VOLUME 2 HIGHWAY STRUCTURE DESIGN: SPECIAL …

November 2001

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 2 HIGHWAY STRUCTUREDESIGN:(SUBSTRUCTURES,SPECIAL STRUCTURESAND MATERIALS)

SECTION 2 SPECIAL STRUCTURES

PART 12

BD 31/01

THE DESIGN OF BURIED CONCRETEBOX AND PORTAL FRAMESTRUCTURES

SUMMARY

This document sets the Standard requirements for andgives advice on the design of buried concrete box andportal frame structures of precast and cast in-situconstruction up to 15 metres long from abutment toabutment and with up to 11m of fill above the roof slab.

In addition the Standard gives requirements forconstruction, installation and procurement of suchstructures.

INSTRUCTIONS FOR USE

This revised Standard is to be incorporated in theManual.

1. This document supersedes BD 31/87 andSB 3/88, which are now withdrawn.

2. Remove existing contents page for Volume 2 andinsert new contents page for Voluem 2 datedNovember 2001.

3. Remove BD 31/87 and SB 3/88, which issuperseded by BD31/01, and archive asappropriate.

4. Insert BD 31/01 in Volume 2, Section 2, Part 12.

5. Archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

BD 31/01

The Design of BuriedConcrete Box and

Portal Frame Structures

Summary: This document sets the Standard requirements for and gives advice on thedesign of buried concrete box and portal frame structures of precast and castin-situ construction up to 15 metres long from abutment to abutment and withup to 11m of fill above the roof slab.

In addition the Standard gives requirements for construction, installation andprocurement of such structures.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT

THE NATIONAL ASSEMBLY FOR WALESCYNULLIAD CENEDLAETHOL CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 2 Section 2Part 12 BD 31/01

November 2001

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments


November 2001

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 2 HIGHWAY STRUCTUREDESIGN:(SUBSTRUCTURES,SPECIAL STRUCTURESAND MATERIALS)

SECTION 2 SPECIAL STRUCTURES

PART 12

BD 31/01

THE DESIGN OF BURIED CONRETEBOX AND PORTAL FRAMESTRUCTURES

Contents

Symbols and Definitions

Chapter

1. Introduction

2. Design Principles

3. Loading

4. Design

5. Materials and Construction

6. References

7. Enquiries

Appendices

Appendix A Diagrams Showing Load Cases to beConsidered

Appendix B Calculation of Backfill Pressures inAccordance with BS8002

Appendix C Calculation of Moments and ShearsDue to Differential TemperatureEffects

Appendix D Design of Corners for OpeningMoments

Appendix E Technical Requirements and DesignChecklist


August 2001


November 2001

SYMBOLS AND DEFINITIONS

SYMBOLS

The symbols used in this Standard are defined asfollows:

As (mm²) Area of tension reinforcement.

b (mm) Width or breadth of section.

C (m) Contact width of a single wheel onthe ground.

d (mm) Effective depth to tensionreinforcement.

dcnr

(mm) Effective depth of cornerreinforcement

D (m) Depth from ground level to point onabutment walls under consideration.

ED (m) Distance of centre of wheel from

inside face of headwall.

ET (m) Distance of centre of traction force

from nearer edge of structure orlongitudinal joint.

FR

(kN) Frictional resistance of foundationbase.

FR1

, FR2

(kN) Frictional resistance of left and rightfoundation bases of a portal frame.

h (mm) Thickness of roof slab.

hna

(m) Depth from top of roof to the neutralaxis of the roof.

hw (mm) Thickness of walls

H (m) Height of cover from top surface ofthe roof to ground level.

K Earth pressure coefficient to be usedfor a given load in the design.

Ka

Coefficient of active earth pressure.

Kmin

Minimum credible coefficient forbalanced earth pressures.

Kp

Coefficient of passive earthpressure.

Kr

Coefficient of partial passive earthpressure resisting sway and sliding

Ko

Coefficient of lateral earth pressure“at rest”.

Kt

Traction factor.

Lj (m) Length of precast segmental unit

(parallel to walls).Distance between longitudinal jointsin in-situ construction.Overall transverse length (or width)of structure (=L

t) if there are no

joints.

LL (m) Overall length of structure

perpendicular to the walls.

Lt (m) Overall transverse length (or width)

of structure parallel to the walls.

L1 to L

5 (m) Dispersion widths of wheels or

axles.

Mmax

(kNm/m) Maximum “free span” moment permetre in the roof of a structure dueto vertical live loads only at theserviceability limit state.

Mf (kNm/m) Differential temperature moment for

encastre roof.

Mroof

“Relaxed” differential temperaturemoment in roof.

Mbase

“Relaxed” differential temperaturemoment in base.

n Number of wheels underconsideration.

Q* Design Loads (Refer to BS5400:Part 1).

psc (kN/m2) Horizontal Live Load surcharge

pressure.


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November 2001

R1, R

2 (kN) Horizontal reactions on the left and

right bases of a portal frame.

R* Design Resistance (Refer toBS5400: Part 1).

S (m) Transverse centre-line spacing ofwheels.

S* Design Load Effects.

Tmax

(oC) The maximum effective temperatureof the roof.

Tmin

(oC) The minimum effective temperatureof the roof.

Tq (kNm) Plan torque applied to foundation.

TR (kNm) Rotational sliding resistance of

foundation base.

V (kN) Wheel Load.

vsc (kN/m2) Vertical Live Load Surcharge

pressure.

Vtot

(kN) Applied Vertical Load.

Xclear

(m) The clear span of a single spanstructure.The maximum clear span in a multispan structure.

X (m) Effective square span of roofmeasured between the centres of thewalls.

Y (m) Effective height of box side-wallmeasured between the centres of theroof and the base slab.

Z (m) Depth below water level.

α (per oC) Coefficient of thermal expansion ofconcrete.

β Superimposed Dead Load Factor.

γ (kN/m3) Average bulk unit weight ofcompacted fill and road constructionmaterials.

γfL

Partial safety factor that takesaccount of the possibility ofunfavourable deviation of the loadsfrom their nominal values and of thereduced probability that variousloadings acting together will allattain their nominal valuessimultaneously.

γf3

Partial safety factor that takesaccount of inaccurate assessment ofthe effects of loading, unforeseenstress distribution in the structure,and variations of dimensionalaccuracy achieved in construction.

γm

Partial safety factor that takesaccount of variability in materialstrength and uncertainties in theassessment of component strength.

δ Design angle of wall friction.

δb

Design angle of base friction.

∆ (m) Midspan deflection of roof.

η Differential Temperature ReductionFactor

θ Skew angle. The acute anglebetween the edge of the structureand the normal to the abutmentwalls.

φ' Effective angle of shearingresistance.

φ'crit

Critical state angle of shearingresistance. (Refer to BS8002).

φ'max

Maximum value of φ' determinedfrom conventional triaxial test orshear box test (Refer to BS8002).

2



November 2001

DEFINITIONS

The following definitions and abbreviations are used inthis standard:

“BS5400 Part 4” means BS5400 Part 4 asimplemented by BD 24 (DMRB 1.3.1) andsupplemented by BD 57 (DMRB 1.3.7).

“BD 37” means BS5400 Part 2 as implemented by BD37 (DMRB 1.3).

“Longitudinal” means perpendicular to the walls.

“Transverse” means parallel to the walls.

“Ground Level” means finished carriageway level, orthe temporary ground level on which traffic can runduring construction.

“Hard material” means material which requires theuse of blasting, breakers or splitters for its removal.

“Cover” means the depth of fill between ground leveland the top of the roof.

“Abutment” means an end wall to which horizontalearth pressure loads are applied.

“Longitudinal Joint” means a break in the lateralstructural continuity of the structure

“HAUDL/KEL Combination” means the combinationof the Uniformly Distributed Load and Knife EdgeLoad described for HA loading in BD 37.

“Traction” means the longitudinal live load describedin BD 37 arising from braking and acceleration ofvehicles.

“Differential Temperature” means variations intemperature through a section at a given moment intime.

“Temperature Range” means the difference betweenthe highest and lowest mean temperatures that anelement is likely to sustain during its life or thespecified return period.

“Self Equilibrating Stress” means the stressesoccurring in a simply supported or continuous memberas a result of a non-linear strain diagram being imposedon a section in which plane sections remain plane.

WaterLevel

Longitudinal Joint

Longitudinal Joint

Edge

Xclear

X

hw

H

Y

D

Ground Level

h hna

Z

Lt

LL

Lj

Lj

Lj

hw

Figure 1 : Symbols for Typical Box Structure

3



November 2001

“SDL” is an abbreviation of “Superimposed DeadLoad”.

“SLS” is an abbreviation of “Serviceability LimitState”.

“ULS” is an abbreviation of “Ultimate Limit State”.

4



November 2001 1/1

Chapter 1Introduction

1. INTRODUCTION

1.1 SUMMARY

This document brings up to date the designrequirements for buried concrete box and portal framestructures of precast segmental and in-situ construction.It sets the Standard requirements for these structuresand gives advice on their design. The design rules aremore comprehensive than those in BD 31/87 and havebeen written taking into account comments receivedover a period of years. Structures with depths of coverup to 11m are included in the scope of this Standardwhich also gives the requirements for construction andinstallation and the procedures to be followed whenprocuring these structures which permit the Contractorto choose a proprietary structure that meets theOverseeing Organisation’s requirements.

1.2 EQUIVALENCE

The construction of buried concrete box and portalframe structures will normally be carried out undercontracts incorporating the Specification forHighway Works (MCHW1). In such cases productsconforming to equivalent standards or technicalspecifications of other states of the EuropeanEconomic Area and tests undertaken in other statesof the European Economic Area will be acceptablein accordance with the terms of Clauses 104 and105 in Series 100 of MCHW1. Any contract notcontaining these Clauses must contain suitableclauses of mutual recognition having the sameeffect regarding which advice should be sought.

1.3 SCOPE

(a) This document sets the Standard for buriedbox structures and portal frames for which:

(i) the depth of cover measured from thefinished ground level to the roof ofthe structure is up to 11.0m

(ii) the length of the structure between theinside faces of the outermost walls(measured perpendicular to the walls)is greater than 0.9m and up to 15.0m.

(b) The structures covered by this document areprecast or in-situ boxes or portal frames,constructed of reinforced or prestressedconcrete. The structures can be single ormulti-span with roof slabs that are eitherintegral with, or pinned to, the abutments.

(c) Structures not covered by this documentinclude:

(i) Structures with inclined abutmentwalls.

(ii) Structures with abutment walls thatare pinned at both top and bottom.

(iii) Any structure that would behave as amechanism when not backfilled.

(iv) Structures with moving bearings ateither abutment.

(v) Proprietary precast arch structures.

(vi) Structures with piled foundations.

(vii) Structures with walls constructed ofcontiguous or similar piling.

(viii) Structures with reinforced earthabutments.

(d) This document gives guidance on theinstalled structure but does not address theloads imposed during construction by thrustboring or jacking structures into place.

