Assessment of Bridge Substructures-Ba5506

May 2006

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 3 HIGHWAY STRUCTURES:INSPECTION ANDMAINTENANCE

SECTION 4 ASSESSMENT

PART 9

BA 55/06

THE ASSESSMENT OF BRIDGESUBSTRUCTURES AND FOUNDATIONS,RETAINING WALLS AND BURIEDSTRUCTURES

SUMMARY

The Advice Note deals with the assessment ofstructures and structure elements where their behaviouris directly influenced by soil-structure interaction. Theadvice is updated to be consistent with BD 21/01, BA79/01 and the Boussinesq method for load dispersal isintroduced.

INSTRUCTIONS FOR USE

1. Remove Contents pages from Volume 3 andinsert new Contents pages for Volume 3 datedMay 2006.

2. Remove BA 55/00 from Volume 3, Section 4,which is superseded by this Advice Note andarchive as appropriate.

3. Insert BA 55/06 into Volume 3, Section 4, Part 9.

4. Please archive this sheet as appropriate.

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

BA 55/06

The Assessment of BridgeSubstructures and Foundations,

Retaining Walls and BuriedStructures

Summary: The Advice Note deals with the assessment of structures and structureelements where their behaviour is directly influenced by soil-structureinteraction. The advice is updated to be consistent with BD 21/01, BA 79/01and the Boussinesq method for load dispersal is introduced.


THE HIGHWAYS AGENCY

TRANSPORT SCOTLAND

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 3 Section 4Part 9 BA 55/06

May 2006

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 3 HIGHWAY STRUCTURES:INSPECTION ANDMAINTENANCE

SECTION 4 ASSESSMENT

PART 9

BA 55/06

THE ASSESSMENT OF BRIDGESUBSTRUCTURES AND FOUNDATIONS,RETAINING WALLS AND BURIEDSTRUCTURES

Contents

Chapter

1. Introduction

2. Present Requirements

3. Assessment by Calculation

4. Interim Measures and Strengthening

5. References

6. Enquiries

Annex A Dispersal of Loads


May 2006


Chapter 1Introduction

1. INTRODUCTION

1.1 The Steady State Assessment Programmeinvolves the assessment of bridge substructures(abutments and wing walls including cantilevered wingwalls and skeletal abutments) and foundations,retaining walls and buried structures. The basicrequirements for the assessment of such elements andstructures are given in BD 21 (DMRB 3.4.3). BD 86(DMRB 3.4.19) gives requirements for assessment ofstructures for the effects of Special Types GeneralOrder (STGO) and Special Order (SO) vehicles. Adviceon the management of structures that fail assessment isgiven in BA 79 (DMRB 3.4.18).

1.2 In general, the structural behaviour ofsubstructures is more complex than that ofsuperstructures. Loading applied to superstructures islargely unaffected by the resulting deformations andmovements, whereas earth pressures at soil/structureinterfaces can be influenced by movement of thestructure, e.g. changes in backfill pressure due to theforward rotation of a retaining wall on its base or soilreaction pressures on an integral-bridge end support dueto temperature generated expansion and contraction ofthe supported bridge deck. A further source ofcomplexity with the design or assessment by calculationof sub-structures is the means of ensuring internal andexternal stability. Internal stability is demonstratedwhen it is shown that the materials comprising thefabric of the structure are capable of safely resisting themost extreme combination of adverse effects. For aproper assessment of external stability the structure isassumed to be a rigid body subjected to the mostadverse combination of applied forces and supportconditions.

1.3 Considerations influencing the means ofassessing the internal and external ability of an exitingstructure differ from those influencing design. Anexisting structure represents a capital investment andthe fact that it has survived until the time of assessmentdemonstrates that the full-scale model has fulfilled itsfunctional purpose. For that reason it can be permissibleto assume that the internal and external stability isacceptable without calculation. In cases where there aresigns of distress indicating problems with internalstability it may be permissible to allow the structure toremain in service without carrying out remedial workswith the risks being managed in real-time in accordancewith the guidance given in BA 79. It may also bepermissible to allow a structure to remain in service

May 2006

where there are signs of external instability e.g. where aretaining wall of robust construction has slid forwardbut where the consequences of that and any furthersliding would not be serious. Again BA 79 would beused to manage the risk in real time.

1.4 Design standards are available for substructuresand foundations, retaining walls and buried structures,such as BD 30 (DMRB 2.1), BD 31 (2.2.12) and BD 42(DMRB 2.1.2). These documents are intended fordesign and not assessment of structures, and as such,they are likely to produce unrealistically conservativeestimates of load carrying capacity.

1.5 Realistic assessment requires that account betaken of the ability of the structure to redistribute load,recognising that the point at which an element within astructure first reaches its capacity, and perhaps firstshows local signs of movement or cracking, does notnecessarily constitute the ultimate limit state,particularly for ductile structures. The ability ofstructures to redistribute load is, therefore, particularlyrelevant in assessment.

Scope

1.6 This Advice Note is intended to cover theassessment of structures and structural elements wheretheir behaviour is directly influenced by soil structureinteraction. It does not deal with the structural aspect ofstems of free-standing piers and columns but it doescover assessment of their foundations.

Clause References

1.7 It is necessary that this Advice Note should referto specific clauses of relevant Standards and AdviceNotes. The particular versions of the documents, whichare referred to by clause numbers in the text, areindicated by including the year of document publicationwith the document reference.

Definitions and Symbols

1.8 The following are definitions of terms used in theAdvice Note:

(i) ‘Abutment’ means an end wall to whichhorizontal earth pressure loads are applied.

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Chapter 1Introduction

(ii) ‘Cover’ means the depth of fill between groundlevel and the top of a structure.

