto 30% between measured and calculated natural frequencies have been recorded. Also, it

is rare that even approximate agreement between calculated values and those from testing

is obtained without some reappraisal of the structural model used in the calculation.

The largest differences in values usually follow from misguided initial qualitative

assessments of likely structural behaviour that then determine the form and extent of

the structural model used in the calculation. However, even with carefully considered

structural models and with the most diligent attention to detail, differences of up to 15%

between calculated and physically determined values are common. These differences can

be expected. The structural model will normally be based on assumed material properties

and idealisations of the connectivity between structural elements comprising the grandstandstructure together with assumptions on how much of the structure needs to be modelled. In

addition, there will be uncertainties in the contribution of mass and stiffness from the non-

structural elements. In contrast to this, the as-built structure responds to excitation, either

in a test environment or due to crowd loading, according to how it is actually constructed

and maintained.

The possible discrepancy between values of dynamic properties used in calculation and

those found by testing may be sufficient to affect an assessment of performance based on the

Route 2 method or a determination of the appropriate category of use by the Route 1 method.

The uncertainty attached to using values obtained by calculation alone can be minimised

by physical testing of the structure while empty of people; either as part of the hand-overprocedures for new or modified structures or as part of a subsequent assessment.

4.2 Structural modelling to determine modal properties of the emptygrandstandAlthough any form of linear elastic dynamic analysis with linear damping may be used,

modal analysis using a finite element representation of the grandstand structure is likely

to be the preferred method except for all but the simplest structures. Appendix 5 provides

details of the theory underlying the method and notes on modelling grandstand structures

for calculation of natural frequencies are given in Appendix 3. These notes indicate

aspects where particular care needs to be taken in developing the finite element model so

that all factors influencing the dynamic characteristics of the grandstand are adequatelyrepresented.

The structure should be analysed to determine the natural frequencies, mode

shapes and related modal masses in the absence of people, but including the mass of all

fixtures and fittings that are involved in motion of the structure. The modes that indicate

significant motion of the seating deck, and are capable of being excited by people on the

deck, need to be identified, together with their natural frequencies. The lowest of these

natural frequencies is taken to be the natural frequency for vertical excitation of the empty

grandstand to be used in the Route 1 design or assessment. (See Section 4.4 on Relevant

Natural Frequency). A Route 2 estimate of performance should be based on all the modes

that are considered to be capable of providing significant motion of the seating deck.




4.3 Values for initial designEngineers undertaking the design of new structures should be aware of the potential

difference between natural frequencies calculated using even the best practice and the

values that will obtain in the completed structure. In design, it will be prudent to allow

for a minimum plus or minus 0.5 Hz variation between calculated values of natural

frequency and those that will influence performance in the actual structure. Variations of

this magnitude are not unusual in comparing calculated and measured values of natural

frequency and can have significant effect on estimates of performance.

4.4 ‘Relevant’ natural frequencyDesign or assessment using the Route 1 approach is determined by a single value of natural

frequency. It is important to note that this is the relevant natural frequency that corresponds

to the mode of vibration with the lowest natural frequency at which people can excite the

seating deck and feel its motion.

It should be noted that the relevant natural frequency may not be the lowest natural

frequency that is determined by testing or analysis. Testing might show that a low frequency

mode exists but this needs to be assessed to determine whether a crowd on the seating deck

can excite the mode significantly.

As an example, consider a roof that vibrates with large amplitude vibrations at low

frequency when excited by wind action. If the roof and the seating deck are connected

so that there is a ‘vibration path’ between them, the effect of the ‘roof mode’ might be

found in tests on the seating deck even though the amplitude of the deck displacement is

small compared to that of the roof. In such cases, decisions on whether a particular mode

is, or is not, ‘relevant’ are better based if the values of natural frequency from testing are

supplemented by additional data on other modal properties. Also, inclusion of ambient

testing (See Appendix 4) in a test programme would provide information that could help to

identify modes driven primarily by sources separate from a crowd on the seating deck.

Testing may also reveal the occurrence of so called global modes of vibration inwhich the whole grandstand, concourses, seating deck and roof may be involved in front-

to-back, sway or torsional motion. Again the ‘relevant’ natural frequency is the lowest

frequency corresponding to those modes by which people can excite the seating deck

through vertical movement and feel its motion.

In some global mode cases, such as with front–to-back ‘nodding’ modes of upper

cantilevers due to flexibility in the main supporting structure, it may not be possible to

decide, from the results of testing alone, which is the appropriate relevant natural frequency

on which to base an assessment or indeed whether it is possible that several modes might

be simultaneously excited by crowd action. In such cases, it will be necessary to use the

Route 2 approach and supplement the assessment with calculations of performance using

more than one mode of vibration.






The tests are to be undertaken on the grandstand empty of people but fully fitted out with

seating and services as would be in place during operation.

In planning testing, it will be convenient for the Client, and also good practice, to

include a programme of testing in the acceptance procedures for new and substantially

modified grandstands before handover to the Owners or Managers. The requirements for

testing should be linked to the specification of the stand and the Design Event Scenario

used in design. (See Section 6.5 on Handover procedures for new and structurally modified

structures).




6 Management responsibilities

6.1 Overall responsibilityThe safety of people using viewing facilities such as grandstands is the responsibility of

the Owners and Managers of the facility.

6.2 Design of new standsWith a new grandstand, it is the Client/Management’s responsibility to make sure, from

the outset, that the Design Team includes a Listed Engineer who will have particular

responsibility for advising on dynamic performance. Management’s responsibility then

continues through subsequent discussions with the Design Team in which the use of

the structure, and how it will be operated, is discussed and agreed. Whilst working as a

member of the Design Team, the Listed Engineer also reports directly to the Management

on matters affecting dynamic performance, including any implications that might arise

from changes proposed following ‘value engineering’ to meet budget constraints.

In principle, all grandstands could be required to be designed to provide both safetyand comfort for all possible uses including those likely to produce the most severe dynamic

crowd loading that can be envisaged. In many cases, depending on the use of the stands,

such designs would not be practical or economic. They would have very high initial cost

and would provide levels of performance that might never be needed in the life of the

structure.

The approach recommended here is for Management to be fully involved in

developing operational strategies to be used with the selected Design Event Scenarios

that become the agreed basis for design for dynamic crowd loading. Besides the physical

characteristics of the grandstand, overall dimensions and capacity, each Scenario should

include the descriptions of the crowd and levels of activity to be used by the Design Teamin estimating performance together with any crowd control measures that are required to

be implemented by Management. It is important that the Management understands its role

in setting this agenda for design and the subsequent continuing responsibility to make

sure that the control measures anticipated at the design stage are made effective during

operation of the stand.

6.3 Change of use and assessment for specific eventsManagement has the responsibility to engage a Listed Engineer and any necessary support,

to form an Assessment Team when changes of use of a grandstand are considered that

could involve the potential for increased dynamic crowd loading. Here the team is led by

the Listed Engineer who reports directly to Management.

Management should accept that the Listed Engineer is engaged to provide an

objective assessment and not necessarily to approve any proposed arrangement. The

Listed Engineer will assess schemes that are proposed and, where this may be helpful,

propose additional measures to mitigate dynamic crowd action and its effects. However,

there could be situations where the Listed Engineer’s advice is that a grandstand should

not be used for certain types of events.

It is important that Management should bear in mind the Design Event Scenario

already used in design or established in a prior assessment and seek the Listed Engineer’s

advice, before proceeding to schedule an event and book a particular Group or Performer.

It will be evident that assessment for specific events should not be overshadowed by

contractual arrangements in which a Music Group or Performer has already been engaged




to appear. In such circumstances, the Listed Engineer should make sure that the terms

of engagement preclude any liability to meet the costs of a cancellation that might be

considered a consequence of the Listed Engineer’s advice.

6.4 Operational strategies to reduce dynamic response and crowd alarmThe following are examples of management strategies that can be used to reduce the effects

of dynamic crowd loading.

• Netting off. Part of a stand, usually the front rows of a cantilever where dynamicloading has the most severe consequences, can be made inaccessible to spectators

by ‘netting off’ the relevant rows.

• No standing areas. Areas can be designated where spectators are required to

remain seated. This would avoid the occurrence of most severe forms of dynamic

crowd loading emanating from that area. This strategy requires practical stewarding

issues to be addressed because dynamic response could build up quickly were a

group of spectators suddenly to become active. For Lively Concerts or Pop-concerts

(Design Event Scenarios 3 and 4 in Table 1), the area of designated seating should

have specific ticketing arrangements and the seated people should have an adequate

view of the stage so that any inclination to rise for a better view is eliminated. Thesight lines for designated areas should be assessed on the basis that people in the

remainder of the stand may be standing, bobbing or jumping. People in designated

seating areas should be advised that they are likely to feel motion of the stand.

• Temporary supports. If demonstrated by appropriate calculations or past

performance, temporary supports (or props) can be used to modify the dynamic

behaviour of the grandstand for specific events. Ideally, the potential use of

temporary supports should be considered at the scheme design stage when it will be

easier to arrange for the supports to be associated with an adequate load path.

• Curtailment of music. Arrangements can be made to cut the visual and/or audio

stimulus to crowd behaviour if the structural response approaches an unacceptablelevel. This strategy provides an ultimate safeguard but there are difficulties in

implementation. Instrumentation is needed to monitor acceleration of the seating

deck. The build up of response can be very rapid so that there is a need for a fixed

response level for curtailment rather than rely on ad hoc decision making. However,

automatic curtailment could be triggered by a single event or sharp transients that have

little relevance to the overall reaction of the crowd. One approach used in practice

employs a traffic light system on stage with green indicating safe operation, amber

showing caution and red indicating danger which should be followed by cut-off.

• Advice to ticket holders. Experience has shown that it is beneficial to advise the

audience that,

a) during a concert, they may become aware of movement of the stand,

b) some structural movement due to crowd action is expected and considered in

the design, and

c) the stand is designed and operated to be safe and not fail structurally even

under extreme movement.

Experience suggests that advance information of this kind provides reassurance,

minimises the likelihood of complaints and reduces a tendency to panic.




6.5 Handover of new and structurally modified grandstandsWhere testing is included in the specified handover procedures for a new or significantly

structurally modified grandstand, Management, representing the Client, should require the

Listed Engineer, in association with the Practice responsible for the design, to:

• Measure dynamic properties to determine the relevant vertical natural frequencies

and their associated mode shapes and damping values for all relevant areas of seating

in the completed structure. These areas of seating are likely to include long back

spans and cantilevers on long props to the rear of large seating decks as well as frontcantilevers.

• Review the test results and corresponding values calculated using the analytical

model. If necessary, refine the analytical model and compare test and recalculated

results. If not already fully allowed for in the calculation of the dynamic properties,

consider the effects of the actual values of material properties in the as-built structure,

actual connection stiffnesses, foundation stiffness and other boundary conditions used

in the analysis (such as the interface with the roof).

• Report on the results of testing and calculation of dynamic properties.

•

For seating decks designed using the Route 1 method, advise Management on the permissible range of use as given in Figure 1. Alternatively, reassess the design in

the light of the test results using the Route 2 method and advise Management on the

appropriate Design Event Scenario and associated management requirements.

• For seating decks designed using the Route 2 approach, confirm that the measured

dynamic properties are consistent with the performance calculated to meet, or

improve on, the specified Design Event Scenario. Alternatively reassess the calculated

performance in the light of the measured properties and advise Management

accordingly.

The test results and results of calculations should be fully documented and included in aninterpretive report to be included in the grandstand Operations Manual together with detailed

structural drawings of the grandstand.

Management should review the report and recommendations provided with the Listed

Engineer/Design Team and agree any changes necessary to achieve acceptable performance.

These should be documented and included in the Operations Manual.

It should be noted that it is the responsibility of Management, acting for the Client,

to make sure that consequences of the measured dynamic performance and any changes

in the Design Event Scenario and associated management requirements are communicated

to the Local Authority safety advisory group and all operational personnel including those

planning events, police and other emergency services, safety managers, ground staff andstewards as necessary.

6.6 Operations ManualManagement should not accept a new or structurally modified stand without being provided

with the Operations Manual that has been prepared for the stand by the principal contractor

with assistance from the designers where required.

The following should be included in the Operations Manual:

• A summary of the dynamic response characteristics including lowest relevant vertical

frequencies, associated mode shapes and damping for each significant area of seating

in the structure incorporating the stand.




• A description of the Design Event Scenarios considered and the anticipated response

of the structure to these together with any crowd management measures required to

be implemented by Management.

• A record of the Engineering Practice and Listed Engineer responsible for the

static analysis of the structure and of the dynamic analysis of the structure and

the Test Agency responsible for the measurement of dynamic performance. The

record should include similar records for any separate sub-analyses of the dynamic

performance of elements within the structure carried out by others such as for precast concrete seating units.