1.4 ENVIRONMENTAL CONSIDERATIONS

The Overseeing Organisation’s requirements forenvironmental design shall be taken into account indesigning buried box and portal structures. Volume10 of DMRB (Environmental Design) gives adviceon the use of underpasses by multiple species ofsmall mammals and fish. It illustrates variousforms of culvert design to facilitate free passage ofthese species. Often these considerations arefundamental to the determination of the span,headroom, cross-section invert and gradient of thestructure.


November 20011/2

Chapter 1Introduction

1.5 IMPLEMENTATION

This document shall be used forthwith on allschemes for the construction and improvement oftrunk roads, including motorways, currently beingprepared, provided that, in the opinion of theOverseeing Organisation this would not result insignificant additional expense or delay progress.Design Organisations shall confirm its applicationto particular schemes with the OverseeingOrganisation. In Northern Ireland, the use of thisStandard will apply on the roads designated by theOverseeing Organisation.

1.6 MANDATORY REQUIREMENTS

Sections of this document which form mandatoryrequirements of the Overseeing Organisation arehighlighted by being contained within boxes. Theremainder of the document contains advice andenlargement which is commended to designers for theirconsideration.


November 2001 2/1

Chapter 2Design Principles

2. DESIGN PRINCIPLES

2.1 LIMIT STATES

(a) In this Standard, limit state principles havebeen adopted for the design of the structuralelements and the foundations. Both anUltimate Limit State and a ServiceabilityLimit State are considered.

(b) The Ultimate Limit State (ULS) is thatrepresented by the collapse of the structuralelement concerned.

(c) The Serviceability Limit State (SLS) is thatrepresented by the condition beyond which aloss of utility or cause for public concernmay be expected and remedial actionrequired. In particular, crack width shall belimited as described in Clause 4.2.1 andthere shall not be excessive movement at thejoints capable of seriously damaging thecarriageway above. (See Clauses 4.2.3 and4.2.4)

(d) Design loads (Q*) are expressed as theproduct of nominal loads and the partialsafety factor γ

fL.

(e) Design load effects (S*) are expressed as theeffects of the product of the design loads(Q*) and the partial safety factor γ

f3.

(f) Design resistance (R*) is expressed as thenominal strength of the component dividedby the partial safety factor γ

m.

(g) The design load effects (S*) at ULS shallnot be greater than the design resistance(R*).

(h) In addition, for precast segments at SLS, thevertical deflection of the roof slab under liveloads (including foundation settlement) shallnot be greater than the limiting value givenin the Standard.

(i) The design life of buried concrete box andportal frame structures shall be 120 years.

2.2 DESIGN PRINCIPLES OFSTRUCTURAL ELEMENTS

The Overseeing Organisation’s requirements forthe design of the concrete structural elements arecontained in BS5400: Part 4 as implemented byBD 24 (DMRB 1.3.1) and supplemented by BD 57(DMRB 1.3.7) except as specified otherwise bythis Standard. These documents are hereafterreferred to collectively as “BS5400 Part 4”.

2.3 DESIGN PRINCIPLES OFFOUNDATIONS

Even when the structural elements are designed totheir required strength, the structure as a whole canfail due to overloading of the soil-structureinterface or excessive soil deformations. In orderto prevent such failures occurring, two situationsshall be investigated prior to carrying out the finalstructural design, to confirm whether or not theproposed geometry and structural form aresuitable.

(a) Sliding

The possibility of failure of the structure bysliding on its base shall be investigated atULS.

(b) Bearing Failure and Settlement of theFoundations.

The maximum net bearing pressure underthe base of the structure under nominal loadsshall be checked against the safe bearingpressure of the foundations to ensure thatthere is an adequate factor of safety againstbearing failure of the foundation and toprevent excessive settlement and differentialsettlement.


November 20012/2

Chapter 2Design Principles

2.4 LOADS

(a) The Overseeing Organisation’s requirementsfor Loading are contained in BS5400: Part 2as implemented by BD 37 (DMRB 1.3),except as specified otherwise by thisStandard. These documents are hereafterreferred to collectively as “BD 37”

(b) The following loads (which are describedmore fully in Chapter 3) shall be used in thedesign:

(i) Permanent Loads

Dead LoadsSuperimposed Dead LoadsHorizontal Earth PressureHydrostatic Pressure andBuoyancyDifferential Settlement Effects

(ii) Vertical Live Loads

HA or HB loads on thecarriagewayFootway and Cycle Track LoadingAccidental Wheel LoadingConstruction Traffic

(iii) Horizontal Live Loads

Live Load SurchargeTractionTemperature EffectsParapet CollisionAccidental SkiddingCentrifugal Load

2.5 LOAD COMBINATIONS

The load combinations to be used in the designshall be as given in BD 37. Only combinations 1, 3and 4 apply to this standard as follows:

(a) Combination 1 Permanent loads, Verticallive loads and Horizontallive load surcharge.

(b) Combination 3 Combination 1 plustemperature effects.

(c) Combination 4 Permanent loads andHorizontal live loadsurcharge plus one of thefollowing:

(i) Traction

(ii) Accidental loaddue to skidding

(iii) Centrifugal loads

(iv) Loads due tocollision withparapets

and the associatedvertical (primary) liveloads in accordance withBD 37.

Details of the load combinations and the associatedpartial safety factors to be used in the design of thestructural elements are given in Table 3.2 at theend of Clause 3.3

Details of the load combinations and associatedpartial load factors to be used in the design of thefoundations are given in Clause 3.4.


November 2001 3/1

Chapter 3Loading

3. LOADING

3.1 PERMANENT LOADS

3.1.1 Dead Load

The nominal dead load consists of the weight ofthe materials and parts of the structure that arestructural elements excluding superimposedmaterials described below.

3.1.2 Superimposed Dead Load

(a) The nominal superimposed dead loadconsists of the weight of the soil cover andthe road construction materials above thestructure. It shall be applied to the roof ofthe structure as a uniformly distributed load.

(b) The possible effects of positive archingreducing this load shall be ignored.

(c) Where consolidation or settlement of the filladjacent to a buried structure will cause

negative arching of the fill above the roof,increased loading will be generated on theroof slab. These effects can be greater if thefoundation is on hard material (seedefinitions). In the absence of reliableestimates of the effects of differentialsettlement between the structure and theadjacent ground, the superimposed deadload intensities to be applied to the roof of astructure with cover H shall be as follows:

(i) The minimum superimposed deadload intensity shall be taken as γH.

(ii) The maximum superimposed deadload intensity shall be taken as βγHwhere:

γ is the average nominal bulk densityof the fill and surfacing and β is takenfrom Figure 3.1

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

0 8 9 10 11

Cover Depth H (m)

Val

ues

of

ββ

For foundations on hard material only β=1.5+0.5(H-8)/3

β=1.15+0.35(H-8)/3

β = 1.5

β = 1.15

Figure 3.1


November 20013/2

Chapter 3Loading

3.1.3 Horizontal Earth Pressure (permanent)

(a) The nominal permanent horizontal earthpressures applied to the side walls of thestructure at a depth D below ground levelshall be taken as follows:

(i) For Combination 1 and 3 loads:

A maximum earth pressure equal toK

oγD applied simultaneously on both

side walls, or

A minimum earth pressure equal to0.2γD applied simultaneously on bothside walls

(ii) For Combination 4 loads withtraction:

A “disturbing” earth pressure equal toK

aγD acting in the same direction as

the horizontal live load, and

A “restoring” earth pressure equal to0.6γD acting in the opposite directionto the horizontal live load

If under the above loading thestructure sways in the oppositedirection to the applied horizontal liveload, this load case need not beconsidered.

(iii) For Combination 4 loads withskidding, centrifugal load or parapetcollision, when relevant:

As (i) or (ii) above to give the mostonerous effect.

(b) Values of Earth Pressure Coefficients

(i) If the backfill properties are notknown but the backfill materialscomply with the requirements ofChapter 5 the following nominaldefault values may be used:

Kmin

= 0.2K

a= 0.33

Ko

= 0.6K

p= 3.0

(ii) If the backfill properties are known,the nominal values of K

a, K

o and K

pmay be calculated from BS8002:1994, as described in Appendix B.However, as the value of K

odetermined using BS8002: 1994 doesnot account directly for effects suchas compaction pressure, thermalexpansion and cyclical loading (strainratcheting) which can lead to asignificant increase in earth pressure,the default value of 0.6 should beused for K

o (with γ

fL = 1.5) unless

such effects are taken into account.

A minimum earth pressure coefficientof not more than 0.2 should be usedwhere earth pressures are beneficial.

(iii) Pressures in excess of Ko but not

exceeding 0.5Kp may be used to resist

sliding (see Clause 4.1.2 (d) (ii) andClause 4.4.2 (c)).

(c) Where the structure is constructed in a steeptrench, the critical horizontal earth pressuresmay be applied by the native ground ratherthan by the backfill (see Clause 5.1.1(d)).This shall be investigated by consideringpotential failure planes in the native groundclose to the edge of the trench.

3.1.4 Hydrostatic Pressure

When appropriate, the effect of hydrostaticpressure and buoyancy shall be taken into account.The increase in pressure on the back of the wallsdue to hydrostatic pressure at a depth Z metresbelow water level shall be taken as 10Z(1-K)kN/m2.

3.1.5 Settlement

The settlement and differential settlement of thesub-soil under unfactored nominal permanent loadsshall be calculated from BS8004 using the siteinvestigation data. Any differential settlement ofthe soil that is likely to affect the structure shall betaken into account.


November 2001

3.2 LIVE LOADS

Vertical Live Loading

3.2.1 HA and HB Carriageway Loading

The nominal carriageway loading shall be HA orHB Loading as described in BD 37, whichever isthe more onerous.

(a) HA Loading

(i) Where the depth of cover (H) is 0.6mor less, HA loading shall consist ofthe HAUDL/KEL combination. Nodispersion through the fill of eitherthe HAUDL or the HA knife edgeload shall be applied.

(ii) For cover depths exceeding 0.6m, theHAUDL/KEL combination does notadequately model traffic loading. Inthese circumstances the HAUDL/KELcombination shall be replaced by 30Units of HB loading, dispersedthrough the fill as described inparagraph (c) below.

(iii) Account shall also be taken of thesingle 100kN HA wheel load,(dispersed through the fill asdescribed in paragraph (c) below),where this has a more severe effect onthe member under consideration thanthe loads described in (i) or (ii) above.

(b) HB Loading

(i) 45 Units of HB loading shall beapplied on structures on Trunk Roadsand Motorways. On structures onother Public Highways, 30 Units shallbe applied unless a higher value isspecified by the OverseeingOrganisation.

(ii) A minimum of 30 Units of HBloading shall be applied to allstructures including those that aredesignated to carry HA loading only.

(c) Dispersal of Wheel and Axle loads throughthe Fill

(i) All wheel loads shall be assumed tobe uniformly distributed at groundlevel over a contact area, circular orsquare in shape, based on an effectivepressure of 1.1N/mm2.