(iii) ‘Effective barrier’ means a system that is in placethat provides defined vehicle restraint againstimpact or against a vehicle crossing the barrier.

(iv) ‘Ground level’ means finished carriageway level,or the temporary ground level on which trafficcan run during construction.

(v) ‘Longitudinal’ means perpendicular to theabutment walls, or in the direction of traffic.

(vi) ‘Substructure’ means the part of the structure thatsupports the superstructure, and includesabutments and wing walls, cantilevered wingwalls and skeletal abutments.

(vii) ‘Superstructure’ means the section of thestructure over which traffic can pass (i.e. thebridge deck).

(viii) ‘Traction’ means the longitudinal live loadarising from braking and acceleration of vehicles.

(ix) ‘Transverse’ means parallel to the abutmentwalls, or perpendicular to the direction of traffic.

The following are symbols used in the Advice Note:

(i) H The depth of cover (m).

(ii) K Earth pressure coefficient to be used for agiven load in the assessment.

(iii) Ka Coefficient of active earth pressure.

(iv) Kp Coefficient of passive earth pressure.

(v) K0 Coefficient of lateral earth pressure ‘atrest’.

(vi) V Weight of one axle (kN).

(vii) Vtot Total weight of combined axles (includingany applied impact factor) (kN).

(viii) Ω Lateral dispersal distance (m)

(ix) Ωeq Lateral dispersal distance for a pair ofaxles of equal magnitude (m).

(x) Ωif Lateral dispersal distance for a pair ofaxles with an impact factor applied to oneaxle only (m).

Im

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1/2

plementation

9 This Advice Note should be used forthwith fore assessment of bridge substructures (abutments anding walls including cantilevered wing walls andeletal abutments) and foundations, retaining wallsd buried structures. The advice should be applied tosessments already in progress provided that, in theinion of the Overseeing Organisation, this would notsult in significant additional expense or delay. Itsplication to particular assessments should benfirmed with the Overseeing Organisation.

May 2006


Chapter 2Present Requirements

TS
2. PRESENT REQUIREMEN
Basic Principles

2.1 The purpose of an assessment is to ensure thatstructure is adequate for the loading applied by presentday traffic when assessed in accordance with currenttechnical standards. Loading has increased since thestructures were built and the requirements may havebecome more onerous. A further objective is todetermine the operational abnormal indivisible loadcapacity of the structure through determining reservefactors in accordance with BD 86 (DMRB 3.4.19).

Summary of Relevant Requirements

[Refer to para 1.7 for the need to cross-refer tospecific versions of DMRB Standards and AdviceNotes]

2.2 BD 21 (DMRB 3.4.3) states that if a foundation,retaining wall or a substructure shows no sign ofmovement or cracking, such items may be assumed tobe adequate and no further assessment is necessary.

2.3 BA 16 (DMRB 3.4.4) deals with spandrel anddry-stone walls, substructures and foundations. It statesthat the adequacy of such items is to be determinedfrom qualitative assessments of their general conditionincluding the significance of any defects.

2.4 BD 34 (DMRB 3.4.17) states that retaining wallsproviding structural support to a road and not designedfor Type HA surcharge or equivalent are to be assessed.Furthermore it states that structures that are thought tohave a reduced load capacity as a result of seriousdeterioration, foundation deficiency, inadequacy ofbackfilling materials or damage are also to be assessed.It also states that bridges, culverts, buried structures etc.of spans less than 1.8m (or 5m total when multi span)and retaining walls of height 1.5m need not be assessed.Additionally, culverts and buried structures of 3m orless span with cover of 1m or more, or buried to anextent that highway loading is only of marginalsignificance when compared to earth pressures, neednot be assessed.

2.5 BA 34 (DMRB 3.4.17) contains advice onsubstructures and foundations. Factors of safetystipulated for design purposes may be relaxed forassessments with the agreement of the OverseeingOrganisation. When a superstructure is to be

May 2006

strengthened or replaced, the adequacy of thesubstructure and foundations should be checked as forany new design.

Approach

2.6 The basic purpose of any assessment is todetermine whether the structure will have adequatestrength for the worst credible combinations of loadingand strength conditions at the ultimate limit state(ULS), but the means of achieving this objective willvary.

2.7 In cases where the final conclusion of theassessment process is that a structure is substandard thestructure may be strengthened, modified, replaced orallowed to remain in service in accordance with BA 79depending upon future functional requirements of thehighway and the duty under the legislation in BA 55 tosafeguard the highway user. For bridge decks, it isessential to check this requirement explicitly since anyfailure is likely to have serious consequences. Due tothis a bridge deck may be considered inadequate evenwithout any signs of movement or cracking. Forsubstructures and foundations, retaining walls andburied structures, failure is likely to be progressive andthere will usually be some warning signs (such asmovement, settlement, foundation erosion, rotation,cracking, evidence of reinforcement corrosion, lockedbearings, etc.) well before final collapse takes place.However, it should be recognised that due to thelocation of the substructure, foundations, retainingwalls or buried structures, these warning signs may notbe readily visible during a structural inspection.Generally, substructures and foundations, retainingwalls and buried structures need not be assessed bycalculation unless there are evident signs of movementor cracking determined from an inspection forassessment or any other inspection of the structure, orwhere traffic loading has a significant effect on thestructure. The structural aspects of piers are excludedfrom the remit of this Advice Note (refer to 1.6).

2.8 When assessment by calculation is deemednecessary for substructures and foundations, retainingwalls or buried structures, realistic parameters (such asearth pressure coefficients) should be used as far aspracticable. An initial assessment may be carried outusing the soil parameters estimated or established fromany available structural records. Alternatively, cautious

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Chapter 2Present Requirements

estimates or default values taken from the designstandards may be used for such parameters. If the initialassessment concludes that the structure is substandardthe OA should be consulted on the way forward.