The Operations Manual should be up-dated following changes in operational procedures

and/or structural modifications.

6.7 OperationThe assessment of the dynamic performance of a grandstand should be seen as part of the

broader risk assessment dealing with the event, the venue and the crowd and the implications

for safe management.

In relation to the structure itself, it is accepted that, with appropriate managementand controls, many existing stands can be operated safely even though the relevant natural

frequency does not satisfy the Route 1 requirements. However, if a stand is to be used in

these circumstances, it is important that control measures adopted are based on the following

principles.

• An existing agreed and recorded Design Event Scenario which should include details

of any measures adopted to reduce dynamic response. (See Section 6.4).

• Sufficient knowledge of the relevant dynamic properties of the structure and the

behaviour of the stand under earlier, and possibly less severe, conditions of crowd

loading.

• Use of a Listed Engineer for detailed assessment and direction on measures needed to

implement Route 2 Recommendations.

In addition, Management should be aware that both design and assessment for dynamic

crowd loading involves uncertainties, particularly in the make up of a crowd at any particular

event and the level of excitation that the crowd provides to the structure. Because of this, the

crowd management requirements included in the Design Event Scenario should be seen as

good guidance based on the best knowledge available but subject to review following each

event.

It is within the responsibilities of Management to build a knowledge base concerning

the performance of its grandstands so that management controls can be revised if this is

necessary to maintain adequate safety levels. Fine tuning of crowd control measures can be

expected following the first use of a stand for different types of events.

The knowledge base should typically contain:

• A detailed description of each event.

• CCTV records of crowd activity preferably synchronised with an audio record.

• Audio tapes from concerts; particularly pop-concerts and high energy events.




• Stewards’ reports noting any signs of structural motion and extremes of crowd

behaviour including signs of panic. – a standard report form is available and can be

downloaded from www.istructe.org/technical/db/277.asp

• Records of complaints, relating to perception of structural motion, from members of

the public attending events. (See Section 8.3 on individuals’ tolerance of motion).

These operational records should be available for reference by the Listed Engineer when

employed to advise on modifications to existing procedures or structural modifications.

Management to monitor

use of incidental musicand maintain records ofaudience feedbackincluding complaints

For seating areas with

f 0 < 3, resonancecould occur with thefirst harmonic of thecrowd loading withconsequent large

structural movements

f 0 > 3.5Minimum for new constructionSuitable for viewing sport and other eventswith predominantly seated audiences.

Scenarios 1 and 2

f 0 > 6Suitable for alltypes of eventsScenarios1, 2, 3 and 4

Existing stands with 3 < f 0 < 3.5 may be deemedsatisfactory for sports viewing (i.e. Scenario 1) onthe basis of past experience and use for less livelysections of the crowd

Management

is advised tomonitor thecrowd’sreactions atall events

OnlyScenario

1

Relevant vertical natural frequency of seating deck, f 0 Hz

2 3 4 5 6 7

Figure 1 Route 1 requirements for different categories of use




7 Route 1: Compliance with natural frequency requirements

7.1 Vertical excitationGrandstand seating decks should meet the requirements of relevant natural frequency

for different categories of use as shown in Figure 1.

7.2 Side-to-side horizontal excitationIn contrast to temporary or demountable grandstands, there has been little cause for

concern over the behaviour of existing permanent grandstands subject to horizontal

excitation due to crowds with individuals swaying from side to side or indulging in

a Mexican wave. No problems associated with crowd comfort should be experienced

for grandstands with natural frequencies for horizontal excitation greater than 1.5 Hz.

However, the connection of the seating deck to the principal members and also the

primary structure should be designed to resist the horizontal loads due to side-to-side

motion of a crowd. Accordingly, it is recommended that the stand should be designed

to withstand the additional horizontal loads in Table 2 as part of the design for static

loading.

7.3 Nodding modes due to front-to-back excitationBecause of the rake of a cantilever seating deck, there will be a horizontal component of

displacement as well as a vertical component due to crowd action. This is accentuated

if the structure supporting the cantilever is itself flexible. The result is a nodding mode

encompassing both the cantilever and its support. This behaviour is distinct from side to

side motion due to sway of crowd and is primarily a result of vertical excitation with the

response being magnified due to lack of stiffness in the structure supporting the seating

deck. (See Appendix 1, Section A1.5.2 on acceleration limits).

As in the treatment of side-side horizontal excitation, it is recommended that thestand should be designed to withstand the additional horizontal loads in Table 2 as part

of the design for static loading. The occurrence of nodding modes and their significance

should also be investigated as part of a Route 2 analysis.




Table 2 Design for horizontal strength and stability

To be used with both Route 1 and Route 2 methods of design and assessment

In addition to the operational wind loading, grandstands should be capable ofwithstanding the following lateral loading due to crowd action. The loads should beincorporated in the static design for ultimate load of the structure in combination withother design loads.

Type of use Additional static horizontal load as a

percentage of the specified static liveloading on the seating deck

Side to side Front to back

All grandstands except those used forpop-conc erts or similar lively activity

±5% ±5%

Grandstands to be used for pop-concertsor other lively events

±7½% ±7½%

Notes

i) The loads are specified as a percentage of the specified live loading on theseating deck. Note only loading on the seating deck needs to be considered.

ii) The horizontal load should be applied in the plane of the seating deck in the waythe people are situated according to the available seating.

iii) The horizontal loads due to c rowd action are additional to loadings from othercauses and so should be applied in combination with operational wind loads.

iv) The partial load fac tors to be used in eac h load case should be those specifiedfor live loads in the appropriate Code of Practice for the structural materialinvolved. A partial factor of 1.5 should be used with the given horizontal loads inload c ombinations with factored values of dead and imposed loads.




8 Route 2: Design for managed events

8.1 OutlineThe Route 1 design method is intended to provide a simple check for safety and serviceability.

The approach is based solely on the physical characteristics of the grandstand and so

does not require analysis of the performance of the grandstand under dynamic crowdloading or consideration of particular measures of crowd control that might be adopted by

Management.

The Route 2 method requires the grandstand to be analysed to estimate its performance

under dynamic loadings specified for different classes of activity and size of crowd on

the seating deck as described in the relevant Design Event Scenario incorporating the

management controls. The recommended method of analysis requires consideration to

be given to human structure interaction due to the grandstand acting in combination with

the crowd treated as load generating structural elements. (See Appendix 1 giving the

background to human structure interaction).

The performance of the seating deck is described by the displacements, accelerationsand stress resultants calculated using the specified crowd loading. The grandstand’s

performance should be assessed on the basis of the given requirements for serviceability

and ultimate load capacity.

For all except the most flexible structures, integrity checks for ultimate load

capacity are likely to be relevant only to connections between structural elements. Also,

displacements can be expected to be within acceptable limits provided acceleration limits

related to tolerance of motion are not exceeded.

8.2 Idealised description of crowd activity

People attending an event do not sit or stand without moving. All individuals in a crowdare likely to move, and if music is played, the movements tend to become synchronised

to the beat. The movements may be an involuntary reaction to the external stimulus and

the behaviour of neighbouring people in the crowd. Even at this level, there will be some

dynamic response that will be felt by individuals if the grandstand is unduly flexible.

However, with events such as pop-concerts, there is an expectation of excitement. The

crowd expects to be involved; participation is deliberate and individual motion may

become extreme with foot-stamping, bobbing (sometimes termed bouncing) and possibly

some jumping on the stand, all in time to a beat.

For the purpose of design, sections of a crowd are idealised as being either

predominantly,

• Standing with dynamic properties given in Appendix A2 and generating loading

due to body motion over a wide frequency range including foot stamping, bobbing

and a proportion of jumping (i.e. active), or

• Seated with the dynamic properties given in Appendix A2 and regarded as inactive

(i.e. passive).

If areas of designated seating are used to reduce the overall level of excitation, these must

be agreed with Management and controls exercised as outlined in Section 6.4.


http://slidepdf.com/reader/full/dynamic-performance-requirements-for-permanent-grandstands-subject-to-crowd 26/6318 Dynamic performance requirements for permanent grandstands subject to crowd action

8.3 Serviceability: Tolerance of motionFor the purpose of design, the maximum root mean square (RMS) acceleration of the

seating deck is used as a measure of what is felt by people in the stand.

Maximum RMS acceleration limits are given in Table 1 for the different Design

Event Scenarios and are to be used with the loadings recommended in Appendix 2 for

Route 2 calculations of performance related to design or assessment.

The design acceleration limit for Scenario 4 is regarded as the maximum that can be

tolerated without the prospect of panic by individuals in a crowd.

For Scenarios 2 and 3, the level of comfort to be provided is essentially a matter for

the grandstand Management. The recommended crowd loading is weighted according to

the expected occurrence of song frequencies over a number of events and the acceleration

limits reflect the situation that seated people will be more sensitive to motion than people

who are standing or moving in time to a rhythmic stimulus.

The tolerance of motion varies between individuals and will not be the same for

different style events. Design according to a given Event Scenario will not necessarily

guarantee that all individuals will react with the same degree of satisfaction to the level of

comfort provided or that there will be a total absence of complaints.More stringent requirements may be appropriate for some sections of a grandstand

or seating deck such as catering areas and hospitality suites where expectations for comfort

and operational convenience will be at a premium.

8.4 Serviceability: Displacement limitsThe maximum dynamic component of displacement due to crowd loading should not

exceed 7mm RMS.

8.5 Ultimate load capacity

No dynamic loading check is required of ultimate load capacity for grandstands designedand managed for Scenarios 1 or 2 since the ultimate load capacity required by current UK

standards for dead and imposed loading for human occupancy, combined with the additional

horizontal imposed loads specified in Table 2, will always be sufficient. The same is true

for grandstands designed and managed for Scenario 3 or 4 other than those where motion

of the deck is significantly affected by a global mode involving the supporting structure

for which a separate check may be advisable.

8.6 FatigueFor stadia with frequent use for Scenario 3 and 4 events, the possibility of fatigue damage

may need to be considered. The precise use of a stand over time can seldom be anticipated

but an initial approximate indication of the potential for fatigue can be based on 20 minutes

exposure per concert to Scenario 3 loading. This estimate should be revised as the pattern

of use develops over time or in response to observed behaviour.




9 Analysis of dynamic performance

9.1 Impulse loadsNo recommendations are made for single sharp increases in load due to sudden

movements of members of a crowd as might occur when a goal is scored at a football

match. Such events may cause motion that can be felt by members of a crowd but arenormally of short duration and so of little significance for crowd comfort. Of much

more importance is periodic loading induced by crowd activity coordinated to coincide

approximately with a given frequency, possibly promoted by an external stimulus such

as a musical beat. In this context, the loading may be more or less severe depending

on the freedom of movement of people within the crowd and whether they are seated,

standing, bobbing or jumping.

9.2 Horizontal loads due to periodic excitationUntil more complete information becomes available, horizontal loads due to crowd action

should be treated in the same way as in the Route 1 method so that Grandstands should

be designed to resist the equivalent static loads given in Table 2. (See also Sections 7.2and 7.3).

It is hoped that, in time, sufficient information will become available for horizontal

actions to be dealt with using a human structure interaction based dynamic analysis as is

recommended for vertical loading.

9.3 Analysis for vertical periodic excitationLinear elastic dynamic analysis with linear damping should be used. In practice, except for

the simplest structures and crowd configurations, this will involve using modal analysis

and a finite element representation of the structural system. Additionally, the effects of

human structure interaction need to be included in the analytical process whatever methodis adopted. (See Appendices 1 and 2).

Except where more precise information is available from testing the as-built structure,

damping should be taken to be 2% critical for each mode of vibration considered.

9.4 Human structure interactionRecent research has shown that people involved in any of the three activities of sitting,

bobbing or jumping interact with moderately flexible structures such as grandstand seating

decks so that the contact forces actually experienced at resonance are significantly different

from those measured in tests on stiff or rigid force plates. The existence of these effects does

not depend on the motion of the structure being extravagantly large but only on the relativestiffness and damping properties of the crowd and the supporting structure. If the effects

due to human structure interaction are ignored in calculations, the response of the structure

will be incorrectly represented in the analysis. For most practically designed grandstands,

accelerations calculated ignoring human structure interaction will be significantly higher

than those calculated taking into account the interaction effects.

The recommended analytical method for treating human structure interaction is

given in the Appendices. This approach has been developed, using the most recent research

and experimental data available, with the aim of reproducing the patterns of behaviour

observed in actual structures subject to dynamic crowd loading. The method cannot deal

with all the variations in human behaviour and physical characteristics that affect theway in which individual people and crowds interact with a structure. Because of this,

the results of analysis should be regarded as indicative of actual behaviour rather than a




precise prediction of performance for a particular event. More positively, the recommended

methods provide a consistent approach to design and assessment that takes account of the

major factors that determine the dynamic behaviour of a stand.