(ii) Dispersion of a wheel load throughthe fill may be assumed to occur bothlongitudinally and transversely fromthe limits of the contact area at groundlevel to the level of the top of the roofat a slope of 2 vertically to 1horizontally as shown in Figure 3.2a.Where the dispersion zones of theindividual wheels overlap, they maybe combined and distributed jointly asshown in Figure 3.2a (Zone 2). Thisapplies to adjacent wheels on thesame axle and to wheels onsucceeding axles.

(iii) As an alternative to the methoddescribed in (ii) the effects of a wheelload on the structure may be derivedusing Boussinesq’s theory of loaddispersion as given in standard textbooks on soil mechanics. TheBoussinesq Theory states that for aninfinite elastic half space the verticalpressure at a horizontal distance R anda depth z from a vertical point load Papplied to the surface is given by:

1 +

(iv) Where however any individual wheelis located close to the edge of thestructure such that its 2:1 dispersalzone is curtailed by a headwall, theincrease in pressure near to theheadwall shall be taken into account.This may be done by assuming thatthe load is dispersed transversely overthe curtailed width of the 2:1 dispersalzone, as shown in Figure 3.2b.

3P

2πz2

Rz

2

2.5

3/3

Chapter 3Loading


November 2001

(v) A wheel load not directly over thepart of the structure being consideredshall be included if its dispersion zonefalls over the part of the structure.

(d) Dispersal of the Wheel and Axle Loadsthrough the Roof Slab.

Where the dispersed width of the wheel oraxle at roof level is less than the spacingbetween adjacent joints (L

j), a further lateral

dispersal of the load may be made at 45o

down to the neutral axis of the roof slab (atdepth h

na) so that:

A single wheel is dispersed over atotal width of C+H+2h

na

An axle is dispersed over a total widthof C+(n-1)S+H+2h

na

where:

n is the number of wheels on an axle

S is the wheel spacing

and

hna

is the depth from the top of theroof to the neutral axis which may forconvenience be approximated to halfthe overall roof depth.

Dispersion through the slab at 45o cannot occurthrough a longitudinal joint. The above approachdoes not account for the distribution properties ofthe structure itself (see Clause 4.1.1(e)).

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Chapter 3Loading


November 2001

Figure 3.2

Wheelload 1

Wheelload 2

L1 L2

L3

2

1

Wheelload 1

Wheelload 2

L4

L5

Face of headwall

2

1

Zone 1: Wheel load 1 and wheel load 2dispersed over length L

1 and L

2 respectively

Zone 2: Wheel load 1 and wheel load 2combined and dispersed jointly overlength L

3

Dispersion of wheel loads through fill(lateral and longitudinal)

Figure 3.2a

Wheel load 2 dispersed over length L4

Wheel load 1 and wheel load 2 combinedand dispersed jointly over length L

5

Example of lateral dispersionthrough fill adjacent to a side wall

(longtudinal dispersion is as in Figure 3.2a)

Figure 3.2b

3/5

Chapter 3Loading


November 2001

3.2.2 Footway and Cycle Track Loading

(a) Footway and cycle track loading shallconsist of a load of 5kN/m2 applied over thetotal area of the footway or cycle trackexcept that this load may be reduced, by afactor of 0.8, to 4kN/m2 for elements thatcarry both footway/cycle track loading andcarriageway loading.

(b) The loading may be assumed to be dispersedat a slope of two vertically to onehorizontally from the edge of the load to atotal width not greater than twice thedistance from the centre of the footway tothe nearer headwall unless a more rigorousdispersion analysis is undertaken.

3.2.3 Accidental Wheel Loading on EdgeMembers

(a) Where the elements of a structure supportingouter verges, footways or cycle tracks arenot protected from vehicular traffic by aneffective barrier, they shall be designed tosustain the local effects of the accidentalwheel loading described in BD 37. Each ofthe accidental wheel loads shall be dispersedthrough the fill using the principlesdescribed in Clause 3.2.1 (c) and (d) andFigures 3.2a and 3.2b.

(b) No other vertical live load nor dispersedload from the adjacent carriageway need beconsidered in combination with theaccidental wheel loading.

3.2.4 Loading on Central Reserves

On dual carriageways the portion of structuresupporting the central reservation shall be designedfor full HA or HB carriageway loading.

3.2.5 Construction Traffic

Under the low cover conditions which prevailduring construction, the structure may be subjectedto load conditions that are more severe than thoseexperienced in normal service. During the designstage therefore, consideration should be given tothe type of construction traffic likely to be relevantat different stages, and details of the live loadcapacities of the structure under various depths ofcover should be recorded on the drawings to

ensure that these are not exceeded duringconstruction.

Horizontal Live Loads

3.2.6 Live Load Surcharge

(a) A horizontal live load surcharge shall beapplied in conjunction with all vertical liveloads. The nominal uniform horizontalpressure (p

sc) to be applied to the external

walls of the structure shall be determinedfrom the equation:

psc = K.v

sc

where K is the value of the nominal earthpressure coefficient from Clause 3.1.3 forthe wall under consideration and v

sc is the

vertical surcharge pressure applied behindthe abutments as follows:

Vertical LL vsc

HA Loading 10 kN/m2

45 Units of HB 20 kN/m2

30 Units of HB 12 kN/m2

Footpath & Cycle Track 5 kN/m2

Accidental Wheel 10 kN/m2

Construction 10 kN/m2 or asotherwisedetermined

For between 30 and 45 units of HB the valueof v

sc shall be linearly interpolated.

(b) The same value of nominal live loadsurcharge with the same partial safetyfactors γ

fL and γ

f3 shall be applied

simultaneously to both external walls exceptas follows:

In conjunction with Combination 4horizontal live loading (for tractionsee Diagrams A/4, A/5 and A/6 inAppendix A).

For calculating the maximum bearingpressure (see Diagram A/7 inAppendix A).

In these cases the live load surchargepressure shall be applied on one face only tomaximise the effect under consideration.

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Chapter 3Loading


November 2001

(c) It should be noted that when the minimumpermanent earth pressure is applied on bothsides of the structure (Clause 3.1.3(a)(i)) nolive load surcharge shall be applied to eitherwall (see Diagram A/3a and 3b).

3.2.7 Traction

(a) The structure shall be designed to resist thetraction forces (longitudinal live loads)described in BD 37.

(b) The traction force shall be appliedperpendicular to the walls of the structurefor precast construction and parallel to thedirection of traffic for in-situ construction.

(c) HA Traction

(i) For structures with cover notexceeding 0.6m, the HA traction forceshall be applied in accordance withBD 37. The loaded length forcalculating the traction force shall beoverall length of the structure in thedirection of the force, except wherethe most onerous effect on themember under consideration occurswith the structure loaded over onlypart of its length.

(ii) For structures with cover greater than0.6m, no traction force need beconsidered in conjunction with the100kN HA wheel load, but where, asin Clause 3.2.1(a)(ii), HA loading isreplaced by 30 units of HB verticalloading, this shall be applied inconjunction with 30 units of HBtraction, with γ

fL at ULS taken as 1.1.

(d) HB Traction

HB traction shall be applied in accordancewith BD 37 when one or more axles of theHB vehicle are on the structure.

(e) All traction forces shall be multiplied by Kt

before they are applied directly to the roofof the structure where:

Kt = (L

L – H)/(L

L – 0.6) but 1 ≥ K

t ≥ 0

(f) The traction force shall be applied directlyto the roof of the structure over thefollowing widths measured perpendicular tothe direction of the traction force.

HA traction - a width equal to thenotional carriageway lane width given inBD 37

HB traction - a width equal to 3 + Cmetres.

(g) In-situ boxes and portal frames are veryeffective in the lateral distribution oftraction because of the in-plane rigidity oftheir roof slabs. For in-situ structuresdesigned on a metre width basis, the tractionforce may therefore be considered to bedistributed transversely through the structureover a width of 2E

T, where E

T is the distance

of the centre of the traction force from thenearer edge of the structure (or from thenearest longitudinal joint) but not less thanhalf the traction width given in (f) above.Alternatively, if consideration is given to thelateral eccentricity of the traction force andthe resistance to plan rotation of thefoundations and walls, the traction may beconsidered to be distributed over the fullwidth L

j.

3.2.8 Load Effects Due to Temperature

(a) Temperature effects may be neglectedwhere:

(i) the cover (H) > 2m and Xclear

< 0.2Lt ,

or,

(ii) the overall length of the structureL

L ≤ 3m.

(b) In all other buried structures the variationsin mean temperature (Temperature Range)and temperature gradients within a section(Differential Temperature) shall be appliedas given below. The coefficient of thermalexpansion (α) shall be taken as 12 x 10-6 per°C for concrete except for concrete withlimestone aggregates where α may be takenas 9 x 10-6 per °C. Interaction between thebackfill and the structure due to temperatureeffects may be neglected (but see Clause3.1.3(b)(ii))

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(i) Temperature Range

The temperature range to be used forboth the in-service stage and theconstruction stages shall be inaccordance with BD 37, except thatthe expansion and contraction of theroof relative to the base of the wallsdue to thermal effects may be basedon the temperature range given below:

Temperature RangeFor ForExpansion Contraction

Box structures 10°C to Tmax

10°C to Tmin

Precast Portal 0°C to Tmax

20°C to Tmin

frames

In-situ Portal 10°C to Tmax

30°C to Tmin

frames

where Tmax

and Tmin

are the maximumand minimum effective temperaturesof the roof given in Table 3.1.

The datum temperatures for boxstructures given above are based onthe assumption that the temperature ofthe base will not be greater than 10°Cwhen the roof is at its coldest and notbe less than 10°C when the roof is atits hottest.

The datum temperatures for precastportal frames assume that when aprecast segment is installed itseffective temperature will not be lessthan 0°C nor more than 20°C. Forinsitu portal frames these figures areboth increased by 10 degrees tocompensate for effects due to the heatof hydration in the roof andsubsequent shrinkage.

The datum temperatures for portalframes may be modified if limits onthe temperature at the time ofconstruction or installation arespecified.

The effects of temperature range shallbe considered at SLS. They need notbe considered at ULS for buriedstructures because expansion orcontraction beyond the serviceabilitylimit state up to the ultimate rangewill not cause collapse of thestructure.

(ii) Differential Temperature

The effects of temperature gradientswithin a section (differentialtemperature) shall be applied to theroof slab only, but the effects of theresulting flexure on other membersshall be considered.

The differential temperature gradientswithin the roof section for both the in-service and construction stages shallbe taken from Table 3.1. Forstructures with both X

clear < 0.2L

t and

H > 0.6m, the moments and shearscalculated using the temperaturedifferences and dimensions given inBD 37, Figure 9, Group 4 should bemultiplied by the reduction factor ηgiven in Table 3.1.

