Learning from failures in timber structures€¦ · Why should we learn from previous failures /...

Learning from failures gin timber structures

Dr. Eva Frühwald, Div. of Structural Engineering, LTHeva.fruhwald@kstr.lth.se

OutlineOutline

• Failure – error ? • Why learning from failures ?Why learning from failures ?• Research project• Explicit cases• What can we learn from failures?• What can we learn from failures?

Definition of failure (1)Definition of failure (1)

• Failure is a deviation from the status quo.

• Failure is not meeting target expectations.

• Failure is any secondary defect.

Definition of failure (2)Definition of failure (2)Ulti t li it t t S i bilit li it t tUltimate limit state• Risk for human life,

negative effect on

Serviceability limit state• No consequence for

safety of structurenegative effect on safety and performance of structure

safety of structure

– Direct collapse of structures

– Vibrating floors– Excessive deformations

– Local cracking– Crushing

degradation

Excessive deformations– Moisture movements– Mould and fungi

– degradation

Why should we learn from previous failures / collapses ?

Hypothesis: All failures are caused by human errorsAll failures are caused by human errors.

• Errors of knowledge (inadequate training in relation to tasks)

• Errors of performance (non-professional performance, carelessness)

• Errors of intent (consciously taking short-cuts and risks to save time/money)

Should be possible to prevent failures

[Kaminetzky]

Learning from failuresLearning from failures

Education and training are the only effective ways to minimize failures.yThe training of an engineer, architect or contractor should provide an understanding notcontractor should provide an understanding not only of the best solutions that may be adopted, but also of practices that should be avoided. (Kaminetzky, 1991)

Previous studies: common failure causesPrevious studies: common failure causes• Concrete

– material qualityq y– work execution– structural design and detailing (joints, openings, supports,…)

• Steel– insufficient temporary bracing during construction– errors in design / construction mainly of connections and details– deficient welding– excessive flexibility and non redundant designexcessive flexibility and non redundant design– Vibration induced failures– stability type failures– fatigue and brittle failure

i d– corrosion damage• Timber

– inadequate behavior of jointseffects of moisture exposure (imposed strains shrinkage)– effects of moisture exposure (imposed strains, shrinkage)

– poor durability performance– inadequate bracing of structural system– inadequate performance of material and products– inadequate appreciation of load

Previous studies: common failure causesPrevious studies: common failure causes• Concrete

– material qualityq y– work execution– structural design and detailing (joints, openings, supports,…)

• Steel– insufficient temporary bracing during construction– errors in design / construction mainly of connections and details– deficient welding– excessive flexibility and non redundant designexcessive flexibility and non redundant design– Vibration induced failures– stability type failures– fatigue and brittle failure

i d– corrosion damage• Timber

– inadequate behavior of jointseffects of moisture exposure (imposed strains shrinkage)– effects of moisture exposure (imposed strains, shrinkage)

– poor durability performance– inadequate bracing of structural system– inadequate performance of material and products– inadequate appreciation of load

Level of safety in timber structuresLevel of safety in timber structures

• Is the level of safety adequate for timber structures compared to other materials?

Implementation of Eurocode 5 (national application rules)– Implementation of Eurocode 5 (national application rules)– Spectacular collapses of timber structures in Europe

Swedish-Finnish project 2005-2007• Research questions investigating collapsed buildingsg g g

– Reasons for failure?– Which type of components are most prone to failure?– Which failure modes are most frequent?– What can be done to avoid or reduce failures?

Survey of failure casesy(Swedish-Finnish survey)

• 4 partners

• 127 cases lit t– literature

– own investigations

• Only cases implying risk for human lives are included (ULS)!included (ULS)!

reportreport1. Introduction2 E i f i2. Experiences from previous

failure investigations3. Present survey of failure y

cases - methodology4. Results and interpretation

of the information collectedof the information collected5. How can we learn from

previous failures6. Summary and conclusions

Annexeexempel

Annexe– Overview failure cases

and classificationand classification – 127 cases, 1-2 pages /

case

Classification of error types causing failureClassification of error types causing failure1. Wood material performance2 Manufacturing errors in factory Materials & products2. Manufacturing errors in factory3. Poor manufacturing principles

4 O it lt ti

Materials & products

4. On-site alterations5. Poor principles during erection Construction work

6. Poor design / lack of design with respect to mechanical loading

7. Poor design / lack of design with respect t i t l ti

Design/planning

to environmental actions

8. Overload in relation to building l ti

Codesregulations

9. Other / unknown reasons

Multiple failure types: estimate of the weight of each failure type

Failure cause (127 cases)( )Wood material performance 1%

Manufacturing errors in factory 5%Other / unknown 5% Manufacturing errors in factory 5%

Manufacturing principles 4%Overload / codes 4%

Poor principles during erection 16%

Design (mechanical loading)