The Appendices also provide the necessary input concerning loads appropriate to

the different Design Event Scenarios together with notes on testing, calculation of natural

frequencies and analysis.




10 Use of the Recommendations

The Recommendations are addressed to both Management and the Design Team. The

Recommendations provide a guide to operational management and a tool for the analysis,

design and assessment of grandstand seating decks for dynamic crowd loading. The

Recommendations have been prepared on the basis that these are related functions with

Management’s role in operating a grandstand having importance similar to that of theDesign Team that provides the details for the stand’s construction.

The Recommendations represent the most considered view now available for the

treatment of dynamic crowd loading on seating decks of permanent grandstands. This is

now an area for continuing research and it is likely that the detailed recommendations for

analysis will be refined over time. It would be helpful if users of the Recommendations,

and researchers, who have relevant material to contribute, would provide the Institution of

Structural Engineers with comment so that the Recommendations can, from time to time,

be refreshed in the light of experience and new knowledge.




Appendix 1 Background to human structure interaction

A1.1 IntroductionExplicit treatment of human structure interaction has not been previously considered

in design guidance for active crowd loading. In the past, it has been assumed that the

load induced by an individual, or a crowd, is an externally applied load unaffectedby the motion of the structure. This assumption led to loads obtained in tests using

people bobbing or jumping on relatively stiff structural elements being presented in

recommendations as being appropriate for all situations. For grandstands with dense

crowd loading and natural frequencies typically less than 7Hz, this approach gives

insufficient consideration to the nature of the loading – due to an individual or a crowd

– or to the effects of the mechanical interaction between individuals and the structure.

These aspects have been recently addressed (Dougill et al., 2006) using a simple structural

model for the active crowd that interacts with the structure during motion. Laboratory

based studies have demonstrated the significance of this interaction for a range of support

natural frequencies, loading and excitation relevant to grandstands (Yao et al., 2004 and

2006). Also, use of the theoretical model (Pavic and Reynolds, 2008), and independentlyderived loading data (Parkhouse and Ewins, 2006), has allowed the performance of

actual grandstands to be calculated and compared satisfactorily with observed data from

stands in service (Pavic and Reynolds, 2008). Both the laboratory tests and full-scale

studies have shown that, for practically designed grandstands and dense crowd loading,

it is necessary to take account of crowd-structure interaction if the structural response

near resonance is not to be significantly overestimated.

A1.2 Basic principlesA1.2.1 Modelling human structure interaction

In calculating the relevant dynamic properties of the empty seating deck, a linear elasticrepresentation of the structure will have already been developed, for example using

a finite element model. In order to represent the effects of crowd loading, this basic

structural model is supplemented by additional elements representing groups of people.

These crowd elements, or body units, are spring-mass-damper systems, as shown in

Figure A.1.1, each energised by an actuator represented by the forces P(t ) that cause

dilation of the crowd or body unit by means of a pair of equal internal forces applied

in opposite directions. The properties of the crowd units depend on whether people are

predominantly standing or sitting whilst the forces P(t ) relate to the type and intensity

of activity in the crowd. In the context of analysis, the body units, with their associated

forcing functions, replace the forces that have commonly been prescribed in design

guidance to represent crowd loading. Use of the body units provides a model of real-lifecrowd loading in which motion of the combined structure/crowd system is caused solely

by forces generated within the system itself.

The motion of each crowd body unit is determined by the relative displacement of

the body mass with respect to the point at which the unit is in contact with the structure. As

a result, each crowd body unit introduces an additional unknown degree of freedom into

the structural system associated with the mass of the crowd body unit.

The forces, P(t ), are taken to be periodic. For practical periodic crowd loading, the

function can be represented by the first three harmonic components. It follows that the

combined structure/crowd system can be analysed using conventional methods of linear

dynamic analysis to determine the separate responses due to each load harmonic. The

separate responses can then be combined to obtain the total response to crowd loading.




Dougill, Wright, Parkhouse and Harrison (2006) provide the governing equations for a

system comprising a single degree of freedom structure energised by a single body unit.Formal solutions are given for harmonic loading – P(t ) being either a sine or cosine

function of time – together with examples of the resulting behaviour of the combined

system. In practice this would correspond to the situation when the unoccupied structure

has a single dominant mode and so can be taken as a single degree of freedom system for

the purpose of dynamic analysis. If the crowd is relatively homogeneous, it can be defined

by a single crowd body unit resulting in a combined two degree of freedom (2DOF) system,

as described in A1.5.1.

A1.2.2 Active and passive behaviourInteraction between a moving structure and a crowd occurs for both active people, who

cause the structure to move by their own efforts through the driving forces P(t ), and passivepeople, who do nothing themselves to cause motion of the structure and for whom P(t )=0.

Besides the difference in loading, the dynamic properties of the body units also differ

between those representing active and passive people.

The recommended standard Design Event Scenarios (given in Table 1) are based on

limiting conditions for various types of events in which everyone is considered to be active

so that there is no passive contribution.

If designated seating areas are specified in the design or as an agreed operational

strategy prior to assessment (as described in Section 6.4 No standing areas) the relevant

part of the structural model would need to be combined with the appropriate passive

elements in addition to using active elements for those parts of the crowd considered to

cause the structure to move.

A1.3 Application of the theoryA1.3.1 Direc t application of the theoryIn principle, the most straightforward and accurate way to use the human structure

interaction theory is to add crowd body units to the finite element model of the empty

structure and to analyse the combined system. No assumptions need be made, beyond

those involved in determining the loading and body unit properties and ensuring that the

internal damping of the empty structure is suitably modelled, so that the results obtained

are analytically correct for the chosen distribution of body units.

P( t )

Crowd body node withassociated nodal displacement

Body unit with mass,spring stiffness anddamping

Internal force pairdriving the system

Contact force with the structure

Basic structure node withassociated nodal displacement

Figure A1.1 Typical body unit for incorporation into the basic structuralmodel of the supporting structure empty of people




Use of the direct approach would allow a comprehensive analysis of the effect of

a particular crowd on a structure. For example, body units, with different properties and

loadings, could be used to represent different groups of people on the seating deck, or

decks, with different levels of activity including passive behaviour. This amount of local

detail is normally unnecessary in design but enough body units need to be used so that the

motion of the crowd is adequately represented.

A1.3.2 Approximate analysis using an assumed mode shape for the

crowd’s motionAn approximate solution can be obtained using an assumed ‘mode shape’ for the

displacement of the crowd (and its associated body units) for each mode of the empty

structure that is being considered for the seating deck or decks. The modal masses,

stiffnesses and internal drivers, P(t ), can then be found using these assumed mode shapes.

The structural response then follows through the usual processes of modal analysis using a

two degree of freedom model for each relevant mode of the empty structure.

The most convenient approach is to assume that the body units adopt mode shapes

that are identical to those of the empty structure in their vicinity. This assumption appears

to be borne out by observations on cantilever decks and can be shown to be exact for

simple structures with constant section properties, fully occupied by a uniform crowd witheach part of the crowd having the same properties. This has led to successful correlation

between calculated and observed motion for plate-like cantilever grandstands (Pavic and

Reynolds, 2008).

A1.4 Body Unit properties and loadingsRecommendations for the body unit properties and loadings appropriate to the different

Design Event Scenarios are given in Appendix 2.

A1.5 Analysis and results

A1.5.1 Modal analysisThe most usual form of analysis will be using modal decomposition. In essence this allows

the separate contributions of each mode to the overall response to be analysed separately

and then combined using the principle of superposition. The process involves identifying

the modes of the unoccupied structure that contribute to the motion of the deck, together

with their associated mode shapes, and then calculating the relevant modal properties for

a two degree of freedom (2DOF) crowd-structure system that can be derived for each

mode using the mode shape of the unoccupied structure and the distribution of mass and

dynamic loading within the crowd-structure system. (See Appendix 3). For a structure, fully

occupied with an active uniform crowd, this leads to a pair of equations corresponding to

each mode that describes the motion of the structure itself. These can then be solved to

obtain the body unit’s and structure’s response as they relate to that particular mode. Aformal solution of these equations for periodic loading is available (Dougill et al., 2006)

that can be used to benchmark numerical methods used in analysis

A1.5.2 Root mean square (RMS) accelerations and acceleration limitsBoth BS 6841(1987) and ISO2631-1-(1997) use Root Mean Square (RMS) acceleration as

the indicator for assessing tolerance of motion. For any function x(t ) the RMS value over

the interval of time T is given by,

RMS , , x t T T

x dt 1

t

t

1/ T

2

2

=

+

^ h

> H #

(A.1.1)




When monitoring actual movements, the underlying periodic motion due to crowd activity

is accompanied by irregular events. The RMS value therefore varies with time and depends

somewhat on the choice of the interval T . A value of 10 seconds is frequently used leading

to ‘10 second rolling RMS’ values over the period of observation.

Calculations used in design based on periodic motion using the harmonic loading

described in Appendix 2 lead to accelerations that are smooth functions of time. T is taken

to be the excitation period for the harmonic considered leading to RMS values that are

independent of time. For a single harmonic with frequency f and amplitude a, such that x = a cos(2π ft ), the RMS value of the acceleration x is then a/√2.

The BS and ISO Standards additionally weight measured RMS acceleration according

to excitation frequency. This has not been considered appropriate for the values calculated

for the range of frequencies encountered with crowd motion. Accordingly, the acceleration

limits in Table 1 of the Recommendations use RMS accelerations calculated for periodic

motion and without frequency weighting.

For most situations it will be sufficient to consider only the vertical component

of acceleration in meeting the limits for acceleration given in Table 1. However, with

‘front-to back’ nodding modes (See Section 7.3) there is the prospect that the horizontal

component of acceleration will be significant even with only vertical excitation. The checkfor acceptable motion should then be based on the limits in Table 1 and the vector sum of

the calculated vertical and horizontal RMS accelerations.

A1.5.3 Analysis with a dominant mode.In general the analysis should cover all modes that are considered to be capable of

providing significant motion of the seating deck. (See Section 4.2). However, in some

situations there will be a dominant mode so that the response of the crowd/structure

system can be determined from a single pair of equations corresponding to the dominant

mode. In general, three loading harmonics will be considered corresponding to excitation

at the activity frequency, f , and the resulting higher harmonics with frequencies 2 f and 3 f .The acceptance criteria in the Recommendations are expressed in terms of the maximum

permitted Root Mean Square, RMS, accelerations of the structure (see Section 2, Table

1) so that the combined effect from the three harmonics must be determined. This can be

done for any given activity frequency from,

R R R RT 1

2

2

2

3

2

= + + (A1.2)

where R1, R

2 and R

3 are the RMS values for the response (acceleration or displacement

as required) due to the 1st , 2nd and third harmonics of the activity frequency and RT is the

RMS value of the total response at that frequency.

In doing the analysis, it should be recognised that the maximum response will not

necessarily occur at the natural frequency of the unoccupied structure or even at a natural

frequency of the combined crowd/structure system due to frequency dependence of the

specified body loading. Accordingly, the analysis will need to include a frequency scan,

with results being obtained over a range of closely spaced frequencies, in order to identify

the maximum response.

A1.5.4 Multi-mode analysisIn general, more than one mode will need to be considered. Again a frequency scan should

be used to determine the RMS responses for a range of frequencies for each loading

harmonic and each mode. The results from the separate modes can be combined in asimilar manner to that indicated in equation A1.1 with R

2

1 , for instance, being the sum




of squares of the RMS contributions to the first harmonic response from all the modes

considered.

A1.6 References

Yao, S., Wright, J.R., Pavic, A. and Reynolds, P. ‘Experimental study of human-induced

dynamic forces due to bouncing on a perceptibly moving structure’, Canadian J. Civil

Engineering, 31(6), 2004, pp1109-1118.

Yao, S., Wright, J. R., Pavic, A. and Reynolds, P. ‘Experimental study of human-induceddynamic forces due to jumping on a perceptibly moving structure’, J. Sound & Vibration,

296, 2006, pp150-165.

Parkhouse, J.G. and Ewins, D.J. ‘Crowd induced rhythmic loading’, Proc. ICE, Structures

and Buildings, 159(SB5), Oct 2006, pp247-259.

Dougill, J.W., Wright, J.R., Parkhouse, J.G. and Harrison, R.E. ‘Human structure interaction

during rhythmic bobbing’, The Structural Engineer, 84(22), 21 Nov 2006, pp32–39.