Simple methods for calculating thenominal moments and shears around astructure arising from the temperaturedifferences in the roof slab taken fromBD 37, Figure 9, Group 4 are given inAppendix C.

Self equilibrating stresses due todifferential temperature (see“Definitions”) need only beconsidered for prestressed concreteroof slabs at SLS.

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November 2001

Span to Cover Minimum and Differential temperatureWidth maximumRatio effective

temperatureTemperature Reduction

difference factor

Xclear

/Lt

H (m) Tmin

Tmax

η

≥ 0.2* All depths In accordance In accordance N/Awith BD 37 with BD 37**

< 0.2 H ≤ 0.6 In accordance In accordance N/Awith BD 37 with BD 37**

0.6 < H ≤ 0.75 0oC 20oC From BD 37, 0.5Figure 9, Group 4

0.75 < H ≤ 1.0 4oC 16oC From BD 37, 0.33Figure 9, Group 4

1.0 < H ≤ 2.0 7oC 13oC From BD 37, ZeroFigure 9, Group 4

H > 2.0m Temperature effects may be neglected

Load effects due to Temperature

TABLE 3.1

* Structures for which the maximum clear span Xclear

is more than 20% of the width Lt are considered to be open to

the atmosphere and the effects of temperature are therefore taken into account in accordance with BD 37

** For fill depths greater than 0.2m the temperature differences given in Table 24 of BD 37 for 0.2m of surfacingmay be used. For roof slabs less than 600mm thick the resulting “fixed end moments” may be approximated as 0.5times the values given in Table C1 of Appendix C.

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3.2.9 Parapet Collision

The effects of parapet collision loading shall beconsidered in accordance with BD 37 where suchloading is transmitted to the buried structure.Particular care should be taken in the design ofsegmental structures to ensure that such loadingdoes not lead to the opening of joints betweensegments. In most cases with segmentalconstruction it will be necessary to designheadwalls and parapets which carry largetransverse loads, such as vehicle impact or earthpressure, as independent structures which do nottransmit these loads to precast units.

3.2.10 Skidding Loads

For structures with cover not exceeding 0.6m,loading due to skidding forces shall be consideredin accordance with BD 37. Skidding loads do notneed to be considered for structures with coverexceeding 0.6m.

3.2.11 Centrifugal Loads

For structures with cover not exceeding 0.6m,loading due to centrifugal forces shall beconsidered in accordance with BD 37. Centrifugalloads do not need to be considered for structureswith cover exceeding 0.6m.

3.3 LOAD COMBINATIONS ANDPARTIAL SAFETY FACTORS FORTHE DESIGN OF THE STRUCTURALELEMENTS

3.3.1 Load Combinations to be used for theDesign of Structural Elements

The loads to be applied simultaneously in any loadcombination for the design of the structuralelements are shown in Table 3.2.

3.3.2 Values of γγγγγfL to be used for the Design of

Structural Elements

To obtain the design loads for a given loadcombination, the relevant nominal loads describedin Clause 3.1 and 3.2 shall be multiplied by thecorresponding value of γ

fL given in Table 3.2,

except that, for the applied loads causing arelieving effect on the element underconsideration, the value of γ

fL shall be taken as 1.0.

Where the same nominal values of horizontal earthpressure or live load surcharge are appliedsimultaneously on both sides of the structure, thesame values of γ

fL (from Table 3.2) shall also be

applied to the relevant loads on each side of thestructure. (See Diagrams A/1 to A/3 in AppendixA).

3.3.3 Values of γγγγγf3 to be used in the Design ofStructural Elements

(a) The value of γf3 at SLS shall be taken as 1.0

(b) The value to γf3 at ULS shall be taken as 1.1

except:

(i) for all relieving effects γf3

shall betaken as 1.0

(ii) For disturbing effects at ULS, whereplastic methods are used in theanalysis, γ

f3 shall be taken as 1.15, as

in BS5400 Part 4.

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LOADS Limit γγγγγfL for Combinations

State1 3 4

PERMANENT LOADSWeight of concrete 3.1.1 ULS* 1.15* 1.15* 1.15*

SLS 1.00 1.00 1.00

Superimposed pavement construction (top 200mm) 3.1.2 ULS** 1.75 1.75 1.75SLS 1.20 1.20 1.20

Superimposed fill including pavement construction ULS 1.20 1.20 1.20in excess of 200mm 3.1.2 SLS 1.00 1.00 1.00

Horizontal earth pressure (using default earth ULS 1.50 1.50 1.50pressure coefficients) 3.1.3 SLS 1.00 1.00 1.00

Horizontal earth pressure (using earth pressure ULS 1.20 1.20 1.20coefficients calculated in accordance with BS8002) 3.1.3 SLS 1.00 1.00 1.00

Hydrostatic pressure and buoyancy 3.1.4 ULS 1.10 1.10 1.10SLS 1.10 1.10 1.10

Settlement 3.1.5 ULS 1.20 1.20 1.20SLS 1.00 1.00 1.00

LIVE LOADSVertical Live Loads

HA carriageway loading 3.2.1 ULS 1.50 1.25SLS 1.20 1.00

HB carriageway loading 3.2.1 ULS 1.30 1.10SLS 1.10 1.00

Footway and cycle track loads 3.2.2 ULS 1.50 1.25SLS 1.00 1.00

Accidental wheel loading 3.2.3 ULS 1.50SLS 1.20

Construction traffic 3.2.5 ULS 1.15 1.15SLS 1.00 1.00

Horizontal pressure due to live load surcharge 3.2.6 ULS 1.50 1.50 1.50SLS 1.00 1.00 1.00

HA traction and associated 3.2.7 ULS 1.25vertical SLS 1.00

HB traction live 3.2.7 ULS 1.10loads SLS 1.00

Temperature range 3.2.8 ULS N/ASLS 1.00

Differential temperature 3.2.8 ULS 1.00SLS 0.80

Parapet collision and 3.2.9 In accordance with BD 37Skidding associated 3.2.10 ULS 1.25

vertical SLS 1.00

Centrifugal load live 3.2.11 ULS 1.50loads SLS 1.00

(1)* γfL

shall be increased to at least 1.20 to compensate for inaccuracies when dead loads are not accurately assessed.(2)** γ

fL may be reduced to 1.2 and 1.1 for ULS and SLS respectively subject to the approval of the appropriate authority.

Loads, Load Combinations and Values of γγγγγfL for the Design of Structural Members

TABLE 3.2

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3.4 LOAD COMBINATIONS ANDPARTIAL SAFETY FACTORS FORTHE DESIGN OF THE FOUNDATIONS

3.4.1 Sliding

The loads to be applied simultaneously forchecking the foundations against sliding shall be asfollows:

γfL

(ULS) γf3

Dead Load 1.0 1.0

Minimum Superimposed 1.0 1.0Dead LoadBuoyancy 1.1 1.1

Traction 1.25(HA)/ 1.11.1(HB)

Vertical Live Load 1.0 1.0associated with TractionDisturbing Earth pressure 1.5/1.2* 1.1(active)

Disturbing Live Load 1.5/ 1.1Surcharge (active)Relieving earth pressure 1.0 1.0(see Clause 4.4.2 (c))

*The value of 1.5 is to be used with the defaultvalue of K and 1.2 for the value of K determinedin accordance with BS8002.

These loads shall be applied at ULS only, using thevalues of γ

fL and γ

f3 given above, (see also Diagram

A/6 in Appendix A).

If the net horizontal force is in the oppositedirection to the traction force, sliding need not beconsidered (except for rotational sliding: seeClause 4.1.2 (d) and Clause 4.4.2 (d))

3.4.2 Bearing Pressure and Settlement

The bearing pressures and settlements under thefoundations shall be calculated for the followingnominal loads as shown on Diagram A/7 inAppendix A:

Dead LoadMaximum superimposed dead loadMaximum horizontal earth pressure on bothsides of the boxHydrostatic Pressure and BuoyancyVertical Live LoadingLive Load surcharge on one side of the boxonly

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Chapter 4Design

4/1

4. DESIGN

4.1 DESIGN OF STRUCTURALELEMENTS

4.1.1 Structural Analysis

(a) The structure shall be analysed as acontinuous frame, with pin joints where thewalls are not continuous or fully integralwith the roof slab or base. The stiffness ofany corner fillets may be taken into account.Both ULS and SLS shall be considered.

(b) For boxes, an elastic compressible supportmay be assumed below the base slab exceptfor structures founded on hard material (seedefinitions). In the former case thefoundation shall be considered to be“flexible” and in the latter case thefoundation shall be considered to be “rigid”.

(c) For portal structures, where the moments inthe frame are sensitive to the rotationalstiffness of the foundations, separateanalyses shall be carried out for footingswhere the foundation shall be considered tobe (a) “rigid” and (b) “flexible”, (see4.1.1(b)), to ensure that the effects of the fullrange of possible foundation stiffnesses areconsidered.

(d) Moments and shears shall be obtained fromthe analysis at critical positions around thestructure. The most critical positions forshear will normally be at a distance “d” fromthe inside edge of the fillets (or from theinternal corners if there are no fillets) andboth shear and coexisting moment shall becalculated at these and other criticalpositions, see also Clause 4.3.3.

(e) Analysis by the Unit Width Method

(i) In most situations it will be adequateto analyse the structure on a “metrestrip” basis using a two-dimensionalframe or similar. For structures wherethe dispersal zones of adjacent wheelsoverlap as shown in Zone 2 in Figure3.2a, it will normally be adequate tobase the load per metre width due tovertical live load on the dispersionwidths determined in accordance withClauses 3.2.1(c) and (d).

(ii) For structures with shallow fill, wherethe dispersal zones of adjacent wheelsdo not overlap (and for the single HAwheel load), the above method maylead to unacceptably conservativeresults. This is because no account istaken of the lateral load-distributionproperties of the structure itself. Amore realistic distribution width forcalculating the live load effect permetre in this situation may be foundby using the Pucher Charts, or themethod for the Distribution ofConcentrated Loads on Slabs given inBS8110, or by other rigorousmethods. It should be noted howeverthat in an elastic analysis, anindividual HB wheel cannot bedistributed over a width significantlygreater than the wheel spacingbecause of the effects of adjacentwheels. Also, a single dispersed wheelload which is narrower than thesegment width (L

j) cannot be

distributed over a width wider thanthe segment width.

(f) If a three dimensional model is usedconsideration shall be given to theinteraction of live loads in adjacent lanes asdescribed in BD 37.

(g) Portal frames shall be designed for the moreonerous effects resulting from assumingthat:

(i) the base of each wall is fullyrestrained against horizontalmovement,

(ii) the base of each wall is restrainedlongitudinally by a horizontal forcenot exceeding the frictional resistanceof the footing under that wall (seeClause 4.4.2).

Where the frictional resistance of the footingis adequate to restrain the wall againsthorizontal movement then only case (i) needbe considered.


November 20014/2

Chapter 4Design

4.1.2 Skew

(a) Precast units shall be rectangular in plan andskew effects can therefore be neglected forprecast construction except that any specialskewed edge units shall be designedseparately.