2.9 In cases where the final conclusion of theassessment process is that a structure is substandard thestructure may be strengthened, modified, replaced orallowed to remain in service in accordance with BA 79depending upon future functional requirements of thehighway and the duty under the legislation in BA 55 tosafeguard the highway user.

May 20062/2


Chapter 3Assessment by Calculation

ATION
3. ASSESSMENT BY CALCUL
3.1 When assessment by calculation is considered tobe necessary, in the absence of assessment standards,design standards may be used. Advice on the use ofdesign provisions for assessment purposes for certaintypes of structures and structural elements are given inthe following sections.

[Refer to para 1.7 for the need to cross-refer tospecific versions of DMRB Standards and AdviceNotes]

Backfilled Retaining Walls and Bridge Abutments

3.2 BD 30 (DMRB 2.1.5) may be used for assessingby calculation all backfilled retaining walls and bridgeabutments including the older types such as massconcrete, mass brick or cellular brick walls andabutments. Clause 5 of BD 30/87 (DMRB 2.1.5) isapplicable, using the following qualifications:

(1) Clause 5.1 – Design: Embedded retaining wallsare assessed to BD 42 (DMRB 2.1.2). Furtherguidance is given in 3.5.

(2) Clause 5.2.2 – Ultimate Limit State of StructuralElements: Instead of BD 24 (DMRB 1.3.1),BD 44 (DMRB 3.4.3) should be used. Thestructure should be considered inadequate ifcalculations confirm any specific deficiencyindicated by visible signs of movement orcracking.

(3) Clause 5.2.3 – Serviceability Limit State ofStructural Elements: Not appropriate; however, ifcracks etc. have been noticed, close monitoringshould be carried out, although it should berecognised that cracking may occur at the buriedface, which is unlikely to be apparent duringinspection.

(4) Clause 5.2.4 – Ultimate Limit State of Soil:Applicable, but with the minimum factors ofsafety specified in CP2. Nominal values of deadand highway live loads should be as given inBD 21 (DMRB 3.4.3) and BD 86 (DMRB3.4.19). 12 kN/m² of vertical live load surchargemay be used in place of the Annex D and Evehicles from BD 21 (DMRB 3.4.3). 20 kN/m2of vertical live load surcharge should be used inconjunction with loading in BD 86 (DMRB

(5)

(6)

(7)

Bu

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(1)

(2)

(3)

May 2006

3.4.19). Alternatively, subject to the agreement ofthe Overseeing Organisation, a rigorous analysisof surcharge pressure accounting for soilstrengths may be appropriate.

The structure should be considered inadequate ifcalculations confirm any deficiency indicated byvisible signs of movement or cracking.

Clause 5.2.5 – Serviceability Limit State of Soil:Not applicable in general; however, ifmovements are noticed, and the structure passesthe ULS checks, close monitoring should becarried out.

Clause 5.3.2: ‘Active’ earth pressure should beused instead of ‘at rest’ earth pressure.

Clause 5.4 – General Design Considerations:Wherever possible, soil parameters should beconfirmed by tests.

ried Concrete Box Structures

For the assessment of buried concrete box andrtal frame structures, for example, culverts andways, the design standard BD 31 (DMRB 2.2.12)y be used, based on the following guidelines. Furtheridance on buried concrete box structures may also bend in BA 88 (DMRB 3.3.5).

Clause 2.1 – Limit States: The assessment ofstructural elements may be carried out using theprovisions of this clause except that assessedinadequacy for serviceability does not in itselfmean that any remedial action has to be taken.BA 79 (DMRB 3.4.18) provides further guidanceon the management of substandard structures.

Clause 2.2 – Design principles of structuralelements: Assessment of reinforced concretesection capacities should be in accordance withBD 44 (DMRB 3.4.15).

Clause 2.3 – Design principles of foundations:Foundations need only be assessed if there isevidence of settlement, tilting or sliding.

3/1



(4) Clause 2.4 – Loads: Loading requirements forassessment should be in accordance with BD 21(DMRB 3.4.3). Assessment for longitudinalforces should only be carried out if there aresigns of tilting, or cracking adjacent to thejunction of the walls and roof, which may be theresult of traction or unbalanced live loadsurcharge forces.

(5) Clause 2.5 – Load combinations: Only LoadCombination 1 (permanent loads, vertical liveloads and horizontal live load surcharge) shouldbe considered except where an assessment fortraction is required as described in 3.3(4) of thisAdvice Note.

(6) Clause 3.1.3 – Horizontal earth pressure(permanent): Where the assessment shows thatthe structure is not adequate to resist the assumedearth pressures, consideration should be given tocarrying out a site investigation to determine thenature of the backfill (including SPT values) andto reassess the structure using earth pressuresderived in accordance with BS 8002 andAppendix B of BD 31.

However, the value of the minimum earthpressure coefficient (K=0.2) given in clause3.1.3(a)(i) and Diagram A/3 should be retainedregardless of the result of site investigation. Alsothe value of the restoring earth coefficient(K = 0.6) given in Clause 3.1.3(a)(ii) andDiagrams A/4 and A/5 should be retainedregardless of the result of the site investigationunless a higher value can be justified by arigorous analysis of the soil-structure interaction.

(7) Clause 3.2.1 – Vertical Live Loading:

(a) The carriageway loading for structureswhere the cover depth is 0.6m or lessshould be Assessment Live Loading basedon the type HA UDL and KEL, singlewheel load and single axle load asdescribed in BD 21 (DMRB 3.4.3). Ifrequired by the Overseeing Organisation,the loads in BD 86 (DMRB 3.4.19) shouldbe used to assess the effects of STGO andSO vehicles.