On-site alterations12%

42%

Design (environmental actions) 11%actions) 11%

Failure cause (127 cases)( )Unknown / other 5%

Materials and productsOverload / codes 4%

Materials and products 11%

Construction work27%

Design / planning53%

27%

Type of buildingsType of buildingsi t fin percentage of cases

Public 51Industrial 23Industrial 23Agricultural 7Apartment 8Apartment 8Other / unknown 11

• NOTE: Failure surveys in general can not be seen as representative for the general population of structures (cover up of mistakes is common, random sampling is impossible)impossible)

Span100

Span

80

90

16% < 10 m

60

70

80

84% > 10 m

50

60

span

[m]

30

40

s

25 m

10

20

0

Age at failureAge at failure2525

2020

15res15ures

10% o

f fai

lur

10of fa

ilu

5

%

5

%

01 2 5

00 1 2 3 4 5 6-10 11-15 16-20 21-25 26-30 31-35 36-40

years

Type of structural elements that failedType of structural elements that failed

in percentage of cases

beam 47truss 34bracing 29bracing 29joint 23

dowel-type 57punched metal plate 10glued 7

arch 8column 4

gother 27

column 4frame 2

Correlated with typical structural elements?!

Failure modesFailure modesi t fin percentage of cases

• instability 30• bending failure 15• bending failure 15• tension failure perp. to grain 11• shear failure 9shear failure 9• drying cracks 9• excessive deflection 7• tension failure 5• corrosion of fasteners / decay 4• withdrawal of fasteners 3• compression (buckling) 2• other / unknown 21

Timber, steel and concrete buildings:failure causes

F il Ti b St l C tFailure cause[in % of cases]

Timber [own survey]

Steel [Oehme, Vogt]

Concrete [Brand, Glatz ]

Design / planning 53 35 40Design / planning 53 35 40

Construction work 27 25 40

Maintenance / reuse 35

t i l 11material 11

other 9 5 20

difficult to compare – definition of categories, number of cases etc.

Q ti A i b tt t d i iQuestion: Are engineers better at designing steel- and concrete structures !?

How can we learn from previous failures?How can we learn from previous failures?

53 % d i / l i human errors53 % design / planning27 % construction work

Errors of intentErrors of knowledge Errors of performance

improved training and education

more efficient Quality Assurance (QA)

more efficient Quality Assurance (QA) ?(Q ) (Q )

Training & educationTraining & educationSh ld f t h i l t hi h t i l• Should focus on technical aspects which are typical causes for failure

• Training of engineers and control in the design / planning phase most important (most errors!)

• Training & education measurementsLectures on good and bad examples for students / practicing– Lectures on good and bad examples for students / practicing engineers

– Database on good / bad examplesC t i t h i l t– Certain technical aspects

– …

Learning from each others mistakesLearning from each others mistakes

Training & education: gexamples for issues to be emphasized

Bracing to avoid instability both during construction and• Bracing to avoid instability both during construction and in the finished structure

• Situations with risk for perpendicular to grain tensile failure (joints, double-tapered beams, curved beams,…)

• Consideration of moisture effects

D i f j i• Design of joints

• Estimation of loading conditionsEstimation of loading conditions

• Estimation of real behavior of the structure

Training & education: building site professionals

• Increasing the competence of building site professionals – Professional trainingProfessional training– Assigned training / certified personnel to perform

certain taskscertain tasks– Continuous courses and seminars

• External quality control by impartial and certified personnelp

ExamplesExamples1 Si A D k1 – Siemens Arena, Denmark2 – Jyväskylä Exhibition Hall, Finlandy y3 – Buckling of trusses

Siemens Arena DenmarkSiemens Arena, Denmark

• Inaugurated February 2002• Failed January 3 2003• Failed January 3, 2003• 8000 seats• Building costs: 6 millions Euros• Calculated cost to rebuild structure:

25% of new price25% of new price

Martin Hansson, 2004

Siemens Arena the fish shapeSiemens Arena – the fish shape

No diagonals! R=380 m

5 m

R 175about 6.5 m R=175 m

72 m(double: center to center 10 m)

FailureFailure

No load at failure2 out of 12 trusses failedout o t usses a ed

Failure initiationFailure initiation

The failure jointThe failure joint

Slotted-in steel plates

Reduced section

F il ti Tension failureFailure section - Tension failure

The failed cross sectionThe failed cross section

glued in spacerglued-in spacer block 45x160533

430

160 160 160

The failed cross sectionThe failed cross section

4 slotted-in 8 mm steel plates in 10533 steel plates in 10 mm wide slots430

160 160 160

The failed cross sectionThe failed cross section

Dowels 12 mm533

430

160 160 160

Experts explanation

Early stage estimation not corrected during detailing!A d ti t id d• Area reduction not considered- depth in bottom chord from 533 to 430 mm- slots for steel platesslots for steel plates- holes for dowels

533

430430

160 160 160

Experts explanation

Early stage estimation not corrected during d t ili !detailing!

