DMRB VOLUME 3 SECTION 4 PART 7 - Standards …

1 June 1993 DESIGN MANUAL FOR ROADS AND BRIDGESVOLUME 3 HIGHWAY STRUCTURES:INSPECTION ANDMAINTENANCESECTION 4 ASSESSMENTPART 7BD 48/93 THE ASSESSMENT ANDSTRENGTHENING OF HIGHWAYBRIDGE SUPPORTSSUMMARYThis Departmental Standard gives the requirements forthe assessment and strengthening of existing bridgesupports for vehicle collision FOR USEThis is a new document to be incorporated into existing contents pages for VOLUME 3 andinsert new contents pages dated BD 48/93 into VOLUME 3, SECTION this sheet as Assessment andStrengthening of Highway BridgeSupportsSummary: This Departmental Standard gives the requirements for the assessment andstrengthening of existing bridge supports for vehicle collision and Published by theabove Overseeing Departments Crown Copyright 1993 Price: THE DEPARTMENT OF TRANSPORT BD 48/93 THE SCOTTISH OFFICE INDUSTRY DEPARTMENTTHE WELSH OFFICEY SWYDDFA GYMREIGTHE DEPARTMENT OF THE ENVIRONMENT FORNORTHERN IRELANDV olume 3 SECTION 4 part 7 BD 48/93 Registration of AmendmentsELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLEDREGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsVolume 3 SECTION 4 Registration of AmendmentsPart 7 BD 48/93 ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLEDJune 1993 REGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsDESIGN MANUAL FOR ROADS AND BRIDGESELECTRONIC COPY - NOT FOR USE

2 OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLEDVOLUME 3 HIGHWAYSTRUCTURES:INSPECTION ANDMAINTENANCESECTION 4 ASSESSMENTPART 7BD 48/93 THE ASSESSMENT ANDSTRENGTHENING OF HIGHWAYBRIDGE and 3 SECTION 4 Chapter 1 part 7 BD 48/93 IntroductionELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED1/11. Standard shall be used for the assessmentand possible subsequent strengthening of existing bridgesupports subject to the enhanced collision loading givenin Table 2/1. For assessment and strengthening purposesthe collision loading given in this document supersedesClause and Table 15 of BD 37/88 ( dmrb ). is intended that existing bridge supports shouldbe made sufficiently strong or be adequately protected, toresist collision impact forces without allowing collapse ofthe supported structure. It is accepted that in doing thisthey may well suffer considerable damage and need to berepaired or Standard is applicable to all road bridges(including accommodation bridges) and footbridges overthe carriageway of roads, with supports located of the edge of the carriageway; the carriagewaybeing defined as in Clause of BD 37/88 ( ).

3 And signal gantries and pipe bridges neednot be assessed for impact loading using analyticalmethods. However, each structure should be individuallyassessed to ensure that it is adequately protected by asafety fence or barrier which has a containment levelequal to or greater than an open sided box beam. In thosesituations where it appears that there is a significant riskof a gantry being hit by an errant vehicle, eg a gantry legclose to the point of a bifurcation, consideration should begiven to providing some extra protection to the supportsuch as a plinth or Standard should be used forthwith forassessments of highway bridge supports. Supports whichfail these assessments should be strengthened (orprotected) in accordance with this Standard. Theprogrammes for assessment and strengthening will bedetermined by the Overseeing programme of research involving theconsideration of the varying probabilities of occurrenceand the modes of application of collision loading, iscurrently under way.

4 Practical results for furtherguidance may not be available for some time. It istherefore recommended that only supports considered tobe particularly at risk or which form part of a criticalconstruction programme, should be strengthened untilfurther guidance is 3 SECTION 4 Chapter 2 part 7 BD 48/93 Collision LoadingELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED2/12. COLLISION LOADINGN ominal nominal collision loads acting horizontallyon bridge supports together with their height ofapplication, are given in Table 2/1. Supports shall beassessed for their ability to resist the main and residualload components acting simultaneously. Loads normal tothe carriageway shall be considered separately from loadsparallel to the protective plinths high are providedto footbridge supports, they should be assessed for thecombined main and residual load components.

5 Thesupports themselves should be designed for the residualload component shown in Table 2/1. all bridge elements except foundations andelastomeric bearings, the effects due to vehicle collisionneed only be considered at the ultimate limit state. Thevalue of the partial safety factor for loading ( shall befLtaken as for both assessment and strengthening. However, for foundations where a permissible stressapproach is used, and elastomeric bearings which areconsidered at the serviceability limit state only, a (fLfactor of , shall be collision loading on supports, no primary orother secondary live loads need be considered. However,following an impact the overall integrity of a damagedstructure shall be assessed at the ultimate limit state,under the primary live loading derived using combination1 serviceability limit state ( factors given in Table 1 offLBD 37/88 ( dmrb ); ( shall be taken as Whenf3HB loading is applied, only 30 units need be normal toLoad parallel toPoint of application onthe carriagewaythe carriagewaybridge supportbelowbelowMain load5001000At the most severe pointcomponentbetween and ground leveladjacent to supportResidual250500At the most severe pointloadbetween 1m and 3mcomponent(100)(100)above ground leveladjacent to supportNote:Figures shown in brackets shall be applied to footbridges in urban locations with robust plinths (see ).))))