(b) Where the depth of cover exceeds 0.6m,the loads given in Annex D or E of BD 21should be used, with a minimumtransverse spacing of 1.5m between wheelcentres of vehicles in adjacent lanes,

3/2

except that the impact factor should bereduced as described in (i) of this clause.If required by the OverseeingOrganisation, the loads in BD 86 shouldbe used to assess the effects of STGO andSO vehicles. The dynamic amplificationfactor for Special Vehicle (SV) loadingshould be reduced as described in 3.3(b)(i)of this Advice Note. The associated HAloading should be replaced by AuthorisedWeight (AW) vehicles as given in AnnexD of BD 21, and applied as a singlevehicle or convoy of vehicles, except thatthe impact factor should be reduced asdescribed in (i) of this clause. A minimumtransverse spacing of 0.7m should beprovided between wheel centres ofvehicles in adjacent lanes. The travellingspeed of SV vehicles may be differentfrom that of the associated AW vehicles.However, if a convoy of vehicles isassumed for the associated AW vehicles,SV vehicles should only be considered atthe “low” speed case.

(i) Provided that there are no abruptsurface irregularities such aspotholes or poorly backfilledtrenches in the carriageway over thestructure, the dynamic effects ofvehicle loading may be reduced toaccount for the damping effect ofthe depth of cover.

For AW vehicles, the reduced impactfactor may be taken as:

1 + 0.8(1 - 0.5H)

but not less than 1.2, where H is thedepth of cover, in metres.

For SV vehicles, the reduceddynamic amplification factor may betaken as:

1 + (DAF – 1)(1 - 0.5H)

but not less than 0.75 + 0.25DAF,where DAF is the dynamicamplification factor specified inBD 86.

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(ii) Dispersal of wheel loads through thefill should be carried out asdescribed in 3.4 of this Advice Note.

(8) Clause 3.2.2 – Live Load Surcharge: 12 kN/m² ofvertical live load surcharge should be used inplace of Annex D and E vehicles. 20 kN/m2 ofvertical live load surcharge should be used inplace of loading in BD 86.

(9) Central Reserves that are not protected fromvehicular traffic by an effective barrier should beassessed for the local effects of the AccidentalWheel Loading. Areas confined for CentralReserves with an effective barrier need not beassessed.

3.4 Dispersal of wheel and axle loads from clause3.3(7):

(1) For buried structures with less than 0.6m depth ofcover, wheel loads may be dispersed using themethod in BD 21 (DMRB 3.4.3).

(2) For assessments based on a 2D frame analysis ofa buried structure with greater than 0.6m depth ofcover, the following method should be used fordispersal of wheel loads. The background to thismethod is given in Annex A. In the event thatthese methods indicate that a structure is under-strength, it should be reassessed using theBoussinesq distribution.

(a) Single lane of vehicles: Where there isonly a single lane of vehicles, the wheelloads should be dispersed from the edgesof the wheels at a slope of two vertically toone horizontally. If the dispersal zones ofwheels on the same axle overlap in thetransverse direction (parallel to the axles),the load per metre width for each axleshould be calculated by treating the axle asa whole and dispersing the load from theedges of the outer wheels. However, wherethe longitudinal dispersal zones foradjacent axles overlap, the axle loadsshould not be combined and distributedjointly. The effects of adjacent axlesshould be considered separately andsuperimposed to give the total effect.

(b) Multiple lanes of vehicles: Where there aretwo or more lanes of vehicles, the load permetre width in the transverse direction(parallel to the axles) should be determined

May 2006

using the method in (i) and (ii) of thisclause for BD 21 (DMRB 3.4.3) loadingand BD 86 loading respectively. In thelongitudinal direction (perpendicular to theaxles), the load due to each axle should bedispersed from the edges of the loaded areaat a slope of two vertically to onehorizontally. Where the longitudinaldispersal zones for adjacent axles overlap,the loads should not be combined anddistributed jointly. The effects of adjacentaxles should be considered separately andsuperimposed to give the total effect.

(i) BD 21 (DMRB 3.4.3) loading

The following method should be used todetermine the load per metre width in thetransverse direction for the vehicle loadsspecified in Annex D and E of BD 21 inboth ‘single vehicle’ and ‘convoy ofvehicles’ configurations, with a transversespacing of 1.5m between wheel centres ofvehicles in adjacent lanes.

Where the axles from vehicles in adjacentlanes have equal load magnitudes, the loadper metre width associated with each pairof axles should be taken as:

eq

V2Ω kN/m

where V is the weight of one axle in kNand Ωeq is given by the minimum of

Ωeq = 1.7H + 3.8

and

Ωeq = 0.75H + 4.75

in metres, where H is the depth of cover inmetres.

Where an impact factor as defined in3.3(7)(b)(i) is applied to only one of theaxles, the load per metre width associatedwith the pair of axles should be taken as:

if

totVΩ kN/m

3/3



where Vtot is the total weight of both axles(including the effect of the impact factoron one of the axles) in kN and Ω is givenby the minimum of

Ωif = 1.9H + 2.8

and

Ωif = 0.9H + 4.3

in metres.

(ii) BD 86 (DMRB 3.4.19) loading

The following method should be used todetermine the load per metre width in thetransverse direction for SV vehicle loads inaccordance with BD 86, with associatedAW loading, with a transverse spacing of0.7m between wheel centres of adjacentvehicles.

Since the axle spacings are different forSV and AW vehicles, each axle must beconsidered separately and thensuperimposed to give the total effect. Theload per metre width for each axle shouldbe taken as:

ΩV

kN/m

where V is the weight of the axle in kNand Ω depends on the axle type, asfollows:

For the axles of the SV-80, SV-100,SV-150, and SV-Train vehicles, and thefront axles of the SV-TT vehicle, Ω isgiven by the minimum of:

Ω = 2.1H + 1.3

and

Ω = 1.3H + 2.9

in metres, where H is the depth of cover inmetres.