Pavic, A. and Reynolds, P. ‘Experimental verification of novel 3DOF model of grandstand

crowd-structure dynamic interaction’. 26 th International modal analysis conference:

IMAC-XXVI, Orlando, Florida, 4-7 Feb 2008, paper 257.






Table A2.1. Recommended properties of crowd body elements

Crowd Event scenario Naturalfrequency

Hz

Dampingpercentcritical

Designated and controlled‘no-standing’ areas

See RecommendationsSection 6.4

5 40

Predominantly

seated

Scenario 2 5 40

Active and mostly standing Event Scenarios 3 and 4 2.3 25

The body spring stiffness, k , is found from the body mass, m, and the natural

frequency, n, as given in Table A2.1, from,

k n m4 2 2r= (A2.1)

The linking member with the basic structure marked in Figure A2.1 as containing the

common node with the structure can be regarded as rigid but with no mass. The concentrated

mass, m, in the element should be calculated from the number of people in the area ofseating deck that affects the node of the structural model to which the element is attached,

using an average person mass of 80kg.

A2.3 Representation of periodic loadingThe internal periodic force pair P(t ) in figure A2.1 is described as the sum of by three

harmonic components as follows,

(A2.2)

wheret is the crowd effectiveness factor that reflects design criteria driven

primarily by serviceability with commonly occurring events for Scenarios

2 and 3, or mitigation of the potential for panic under extreme motion

considered in Scenario 4.

m is the mass of the crowd associated with the particular body element

considered. This is to be taken as 80 kg times the number of people.

g is acceleration due to gravity 9.81 m/s2.

Gi

is the ith ‘generated load factor’ GLF defining the load generated by

activity of the crowd.

f is the fundamental frequency of the crowd activity in Hz. This

corresponds to the musical beat (or frequency of an alternative prompt) in

beats per second.

t is the time in seconds.

ii

is the phase difference of the ith harmonic. These phase differences can be

set to zero in calculations if only RMS values of force, displacement oracceleration are required.




The Generated Load Factors, GLFs, replace the Dynamic Load Factors used in analyses

that ignore the effects of human structure interaction. The differences in response are small

when the crowd is sparse (that is for low crowd mass to structure mass ratios) and for

structures with natural frequencies that are high compared with those of the crowd body

elements. In other situations, human structure interaction needs to be considered.

The principal assumption in developing the GLFs from available data is that the internal

forces P( f,t ) used by groups of people to move on a flexible platform will be the same as

would be involved when undertaking the same activity on a rigid base. However, crowdsengage in different types of activity and most research has been concerned with jumping

that can be related more to aerobics and vigorous dancing than to crowd behaviour in stadia

where people react to music, over a wide frequency range, with handclapping, stamping,

bobbing and occasional jumping. Accordingly, for the purpose of design, the loading for

Scenarios 3 and 4 is idealised and taken to be equivalent to a multiple of loading due to

bobbing by groups of 50 people or more modified by an effectiveness factor. The factor takes

some account of activity that might occur over a wider frequency range than is covered by

the testing and, for the commonly encountered events of Scenarios 2 and 3, also provides a

weighting based on the frequency of occurrence of songs with different tempi.

The basic loads, Gi, (i = 1,2,3) before modification by an effectiveness factor,t, are based on results for bobbing from extensive recent testing by Parkhouse and

Ewins (2006) that also provide data to enable the body unit properties to be determined

(Dougill et al., 2006).

A2.4 Internal ‘drivers’- G i – producing dynamic crowd loading

The recommended values for the Generated Load Factors are given in Table A2.2. These are

derived from the synthesised results for a group of 50 people bobbing on a rigid platform.

The values can be taken as constant for larger groups of people. For smaller groups, there

is a significant increase in Gi with group size, together with increasing variations from

the mean. In modelling a crowd, it is recommended that the crowd is not subdivided into

groups of less than 50 people.

Table A2.2 Recommended values of the Generated Load Factors, G i, for use in

calculations of performance for design or assessment.

ScenarioHarmonic number

Typical activity representedEffectiveness

factori = 1 i = 2 i = 3

1 ------- ------- -------

Not required.

Route 1 only, but with discretionavailable by Listed Engineer(Recommendations Table 1)

Not required.

2 0.12 0.015 Zero

Predominantly seated withoccasional coordinatedrhythmic movement fromstanding people

Eqn. A2.3

3 0.188 0.047 0.013

All crowd considered ac tive.Moderate bobbing at threequarters Parkhouse and Ewins’50 person level

Eqn. A2.3

4 0.375 0.095 0.026

The whole crowd active.Loading taken to be twice thatfor the commonly occurring

events of Scenario 3

Eqn. A2.2




A2.5 The crowd effectiveness factor ‘t’.A2.5.1 Scenario 4Scenario 4 is concerned with high energy events. The principal concern is safety against the

prospect of crowd disturbance, or panic, as a consequence of the motion of the grandstand.

Experience suggests that the probability of such a situation occurring is small. However,

the consequences of panic in a crowd confined in a grandstand with fitted seats could be

dramatic and possibly life threatening. For such an extreme situation it is appropriate to

consider an unrestricted frequency range of possible excitation but with some allowance

for reduced effectiveness of the loading at low and high excitation frequencies. This isprovided by an effectiveness factor shown in Figure A.2.2 and given by the equation,

( ) f f 2secht = -^ h (A2.3)

where f is the fundamental frequency of the crowd’s activity in Hz and the factor is

used in equation A2.2 with the recommended values of Gi in Table A2.2 and the crowd

body unit properties of Table A2.1.

A2.5.2. Scenarios 2 and 3Scenarios 2 and 3 are relevant to less energetic events than those of Scenario 4 so that

the occurrence of panic due to crowd motion can be discounted. It follows that criteria

for structural performance can be set in terms of what is needed to meet a crowd’s

expectations of comfort over a period of time and for many events. This element of

repeated exposure to crowd action and a cumulative experience of comfort relative to

motion of the structure is in direct contrast to conditions for the single extreme event

in which safety is the prime consideration. In order to take account of the continuing

exposure to a variety of songs of different beat frequencies, the effectiveness factor

0

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3 4 5 6

Activity frequency, f , Hz

t( f )

Figure A2.2 Effectiveness factort(f ) for use with Scenario 4 inconsideration of safety




for comfort is weighted, using data from Littler (2003), according to the probability

of occurrence of songs in the overall pop repertoire. The probability distribution is

approximately Normal with mean 1.8 and standard deviation 0.5, so leading to the

effectiveness

e

(A2.4)

The resulting effectiveness factor is shown in Figure A2.3. This covers the beat frequencyrange of commonly occurring songs with a maximum near to that for the most frequently

occurring songs, Littler, (2003).

A2.6 Monitoring and back analysisEvent monitoring and subsequent back analysis of performance is most likely to be

undertaken under conditions similar to Scenario 3. However, in examining a specific

event, a view should be taken of the proportion of the crowd that is actively involved

with the remainder being considered as passive. For most events, the loading from active

people will correspond to that for bobbing. Appropriate values for the internal drivers Gi are given in Table A2.3. These should be used with the crowd body properties for active

and passive crowd body elements in Table A2.1 and the single event effectiveness factor

of equation A2.3. An example of monitoring and back analysis is provided by Pavic and

Reynolds (2008).

0

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3 4 5 6

Activity frequency, f , Hz

t( f )

Figure A2.3 Effec tiveness factor t(f ) for use with Scenarios 2 and 3 inconsideration of comfort




Table A2.3 Suggested values of the Generated Load Factors, Gi, for use in back

analysis of specific events.

CrowdHarmonic Number

Typical activity in identifiedsections of the crowd

EffectivenessFactor

i = 1 i = 2 i = 3

Active 0.25 0.063 0.018 Active crowd, mainly bobbing. Eqn. A2.3

Passive All zero Inactive, standing or seated

A2.7. References

Dougill, J.W., Wright, J.R., Parkhouse, J.G. and Harrison, R.E. ‘Human structure interaction

during rhythmic bobbing’, The Structural Engineer, 84(22), 22 Nov 2006, pp32–39.

Littler, J.D. ‘Frequencies of synchronised human loading from jumping and stamping’,

The Structural Engineer, 81(22), 18 Nov 2003, pp27–35.

Parkhouse, J.G. and Ewins, D.J. ‘Crowd induced rhythmic loading’, Proc. ICE, Structuresand Buildings, 159(SB5), Oct. 2006, pp247-259.

Pavic, A and Reynolds, P. ‘Experimental verification of novel 3DOF model of grandstand

crowd-structure dynamic interaction’, 26 th international modal analysis conference:

IMAC-XXVI, Orlando, Florida, 4-7 Feb 2008, paper 257.




Appendix 3: Calculation of modal properties

A3.1 IntroductionThe Recommendations provide alternative routes to design or assessment of a

grandstand.

Route 1 uses natural frequency as an index of quality for a stand and requiresknowledge of the natural frequencies of the stand and identification of the lowest value

corresponding to a mode that can be excited by and felt by people on the seating deck.

The approach is simple but rendered useless if natural frequencies are not determined

to sufficient accuracy. As outlined in the Recommendations, (Section 4), calculation of

natural frequencies may appear to be a straightforward task but, in reality, is complicated

by uncertainties in setting up the analytical model with almost inevitable differences

between assumptions made for purposes of calculation and the actual behaviour of the

as-built structure.

The Route 2 approach provides more flexibility for the designer by using calculation

to obtain an estimate of performance of a grandstand under prescribed loading appropriateto a given idealised Design Event Scenario. A full dynamic model of the structure is

needed for such calculations and the opportunities for error and miss-match between the

analytical model and real structural behaviour are certainly not less than in calculations of

natural frequencies for Route 1. Clearly, if the calculated dynamic properties are seriously

in error, the resulting estimate of performance will have little relation to the behaviour of

the as-built structure.

To deal with this potentially difficult situation, engineers need to be aware of

the assumptions or simplifications made in analysis and how these affect the result of

dynamic analysis. They should also recognise that even the most careful attention to detail

in analysis cannot guarantee that the analytical model will match the behaviour of thephysical as-built structure. As a general rule, it is advisable to check properties obtained by

calculation by physical testing so that the analytical model can be refined and made more

relevant to the actual structure.

This Appendix deals with calculation of modal properties and aims to point out some

of the more common sources of error. In doing this, it extends and replaces the Advisory

note on calculation of natural frequencies of grandstand seating decks published in The

Structural Engineer, Vol. 81, No.22, November 2003. The need to check calculations by

testing grandstands is dealt with in Section 5 of the Recommendations whilst Appendix A4

provides advice on specification, procurement and reporting.

A3.2 Modal analysis and natural frequenciesWhen a linear elastic structure vibrates under arbitrary loading, it does so in a way that

its deflection shape at every moment in time can be presented as a linear combination

of deformed shapes called mode shapes (Clough and Penzien, 1993). Therefore, for a

system with N degrees of freedom, the vector of N unknown displacement functions xi(t )

can be expressed as:

x t q t r

N

r r

1

z==

] ]g g" ", ,/ (A3.1)

where {zr } is the rth mode shape and qr (t ) is the rth time-dependent scaling factor.




In other words, when calculating the unknown response, xi(t ), equation A3.1 can

be written as:

x t q t ir

r

N

i r

1

z==

] ]g g/ (A3.2)

where zir is the mode shape amplitude of the r th mode at the point and in the direction

of the displacement xi.

The mode shapes {zr } are properties which depend only on the mass and stiffness ofthe structure and do not depend on the dynamic loading, so they do not change with time.

The scaling factors qr (t ), also known as generalised or modal coordinates, are

functions of time and depend on the dynamic loading. The generalised coordinates qr (t )

are solutions of the following modal equations of motion:

,m q t c q t k q t F t r N 1r r r r r r r + + = =p o] ] ] ]g g g g (A3.3)

where mr , is modal mass for the r th mode, c

r is modal damping, k

r is modal stiffness

and F r(t ) is modal force given by:

F t f t ir

1i

i N

r iz==

=

] ]g g/ (A3.4)

and where f i (t ) is the time-varying physical force acting at the point and in the

direction of the displacement xi.

For many practical systems in which mass is modelled using only translationally

moving lumped masses, mi the modal mass m

r can be calculated as:

ir m mi

N

r i

1

==

z2/ (A3.5)

Equation (A3.3) will be recognised as a group of r independent equations. Each describes

a single degree of freedom system having its own distinct displacement mode. In these

terms, each equation requires its own modal properties: mr , c

r and k

r associated with the rth

mode. An equation can be set up for each of N modes of vibration. Therefore, there will be

N natural frequencies ωr:

mk

2

1r

r

r

~r

= (A3.6)

Natural frequencies and the corresponding mode shapes are properties of the structure. All

flexible structures have natural frequencies.