(b) In-situ skewed structures which are long(transversely) relative to their span may bedesigned on the basis of either the square orthe skew span. For structures designed onthe basis of the square span however,structural elements within a width of Xsinθfrom the edge of the structure shall bedesigned on the basis of the skew span.

(c) Alternatively skewed in-situ structures maybe analysed by more rigorous methods suchas a three dimensional computer analysis.

(d) In skewed, in-situ structures the line ofthrust of the horizontal earth pressure forceson one abutment is offset laterally from theline of thrust of the earth pressure forces onthe opposite abutment. This results in a plantorque which will be resisted by torsionalfriction on the base, and, if this isinsufficient (and the skew is not too great),by a build up of passive pressure towards theobtuse corners of the abutment walls.

(i) If the line of thrust of the earthpressure forces on one wall passeswithin the middle third of the otherwall (that is if L

Ltanθ < L

j/6) this plan

twisting effect may be ignored.

(ii) If LLtanθ ≥ L

j/6 and the applied torque

(Tq) is greater than the frictional

resistance torque of the base (TR),

consideration shall be given toresisting the unbalanced torque byincreasing the horizontal earthpressure on the walls towards theobtuse corner. In this case, themaximum earth pressure anywhere onthe wall at ULS at depth D (excludinglive load surcharge) shall not exceed0.5γDK

p and the structural elements

must be designed to resist thisincreased pressure.

(iii) It should be noted that on skewedstructures where the line of thrustfrom one abutment passes close to, oroutside, the obtuse corner of theopposite abutment, passive pressurewill tend to increase, rather than resistthe tendency of the structure to rotatein plan and the danger of failure dueto rotational sliding needs to becarefully examined.

4.1.3 Stages to be Analysed

Three stages shall be considered:

(i) The completed structure backfilled up to thetop of the roof.

(ii) The structure backfilled to an intermediatelevel between roof level and finished surfacelevel, at which it is proposed to use thestructure for construction traffic.

(iii) The structure, fully backfilled, in service.

4.1.4 Load Cases to be Considered

(a) Each element of the structure shall bedesigned for each of the three stages listedabove, using the most onerous of thefollowing Combinations 1 and 3 effects:

(i) Permanent loads with maximum orminimum dead load surcharge(excluding differential settlement inStages i and ii)

(ii) Maximum or minimum horizontalearth pressures

(iii) The appropriate Combination 1 and 3live loads positioned to give the mostsevere effect to the element underconsideration.

(iv) Temperature effects (Combination 3only)

(b) In addition, for the “In Service” stage (Stageiii of Clause 4.1.3), the structure shall bedesigned for the most onerous of thefollowing Combination 4 effects:


November 2001

(i) Permanent loads with maximum orminimum dead load surcharge.

(ii) The Combination 4 horizontal earthpressures described in Clause 3.1.3.(a) (ii) or (iii).

(iii) Either traction, skidding, centrifugalor parapet collision loading.

(iv) The associated Combination 4 verticallive loads positioned to give the mostsevere effect to the element underconsideration.

(c) For Combinations 1 and 4 with traction, theload cases to be applied for the design of thestructural elements are showndiagrammatically in Appendix A diagramsA/1 – A/5.

4.2 SPECIAL REQUIREMENTS AT THESERVICEABILITY LIMIT STATE

4.2.1 Crack Control

In Combination 1 at SLS, crack widths shall belimited in accordance with BS5400 Part 4, exceptthat where the cover (H) is greater than 0.6m, thecrack width should be checked for 30 Units of HBrather than HA loading.

4.2.2 Early Thermal Cracking

Early thermal cracking need not be considered forprecast segmental construction where the segmentsare monolithic and of 3m length (L

j) or less. For

other buried structures the requirements for thecontrol of early thermal cracking are as specifiedin BS5400 Part 4, BD 28 (DMRB 1.3) and BD 37(DMRB 1.3), except that the horizontal steelrequired to resist early thermal cracking need notbe placed outside the primary longitudinalreinforcement. Guidance is also given in BA 57(DMRB 1.3.8).

4.2.3 Deflection

(a) In precast construction, and in in-situstructures with longitudinal joints that donot comply with 4.2.4 (c)(ii), the net verticaldeflection at the midspan of the roof underthe combined effects of the elastic deflectionof the structure and the short-term settlement

of the foundations under the application ofvertical live loads at SLS shall be less than0.015H. This limitation is required toprevent the occurrence of excessivemovements at longitudinal joints instructures with low covers, which canseriously damage the overlying carriageway.

(b) Where an assessment of the live loaddeflection of the roof is required, it will besufficiently accurate to estimate the midspandeflection of the roof using the empiricalformula:

∆ = 20Mmax

X2/h3 metres

where X is the effective span in metres, h isthe overall depth of the roof in millimetresand M

max is the maximum “free span”

moment in kNm/m in the roof due to verticallive load only, at SLS. The free spanmoment is calculated assuming the roof slabto be simply supported over its effectivespan (X).

(c) The foundation settlement at SLS shall betaken to be the nominal live load settlementderived as in Clause 3.1.5.

4.2.4 Longitudinal Joints

(a) The structures shall be designed toaccommodate all differential movements orto resist the forces set up by suchmovements.

(b) In precast construction the joints betweenthe segments shall be designed toaccommodate the anticipated settlements.The joints need not be designed to transferload between the segments.

(c) In most cases, where cast in-situconstruction is used, the structure acting as adeep beam is capable of accommodatingcurvatures induced by differentialsettlements and longitudinal joints should beavoided where possible, for reasons ofdurability. Where, however, the predictedmovements are so large that articulation inthe structure is required, the longitudinaljoints shall be designed either:

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(i) to accommodate all movementsresulting from the differentialsettlement of the soil as well as themaximum differential live loaddeflection between sections which arenot similarly loaded, or

(ii) to allow for the transfer of forcesbetween units or sections joints. Thisshall be checked at both ULS andSLS.

4.3 ASPECTS OF REINFORCEMENTDESIGN

4.3.1 Primary Longitudinal Reinforcement(vertical, perpendicular to the walls or parallel tothe edges of the structure).

The longitudinal reinforcement shall be providedto resist the moments and shears determined fromthe analysis in accordance with BS5400 Part 4.

4.3.2 Transverse Reinforcement in the Soffit ofthe Roof

Where the dispersed width of the applied load isless than the distance between joints (L

j),

transverse reinforcement shall be provided in thesoffit of the roof to resist the transverse bendingcaused by the local wheel effect. In this case, in theabsence of a rigorous analysis, sufficient transversesoffit reinforcement shall be provided per metre toresist a moment equal to half the longitudinalsagging moment per metre caused by the verticallive load at ULS.

Where the dispersed width of the applied load isgreater than or equal to L

j, the transverse

reinforcement shall be in accordance with Clause4.3.6.

4.3.3 Longitudinal Steel for Shear

The critical position for shear in a buried box orportal frame structure is frequently close to a pointof contra-flexure, especially in the deck, so that,over a small distance, the tension face may changefrom one face of the member to the other while theshear force stays sensibly constant. In this case thevalue of the parameter 100A

s/bd (from BS5400

Part 4) used in the calculation of the shear

resistance of the member shall be based on the areaof longitudinal steel in the less heavily reinforcedface.

4.3.4 Anchorage of Longitudinal Steel

Designers shall pay particular attention to theprovision of adequate anchorage to thelongitudinal reinforcement in accordance withBS5400 Part 4.

4.3.5 Reinforcement Details at Corners

(a) For Opening Moments (tension on the insideface)

(i) Where the bending moment applied to thecorner of a buried structure causes thatcorner to open, the resolved component ofthe compressive and tensile bending forcesin the members on either side of the cornerproduce a tensile force along the diagonal ofthe corner which tends to split the outersection of the corner from the mainstructure, as illustrated in Figure 4.1. As aresult, the use of many conventionalreinforcement details can lead to the flexuralstrength of the corner being significantlyless than the strength of the adjacentmembers. These effects shall be taken intoaccount in designing corner reinforcement.

The behaviour of opening corners isdescribed by Somerville and Taylor, Nilssonand Losberg, Jackson and Noor (see Clause6.4). A method for the design of cornerreinforcement for opening moments is givenin Appendix D.

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(b) For Closing Moments (tension on theoutside face)

(i) Where the applied moments tend toclose a corner, the area of tensionreinforcement provided around theoutside of the corner to resist the peakcorner moment may been determinedon the assumption that the effectivedepth of the section, d

cnr, is the

effective depth of the smaller adjacentmember plus half the nominal filletsize.

(ii) Where a Type 2 corner detail isprovided as shown in Figure D/1b inAppendix D, the area of the outsidelegs of either the vertical or thehorizonal hairpin bars (but not both)may be considered as contributing tothe moment of resistance providing

Fc

Fs

M

Fc FsM

A

B

A

B

(√2)Fs

(√2)Fc

Flexural crack

Crack

Figure 4.1: Cracking in an Opening Corner

they extend at least an anchoragelength beyond the end of the fillet.Where the area of the horizontal andvertical hairpin bars differ, the smallerarea should be used in the calculation.

(iii) Care should be taken to ensure thatthe bearing stress inside the bend ofthe corner bar does not exceed thelimits allowed in BS5400 Part 4 (seeClause D.4 in Appendix D). For thispurpose the mean radius of the bendmay be increased to a value notexceeding d

cnr. If a radius in excess of

this value is required to satisfy thebearing stress requirements, then thesize of the fillet or the area of tensilereinforcement should be increased.

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4.3.6 Minimum Areas of Reinforcement

The minimum area of reinforcement to be providedin the structural members shall be the greatest ofany of the following:

Longitudinal steel on any 0.15% of the net sectiontension face. per face

Transverse steel on any 0.12% of the net sectiontension face. per face

Transverse steel on the soffit 0.15% of the net sectionof the roof when the cover but see Clause 4.3.2.is less than 600mm.

Vertical steel on an internal Enough vertical steel tovertical face. resist a moment of not

less than 0.8Y3 kNm/mat ULS

Any face. Early thermal crackingsteel in accordance withBD 28 (DMRB 1.3),except that transverseearly thermal crackingsteel is not required inmembers where L

j does

not exceed 3m.

4.3.7 Cover to Reinforcement

(a) The nominal cover to reinforcement to beused for precast segments shall be inaccordance with Table 13 in BS5400 Part 4.

(b) The nominal cover to be used for cast in-situstructures shall be based on Table 13 inBS5400 plus 10mm as specified in BD 57(DMRB 1.3.7).

(c) Where the concrete is cast directly againstthe ground (as opposed to on blinding) thenominal cover shall be based on Table 13 inBS 5400 Part 4 plus a further 40mm.

(d) For cast in-situ concrete, where the surfaceis subject to flowing water the cover shall beincreased by a further 10mm to allow forerosion.