For the rear axles of the SV-TT vehicle, Ωis given by:

Ω = 1.14H + 3.69

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For the axles of an associated AW vehicle,Ω is given by the minimum of:

Ω = 1.6H + 2.2

and

Ω = 1.2H + 2.8

For the assessment of STGO and SOvehicles that are outside of the scope of theSV models defined in BD 86, theBoussinesq distribution may be used.

(c) For assessments based on a 3D analysis ofan in situ concrete buried structure withgreater than 0.6m depth of cover, unless amore rigorous method is used, the wheelloads should be dispersed from the edgesof the wheels at a slope of two vertically toone horizontally, and the effects of eachwheel should be considered separately andsuperimposed to give the total effect.

mbedded Retaining Walls

.5 BD 42 (DMRB 2.1.2) should be used for thessessment of embedded retaining walls, embedded cutnd cover tunnel walls and bridge abutments. Thistandard is applicable to retaining structures whoseain stability is provided by having a significant length

f wall stem embedded in the ground. The wall may beantilevered, propped at either the top or at excavationevel, doubly-propped or anchored.

uidance is given for retaining walls embedded in overonsolidated stiff or firm clay and also granularaterials. Walls in soft clay are not covered by thistandard.

ssessment of embedded retaining walls should bearried out using the limit state design principlesescribed in BD 42 (DMRB 2.1.2). Assessment shoulddditionally consider the serviceability limit state foralls embedded in over-consolidated soils as this isften more onerous than the ultimate limit state. Theollowing adaptations to BD 42 (DMRB 2.1.2) apply tots use for assessment purposes:

1) Clause 2.6: Only necessary to consider globaland local movements that are due to postconstruction changes and in the long term.Adjacent and supported structures should beexamined for signs of movement or cracking.

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Volume 3 Section 4 Chapter 3Assessment by Calculation
Part 9 BA 55/06
(2) Clause 2.11: Not appropriate.

(3) Clause 3.3: Applicable. First assessment ofground movements should be based uponrelevant field data and from experience of similarstructures in similar ground conditions. Adjacentstructures and buried services should beinspected for signs of movement or cracking andmonitored closely where appropriate.

(4) Clause 3.4: Not applicable for construction stage.Deformation analysis may be required forconsideration of the long-term condition.

(5) Clauses 3.5 to 3.8 – Ultimate Limit State ofStructural Elements: Confirm any specificdeficiency.

(1) Clause 3.5: Replace reference to BD 13 (DMRB1.3.14), BD 24 (DMRB 1.3.1) and BD 16(DMRB 1.3.14) with reference to BD 56 (DMRB3.4.12), BD 44 (DMRB 3.4.14) and BD 61(DMRB 3.4.16).

(7) Clause 3.7: Replace reference to BD 37 (DMRB1.3.14) with reference to BD 21 (3.4.3).

(8) Clause 3.9: Wherever possible, soil parametersand pore pressure distributions should beconfirmed by investigation and testing.

(9) Clause 3.10: Care must be taken to assess the K0likely to be present at the time of assessment. Thevalue of K0 will vary from the in situ valueinitially present. It will have been influenced bythe construction process, the flexibility of theretaining wall system and the in service period.Assessment by calculation should use the mostappropriate limit equilibrium approach describedin Clause 3.10.

(10) Clause 4.3: Drainage systems should beexamined to assess their effectiveness. Accountshould be taken of any malfunction of thedrainage system.

(11) Chapter 5: Assessment for durability should takeaccount of possible member deficiencies due tocorrosion, cracks, damage, etc. observed duringthe inspections required by BD 21 (DMRB3.4.3).

(12) Clause 6.8: Where a hard-soft piling system isused, a visual examination of the structure shouldbe made to assess if there is evidence of anyundue seepage through the soft piles.

May 2006

(13) Clause 6.9: Integral bridges are designed usingthe guidance given in BA 42 (DMRB 1.3). Forsuch structures, the abutments should beexamined for signs of cracking caused bymovements resulting from the thermal expansionand contraction of the bridge deck.

(14) Clause 7.5: A visual inspection of the prop slab(or the carriageway over it) should be carried outto assess if there is any sign of movement.

(15) Clause 8.4: For doubly-propped structures, suchas cut-and-cover tunnels, a visual inspection ofthe tunnel roof and the lower prop slab (or thecarriageway over it) should also be carried out.

(16) Clause 9.5: For structures with a stabilising base,a visual inspection of any carriageway over thebase should be carried out. If there is movement,cracking is likely to develop above the end of thebase remote from the wall.

(17) Chapter 10: Use of the Observational Method isonly applicable if there are signs of movementwhich need to be monitored and controlled.

(18) Chapter 11: Not appropriate.

Foundations

3.6 Foundations in general should be assessed byusing BD 74 (DMRB 2.1.8) with the followingqualifications:

(1) Structural elements of concrete and steel pilesshould be assessed using BD 44 (DMRB 3.4.14)and BD 56 (DMRB 3.4.11), the assessmentversion of BS 5400: Part 4 and Part 3,respectively.

(2) Load transferred from the structure above, andthe appropriate load factors, should be inaccordance with BD 21 (DMRB 3.4.3). OnlyLoad Combination 1 should be considered.

Reinforced Concrete Arch Bridges

3.7 Reinforced concrete, spandrel filled, arch bridgesshould be assessed using the same requirements as anyother type of concrete bridges except that therestraining action of the surrounding fill should betaken into account in the analysis as appropriate. Theapproach used to account for soil-structure interactionshould be agreed with the Overseeing Organisation.