Equation (A3.6) shows that the natural frequencyωr that relates to a particular mode

of vibration depends on terms contributing to the modal stiffness k rof the structure and

terms contributing to the modal mass mr of the material that moves during vibration in a

particular mode.

From Equations (A3.3) and (A3.4), it follows that if the external harmonic force

f i (t ) has a frequency identical or close to the natural frequency ω

s there will be a strong

resonant response qs(t ) of mode s. Equation (A3.1) then suggests that this would cause the

physical response to be dominated by mode s.

Equation (A3.4) also suggests that a mode of vibration will be excited by an external

force only if this acts at a point where the amplitude of the mode shape is non-zero.




A3.3 Basic errors: two prime suspectsA3.3.1 Distinction between force and massEquations (A3.5) and (A3.6) make clear that natural frequency involves mass and

not force.

Accordingly, the input to a dynamic analysis for natural frequency should include all

the mass that would be involved in free vibration. This will include the mass of the basic

structure plus the mass of all the attachments including seating, partition walls or barriers,

hospitality boxes and equipment supported by the seating deck.Note that, in a static analysis to determine forces or bending moments used in strength

calculations, the weight of some of the above elements would be treated as forces acting

on the primary structure rather than as masses that affect dynamic behaviour.

It follows that it is not generally possible to use the analytical model used in a static

analysis by merely adding the dynamic loading. The model will need to be reassessed to

include lumped masses associated with self weight of both the structure and its appendages.

Almost invariably, forces used in static analysis that derive from weight will correspond to

mass to be considered in a dynamic analysis.

It is easy to overlook the contribution of mass due to a non-structural element or,in the as-built structure, additions made during fit-out. If there are significant differences

between calculated results for natural frequency and those obtained from testing, it is

suggested that one of the first tasks in checking should be a ‘mass audit’ to see if all the

mass that moves is included in the analytical model.

A3.3.2 StiffnessThe stiffness of so-called non-structural elements is often ignored in a static analysis for

forces and bending moments. Errors result if this is done in a dynamic analysis for natural

frequency. In particular:

•

The stiffness of a seating deck may be underestimated if the stiffness of wing walls,vomitories, partitions and glazing attached to the deck are ignored.

• The stiffness of a seating deck may be overestimated if a rigid or fixed boundary

condition is assumed at the interface with the remainder of the structure.

• The stiffness of a cantilever deck is very much dependent on the boundary

conditions of the cantilever. Therefore, it depends on the stiffness of the connection

with the structure supporting the cantilever and by the displacements/rotations in the

supporting structure itself at the location of the connection. Clearly, if the access or

main supporting structure for a cantilever grandstand is itself flexible, the motion of

the cantilever could be determined as much by the motion of the support structure

as by the flexibility of the cantilever. Here the need is to check that the analyticalmodel is sufficiently extensive to include the influence of the supporting structure.

It will be recognised also that whole-body movement of the support structure can

occur due to foundation movement or flexible tie-backs and may not be solely

determined by the elements of the main structure.

It will be noted also that the mode shapes obtained from testing can be particularly

informative in guiding revisions of an incomplete analytical model.




A.3.4 Methods for calculation of natural frequenciesA3.4.1 General commentAll methods for calculating natural frequencies fit into one of two groups: the approximate

method suitable for hand calculations (Blevins, 1995), and numerical methods, suitable for

computer applications (NAFEMS, 1992).

The two methods are complementary and should be used in parallel as a cross-check

whenever possible. Rules of thumb provide a useful check rather than a reliable tool for

analysis. Even for simple structures, such rules are unlikely to be sufficiently accurate forassessment or design but can provide a useful check for gross errors in natural frequencies

obtained by numerical, typically finite element, analysis or other more involved approximate

methods.

A3.4.2 Approximate analysisThese methods typically yield the natural frequency of the fundamental mode of vibration

and are based on treating this mode as a single degree of freedom system having an

assumed, rather than calculated, mode shape. Usually the mode shape is assumed to be

identical to the static deflection profile due to self weight applied in the direction of the

required mode (vertical or horizontal).

The approximation can work quite well for simple single-span or cantilever structures.

More complicated structures require a full numerical analysis because of difficulties in

estimating the relevant mode shape.

Typically, the lowest natural frequency in vibration cycles per second (i.e. in Hz)

can be approximated by a non-dimensional constant A divided by the square root of the

deflection under dead loading, D, i.e.

f A

1

D= (A3.7)

For simple beam or cantilever structures, the ‘rule of thumb’ is that A usually lies between15 and 20 when D is expressed in millimetres. More complex structures may have an A

value outside this range.

A3.4.3 Computer based analysisThis is typically done using standard commercial computer software based on finite element

analysis. Natural frequencies are found as a result of the so called modal analysis or eigen-

value extraction resulting in mode shapes and the corresponding natural frequencies.

For stands comprising nominally identical frames, a 2D ‘plane frame’ or a 3D ‘slice

frame’, finite element analysis can be performed taking into account contributory mass

and stiffness corresponding to the frame. If the frames making the stand are not nominallyidentical (e.g. they change width or height, or have generally different mass and stiffness

properties) a full 3D multi-frame analysis is recommended.

Uncertain modelling parameters (e.g. boundary conditions, composite action,

material properties, and effects of cracking) should be studied parametrically to explore

the uncertainty in the calculated values of dynamic properties. If wide variations occur in

the calculated values, the analytical model may need to be revised.

A3.5 Consequences of mistaken idealisationsIt is clear that the engineer should be aware of the effects of the various assumptions made

in calculating natural frequencies and, in analysis, should attempt to represent as closely aspossible the geometry of the entire structure and the effect of the non-structural elements.




In general, assumptions should be made so that the calculated natural frequency will be on

the low side, so providing a conservative estimate for assessment purposes.

Mistaken idealisations may have the effects shown in Table A3.1 on calculated

values of natural frequency.

Table A3.1: Idealisation and its effects on the calculated natural frequency

Idealisation Calculated natural frequency

Neglect of significant mass including neglec t of massof non-structural elements

Too high

Connec tions taken as rigid when flexible Too high

Only part of the structure considered with theremainder taken as motionless and effectively rigid

Usually too high: with prospectof missing a significant mode

Stiffening effects of the ‘not considered part’ of thestructure more important than the effects of its mass

Too low

Wrong/ inappropriate mode shape used inapproximate method

Too high

Neglect of foundation flexibility Too high

Use of too coarse a finite element grid/mesh Potentially too high

Concrete assumed to be uncracked Too high

Neglect of stiffness arising from interaction betweenrakers and seating deck

Too low

Assume perfect supports and connections andneglect of physical slack due to tolerance

Too high

Neglect of stiffness of non-structural elements Too low

A3.6 CommentIt should be recognised that dynamic analysis to obtain modal properties of a real structure,

or to estimate its performance under dynamic loading, is a more challenging task than

checking for strength under essentially static loading.

In calculations for strength, assumptions can be made on structural behaviour

concerning connections, ductility and load redistribution that can be made real in design

through appropriate detailing. Also, high accuracy is not essential provided the assumptions

made are conservative and the resulting structure has a reserve of strength over that called

for in the specification. This is not the situation with dynamic analysis. The structure

behaves linearly and as it is constructed. There are no opportunities for alternative load

paths and the simplifications of behaviour based on ductility, that provide safe routes to

simpler design when considering strength under static loading, are not available.

Experienced engineers recognise these difficulties and how they are compounded

by the task of creating an analytical model that will predict adequately the performance

of a ‘still to be constructed’ grandstand. The analytical process itself is almost routine.

However, the modelling is often likely to be less than precise and open to surprise for all

but the simplest structures. In these circumstances, engineers should normally welcome the

opportunity for an independent check of the modal properties by testing and the additional

insight this can provide. (See Appendix 4).




A3.7 Further information

NAFEMS. A finite element dynamics primer. Glasgow: National Agency for Finite Element

Methods & Standards, 1992.

Clough, R.W. and Penzien, J. Dynamics of Structures. 2nd ed. New York: McGraw-Hill,

1993.

Blevins, R.D. Formulas for natural frequency and mode shape, Malabar, FL: Robert E

Kreiger Publishing Company, 1995.




Appendix 4 Dynamic testing ofgrandstands and seating decks

A4.1 IntroductionThe Recommendations refer to the need to determine or check values of natural frequencies

and other modal properties by testing grandstands as fitted out for use, but empty of people.(See Sections 4 and 5 dealing with dynamic properties and testing).

The Recommendations put particular reliance on Listed Engineers to advise on the

form of testing required. (See Listed Engineers, Section 3.4). This Appendix provides

additional guidance to the Listed Engineer and also to Management, who may require

testing as part of the hand-over procedures for new structures, and for Local Authority

Engineers with responsibilities for Building Control and Safety Certification. The

Appendix extends and replaces the Advisory Note, Dynamic testing of grandstands and

seating decks published by the Institution of Structural Engineers in 2002.

No guidance is given on monitoring grandstands in service. References to monitoring

are given in the Bibliography, (Appendix 5).

A4.2 What should be tested and what results are needed?The form and extent of testing needed will depend on choice of method – Route 1 or

Route 2 – used in design or assessment.

The Route 1 method needs only the value of the relevant natural frequency for

vertical excitation by crowd action to be determined. (See Section 4.4). Here testing must

be concerned with identifying and determining the lowest natural frequency at which

people can excite the seating deck and feel its motion. For simple structures with easily

identified modes of vibration relatively simple test methods can be used.

If Route 2 (based on performance calculations and satisfying a response based

criterion) is used, all the frequencies that are judged to contribute significantly to the

dynamic response need to be measured so that the dynamic model of the grandstand can

be made as accurate as possible. This requires greater coverage from the test programme

with initial calculations from the theoretical model being used to indicate which modes are

important and therefore which frequencies need to be confirmed experimentally.

Broadly, two types of test are available, corresponding to different levels of

information obtained from the tests.

• Type 1 Tests provide basic information concerning the minimum relevant natural

frequency.

• Type 2 Tests provide more detailed information than Type 1 tests that, in principle,

could provide a full modal description comprising natural frequencies, mode shapes,

modal damping ratios and modal masses for all the modes that are considered to be

important in the dynamic performance calculations. Tests of this type may be required

to check the values of modal properties used in a Route 2 analysis of performance or

when the results of Type 1 testing cannot be reconciled with calculated values.

The Listed Engineer will need to specify the type of testing required and which modes

of vibration need to be investigated. In addition, it will normally be useful if the testing

provides additional information on mode shapes to assist in assessing the significance ofany differences between test and calculated results in a Route 2 approach and also the

degree to which any mode is likely to be excited by crowd movement.




The Listed Engineer should also consider where additional information may be

needed. This could occur if the Management had set particular performance specifications

for parts of the structure or if information were needed in the context of a possible upgrading

of the structure or for introducing damping systems to reduce the effects of vibration.

Type 1 testing will normally be sufficient to meet the requirements of the

Route 1 approach to satisfying the Recommendations and so enable the Listed Engineer to

recommend a category of use. Moreover, if Type 1 testing is conducted in such a way that

mode shapes are determined in addition to values of natural frequency, comparisons canbe made with the calculated mode shapes as a check that natural frequencies obtained by

test and calculation are being compared on a like-for-like basis.

Type 2 testing will normally be required if a performance based analysis using the

Route 2 approach is undertaken. Such testing will allow the designer to assess the accuracy

of the natural frequencies and also the mode shapes in the theoretical model. Testing will

also allow the experimentally determined modal damping values to be used directly in the

performance calculations since damping cannot be determined theoretically.

Type 2 testing requires more time on site, more specialist equipment and more

information processing than Type 1 testing. As a consequence, use of Type 2 testing is

likely to cost more than a basic Type 1 test programme. In deciding the form of testingto be used, the additional cost of Type 2 testing needs to be considered in relation to

the increased detail and quality of information that can be obtained and the consequent

increased surety in achievement of safety and better informed management procedures.

To summarise, a Route 1 approach will most often use a Type 1 test with the aim of

determining the lowest relevant natural frequency whereas a Route 2 approach will require

a Type 2 test and aim to determine the natural frequencies, likely to be significant in a

dynamic response calculation, together with their associated modal properties. It should

be noted though that, for some structures, it may not always be possible to identify the

‘relevant’ natural frequency using only Type 1 testing.