4.3.8 Fatigue

(a) Fatigue due to repeated live loading neednot be considered for structures where thecover depth (H) is more than one metre. Forother structures the requirements arecontained in BS5400 Part 4 as amended byInterim Advice Note IA.5.

(b) For cover depths greater than 0.6m theeffective stress range in unweldedreinforcing bars under Load Combination 1for the SLS shall be checked for 30 units HBloading instead of HA loading.

(c) The fatigue strength of tack weldedreinforcing bars shall be checked inaccordance with BA 40 (DRMB 1.3.4).

4.4 DESIGN OF FOUNDATIONS

4.4.1 Requirements

Consideration of sliding, global settlement anddifferential settlement is required to confirmwhether or not the proposed geometry andstructural form are suitable.

4.4.2 Sliding

(a) Requirements

Checks are required to ensure that thestructure as a whole does not fail by slidingwhen subject to traction forces and/or skeweffects. Furthermore, where the structure issupported on a number of individualfoundations, as opposed to a singlecombined base slab, each individualfoundation is required to remain stable (seeparagraph (c) below).

(b) Sliding Resistance

This is a ULS check. The loads and partialload factors to be applied are as given inClause 3.4.1 and Diagram A/6a in AppendixA for box structures and Diagram A/6b forportal frames.

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The friction force (FR) that can be developed

on the base can be determined from BS8002as follows:

FR = V

tot tan δ

b

where

Vtot

is the total applied verticalforce on the footing due topermanent loads less any uplift dueto Combination 4 loading andbuoyancy.

δb is the design angle of base

friction described in Clauses 2.2.8and 3.2.6 of BS8002: 1994.

In the absence of tests δb may be

determined from

tan δb = 0.75 tan φ'

where φ' is the design angle ofshearing resistance of the materialunder the slab defined in Clause3.2.5 of BS8002: 1994, which maybe determined using Appendix B.

For a precast structure on granularbedding δ

b shall not be taken as

greater than 20o.

No effective adhesion shall betaken between the base andfoundation material.

(c) The following relationship shall be satisfiedfor the structure as a whole:

(Traction + Disturbing Earth Pressure).γ

fL. γ

f3 < (Relieving Earth Pressure + F

R)

where FR is the sliding resistance of the

whole base for box structures, and thesum of the sliding resistances of theindividual bases for portal frames.

The relieving earth pressure shall be basedon a partial passive resistance coefficient,K

r, of 0.6, but in the event of the above

criterion for sliding not being satisfied withthis value of K

r, higher values of K

r, not

exceeding 0.5Kp, may be used if the

structural elements are designed to carry theeffects of these increased pressures.

In addition, portal frames should be checkedto ensure that they are structurally capableof carrying the applied loads shown inDiagram A/6b in Appendix A in conjunctionwith the frictional resistance forces.

If it is necessary to increase the resistanceagainst sliding this may be achieved byextending the footings of the abutments intothe backfill to mobilise a greater weight offill, or by extending the walls below theunderside of the base.

(d) Rotational Sliding

Rotational sliding tends to occur withskewed in-situ structures as discussed inClause 4.1.2(d) and also with eccentrictraction. Checks are required to ensure thatthe structure as a whole does not fail byrotational sliding when subject to eccentrictraction forces and/or skew effects.

4.4.3 Bearing Pressure

(a) The design shall ensure that the maximumnet bearing pressure under the foundationsdue to nominal loadings does not exceed theallowable net bearing pressure determined inaccordance with BS8004 as implemented byBD74 (DRMB 2.1.8). The purpose of thischeck is to ensure that there is an adequatefactor of safety against failure of thefounding soil and that settlements are keptwithin acceptable limits.

(b) The structure shall be able to accommodateany settlements either through movements atthe structural joints or through adequatestructural strength. In the latter case thestresses arising from differential settlementshall be considered at ULS and SLS as aCombination 1 load using the partial factorsof safety given in Table 3.2. (see also Clause4.2.4).

4/7

Chapter 4Design


November 2001

5.1 EXCAVATION

5.1.1 Trench Condition

(a) For a precast structure, excavation inmaterials other than hard material (seedefinitions) shall extend to at least 200mmfor a granular bedding, or to at least 125mmfor a concrete blinding with a granularoverlay, below the base of the structure. Theexcavation shall extend at least 300mmbeyond the outside wall faces.

(b) For a cast in-situ structure, excavation inmaterials other than hard material (seedefinitions) shall extend to at least 75mmbelow the base of the structure and shallextend at least 300mm beyond the outsidewall faces.

(c) Excavation in hard material (see definitions)shall extend to at least 125mm for a precaststructure, or 75mm for a cast in-situstructure, below the base of the structure,and at least 225mm beyond the outside wallfaces.

(d) For both types of structure, excavation shall,where possible, be benched to a slope nosteeper than 1.0 horizontally to 1.0 verticallyto a height of not less than 500mm above thetop of the structure or to the carriagewayformation level, whichever is lower. Wherethe side slopes are steeper and in closeproximity to the finished structure,consideration shall be given to the effectsthe native ground might have on horizontalearth pressures and thus to the earth pressureparameters that are appropriate for design.(See Clause 3.1.3(c)).

5.1.2 Embankment Condition

In the embankment situation, when theembankment is built before the structure, theembankment fill shall be benched to a slope nosteeper than 0.6 horizontally to 1.0 vertically to aheight of not less than 500mm above the top of thestructure or to the carriageway formation level,

whichever is the lower. When the structure isbackfilled before or at the same time as theconstruction of the embankment, in addition to theabove benching requirement the top of thebackfilling shall have the same width as thatrequired when the embankment is built first.

5.2 BLINDING AND BEDDING

(a) All box structures shall be founded on asuitably prepared blinding or bedding layerthat shall extend at least 300mm, or 225mmwhen the excavation is in hard material,beyond the outside wall faces of thestructure.

(b) Cast in-situ structures shall be constructedon a blinding layer of Class 20/20 concrete,as described in MCHW1 Series 1700, with aminimum thickness of 75mm.

(c) Precast units founded on other than hardmaterial shall be laid on either a two layergranular bed which shall have a minimumthickness of 200mm or a concrete bed with agranular overlay. In the granular bed thelower 150mm shall be of selected well-graded Class 6N material and the upper50mm shall be of Class 6L material asdescribed in MCHW1 Series 600, exceptthat for Class 6L, only the gradingrequirement applies and not the othermaterial properties listed in Table 6/1 ofMCHW1 (but the sulphate requirements ofClause 601 still apply). Alternatively thelower 150mm may be replaced by a 75mmminimum blinding concrete layer asdescribed in (b) above.

(d) Precast units founded on hard material shallbe laid on a two layer bed consisting of alower 75mm minimum blinding concretelayer as described in (b) above covered byan upper 50mm minimum granular layer ofClass 6L as described in (c) above.

5. MATERIALS AND CONSTRUCTION

5/1

Chapter 5Materials and Construction


November 2001

5.3 FILLING AND COMPACTION

Backfilling for either trench or embankmentcondition shall be in accordance with Clause 610(Fill to Structures) of MCHW1 except that Classes7A and 7B shall not be used. The backfillingmaterial shall be used to a height of 500mm abovethe structure or to the carriageway formation level,whichever is lower.

5.4 REINFORCED AND PRESTRESSEDCONCRETE

The Overseeing Organisation’s requirements forreinforced and prestressed concrete are containedin Series 1700 of MCHW1. The concrete mix shallbe Grade 40 or higher. In Scotland, all structuralconcrete above foundation level shall be airentrained. Further guidance is given in SeriesNG1700 of Notes for Guidance on theSpecification for Highway Works MCHW2.Detailed guidance on the assessment of groundaggressivity to concrete is given in Part 1 of BRESpecial Digest SD1 “Concrete in aggressiveground” (2001). The ground to be assessed shouldinclude the surrounding ground, the groundwater,the general embankment fill and the backfillmaterial, and any contained water or effluent to becarried by the structure. Detailed guidance on thespecification of concrete for foundations inaggressive ground is given in parts 2 and 3 of BRESpecial Digest SD1. Provision is made in theDigest for the use of various cement and cementcombinations in conjunction with aggregates ofdiffering carbonate content. The Digest alsorecommends additional protective measures forconcrete where ground water is mobile and sulfateconcentrations are high.

5.5 WATERPROOFING

(a) For both precast and in-situ construction, thetop surface, and the top of the adjoiningvertical external surfaces to a level of200mm below the soffit of the top slab, shallbe protected with a suitable bridge deckwaterproofing system in accordance withMCHW1 Series 2000. The OverseeingOrganisation’s requirements for thewaterproofing system are contained inMCHW1 Clause 2003, the preparation ofthe concrete surfaces to receive the

waterproofing system, and the laying of thewaterproofing system are contained inMCHW1 Clause 2005.

(b) For precast construction only, all otherconcrete surfaces of the box structure incontact with soil, backfill, or bedding shallbe waterproofed in accordance with therequirements for Below Ground ConcreteSurfaces. The Overseeing Organisation’srequirements for waterproofing thesesurfaces are contained in MCHW1 Clauses2004 and 2006.

(c) For in-situ construction all other concretesurfaces in contact with soil or backfill shallbe waterproofed in accordance with therequirements for Below Ground ConcreteSurfaces as given in MCHW1 Clauses 2004and 2006.

5.6 PERMEABLE DRAINAGE LAYER

A permeable drainage layer in accordance withClause 513 (Permeable Backing to Earth RetainingStructures) of MCHW1 shall be provided adjacentto all vertical buried concrete faces of boxstructures which do not carry water or effluent. Aperforated drainage pipe, not less than 150mmdiameter, with adequate facilities for rodding shallbe incorporated at the bottom of the drainage layer.This drainage pipe shall be connected to a positiveoutfall. All drainage to comply with therequirements of MCHW1, Series 500.

5.7 JOINTS

(a) Joints in structures which are to bemaintained in a dry condition internally,such as subways, shall be made watertightthrough the use of a continuous waterstop orby placing a suitable hydrophillic swellablewaterstop or compression sealant strip in thejoints and/or pointing internally withelastomeric or bitumen-based sealant wherethe units are of sufficient size to allow entry.In the case of precast segmental units sealantstrips shall be placed in the joints prior tothe butting of adjacent box segments. Theseprovisions are also applicable whengroundwater is to be protected from anyleakage of the effluent carried by thestructure or where the leakage of water or

5/2



November 2001

effluent carried by the structure could leadto erosion of the backfill. For all structuresany gaps in the joints at both internal andexternal surfaces, where not covered by awaterproofing membrane, or concreteblinding, shall be filled with a flexiblesealant, to prevent the entry of backfillingmaterials and other debris into the joints.

(b) A supporting compressible joint filler shallbe inserted prior to laying the waterproofingmembrane or flexible sealant.

(c) The Overseeing Organisations’ requirementsfor the sealing of gaps and joints are givenin MCHW1 series 2300.

Note: in-situ structures should be jointless unlessthere are unavoidable construction reasons.