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Reinforced Soil Structures

3.8 As there are only two resistance elementsproviding global stability in reinforced soil structures(the soil and the reinforcement) it is unlikely that therewill be signs of movement or cracking at the facingunless there is loss of strength in the soil or thereinforcement. It is anticipated that reinforced soilstructures could have potentially serious global failuremodes, although this opinion is not based uponexperience because the reinforced soil form of structureis relatively new and there has not been sufficient timefor these structures to deteriorate to the extent wherefailure occurs. Reinforced soil structures can sufferlocal failure of the facing, without danger of globalfailure, where the reinforcement attached to the rear ofthe facing fails due to local corrosion or some otherform of deterioration in non metallic elements.

Reinforced soil structures that show no signs ofmovement or cracking need not be assessed bycalculation unless there is evidence of corrosion orsome other form of deterioration of the reinforcementthat exceeds that allowed for in the design. It should beappreciated that the original design should have usedthe deteriorated material properties appropriate to thosethat were calculated to exist at the end of the designlife, e.g. the cross-sectional dimensions of steelreinforcing straps should have been reduced to allowfor corrosion over the design life. When assessing suchstructures, special guidance should be sought from themanufacturers of such items or other specialists. It willnot be possible to assess the structure without ‘as built’information on the fill and reinforcement properties andlayout. While the factors of safety used in assessmentshould not be lower than those used in design realistic,rather than design, values of soil parameters may beused based upon the findings of soil investigation.

For anchored earth structures, the facing panels play amajor role in providing global stability.

Corrugated Steel Buried Structures

3.9 Corrugated steel buried structures need not beassessed by calculation unless there is evidence ofcorrosion or deterioration of the corrugated steel ormovement of the structure’s profile. When assessingsuch structures, special guidance should be sought fromthe manufacturers of such items or other specialists. Itshould be noted that these structures have potentiallyserious failure modes, although such events are rare andextremely unlikely for structures designed to BD 12
(DMRB 2.2.6).
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When assessing corrugated steel buried structures up to8m span the analysis may follow the design method ofBD 12, if considered applicable, although theassessment loading should be agreed with theOverseeing Organisation. The ‘two-to-one’ method forthe dispersal of wheel and axle loads through the fillgiven for HB loading in BD 12 may not be appropriatefor vehicles with unequal wheel loads or spacings.Guidance on an alternative approach is given in 3.3(7),3.4 and Annex A of this Advice Note.

The structure should be assessed based on the presentremaining steel thickness, which should be measured.No deductions should be made for future corrosionalthough the method of BD 12 (DMRB 2.2.6) may beused to predict the future remaining life of the structure.The factors of safety used in assessment should not belower than those used in design. Realistic, rather thandesign, values of soil parameters may be used basedupon the findings of soil investigation.

Further guidance on corrugated steel buried structuresmay be found in BA 87 (DMRB 3.3.4).

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Chapter 4Interim Measures and Strengthening

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4. INTERIM MEASURES AND STRENGTHENING

4.1 In general, bridge substructures and foundations,retaining walls and buried structures covered by thisAdvice Note need not be assessed by calculation unlessthere are evident signs of movement or crackingdetermined from an inspection for assessment or anyother inspection of the structure, or where trafficloading has a significant effect (>25%) on the structure.

If the structure is assessed to have adequate capacity, itshould be observed carefully for signs of progressivedeterioration. Inadequate structures should bestrengthened or replaced as necessary. In some cases, itmay be permissible not to carry out any remedial workand to manage the structure in accordance with BA 79(DMRB 3.4.18). This should be agreed with theOverseeing Organisation who will be responsible forimplementing this option.

4.2 Before strengthening or replacing a concrete boxstructure that has failed an assessment, the guidance inBA 79 (DMRB 3.4.18) should be followed. For thisform of structure, a further assessment using some or allof the following may be beneficial:

(a) reassessment to the current version of BD 21(DMRB 3.4.3) and consider Departures fromstandards where appropriate, if the originalassessment was undertaken in accordance with anearlier version;

(b) use of soil parameters obtained from testing;

(c) moment redistribution;

(d) yield line analysis;

(e) use of worst credible strength (WCS) forconcrete and reinforcement obtained fromtesting.

4.3 The form of any interim measures necessaryfollowing an apparent assessment failure should bedetermined on the basis of the severity of the signs ofmovement or cracking, the nature of the deficiency andthe factors of safety (i.e. reserve strength) available forthe particular relevant aspects of structural response, inaccordance with BA 79 (DMRB 3.4.18).


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Chapter 5References

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5. REFERENCES

1. Design Manual for Roads and Bridges, TheStationery Office

Volume 1: Section 3: General Design

BD 13/06 (DMRB 1.3.14) – Design of Steel Bridges.Use of BS 5400-3:2000.

BD 16/82 (DMRB 1.3.14) – Design of CompositeBridges. Use of BS 5400: Part 5: 1979. [IncorporatingAmendment No. 1 dated Devember 1987]

BD 24/92 (DMRB 1.3.1) – The Design of ConcreteBridges. Use of BS 5400: Part 4: 1990.

BD 37/01 (DMRB 1.3.14) – Loads for HighwayBridges.

BA 42/96 (DMRB 1.3.12) – The Design of IntegralBridges [Incorporating Amendment No. 1 dated May2003]

BD 74/00 (DMRB 2.1.8) – Foundations.

Volume 2: Section 1: Substructures

BE 3 Reinforced and Anchored Earth Retaining Wallsand Bridge Abutments for Embankments [Revised1987] replaced by BD 70/03 (DMRB 2.1.5),Strengthened/Reinforced Soils and other Fills forRetaining Walls and Bridge Abutments.