A4.3 Analysis and testingWhether following a Route 1 or 2 approach, the Listed Engineer will be concerned that the

values of the relevant natural frequencies have been determined with sufficient accuracy

for an appropriate Design Event Scenario to be selected on the basis of natural frequency

alone or for a performance based analysis to be relevant to behaviour of the as-built

structure. In doing this, and bearing in mind the idealisations made in even a sophisticated

analysis, it should be realised that exact correspondence between measured and calculated

values is extremely unlikely. However, if the difference in results is substantial, the Listed

Engineer could be expected to review the results to check inter alia that:

• the mode shapes corresponding to the relevant natural frequencies are the same forthe calculated results as those found from physical testing,

• the mass and stiffness of the structure, and all the non-structural elements associated

with the grandstand or seating deck, has been properly represented in the dynamic

analysis, and,

• the support conditions, including the continuity and fixity of the elements of the

structure, are appropriately represented in the dynamic analysis.

Such a review could indicate whether the test programme had missed the mode of vibration

corresponding to the minimum natural frequency in a Route 1 approach or a relevant mode

in a Route 2 approach. The review could show whether the analysis should be refined to




include some mass or stiffness that had been ignored in an earlier calculation or extended

to examine the significance of assumptions made in modelling the structure. Depending on

the circumstances, the Listed Engineer might decide that further Type 1 testing is necessary

or that more detailed testing is required and so review the testing specification to provide

for some form of Type 2 testing.

It is recommended that an analysis to give an estimate of natural frequencies and

mode shapes should be undertaken before testing is commissioned or undertaken. With

the Route 2 approach, preliminary dynamic response calculations should be performedto indicate which modes are likely to be significant and the sensitivity to errors on the

frequencies could be explored. In doing this, it will be important to check that the available

drawings properly represent the existing structure with any differences being noted for

future reference. Only by having the results of an analysis available can additional tests

be requested to find a missing mode within a single programme of testing, so avoiding the

Test Agency having to make a second visit to site. Information on the likely mode shape is

also helpful in informing the choice of test points to be used in the test programme.

As noted in the Recommendations, (See Section 4.4 on ‘relevant’ natural frequency

and Section 5.2 on Aims of Testing) it is important to recognise the existence, for some

stadia, of so called ‘global modes’ of vibration. These are typically low frequency modesinvolving motion of the entire grandstand structure in sideways sway, twist or front to back

movement causing ‘nodding’ of the seating decks. Care needs to be taken to identify global

modes in the test process, particularly since some excitation methods (e.g. heel-drop) are

unlikely to provide the energy required to excite such modes.

A4.4 Principles of dynamic testingA variety of excitation methods may be used in dynamic tests on grandstands. These include

the use of ambient excitation due to wind or other disturbance, impact testing by heel-drop

or calibrated hammer or the use of shakers to provide excitation at a given frequency.

Whichever method of testing is used, it is vitally important that the level of excitation

available is sufficient to excite all the modes of interest. Also, the instrumentation mustbe appropriate to record the response of the grandstand with sufficient accuracy to enable

meaningful results to be derived.

Different testing procedures may be adopted depending on the type of excitation

being used, the availability of instrumentation and the amount of detail required in the

results. For instance, testing may be undertaken with accelerometers at a number of fixed

locations with the excitation sources being moved, from test point to test point, in order to

excite different modes and explore the sensitivity of the structure to vibration. Alternatively,

the excitation source can be used in one place and the measuring points changed from test

to test. A combination of these two approaches may also be appropriate. Also, if mode

shapes are to be determined, testing should be performed across a range of test points thatare sufficiently closely spaced that mode shapes are uniquely defined, even when there are

neighbouring modes with similar shape.

In contrast to ambient vibration surveys, heel-drop tests and some forms of measured

impact testing, shakers provide a consistent and reproducible source of excitation. This

means that, besides providing good quality results for Type 1 testing, shakers can be used

for Type 2 testing with the scope, or range of results, being dependent on the experience

of the operators and the particular techniques and instrumentation employed. As with all

forms of excitation, it is important that the shaker provides sufficient energy to excite the

structure at the frequencies of interest.

An overview of the different techniques is given in Table A4.1. A more detailed

assessment of the different excitation options is given in the following section.




T a b

l e A 4 . 1 .

T e c h n i q u e s f o r d y n a m

i c t e s t i n g o f g r a n d s t a n d s a n d s e a t i n g d e c k s

T e s

t c h a r a c t e r i s t i c s

T e s t o u t c o m e s

E s s e n t i a l o u t c o

m e

D e s i r a b l e o u t c o m e A

d d i t i o n a l i n f o r m a t i o n r e l e v a n t

t o R o u t e 2 a n a l y s i s

T e s

t T y p e

E x c i t a t i o n

F

o r c e

m

e a s u r e m e n t

N a t u r a l f r e q u e

n c i e s

M o d e s h a p e s

D

a m p i n g r a t i o

F r e q u e n c y

R e s p o n s e F u

n c t i o n

M o d a l m a s s

T y p

e 1

A m b i e n t

N

o t p o s s i b l e .

Y e s , b u t c a r e n

e e d e d

w i t h i n t e r p r e t a

t i o n .

C a n c o m b i n e

w i t h o t h e r T y p e 1

t e c h n i q u e s t o

a s s i s t

i n t e r p r e t a t i o n .

Y e s , i f e x c i t a t i o n

e n e r g y i s s u f f i c i e n t .

N

o t r e l i a b l e

N o

N o

T y p

e 1

H e e l - d r o p

N

o t n o r m a l l y

d

o n e .

S u i t a b l e f o r s i m

p l e

s t r u c t u r e s . D i f f i c u l t i e s

w i t h c o m p l e x

s t r u c t u r e s o r c l o s e l y

s e p a r a t e d v i b r a t i o n

m o d e s . N o t s u i t a b l e

f o r g l o b a l m o d

e s .

P r o v i d e s c o a r s e

i n d i c a t i o n

s u f f i c i e n t f o r

s i m p l e s t r u c t u r a l

a r r a n g e m e n t s .

N

o t r e l i a b l e

N o

N o

T y p

e 1

D r o p - w e i g h t

o r s l e d g e

h a m m e r

M

e a s u r e d .

Y e s

Y e s

B

e t t e r t h a n

h

e e l - d r o p

P o s s i b l e w i t h

f u r t h e r p r o c e s s i n g

o f m e a s u r e d

d a t a i f e x c i t a t i o n

e n e r g y i s a d e q u a t e .

T y p

e 1 o r 2

a c c o r d i n g t o

t e c

h n i q u e s a n d

i n s t

r u m e n t a t i o n

e m

p l o y e d

S h a k e r w i t h

v a r i e t y o f

p o s s i b l e

t y p e s a n d

t e c h n i q u e s

M

e a s u r e d

o

r i n f e r r e d

d

e p e n d i n g o n

t e c h n i q u e .

Y e s , a n d p r o v i d e s

b e t t e r q u a l i t y r e s u l t s

t h a n h e e l - d r o p

,

i m p a c t o r A V S .

Y e s

Y e s

Q u a l i t y o f r e s u l t s d e p e n d e n t o n

t e c h n i q u e a n d i n s t r u m e n t a t i o n .

M o s t r e l i a b l e

r e s u l t s o b t a i n e d w i t h

i n s t r u m e n t e d

s h a k e r g i v i n g d i r e c t

m e a s u r e m e n t o f f o r c e t i m e h i s t o r y

a n d u s i n g m

u l t i p l e - d e g r e e o f

f r e e d o m c u r

v e f i t t i n g p r o c e d u r e s .

N o t e

T h e

T a b l e p r o v i d e s a n i n i t i a l g u i d e

t o t h e c h o i c e o f t e s t m e t h o d . S e c t i o n A 4 . 5 p r o v i d e s m o r e d e t a i l e d i n f o r m a t i o n o n t h e u s e

o f t h e d i f f e r e n t

m e

t h o d s . H o w e v e r , i t i s i m p o r t a n t

t o d i s c u s s w i t h a p r o s p e c t i v e T

e s t A g e n c y t h e m e t h o d s t h a t m i g h t b e a p p r o p r i a t e f o r a p a r t i c u l a r s i t u a t i o n , h o w

t h e

s e w o u l d b e i m p l e m e n t e d a n d

t h e t y p e a n d q u a l i t y o f r e s u l t s t h a t t h e p a r t i c u l a r T e s t A g e n c

y c a n p r o v i d e f o r a g i v e n m e t h o d a n d p r o g r a m m e

o f t e s t i n g .




A4.5 Excitation sources and testing techniquesA.4.5.1 Ambient vibration survey (AVS)This method relies on ambient excitation – typically wind or passing traffic – to excite the

structure. The response is measured and spectra are calculated to yield vibration property

estimates, either by visual inspection of the spectra or by some form of curve fitting. The

method can yield useful results for natural frequencies and mode shapes, if the ambient

excitation is able to excite the modes of interest adequately, but results for modal damping

can be unreliable.

Care is needed in interpreting the results of an ambient vibration survey so that the

relevant modes are correctly identified. (See Recommendations, Section 4.4 Relevant Natural

Frequency). For example, it would be misguided to focus on ‘roof modes’, which engage

only slightly with the seating deck, rather than those modes that engage the seating deck

strongly and so could be excited significantly by crowd loading. Also, care must be taken to

avoid misinterpreting peaks in the AVS response spectra that are not primarily due to resonant

response but correspond to dominant frequencies in the ambient excitation spectrum such

as might occur due to vortex shedding or the influence of machinery in the vicinity. This

is because the method is based upon the excitation spectrum being flat, or at least smooth,

across the frequency range of interest. Also, as the AVS approach depends on the ability

of the ambient excitation to vibrate the empty stand, its application may be limited whenmeasuring vertical modes of seating decks that are protected from the wind.

Bearing these reservations in mind, AVS should be regarded as a Type 1 test. The

test can be used in isolation but, because of the potential difficulties of interpretation, it

is normally better used in combination with other techniques. However, because of the

difficulties of providing horizontal excitation with significant energy input, it is useful to

recognise that an ambient vibration survey may be the only practicable method of detecting

the presence of global horizontal modes of grandstand vibration.

A.4.5.2 Heel-drop testing

This method is suitable for Type 1 testing of moderate size, simple structural arrangements.Heel-drops should be performed across a grid of test points making sure that all relevant

modes of vibration are excited. The natural frequencies of excited modes of vibration will

usually show as peaks in the spectra in the response to the heel-drop.

The signal received from a heel-drop test usually contains significant noise which

overlays the primary effect of the impact. This leads to a poor signal-to-noise ratio and

spikes in the spectra that may obscure the peaks corresponding to the modes of vibration.

As a consequence, there are difficulties in identifying peaks relating to different modes if

these occur at frequencies that do not differ very much in value.

It is possible to use one or more heel-drop tests to establish mode shapes by measuring

the response across a series of test points. However, if results from several heel-drops are

combined, the results may be too crude to enable useful comparison to be made with

results from analysis because of variations in excitation between heel-drops. It is unlikely

that heel-drop testing by an individual will excite ‘global’ modes involving significant

motion of the whole of a large stand.

A.4.5.3 Measured impact testingSimultaneous measurement of an impact force pulse and the corresponding structural

response would enable the full set of modal properties to be determined. The technique is

commonly used in laboratory testing and testing of smaller structures, but does not appear

to have been used on grandstands where the energy required to excite the structure is muchgreater. It is possible that, for large structures, an instrumented sledge-hammer or a drop




weight and force plate could be used. However, as for a heel-drop, sledge-hammers may

not be sufficiently large to excite the structure sufficiently and drop weight devices may be

difficult to install on a grandstand and could damage the seating deck.

Bearing these difficulties in mind, it is considered that measured impact testing

should be considered as a Type 1 test being, in effect, an upgraded heel-drop test in which

the excitation is both measured and more repeatable.

A4.5.4 Shaker testing of different types and complexityThere is a wide variety of equipment and techniques available using shakers for dynamic

testing. As might be expected, these have been developed furthest in the context of

mechanical and aerospace engineering. The latest techniques are now becoming more

widely used for site testing structures of significant size so that, from a structural engineering

viewpoint, it can be anticipated that the near future will bring greater choice in the type of

testing that can be employed and the quality of results that can be obtained.

In the past, most dynamic testing of grandstands using shaker excitation has used

rotating mass shakers. Typically, a constant speed of rotation is used to develop sinusoidal

excitation at a particular frequency. The tests are normally repeated for a range of

increasing speeds, so providing excitation at a range of discrete frequencies, a procedurereferred to as stepped sine excitation. The rotating mass shakers that have been used for

grandstands are sufficiently large (i.e. the rotating mass produces sufficient force) to excite

a cantilever deck for all the modes of interest. The amplitude of the excitation force is

easily calculated knowing the rotating mass, its eccentricity and the speed of rotation.