• Moment caused by very stiff connection –y yassumed as hinged

• Angle between stresses at surface andAngle between stresses at surface and grain direction

• Used tensile strength about 50% higher• Used tensile strength about 50% higher than code value

Experts explanationTotal effect:The actual strength in joints was about 25% of theThe actual strength in joints was about 25% of the

“designed strength”The actual stresses were 40% higher than the characteristic 5% strengthcharacteristic 5% strengthAll trusses will fail sooner or later…

Why those two? the trusses had large knots and low quality glueline glulam quality determined which of the trusses failedthe trusses failed

Why just then? Duration of loady j

Restauration- cables

Design related causesX strength design

environmental actionsConstruction related causes

X

Poor principles during constructionAlterations on-site of intended design or products

Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading

Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood products

(X)g p p p

Manufacturing errors in factoryCauses related to utilisation of the structure

Misuse or lack of maintenance of the structure

(X)

Misuse or lack of maintenance of the structureOther causes

Jyväskylä Exhibition hall, Finland

• Glulam truss, about 55 meter span• Inaugurated January 17, 2003• Failed February 1 2003 (the day after an• Failed February 1, 2003 (the day after an

exhibition)• Number of people in building: 10• Nobody hurt• Nobody hurt

Martin Hansson, 2004

StructureStructure

Double trusses

4.8 m

55 m(center to center 9 m)

The failureThe failure

Failure loadFailure load

• 52 kg/m2 snow load (25 % of design value)• Failure load = 51% of expected

characteristic 5% capacitycharacteristic 5% capacity

Failure initiationFailure initiation

Failure investigationFailure investigation

Slotted-in steel plateDowels (hidden, no inspection!)Dowels (hidden, no inspection!)

Extremely poor manufacturing of dowel jointsof dowel joints• steel plate was placed incorrectly

compared to center line of beams• Positioning of dowels was poor: Some not penetrating the steel plate

• Drilled holes varied in depth andDrilled holes varied in depth and were oversized

• Partly too short embedding length f d lfor dowels

Failure investigationFailure investigation

Slotted-in steel platedoweldowel

In failure joint only 7 of 33 dowels were found !

Inadequate formula in old draftof Eurocode 5 for block shear failure Corrected years beforefailure. Corrected years before the incident.

Failure investigationFailure investigation

• Roof elements constructed as two-span, but support reactions on trusses designed for single-span (+25%)

• Lateral stability of trusses not sufficient progressive failure

Design related causes(X)strength design

environmental actionsConstruction related causes

(X)

Poor principles during constructionAlterations on-site of intended design or products

Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading

Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood productsg p p pManufacturing errors in factory

Causes related to utilisation of the structureMisuse or lack of maintenance of the structure

X

Misuse or lack of maintenance of the structureOther causes

Buckling of trussesBuckling of trusses

Buckling of trussesBuckling of trusses

Design related causesstrength design environmental actions

Construction related causes

X

Poor principles during constructionAlterations on-site of intended design or products

Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading

Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood productsg p p pManufacturing errors in factory

Causes related to utilisation of the structureMisuse or lack of maintenance of the structureMisuse or lack of maintenance of the structure

Other causes

What can we learn from failures?What can we learn from failures?

• All materials are different: steel, concrete and timber have common but also their special problems and failures types

An engineer good at designing steel and / or concrete structures is not automatically excellent at designing timber structures

What can we learn from failures?What can we learn from failures?

E t d ti / t i i h ld b id d tExtra education / training should be provided to introduce engineers to the special problems in timber engineeringtimber engineering

ImplementationImplementationdatabase / books on failure cases – learn from each others mistakesH db k/ t i l f d i f ti b t tHandbook/ material for design of timber structures including good and bad solutions, focus on typical problems Courses in design of timber structures

t l t l i t l t lexternal control – internal control

LiteratureLiterature• Kaminetzky, D.: Design and Construction failures - lessons from

forensic investigations McGraw-Hill 1991forensic investigations, McGraw-Hill, 1991• Martin Hansson: presentations on failures in timber structures, 2004• Accident Investigation Board Finland

http://www.onnettomuustutkinta.fi/2601.htmp• http://www.maintenanceresources.com/ReferenceLibrary/FailureAnalys

is/FailureModes.htm• Dröge, G., Dröge, T.: Schäden an Holztragwerken, Schadenfreies

Bauen Band 28 Fraunhofer IRB Verlag 2003Bauen, Band 28, Fraunhofer IRB Verlag, 2003• Brand, B., Glatz, G.: Schäden an Tragwerken aus Stahlbeton,

Schadenfreies Bauen, Band 14, Fraunhofer IRB Verlag, 2005 • Oehme, P., Vogt, W.: Schäden an Tragwerken aus Stahl,Oehme, P., Vogt, W.: Schäden an Tragwerken aus Stahl,

Schadenfreies Bauen, Band 30, Fraunhofer IRB Verlag, 2003• Frühwald et al, 2007: Design of safe timber structures – how can we

learn from structural failures in concrete, steel and timber? Division of Structural Engineering Lund University Report TVBK 3053Structural Engineering, Lund University, Report TVBK-3053

• STEP, Timber Engineering, 1995