BD 30/87 (DMRB 2.1.5) – Backfilled Retaining Wallsand Bridge Abutments.

BD 42/00 (DMRB 2.1.2) – Design of EmbeddedRetaining Walls and Bridge Abutments.

BD 74/00 (DMRB 2.1.8) – Foundations.

Volume 2: Section 2: Special Structures

BD 12/01 (DMRB 2.2.6) – Corrugated Steel BuriedStructures with Spans Greater than 0.9 Metres and up to8.0 Metres.

BD 31/01 (DMRB 2.2.12) – The Design of BuriedConcrete Box Type and Portal Frame Structures.

Volume 3: Section 3: Repair

BA 87/04 (DMRB 3.3.4) – Management of CorrugatedSteel Buried Structures.

BA 88/04 (DMRB 3.3.5) – Management of BuriedConcrete Box Structures.

Volume 3: Section 4: Assessment

BA 16/97 (DMRB 3.4.4) – The Assessment of HighwayBridges and Structures.

BD 21/01 (DMRB 3.4.3) – The Assessment of HighwayBridges and Structures.

BD 34/90 (DMRB 3.4.17) – Technical Requirementsfor the Assessment and Strengthening Programme forHighway Structures. Stage 1 – Older Short SpanBridges and Retaining Structures.

BD 44/95 (DMRB 3.4.12) – The Assessment ofConcrete Highway Bridges and Structures.

BD 56/96 (DMRB 3.4.12) – The Assessment of SteelHighway Bridges and Structures.

BD 61/96 (DMRB 3.4.16) – The Assessment ofComposite Highway Bridges and Structures.

BA 79/01 (DMRB 3.4.18) – Management of Sub-Standard Highway Structures.

BD 86/04 (DMRB 3.4.19) – Assessment of highwaybridges and structures for the effects of special typesgeneral order (STGO) and special order (SO) vehicles.

2. British Standards

BS 8002: Earth Retaining Structures.

3. Code of Practice

Institution of Structural Engineers (Joint Committee)Civil Engineering Code of Practice No. 2 (1951) CP 2 -Earth Retaining Structures. (Withdrawn)

4. Legislation

The legislation in BA 55.


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6. ENQUIRIES

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

Divisional DirectorSafety Standards & ResearchAsset Performance DivisionRoom D2No 4 BroadwayBroad Street J ORDBirmingham B15 1BL Divisional Director

Chief Road EngineerTransport ScotlandVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport WalesWelsh Assembly GovernmentCathays Parks M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Wales

Director of EngineeringThe Department for Regional DevelopmentRoads Service HeadquartersClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 6Enquiries


OADS

Annex ADispersal of Loads

ANNEX A DISPERSAL OF L

1 Introduction

1.1 This Annex provides background to thesimplified dispersal method given in 3.4.

1.2 Previously, the standard method of dispersal, asgiven in BD 31 and BD 21, was to disperse the loads ata slope of 2 vertically to 1 horizontally. Where thedispersal zones for adjacent wheels overlapped, theloads were to be considered as a group and dispersedfrom the edges of the group. This ‘BD 31 method’ isappropriate for design loading where the wheel loadsare uniform, as for the HB vehicle. However, it can beunsafe where the wheel loads vary in magnitude andspacing. It is therefore inappropriate for the assessmentvehicle loading specified in BD 21 and BD 86 forburied structures.

2 Dispersal Method for 2D Analysis

2.1 Since buried structures are frequently modelledwith a 2D frame analysis, it is particularly important toensure that the critical load per metre width to beapplied to the model is accurate, and so the transversedispersal method has a significant effect on the

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Figure 1: Illustration of

analysis. However, the precise modelling of thelongitudinal dispersal has a much smaller effect on theanalysis of the structure. The simplified dispersalmethod therefore comprises separate processes fortransverse and longitudinal dispersal.

2.2 The transverse dispersal method allows a criticalload per metre width to be calculated for each axleposition. The method has been calibrated using theBoussinesq analysis method, and is described in detailfor each configuration in the following section.

2.3 This load per metre width for each axle positionis then dispersed in the longitudinal direction using asimple 2:1 rule. However, the effects of adjacent axlesare not combined into a group where the dispersal zonesoverlap because the axle loads will have differentmagnitudes and combining them could be unsafe.Instead, each axle is considered separately and theeffects superimposed to give the total effect. Thisapproach also ensures that the centroid of the dispersedload is aligned longitudinally with the centroid of thevehicle loads. Where the dispersal zones overlap therewill be a step in the load intensity, as illustrated inFigure 1.

Longitudinal Dispersal Method

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3. BD 21 Loading

3.1 The loading specified in BD 21 for buriedstructures with greater than 0.6m fill requires vehiclesfrom Annex D or E of BD 21 to be applied, in either a‘single vehicle’ configuration or a ‘convoy of vehicles’configuration. The transverse separation of vehicles inadjacent lanes is 1.5m between wheel centres.

Figure 2: Equal Axles in Adj

3.3 The loading in Figure 2 is applied to a modelusing the Boussinesq method at various depths. Foreach depth, the vertical pressures due to these loads arecalculated. Then the total load acting on a metre-widestrip at any transverse position is found by integratingthe vertical pressures over the area of the strip. In thisway the critical load per metre width VL is calculated.

3.4 The Ωeq parameter for the ‘equal axles’ situationillustrated in Figure 2 is calculated by dividing the sumof the two axles, 2V , by the critical load per metrewidth, VL.