5.8 SCOUR PROTECTION

In the case of culverts the design shall containadequate provision for preventing scour at the inletand outlet of the structure. This may include theuse of cut off walls below the base slab level andlateral training walls.

5/3



November 2001

Chapter 6References

6/1

6. REFERENCES

6.1 DESIGN MANUAL FOR ROADS ANDBRIDGES

Volume 1: Section 1 Approval Procedures

BD 2: Part 1 - Technical Approval of HighwayStructures (DMRB 1.1)

Volume 1: Section 3 General Design

BD 24: Design of Concrete Highway Bridges andStructures, Use of BS5400 Part 4 (as amended byInterim Advice Note IA.5) (DMRB 1.3.1)

BD 28: Early Thermal Cracking of Concrete [andAmendment No 1] (DMRB 1.3)

BD 37: Loads for Highway Bridges (DMRB 1.3)

BD 57: Design for Durability (DMRB 1.3.7)

BA 40: Fatigue Strength of Tack Welded ReinforcingBars (DMRB 1.3.4)

BA 57: Design for Durability (DMRB 1.3.8)

Volume 2: Section 1 Substructures

BD 74: Foundations: Use of BS8004: 1986 (DMRB2.1.8)

Volume 10: Environmental Design

6.2 MANUAL OF CONTRACT DOCUMENTSFOR HIGHWAY WORKS

Volume 0: Section 2 Implementing Standards

SD4: Procedures for Adoption of ProprietaryManufactured Structures (MCHW 0.2.4)

Volume 1: Specification for Highway Works(MCHW1) HMSO 1998 with revisions to 2001

Volume 2: Notes for Guidance on the Specificationfor Highway Works (MCHW2) HMSO 1998 withrevisions to 2001

Volume 4: Bills of Quantities for Highway Works(MCHW4) HMSO 1998 with revisions to 2001

6.3 BRITISH STANDARDS

BS5400: Steel, Concrete and Composite Bridges.Part 2: 1978: Specification for Loads.Part 4: 1990: Code of Practice for Design of ConcreteBridges

BS8002: 1994 Code of Practice for Earth RetainingStructures

BS8004: 1986 Foundations

BS8110: 1997 Structural Use of Concrete

6.4 BIBLIOGRAPHY

BRE Special Digest SD1. Concrete in aggressiveground. (2001).

Jackson, N. “Design of Reinforced Concrete OpeningCorners”, The Structural Engineer, Vol. 73, No. 13,pp209-213, 1995.

Jackson, N. “Design of Reinforced Concrete Cornerswith Diagonals”, The Structural Engineer, Vol. 75, Nos.23 & 24, pp417-420, 1997.

Nilsson, I.H.E and Losberg, A., “Design of ReinforcedConcrete Corners and Joints Subjected to BendingMoments”, ASCE National Structural EngineeringConvention, New Orleans, US, April 14-18, 1975.

Noor, F.A. [1977], “Ultimate Strength and Cracking ofWall Corners”, Concrete, Vol. 11, No. 7. pp 31-35,1977.

Pucher, A., “Influence Surfaces of Elastic Plates”,Springer-Verlag Wien, New York, 1977.

Somerville, G. and Taylor, H.P.J., “The Influence ofReinforcement Detailing on the Strength of ConcreteStructures”, The Structural Engineer, Vol. 50, No. 1,January 1972, pp 7-19, and discussion in The StructuralEngineer, Vol. 50, No. 8, August 1972, pp 309-321.


November 2001 7/1

7. ENQUIRIES

All technical enquiries or comments on this Standard should be sent in writing as appropriate to:

Chief Highway EngineerThe Highways AgencySt Christopher HouseSouthwark Street G CLARKELondon SE1 0TE Chief Highway Engineer

Chief Road EngineerScottish Executive Development DepartmentVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerThe National Assembly for WalesCynulliad Cenedlaethol CymruCrown BuildingsCathays Park J R REESCardiff CF10 3NQ Chief Highway Engineer

Director of EngineeringDepartment for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 7Enquiries


November 2001

The diagrams give a graphical representation of theload cases to be applied and the associated partial loadfactors to be used with these load cases. The diagramsare not fully comprehensive and should be read inconjunction with Table 3.2 and Chapters 3 and 4.

Each diagram is presented in two sections, the uppersection relating to closed box structures and the lowerto portal structures. The diagrams are also relevant tomulti-celled boxes and multi-span structures onindividual foundations, respectively.

It should be noted that where γf3 is shown as equal to

1.1 on the diagrams, it should be increased to 1.15 whenthe design is being carried out by plastic analysis (seeClause 3.3.3(b)(ii)).

In diagrams A/1 to A/5, for portal frames R1 and R

2should be calculated in accordance with Clause4.1.1(g).

APPENDIX A - DIAGRAMS SHOWING LOAD CASESTO BE CONSIDERED

A/1

Appendix ADiagrams Showing Load Cases to be Considered


November 2001

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1

Maximum SDL(βγH per m2)

Live Load in most onerous position: γfL varies, γf3 = 1.1

LLSC Earth Earth LLSCK = 0.6γfL = 1.5γf3 = 1.1

K = 0.6γfL = 1.5γf3 = 1.1

Default valuesK = 0.6γfL = 1.5γf3 = 1.1

K = 0.6γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.2γf3 = 1.1

BS 8002 coefficientsK = Ko

γfL = 1.2γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

Diagram A/1a

γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

Box Structures




K = 0.6γfL = 1.5γf3 = 1.1


K = 0.6γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.2γf3 = 1.1


γfL = 1.2γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

Diagram A/1b

γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1

R1 R2

Portal Frames

MAXIMUM VERTICAL LOAD WITH MAXIMUM HORIZONTAL LOAD

Diagram A/1

A/2



November 2001

MINIMUM VERTICAL LOAD WITH MAXIMUM HORIZONTAL LOAD

Diagram A/2

DL Structure

γfL = 1.0

γf3 = 1.0

No Live Load


K = 0.6γfL = 1.5γf3 = 1.1


K = 0.6γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.2γf3 = 1.1


γfL = 1.2γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

Diagram A/2a

γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

Box Structures

DL Structure

γfL = 1.0

γf3 = 1.0

No Live Load


K = 0.6γfL = 1.5γf3 = 1.1


K = 0.6γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

K = Ko

γfL = 1.2γf3 = 1.1


γfL = 1.2γf3 = 1.1

K = Ko

γfL = 1.5γf3 = 1.1

Diagram A/2b

γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

R1 R2

Portal Frames

A/3



November 2001

MAXIMUM VERTICAL LOAD WITH MINIMUM HORIZONTAL LOAD

Diagram A/3

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1


Earth EarthK = 0.2γfL = 1.0γf3 = 1.0


K = 0.2γfL = 1.0γf3 = 1.0

BS 8002 coefficientsK = 0.2γfL = 1.0γf3 = 1.0

Diagram A/3a


γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

Box Structures

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1


Earth EarthK = 0.2γfL = 1.0γf3 = 1.0


K = 0.2γfL = 1.0γf3 = 1.0


Diagram A/3b


γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

R1 R2

Portal Frames

A/4



November 2001

TRACTION WITH MAXIMUM VERTICAL LOAD

Diagram A/4

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1


LLSC Earth EarthK = 0.33γfL = 1.5γf3 = 1.1

K = 0.33γfL = 1.5γf3 = 1.1


K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1


Diagram A/4a

HA or HB TractionγfL = 1.25 or 1.1, γf3 = 1.1


γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

Box Structures

DL Structure

γfL = 1.15 or1.2

γf3 = 1.1



K = 0.33γfL = 1.5γf3 = 1.1


K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1


Diagram A/4b



γfL = 1.75, γf3 = 1.1

γfL = 1.2, γf3 = 1.1

200 mm

R1 R2

Portal Frames

A/5



November 2001

TRACTION WITH MINIMUM VERTICAL LOAD

Diagram A/5

DL Structure

γfL = 1.0

γf3 = 1.0

Live Load associated with Traction in most onerous position: γfL = 1.0, γf3 = 1.0


K = 0.33γfL = 1.5γf3 = 1.1


K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1


Diagram A/5a


γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

Box Structures

DL Structure

γfL = 1.0

γf3 = 1.0


K = 0.33γfL = 1.5γf3 = 1.1


K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1


Diagram A/5b


γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

R1 R2

Live Load associated with Traction in most onerous position: γfL = 1.0, γf3 = 1.0

Portal Frames

A/6



November 2001

SLIDING

Diagram A/6

DL Structure

γfL = 1.0

γf3 = 1.0

Live Load associated with Traction in most onerous position : γfL = 1.0 γf3 = 1.0


K = 0.33γfL = 1.5γf3 = 1.1

Default valuesKr < 1.5γfL = 1.0γf3 = 1.0

K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1

BS 8002 coefficientsKr < 0.5Kp

γfL = 1.0γf3 = 1.0

Diagram A/6a


FrictionBuoyancy: γfL = 1.1, γf3 = 1.1

γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

Box Structures

See clause 4.4.2 (c)


DL Structure

γfL = 1.0

γf3 = 1.0

Live Load associated with Traction in most onerous position : γfL = 1.0 γf3 = 1.0


K = 0.33γfL = 1.5γf3 = 1.1

Default valuesKr < 1.5γfL = 1.0γf3 = 1.0

K = Ka

γfL = 1.5γf3 = 1.1

K = Ka

γfL = 1.2γf3 = 1.1

BS 8002 coefficientsKr < 0.5Kp

γfL = 1.0γf3 = 1.0

Diagram A/6b


Buoyancy: γfL = 1.1, γf3 = 1.1

γfL = 1.0, γf3 = 1.0

γfL = 1.0, γf3 = 1.0

200 mmMinimum SDL

(γH per m2)

R1 £ FR1 R2 £ FR2

Portal Frames



A/7



November 2001

BEARING PRESSURE (NOMINAL LOADS)

Diagram A/7

DL Structure

Live Load in most onerous position

LLSC Earth Earth

K = 0.6 K = 0.6 Default values K = 0.6

K = Ko K = Ko BS 8002 coefficients K = Ko

Diagram A/7a


Buoyancy

All loads are “nominal”

Box Structures

DL Structure

Live Load in most onerous position

LLSC Earth Earth

K = 0.6 K = 0.6 Default values K = 0.6

K = Ko K = Ko BS 8002 coefficients K = Ko

Diagram A/7b


BuoyancyR1 £ FR1 R2 £ FR2

All loads are “nominal”

Portal Frames

A/8



November 2001

B.1 The backfill materials permitted by this standardare granular and cohesionless as described in Chapter 5.

B.2 In BS8002 the earth pressure coefficients Ka, K

oand K

p are related to φ'

crit and φ'

max. However, φ'

max is

dependent on the SPT which, in most cases is notknown. In this case the pressure coefficients may bebased on φ'

crit alone as follows:

Calculate the “representative value” of φ'crit

fromBS8002 Clause 2.2.4.