Together with measurements of acceleration from locations on the structure, this enables

natural frequencies, mode shapes, modal mass and damping values to be estimated for

well-separated modes where single degree of curve fitting is appropriate. This capability

goes significantly beyond what is needed for Type 1 testing.

As yet, it has not been the practice to instrument the rotating mass shaker so as to

record directly the excitation force/time history required for correct estimation of modalproperties when there are closely spaced modes of vibration that often occur in cantilevered

seating decks. However, some Test Agencies have developed procedures to derive the phase

difference between the excitation and response and so improve the identification of modes

corresponding to closely-spaced natural frequencies. This allows the full range of modal

properties to be determined and, in these terms, tests using a non-instrumented rotating

mass shaker meet all the requirements for Type 2 testing. However, the processing of the

results involves curve fitting to establish the modal parameters. The accuracy of the results

of curve fitting depends on the quantity and quality of information available to define the

relationships being described. Accordingly, there will be circumstances when the results

of Type 2 tests undertaken with a rotating mass shaker, without additional instrumentation,

can be less accurate than if the force/time history had been obtained by direct measurementand the results used when processing the data to determine modal properties.

Testing with fully instrumented shakers providing a direct measurement of the

force/time history has been standard practice for some time in mechanical and aerospace

engineering and is now being used for large–scale civil structural engineering applications.

This has led to the use of electrical or hydraulic shakers that provide excitation using

an inertial mass oscillating in the direction of excitation. These tend to be smaller than

rotating mass shakers but, being portable, can be moved around a structure to provide

excitation at different locations.

Simultaneous measurement of the shaker excitation force and the corresponding

response is used when estimating the Frequency Response Function (FRF) between the




response and excitation on a test structure. In this case, both modulus and phase information

for the FRF is determined directly and used in a curve fitting approach that yields the

natural frequency, mode shape, modal damping ratio and modal mass for all the modes

of interest. The availability of a complete FRF makes this method the most reliable of all

those available, particularly where modes are closely spaced in frequency. Stepped-sine

and slow-sweep sine as well as broadband random excitation can be used, depending on

the time available for testing, the shaker employed (typically inertial, with acceleration of

the mass measured so as to derive the force) and the facilities for information processing.

Always providing that the shaker generates sufficient force to excite all the modes ofinterest, instrumented shakers provide high quality information for both Type 1 and

Type 2 testing.

Because of the substantial size of a grandstand seating deck, not all commercial

shakers will be suitable for testing grandstands. More particularly, the shaker has to be of

sufficient size that the oscillatory or rotating mass develops the force necessary to excite

all relevant modes of the structure. In practice, this difficulty is avoided by using more

than one shaker to achieve the necessary excitation. Besides increasing and distributing

the energy input, simultaneous use of shakers at more than one location helps to improve

the identification of modes with closely spaced natural frequencies and to minimise the

possibility of missing a mode of vibration as could occur with a shaker used in a singlelocation.

A.4.5.5 Future developmentsIt will be evident that there are techniques available in other disciplines that, if properly

implemented, can enhance the general capability for dynamic testing of grandstand

structures. Almost certainly, greater choice of testing techniques for use on grandstands

will become available to the Listed Engineer responsible for procuring testing and using

the results. In making this choice, the Listed Engineer will need to consult the proposed

Test Agency to ascertain its range of expertise and preferred way of working. In a climate

of change and new developments, an established track record of on-site dynamic testing ofstructures of significant size will be a useful recommendation.

A4.6 Specification and procurementThe role of the Listed Engineer in preparing the specification, procuring testing and

reporting to management is illustrated in Figure A4.1. Note that the need for Type 2

testing may emerge from the procedures indicated if additional information is considered

necessary in order to reach a satisfactory conclusion.

Prior to commissioning a Test Agency to undertake work, the Listed Engineer

should decide whether the Route 1 or 2 approach is to be followed, the extent and types

of information required from testing and take a preliminary view of the techniques to be

employed. At this stage, it will often be useful to discuss the programme and methods

with a possible Test Agency and agree a specification for the work required. The Test

Agency should have the necessary experience and capability to undertake the chosen type

of testing and deal with the logistical difficulties of on-site testing of major structures. It

should also be noted that, although Type 1 tests are simpler than Type 2, there is the same

need for experienced personnel to undertake the testing and reporting. For instance, the

equipment required for heel-drop tests or ambient vibration monitoring is quite widely

available but can be used by inexpert operators so giving results of little value. It is also

useful if the Test Agency is able to process and analyse results, at least partially, on site

so allowing some flexibility in the programme and avoiding the need for repeat testing on

another occasion.




In the particular circumstances of thestructure under consideration, is testing

required to confirm the assumptionsmade in design or assessment?

( See Section 3.4 )

Has dynamic testing beenpreviously undertaken on the

stand in its present form andis the report of the testing

available?

Does the comparison give confidencethat the figures are sufficiently accurate

to support a recommendationconfirming the assumptions made in

design or assessment?

Calculate relevant natural frequencies.

Compare test andcalculated values.

Decide information

required from testing oradditional testing.

Agree method and

extent of testing withTest Agency.

Write requirementspecification and

recommend experiencedTest Agency to Management.

Testing undertaken by Test Agency. The Listed Engineer

should be available to reviewresults and revise programme

if necessary during testing.

Prepare report formanagement

(See Appendix 4Section 4.7 )

Review test results and comparewith calculated values. Refine

the analytical model if necessaryand compare test andrecalculated results.

Start

End

No

No

No

Yes

Yes

Yes

Figure A4.1 The role of the Listed Engineer




The specification should be written as a ‘performance’ document, based on the

agreed types of testing and the properties required from the tests, with the details of the

test programme being left to the Test Agency to decide. However, the specification should

include:

• A description of the properties required from testing.

• Confirmation of the agreed type of testing and the form of presentation of results.

•

A requirement for the work to be undertaken to a recognised quality standard suchas BS ISO Standard 14964:2000, ‘ Mechanical vibration and shock – Vibration

of stationary structures – Specific requirements for quality management in

measurement and evaluation of vibration’.

• The time agreed for the delivery of the results and report on the testing.

• Requirements for reporting, bearing in mind that the Engineer’s report to

Management should include ‘an account of the procedures used and the detailed

results’.

• A requirement for a method statement to meet the requirements of Health and

Safety, and also CDM Regulations where these are applicable.

The Listed Engineer should be available while testing is in progress in order to review

results as they are obtained and, if necessary and possible to arrange, modify the instructions

to the Test Agency

A4.7 ReportingThe Listed Engineer is required to make a report to Management. This should include:

• An explanation of the use of personal judgement in requiring additional testing in

situations where test records are already available or of a decision not to test (as

outlined in Section 3.4).• A note on the choice of Test Agency including the Agency’s track record of on-site

testing of structures.

• The agreed specification for the test programme.

• The Test Agency’s report of the testing including all results.

• An interpretive appraisal of the results including comparisons with values used in

design and the significance of any differences in estimates of performance.

• Recommendations on any further action required.

A4.8. Further InformationMore information on methods of testing may be obtained from the following sources:

Dynamic Test Agency. Primer on best practice in dynamic testing. London: Chameleon

Press, 1993.

Maia, N.M. and Silva, J.M.M. eds. Theoretical and experimental modal analysis. Baldock:

Research Studies Press, 1997.

Ewins, D.J. Modal testing: theory, practice and application. 2nd ed. Baldock: Research

Studies Press, 2000.




Appendix 5 Bibliography

The Bibliography is a limited selection from published material. Papers are referenced

under the following categories.

A Analytical methods

B Behaviour of grandstands in serviceD Loading, dynamic load factors, crowd behaviour and tolerance of motion

G Overviews and general interest

H Human structure interaction

T Testing and monitoring of grandstands

M Management, risk assessment and liability

S Codes, Standards and Guidance.

∗

Referred to in other Appendices

The entries are given in date order by year. References to papers of particular interest or

relevance are annotated with a comment in italics.

G/D Bachmann, H. and Ammann, W. Vibration in structures: induced by man and

machines. Zurich: IABSE, 1987.

Widely used source of information that has stimulated research and practice

S BS 6841:1987: Guide to measurement and evaluation of human exposure to

whole-body mechanical vibration and repeated shock . London: BSI, 1987

G/D Allen, D.E. ‘Vibrations from human activities’, Concrete International: Designand Construction, 12(6), June 1990, pp66-73.

Wide coverage of Canadian Practice as recommended in successive revisions of Commentaries

to the NBC Building Code

A* NAFEMS. A finite element dynamics primer. Glasgow: National Agency for

Finite Element Methods & Standards, 1992.

A* Clough, R.W. and Penzien, J. Dynamics of structures. 2nd ed. New York: McGraw-

Hill, 1993.

A/T* Dynamic Test Agency. Primer on best practice in dynamic testing. London:

Chameleon Press, 1993.

B Batista, K.C. and Magluta, C. ‘Spectator-induced vibration of Maracana football

stadium’, Proceedings of the 2nd European conference on structural dynamics:

EURODYN ‘93, Trondheim, 21-23 June 1993, vol 2. Rotterdam: Balkema, 1993,

pp985-992.

B Kasperski, M. and Niemann, H.J. ‘Man induced vibration of a stand structure’,

Proceedings of the 2nd European conference on structural dynamics: EURODYN

‘93, Trondheim, 21-23 June 1993, vol 2. Rotterdam: Balkema, 1993, pp977-983.

Full coverage of design issues set in the context of determining remedial measures for a grandstand

that vibrated excessively under crowd loading. Recommends acceleration limits for design and

proposes a 7Hz minimum natural frequency limit for grandstands without additional damping




B Van Staalduinen, P. and Courage, W. ‘Dynamic loading of Feyenoord Stadium

during pop concerts’, Places of assembly and long-span structures, IABSE

Symposium, Birmingham, 1994, IABSE Reports, vol 71, 1994, pp283-288.

A* Blevins, R.D. Formulas for natural frequency and mode shape. Malabar, FL:

Robert E Kreiger Publishing Company, 1995.

D Ji, T. and Ellis, B.R. ‘Floor vibration induced by dance-type loads: theory’, The

Structural Engineer, 72(3), 1 Feb 1994, pp37-44.

Highly influential paper in deriving DLFs for individual jumping in terms of contact ratio for a

repeated half sine load pulse

D Ellis, B.R. and Ji, T. ‘Floor vibration induced by dance-type loads: verification’,

The Structural Engineer, 72(3), 1 Dec 1994, pp45-50.

Includes experimental confirmation of the half sine load pulse assumption for jumping on a very

stiff support

D Kasperski, M. ‘Actual problems with stand structures due to spectator induced

vibrations’, Proceedings of the 3rd European conference on structural Dynamics:

EURODYN ‘96, Florence, 5-8 June 1996 , vol 1. Rotterdam: Balkema, 1996,

pp455-461.Overview including DLFs for different activities including hand clapping and stamping, review

of tolerance of motion including potential for panic and example of use of tuned mass dampers

S BS 6399-1: 1997: Loadings for buildings. Part 1: Code of practice for dead and

imposed loads. London: BSI, 1996.

First inclusion in UK Code of requirements concerning dynamic loading due to people in

buildings. Ji and Ellis (1996) contact ratios identified with specific activities and so DLFs. Also

set out natural frequency trigger values as alternatives to assessing performance by calculation

A/T* Maia, N.M. and Silva, J.M.M eds. Theoretical and experimental modal analysis.

Baldock: Research Studies Press, 1997.

H Ellis B.R. and Ji, T. ‘Human–structure interaction in vertical vibrations’, Proc. ICE, Structures and Buildings, 122(1), Feb 1997, pp1-9.

Recognition of role of passive people in moderating motion

S/M Scottish Office and Department of National Heritage. Guide to safety at sports

grounds. 4th ed. London: The Stationery Office, 1997 [the Green Guide].

Mainly concerned with spectator management but includes a brief section on dynamics with

different frequency limits for structures at sports grounds to those given in BS 6399 (1996)

S ISO 2631-1: 1997: Mechanical vibration and shock: evaluation of human exposure

to whole-body vibration. Part 1: General requirements. Geneva: ISO, 1997.

G Reid, W.M., Dickie, J.F., and Wright, J. ‘Stadium structures: are they excited?’The Structural Engineer, 75(22), 18 Nov 1997, pp383-388.

Useful overview of factors influencing structural design of cantilever grandstands and roofs.

Emphasises difficulty of designing practical grandstands to BS6399(1996) frequency limits

M Chapman, J.C. ‘Collapse of the Ramsgate Walkway’, The Structural Engineer,

76(1), 6 Jan 1998, pp1–10.

Although not concerned with stadia, presents important messages to operators of facilities used

by the public on their responsibilities in procuring services and overseeing the work of specialists

H Wei, L. and Griffin, M.J. ‘Mathematical models for the apparent mass of the

seated human body exposed to vertical vibration’, J. Sound & Vibration, 212(5),

1998, pp855-874.