Ωeq =2V/ VL

3.5 Figure 3 illustrates the variation of Ωeq plottedagainst the depth of cover H, for the results of theBoussinesq analysis. It is possible to approximate thisgraph using two straight lines that lie on the safe side ofthe Boussinesq results, as shown in Figure 3, and taking

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Convoy of Vehicles

3.2 For the convoy situation, no impact factor is to beapplied. The critical loading is assumed to compriseidentical vehicles in adjacent lanes, with the axles ofthe vehicles aligned. It is therefore necessary todetermine the transverse dispersal for each axleposition, based on two equal axles side by side, asillustrated in Figure 2. The effects of vehicles in twolanes only are modelled, since vehicles in other laneshave a negligible effect on the critical load per metrewidth.

acent Lanes for BD 21 Loading

the minimum value from the following equations:

8.37.1 +=Ω Heq

and

75.475.0 +=Ω Heq

Figure 3 also shows the equivalent results using the 2 to1 method in BD 31, combining the wheel loads. Fordepths greater than around 1m, the critical load permetre width using the 2 to 1 method would be on theunsafe side of the Boussinesq results.



Figure 3: Transverse Dispers

Single Vehicle

3.6 As well as the ‘convoy’ loading configuration,BD 21 requires the ‘single vehicle’ configuration to beapplied. In this situation, one vehicle has an impactfactor applied to one axle, while vehicles in other lanesdo not have any impact factors applied to them. Aloading situation is assumed where the vehicles inadjacent lanes have axles aligned, with 1.5m transversespacing between wheel centres of vehicles in adjacentlanes. As before, only two lanes are modelled explicitly.At each axle position, there is either a pair of equalaxles, or, at the critical axle position, a pair of axles ofdifferent magnitudes (because only one vehicle has animpact factor applied to it).

3.7 The load per metre width for each pair of equalaxles can be calculated using the Ωeq values describedpreviously. However, a slightly different method isneeded for the critical axle position, where the axleshave unequal magnitudes. Using the impact factor asdefined in BA 55, which reduces linearly with depth ofcover, it is possible to repeat the Boussinesq analysisfor a pair of axles with impact factor on one axle only,at various depths.

3.8 The Ωif parameter for the ‘impact factor’ situationis calculated by dividing the sum of the two axles, Vtot ,by the critical load per metre width, VL.

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al for the Equal Axle Scenario

Ωif =Vtot/ VL

The variation of Ωif with depth of cover H is illustratedin Figure 4. The 2 to 1 method is on the unsafe side ofthe Boussinesq results, as illustrated in Figure 4. Thecurve derived from the Boussinesq analysis can beapproximated by the minimum of the followingequations:

8.29.1 +=Ω Hif

and3.49.0 +=Ω Hif

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Figure 4: Transverse Dispersal fo

For the ‘single vehicle’ loading situation, the transverselocation of the critical load per metre width for eachpair of equal axles might not be aligned with the criticalload per metre width for the critical axle position,where the impact factor is applied to one axle.However, it is conservative to base the design loadingon the worst case for each axle position.

4 BD 86 Loading

4.1 The loading specified in BD 86 comprises an SVvehicle (or STGO or SO vehicle) with associated HAloading. However, for buried concrete box structureswith greater than 0.6m depth of cover, the associatedHA loading is replaced with AW vehicles from Annex Dof BD 21.

4.2 The AW vehicle loading does not have the sameaxle spacings as the SV loading, and so the approachused for BD 21 loading (where the axles are assumed tobe aligned) cannot be used. Instead, each axle isconsidered separately, and the results are superimposed.

4.3 As specified in BA 55, the transverse spacingbetween the SV and AW vehicles is 0.7m betweenwheel centres.

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r the Impact Factor Scenario

4.4 The position of the critical load per metredepends on a large number of variables, including therelative magnitude and geometry of the vehicle loading,and the structure span. For the purposes of developing asimplified method, it has been assumed that the criticalload per metre width occurs at some position betweenthe edge wheel of the SV vehicle and 0.35m from thecentre of the edge wheel of the SV vehicle (i.e. midwaybetween the edge wheels of adjacent vehicles). It thenfollows that the critical load per metre width can beconservatively estimated by considering the load permetre width of each SV axle under the edge wheel, andthe load per metre width of each AW axle at a position0.35m from the edge wheel.

4.5 For each axle type, the load per metre width atthe relevant location, VL, is calculated using aBoussinesq analysis, and the Ω parameter for that axletype is then calculated as

Ω =V/ VL

where V is the axle load. Four axle types areconsidered, as illustrated in Figures 5-8. Figure 5illustrates the axle layout and analysis for an AWvehicle axle.

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Figure 5: AW Axle Analysis

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Figure 6: Standard SV Axle Analysis

4.6 Figures 6 and 7 illustrate the analyses for thestandard SV axle (i.e. the axle for SV-80, SV-100,SV-150 and SV Trailer Train vehicles) and the frontaxles of the SV-TT vehicle, respectively. Since theresults for these axle types are similar, the results forthe standard SV axle are used for both axle types inBA 55.

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4.7 The analysis for the SV-TT rear axles isillustrated in Figure 8. The axle is similar to an HB axlein that there are 4 equal wheel loads at approximately1m spacing. The transverse dispersal of this axle issimilar to a 2:1 dispersal of an HB axle, since Ω isapproximately linear with increasing H. The 2:1dispersal would give:

Ω=H+3.5

w

4sBsn

Figure 7: SVTT Fro

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hereas the Boussinesq approach approximates to:

Ω=1.14H+3.69

.8 In this case, the 2:1 method results, woulduggest that these were on the safe side of theoussinesq output, as for HB loading. However, the

implified Boussinesq approach in Figure 8 isevertheless used in BA 55 for consistency with the

other axle types and for improved accuracy.

nt Axle analysis

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Figure 8: SVTT Rear Axle Analysis

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Assessment of Bridge Substructures-Ba5506

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