6 TABLE 2/1 Collision Loads on Supports of Bridges Over Highways3030%mVolume 3 SECTION 4 Chapter 3 part 7 BD 48/93 AnalysisELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED3/13. two methods of analysis which can beadopted for impact loading quasi-static method in which theimpact force is replaced by an equivalent staticload. (See ). rigorous dynamic quasi-static approach is simpler to apply than thedynamic analysis but it may yield more conservativeresults. For the assessment of existing bridge supports, aquasi-static analysis should be carried out in the firstinstance. If the structure fails this assessment dynamicanalysis may be assessment of bridge supports, the nominalloads given in Table 2/1 can be multiplied by a reductionfactor of:where m is the mass of the support member in tonnes. This reduction is based on momentum conservation andassumes that the support member alone participates in thedynamic response.

7 Hence the deck loading or weight offoundation cannot be included when calculating m. Forthe assessment of bridge supports this reduced value ofimpact loading shall be applied statically. has been shown by laboratory impact tests (Ref2), that a considerable amount of the impact energy is lostthrough local damage and vibration. Therefore, for theassessment of foundations, deck slabs and other membersdirectly connected to the support member, the loads inTable 2/1 can be reduced by 50% and treated as actingstatically. For more remote members, for example pilingsystems, the loads shown in Table 2/1 can be reduced by75% and treated as acting 3 SECTION 4 Chapter 4 part 7 BD 48/93 Assessment and StrengtheningELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYJune 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED4/14. ASSESSMENT AND bridge supports are to be assessed inconjunction with Departmental Standard BD 44 ( ).

8 However, when considering shear, flexure andbending the values of (, ( and ( may be reduced bymvmcmb10%, for both characteristic and worst credible strengthapplications. Steel supports shall be assessed inconjunction with BD 13 ( dmrb ) and foundations inconjunction with current Departmental Standards andwith relevant British Standards in so far as they have beenimplemented by the Overseeing Department. However,when checking for bearing, the ultimate bearing capacityshall be an assessment it should be recognised that inmany instances considerable damage may be sustained byan individual support member including failure of themember, its bearings or foundations, without the structureitself failing. Large rotations, lateral displacements andlocal damage may also occur. However under thesemovements the support system as a whole must still becapable of carrying the imposed load from the deckabove. (See ). Non-linear methods such as plasticanalysis may be supports, including ramps and staircasesof footbridges, whose removal would not affect theoverall integrity of the structure (see ) need not beassessed for collision loading.)))

9 However, engineersshould satisfy themselves that after impacting andremoving one support, a vehicle will not damage adjacentsupports and thereby cause a collapse of the bridge which fail assessments will requireadditional protection or strengthening so as to be able tosustain the collision loads given in Table 2 schemes for structures which failassessment shall be designed in accordance with currentdesign Standards and Codes of Practice. However, non-linear methods of analysis may be used whereappropriate. In the case of foundations, when checkingfor bearing the ultimate bearing capacity shall be should be devised on an individual basisand take account of construction and traffic managementcosts as well as aesthetics. Strengthening shouldwherever possible be carried out with other maintenanceor improvement works in order to minimise delays to considered for strengthening bridgesupports should include the structural continuity by tyingtogether slender supports to form a more the size of the supports.

10 Thismay be achieved by casting a mass concretecollar or plinth around an existing slendermember, or casting vulnerable individualcolumns into a leaf continuity to prevent collapse. It may be possible in some cases to providecontinuity at the deck which would prevent acollapse in the event of the loss of one an articulated pier. Sufficientstrength may be achieved by the locking up oflow level bearings and providing for movementat the deck. However, the consequence ofthermal and future foundation movement shouldbe fully taken into account. It may be necessaryto replace the upper bearing by a sliding one. When resulting movements at the pier top arelarge, local strengthening of the bridge deck maybe structural redundancies byproviding additional supports or strengtheningdeck an alternative to strengthening, supports thatfail assessment may be protected by means of anappropriate vehicle restraint system. Advice on a suitablesystem may be obtained from the Overseeing 4 VOLUME 3 SECTION 4 Assessment and StrengtheningPart 7 BD 48/93 ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLEDJune 19934 should be aware that, with somevehicle restraint systems, there is a tendency for thevehicle to climb the system and tilt over.