Major study of passive action leading to body unit properties for seated people




T Littler, J.D. ‘The dynamic response of a three tiered cantilever grandstand’,

Proceedings of the 4th European conference on structural dynamics: EURODYN

‘99, Prague, 7-10 June 1999, vol 1. Rotterdam: Balkema, 1999, pp623-628.

G/M Standing Committee on Structural Safety. Structural Safety 1997-99: review

and recommendations. 12th report of SCOSS . London: SETO, 1999, Section 3.2:

‘Safety of sports stadia structures’, pp28-29.

Authorative independent expression of concern on procurement, inspections and maintenance of

stadia structures. Also suggests independent checks on structural designs

M Health and Safety Executive. The event safety guide: a guide to health, safety,

and welfare at music and similar events. 2nd ed. Sudbury: HSE Books, 1999.

Key document providing more information than the ‘Green Guide’ for non- sporting events

T Littler, J.D. Permanent cantilever grandstands: dynamic response. BRE

Information Paper IP 5/00. Garston: BRE, 2000.

D Ellis, B.R., Ji, T. and Littler, J.D. ‘The response of grandstands to dynamic crowd

loads’, Proc. ICE, Structures and Buildings, 140(4), Nov 2000, pp355-365.

Notes human structure interaction due to presence of a passive crowd and corresponding effects

on natural frequency of the combined system

A/T* Ewins, D.J. Modal testing: theory, practice and application. 2nd ed. Baldock:

Research Studies Press, 2000.

D Kasperski, M. ‘Safety assessment of stadia in regard to human induced vibrations’,

Safer solutions in sport and leisure: responsibilities for crowd management

at major events, Manchester, 5 April 2001 [unpublished Institution of Civil

Engineers seminar].

Besides loading, and target reliability over life-time use, discusses tolerance of motion and

potential for panic due to excessive motion of a stand

S/D Willford, M. ‘Stadium Dynamics’, Safer solutions in sport and leisure:responsibilities for crowd management at major events, Manchester, 5 April

2001 [unpublished Institution of Civil Engineers seminar].

Overview including statistical appreciation using Monte Carlo modelling and assumed

distributions of input variables to achieve probabilistic acceleration predictions for different

frequency stands. Concludes deterministic design leads to an overestimate of risk of exceeding

any given acceleration limit due to neglect of song frequency input and other factors

G/M Standing Committee on Structural Safety. Structural Safety 2000-01: 13th report

of SCOSS . London: IStructE, 2001, chapter 3: Dynamic response of structures,

pp23-26.

On cantilever decks at sports grounds, expresses concern on the “consequences of a structural

collapse or disturbing movement causing panic amongst an occupying crowd…”. Concludesthat specifically targeted research needed to resolve uncertainties in design for dynamic crowd

loading

D Ginty, D., Derwent, J.M. and Ji, T. ‘The frequency ranges of dance type loads’,

The Structural Engineer, 79(6), 20 Mar 2001, pp27–31.




S/M Institution of Structural Engineers, Department for Transport, Local Government

and the Regions and Department for Culture, Media and Sport. Dynamic

performance requirements for permanent grandstands subject to crowd action:

interim guidance on assessment and design. London: IStructE, 2001.

Uses natural frequency for vertical excitation of the stand to set limits on categories of use.

Testing required to check calculated natural frequencies. Knowledge base considered inadequate

to enable recommendations to be formulated for calculating grandstand performance under

crowd loading. Recommendations accepted by UK Government re Building Control and Safety

Certification

D Ellis, B.R. and Ji, T. Loads generated by jumping crowds: experimental

assessment. BRE Information Paper IP 4/02. London: CRC, 2002.

Only study of large group (up to 64 people) loading and reduction in DLFs for jumping with

increasing group size

H Sachse, R., Pavic, A. and Reynolds, P. ‘The influence of a group of humans on

modal properties of a structure’, Proceedings of the 5th European conference on

structural dynamics: EURODYN ‘02, Munich, 2-5 September 2002. Rotterdam:

Balkema, 2002, pp1241-1246.

Wide ranging study providing body unit models for passive human structure interaction effects

D* Littler, J.D. ‘Frequencies of synchronised human loading from jumping and

stamping’, The Structural Engineer, 81(22), 18 Nov 2003, pp27–35.

Reviews song frequencies experienced at concerts and challenges received wisdom on limits to

possible excitation ranges with tests involving jumping and stamping

H Matsumoto, Y. and Griffin, M.J. ‘Mathematical models for the apparent masses

of standing subjects exposed to vertical whole-body vibration’, J. Sound &

Vibration, 260(3), pp431-451.

Passive action and body unit properties for erect people

S/G ISO/CD/10137: 2004: Bases for the design of structures: serviceability of

buildings and pedestrian walkways against vibration [Committee Draft]. Includes proposals for stadia with loading corresponding to an extreme event involving everyone

jumping. Proposes separate acceleration limits for the onset of panic and an upper limit for

comfort. It is doubtful that practical cantilever grandstands would meet these requirements

A/D/S Ellis, B.R. and Ji, T. The response of structures to dynamic crowd loads. BRE

Digest 426. Garston: BRE Bookshop, 2004.

Provides methods for calculating dynamic response in accordance with BS 6399-1 (1996). Useful

guidance for low crowd densities and vigorous activity on relatively stiff structures, (dancing and

aerobics). Not applicable to dense crowd loading on flexible grandstands

D Ellis, B.R. and Littler, J. D. ‘The response of cantilever grandstands to crowd

loads. Part 1: Serviceability evaluation.’ Proc. ICE, Structures and Buildings,

157(SB4), Aug 2004, pp235–241.

Explores the use of the vibration dose value (VDV) approach to assessing tolerance of motion for

people in grandstands

A/D Ellis, B.R. and Littler, J.D. ‘The response of cantilever grandstands to crowd

loads. Part 2: Load estimation’, Proc. ICE, Structures and Buildings, 157(SB5),

Oct 2004, pp297-307.

Back analysis of two cantilever grandstands tiers to determine effective DLFs for crowd loading

at concerts with modest excitation levels. Conventional analysis with allowance for crowd size

(Ellis & Ji 2002) led to a value of 16% effective damping which was attributed to crowd action.

Results applied to other grandstands. High damping clearly the result of human structure

interaction




H Yao, S., Wright, J. R., Pavic, A. and Reynolds, P. ‘Experimental study of human-

induced dynamic forces due to bouncing on a perceptibly moving structure’,

Canadian J. Civil Engineering, 31(6), Dec 2004, pp1109-1118.

Unambiguous demonstration of effects of human structure interaction due to an active (i.e.

moving) person on a flexible support for a range of natural frequencies and mass ratios typical

of practical cantilever grandstands. Significant reductions in contact force at resonance (drop-

out) observed leading to lower accelerations than would be predicted from conventional theory

ignoring active human structure interaction

T Reynolds, P., Pavic, A. and Ibrahim, Z. ‘A remote monitoring system forstadium dynamics’, Proc. ICE, Structures and Buildings, 157(SB6), Dec 2004,

pp385-393.

Report of long-term monitoring of moderately flexible grandstand mainly used for viewing

soccer

A/H Sachse, R., Pavic, A., and Reynolds, P. ‘Parametric study of modal properties of

damped two-degree-of-freedom crowd-structure dynamic systems’, J. Sound &

Vibration, 274(3-5), 2004, pp461-480.

T Reynolds, P., Pavic, A. and Willford, M. ‘Prediction and measurement of stadia

dynamic properties’, 23rd International modal analysis conference: IMAC- XXIII ), Orlando, Florida, 31 Jan-3 Feb 2005.

Account of dynamic testing to obtain modal properties of a curved multi-tier cantilever grandstand

with comparison of results from pre test analysis using single frame and 3-D analysis and post

testing results from 3-D Finite Element modelling. The post testing analysis satisfactorily

reproduced the family of closely spaced modes but with discrepancies in values of natural

frequency considered due to the treatment of non-structural elements and omission of the roof in

the FE model

G/H Willford, M. ‘Dynamic performance of stands’, in Culley, P. and Pascoe, J. eds.

Stadium Engineering. London: Thomas Telford, 2005, pp47-54.

Broad coverage of design of stadia for dynamic loading with particular consideration given to

human structure interaction due to passive /inactive people in an otherwise active crowd. Active participation treated using conventional DLFs

S ‘Commentary D: deflection and vibration criteria for serviceability and fatigue

limit states’, in National Research Council of Canada. User’s guide - NBC 2005:

structural commentaries (Part 4 of Division B). Ottawa: NRC, 2005, ppD1-

D10.

Widely used Code based on performance design principle. 2005 revision includes

recommendations for stadia based on serviceability criteria related to Canadian practice and

“commonly encountered events”. Does not address concerns on possibility of an extreme event

and consequences for safety related to panic due to excessive motion. Not recommended for

major stadia with open-ended use

T Reynolds, P. and Pavic, A. ‘The dynamic performance of sports stadia under

crowd dynamic loading at concert events’, Proceedings of the 6 th European

conference on structural dynamics: EURODYN ‘05, Paris, 4-7 September 2005.

Rotterdam: Millpress, 2005, pp473-478.

T/H Reynolds, P. and Pavic, A. ‘Vibration of a large cantilever grandstand during an

international football match’, ASCE J. Performance of Constructed Facilities,

20(3), Aug 2006, pp202-212.

Presents modal properties obtained by testing the empty stand and compares these with properties

obtained whilst monitoring the stand during the match. Significant differences were recorded

showing the crowd interacted structurally with the basic structure




T Mohanty, P. and Reynolds, P. ‘Modelling of dynamic crowd-structure interactions

in a grandstand during a football match’. 24th International modal analysis

H Yao, S., Wright, J.R., Pavic, A. and Reynolds, P. ‘Experimental study of human-

induced dynamic forces due to jumping on a perceptibly moving structure’,

J. Sound & Vibration, 296, 2006, pp150-165.

Demonstration thatactive human structure interaction does not depend on uninterrupted contact

with the support with results for jumping similar to those in the 2004 paper by the same authors

D/H Sim, J., Blakeborough, A. and Williams, M. ‘Modelling effects of passive crowds

on grandstand vibration’, Proc. ICE, Structures and Buildings, 159(SB5), Oct

2006, pp261-272.

D* Parkhouse, J.G. and Ewins, D.J. ‘Crowd-induced rhythmic loading’, Proc. ICE,

Structures and Buildings, 159(SB5), Oct 2006, pp247-259.

Results of 1000 tests involving individual bobbing and jumping on force plates at excitation levels

comparable to pop-concert participation and leading to synthesis of Dynamic Load Factors for

groups of different size

H* Dougill, J.W., Wright, J.R., Parkhouse, J.G. and Harrison, R.E. ‘Human structure

interaction during rhythmic bobbing’. The Structural Engineer, 84(22), 21 Nov2006, pp32–39.

Theoretical development of an active human structure interaction model with properties derived

from independent experiments and validation through comparison with results from individual

bobbing/bouncing on a flexible platform. Includes discussion of relevance to grandstands

H Alexander, N.A. ‘Theoretical treatment of crowd-structure interaction dynamics’,

Proc. ICE, Structures and Buildings, 159(SB6), Dec 2006, pp329-338.

Theoretical treatment of similar model to Dougill et al (2006 )

M The Construction (Design and Management) Regulations. Norwich: The

Stationery Office, 2007 (SI 2007/320).

Important implications for a client requiring construction works

T Reynolds, P., Pavic, A. and Carr, J. ‘Experimental dynamic analysis of the

Kingston Communications Stadium’, The Structural Engineer, 85(8), 17 Apr

2007, pp33–39.

Includes comparison of calculated and measured dynamic properties

C Institution of Structural Engineers. Temporary demountable structures: guidance

on procurement, design and use. 3rd ed. London: IStructE, 2007.

Comprehensive guidance on demountable structures. Guidance on horizontal motion is adopted

in the present Recommendations (Table 2) but without allowance for geometrical imperfections

T/B/H* Pavic, A. and Reynolds, P. ‘Experimental verification of novel 3DOF modelof grandstand crowd-structure dynamic interaction’, 26 th International modal

analysis conference: IMAC-XXVI, Orlando, Florida, 4-7 Feb 2008, paper 257.

Includes details of testing a stadium before and during a pop-concert with moderate crowd

excitation. Calculations of performance using finite element analysis and the human structure

interaction model recommended in Appendices 1 and 2 show acceptable agreement with the

measured values of RMS acceleration






Documents

Dynamic Performance Requirements for Permanent Grandstands Subject to Crowd Action