Take the design value of φ' equal to φ'

crit

Ko is given by:

Ko = 1 – sin φ'

Take Ka and K

p from Figures A/1 and A/2 in

Annex A to BS8002 using an angle of wallfriction δ, not exceeding 20o and interpolating forδ/φ'.

Alternatively if wall friction is to be ignored(δ = 0), K

a and K

p can be calculated as follows:

Ka = (1 – sin φ')/(1 + sin φ')

Kp = 1/K

a

Earth pressures based on the above values of Ka,

Ko and K

p are nominal

B.3 If however the backfill is already in place (in anassessment for example), or if for some other reason theSPT value of the backfill is known, the procedure forcalculating the earth pressure coefficients is as follows:

Calculate the “representative value” of φ'crit

andφ'

max from BS8002 clause 2.2.4 and Table 3.

Take the design value of tan φ' equal to the lesserof tan φ'

max /1.2 or tan φ'

crit

Thus find φ'

Calculate Ka, K

o and K

p using φ'

as described in

B.2 above.

APPENDIX B - CALCULATION OF BACKFILLPRESSURES IN ACCORDANCE WITH BS8002

B/1

Appendix BCalculation of Backfill Pressures in Accordance with BS8002


November 2001

C.1 In buried box and portal frame structuresDifferential Temperature is considered to occur in theroof slab only.

C.2 Fixed End Moments In Roof

If the roof slab was encastre at both ends and subject tothe differential temperatures specified in BD 37,Figure 9, Group 4, the nominal “fixed end moments”,M

f, occurring in the roof (based on α = 12 x 10-6 and

Youngs modulus = 32 kN/mm2) would be as shown inTable C1.

APPENDIX C - CALCULATION OF MOMENTS ANDSHEARS DUE TO DIFFERENTIAL TEMPERATUREEFFECTS

Roof Thickness h (mm) 200 400 600 800

Mf with Positive Temperature Difference (kNm/m) 11 50 104 154

Mf with Reverse Temperature Difference (kNm/m) -1 -4 -12 -21

Encastre Differential Temperature Moment, Mfbased on BD 37, Figure 9, Group 4

TABLE C1

C.3 Single Span Integral Structures

If the fixed end moment restraints are then relaxed, theresulting nominal moments in single span integralstructures may conservatively be taken as follows:

Roof Slab: Mroof

= Mf /(1+λ)

where λ = 0.5 (h/hw)3 (Y/X)

Base Slab: Mbase

= -Mroof

/2

Walls: Vary linearly from Mroof

at the top toM

base at the bottom.

Positive moments are those which cause tensionon the inner face of the member.

C.4 Other Structures

For other structures the differential temperaturemoments should be calculated from M

f using moment

distribution, or by other rigorous methods.

C.5 Reduction Factors

Where Xclear

< 0.2Lt the moments and shears found from

the above analyses should be multiplied by thereduction factor η given in Table 3.1 and Clause 3.2.8.

C/1

Appendix CCalculation of Moments and Shears Due to Differential Temperature Effects


November 2001

D.1 As described in Clause 4.3.5(a), the use of someconventional reinforcement details can lead to theflexural strength of opening corners (i.e. those wherethe applied moment produces tension on the insideface) being significantly less than the strength of theadjacent members. The following method may be usedfor the design of reinforcement for opening corners.

D.2 The tension reinforcement on the inside face ofthe members on either side of an opening corner shouldbe extended across the full width of the corner andanchored using U-bars or “hairpin” bars in the Type 1or Type 2 configurations shown in Figure D/1a and b.For the Type 2 corner detail the legs of the U-bars or“hairpin” bars in the outer face should be extended atleast 20 bar diameters beyond the face of the adjacentmember as shown in Figure D/1b. This dimension mayneed to be increased if the bar is also considered toform part of the tension steel for a closing moment (seeClause 4.3.5(b)(ii)).

D.3 Fillet bars should be provided to resist the peakopening corner moment, based on an effective depth d

fas shown in Figure D/2, which assumes that a crack hasdeveloped along BB and the concrete beyond it isineffective. The area of the fillet bars should not be lessthen half the area the main inner face tension bars ineither adjacent member.

D.4 In the design of tension reinforcement to resistcorner moments, the maximum tension that may beconsidered to exist in any bar under ultimate load at adistance L

st from the start of a bend should not exceed

fbπφL

st + F

bt or f

y/γ

ms where:

fb is the allowable bond stress in accordance with

BS5400 Part 4.

φ is the bar diameter.

Fbt is the allowable tensile force in a bend in a bar

in respect of the bearing stress inside that bend,as given in Clause 5.8.6.9 of BS5400 Part 4.

fy is the characteristic strength of the

reinforcement

D.5 When a Type 1 or Type 2 corner detail is used,the resolved component of the tension in the verticaland horizontal U-bars or “hairpin” bars may be taken tosupplement the fillet bars in resisting the cornermoment as shown in Figure D/2 provided the tensionsin the bars are limited in accordance with Clause D.4.

D.6 When for practical reasons it is essential to havea buried structure without a fillet, inclined “fillet bars”should be provided inside the corner (if there is anopening moment) and the corner should be designed asabove assuming the fillet has zero size.

APPENDIX D - DESIGN OF CORNERS FOR OPENINGMOMENTS

D/1

Appendix DDesign of Corners for Opening Moments


November 2001

Dimension B* (20 bardiameters, minimum)

E

Dimension A*(Full anchorage)

E

Fillet barVertical hairpin bar External corner bar

Horizontalhairpinbar

A

A

Fillet bar

Vertical hairpinbar

Horizontalhairpin bar

B

B

SECTION AA SECTION BB

Figure D/1a: Type 1 Corner Detail

* If the outer leg of the U-bar or “hairpin” is used to contribute to the tension steel of the closing corner momentthe location of the end of the leg, E, is determined by Dimension A (as Clause 4.3.5 (b)(iv)). Otherwise, E isdetermined by Dimension B (as Clause D.2).

Figure D/1b : Type 2 Corner Detail

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November 2001

The figure shown is a Type 2 corner detail but the design procedure is the same for a Type 1 detail. Thecompression face of the effective corner is assumed to lie along BB.

df

is the distance from the fillet bar to BB

Laf

is the lever arm to the fillet reinforcement

La

is the lever arm to the vertical and horizontal reinforcement at C

Fsh

is the tensile force in the horizontal bars at point C determined in accordance with Clause D.4

Fsv

is the tensile force in the vertical bars at point C determined in accordance with Clause D.4

Fsf

is the design strength of the fillet bars.

Moment of resistance of the corner is given by

Mr = Fsf.L

af + (F

sh + F

sv).L

a/√2

The moment of resistance of the members should also be checked at the ends of the fillet

Figure D/2 : Design Method for Opening Corners

A

B

B

A

FcFsv

Fsh

Fsf

La

df

Laf

M

M

Main bars

Fillet bar

C

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November 2001

E.1 Cast in-situ buried concrete box type structuresare non-proprietary and the Overseeing Organisationdoes not have any special requirements for theprocedure to be followed, including technical approval,when they are specified. The complete design is to becarried out by the Design Organisation.

E.2 Precast segmental buried concrete box typestructures are normally proprietary products and theOverseeing Organisation’s procedures to be followed,when specifying a proprietary manufactured structureare given in Standard SD4 (MCHW 0.2.4).

E.3 The Design Organisation, prior to the invitationof tenders, normally submits an Outline Approval inPrinciple (O/AIP), as part of the Technical Approval/Technical Appraisal procedure, described in BD 2(DMRB 1.1), for each alternative type of proprietarystructure which satisfies the requirements of thescheme, (including corrugated steel buried structureswhere appropriate) and contains a Schedule ofEmployer’s Requirements. (Note: Currently BD 2 is notapplicable in Northern Ireland, however, therequirements as outlined in this Appendix may befollowed as appropriate. An update to BD 2 is plannedfor issue during 2002 and is intended to be applicable inNorthern Ireland.) For precast buried concrete box typestructures the Schedule of Employer’s Requirementsshould include the following information:

i. Location plan and name of structure.

ii. Environmental considerations (see clause 1.4).

iii. Long Section along centre-line of structure.

iv. Finished levels of carriageways and side slopeswithin designated outline.

v. Skew of structure.

vi. Minimum internal span of structure.

vii. Minimum headroom of structure.

viii. Hydraulic requirements or clearance envelope, ifany and requirements for invert protection.

ix. Gradient of invert.

x. End detail requirements, including requirementsfor headwalls.

xi. Highway loading requirements.

xii. Details of existing soils and other materialswithin the designated outline.

xiii. Requirements to satisfy any anticipateddifferential settlement.

xiv. Details of ground water, anticipated hydrostaticpressure and buoyancy effects (if applicable).

xv. Aggressivity of existing soil, ground water andcontained water/effluent and of any requirementfor sulphate-resisting or other type of cement.

xvi. Drainage requirements including provision forpumping.

xvii. Public safety requirements including lighting andprotection for pedestrians around headwalls.

xviii. Aesthetic requirements.

xix. Protection of structures against vehicle impact.

xx. Identification of any design elements outside thescope of the Proprietary Manufacturer’sResponsibilities, and/or identification of anyelements in addition to the proprietary boxstructure requiring design.

xxi. Any other essential requirements.

Special requirements should be avoided. Howeverwhere the circumstances are such that they are justifiedthen care should be taken to avoid requirementsimplicitly favouring the system of a particularmanufacturer.

E.4 Subsequent to the award of contract and prior tothe commencement of construction, the Contractornormally completes the AIP form (containing theSchedule of Employer’s Requirements), the design andthe design certificate (including check certificate whereappropriate) and submits these for approval/appraisal.The full design normally contains the followingadditional information relating to the particularproprietary product which forms the basis of the design.

APPENDIX E - TECHNICAL REQUIREMENTS ANDDESIGN CHECKLIST

E/1

Appendix ETechnical Requirements and Design Checklist


November 2001

a. Structure Geometry - Structure type- Internal span- Internal height- Length between joints

b. Materials - Whether reinforced orprestressed concrete

- Concrete mix and grade- Concrete sections

adopted- Details of

reinforcement/prestressing

c. Confirmation of foundation depth and material

d. Bedding and/or blinding details

e. Joints and joint treatment including sealing forwater-tightness (if required)

f. Details of waterproofing system for roof andwalls and of other waterproofing materials to beused

g. End treatment - Geometry- Concrete type and

reinforcement

h. Excavation and construction sequence withdrawings

E.5 The further stages in the post award contractprocedures are given in Chapter 4 of Standard SD4(MCHW 0.2.4).

E.6 Construction copies of the following will berequired for inclusion in the maintenance manual andhealth and safety file:

The completed AIP used for design.A set of as constructed drawings.A design summary for information listed inClause E.4.Design and Check certificates.Other relevant information.

E/2

Appendix ETechnical Requirements and Design Checklist

Documents

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