Atkins Technical Journal 04

8/10/2019 Atkins Technical Journal 04

1/164

04

Technical JournalPapers 051 - 066


2/164

Welcome to the fourth edition of the Atkins Technical JournThis edition of the Journal is dedicated to Cressida Spachis away earlier this year. Her paper, Delivery of bus priority

partnership approach, was submitted to the Technical Journthe success of working in partnership on the Route 38 ProjeProject Manager for the Route 38 Corridor Management Pilhas developed principles for intensified bus priority now beLondon. The project received many accolades and Cressidapart of the client/consultant team receiving the Improvemenaward at the London Transport Awards earlier this month. ACressida is made by Andy Southern, Managing Director forand Management, on page 69 immediately prior to the text o

Once again there has been a great response to the call for paall businesses and it is particularly encouraging to have beepublish papers from a wide range of staff grades covering GEngineers to Technical Directors. The Journal continues to extensive range of technical disciplines Atkins can offer its Defence and Offshore Energy capabilities are prominent in With the recent appointment of three new Network Chairs wforward to even more effective showcasing of the diverse anproblems that Atkins staff solve so well in their everyday aIt is also particularly pleasing to see the excellent working rwe have developed with our clients and partners being manico-authored papers such as those written with the UniversityUniversity of Southampton and English Heritage. This demAtkins creates solutions that both we and our clients are pro

I hope you enjoy the selection of Technical papers included

Chris HendyChair of H&T Technology Board

Highways & Transportation


3/164

Technical Journal 4Papers 051 - 066

051 - StructuresThe launching of the River Esk Bridge

052 - StructuresRecommendations for assessment Eurocodes for bridges

053 - StructuresLimit equilibrium assessment of drystone retaining structures

054 - Intelligent Transport SolutionsITSO smart cards in Welsh public transport

055 - Intelligent Transport SolutionsOn balance - the see-saws of congestion charge business cases

056 - Intelligent Transport SolutionsImprovements to ramp metering system in England:VISSIM modelling of improvements

057 - HighwaysScrambled pedestrian crossings at signalcontrolled junctions - A case study

058 - HighwaysDelivery of bus priority projects - A partnership approach

059 - Water & EnvironmentWater quality modelling as a tool for assessing new water resourcemanagement options: The case of the River Stour, Kent

060 - Water & EnvironmentHigh accuracy recording for heritage applications - Dover Castle graffiti

061 - AerospaceConjugate heat transfer study of a spin pit rig:Application to the lifing of HP turbine disc firtrees

062 - AerospaceNon-deterministic thermo-fluid analysis of a compressor rotor-stator cavity


4/164

Abstract


Senior Group Engineer

Rob Liddle

The launching of River Esk Bridge

IntroductionThis paper covers the approach forcarrying out the analysis, designand detailing of the River Esk Bridgeon the M6 Carlisle to Guards MillMotorway Extension. The Esk bridgeis a four span 180m long steelcomposite viaduct over the RiverEsk and carries the southboundcarriageway of the new section ofM6 Motorway. The span arrangementis 31.4m/51.9m/51.9m/44.9m.The south abutment is fullyintegral, forming the point offixity, and the superstructure is

The River Esk Bridge carries the southbound carriageway omotorway over the River Esk estuary and forms part of theconstructed M6 Carlisle to Guards Mill Motorway Extensiothe missing link in the motorway network between EnglandThe bridge is a 180m long four span steel composite viaducto site constraints the most economical and practical methoconstruction was to launch the steelwork into position ratheto lift the steelwork in by crane. The bridge steelwork, weigover 800 tonnes, was assembled on the northern approach tbridge and launched southwards over the River Esk estuaryThis paper demonstrates that the design and construction obridge can be significantly more complicated than the desig

comparable bridge which is constructed using conventionathat there are many more load cases and construction detailThis paper covers the approach for carrying out the analysidetailing and construction of the structure and focuses specthe key issues relating to launching steelwork. It serves as aand quick point of reference for the design of launched stee


5/164

51 The launching of

Analysand bui

Line bealaunch p

Launch PThe bridgsouthwarPhase 1 - the steelwtemporarya launch nlaunched platform spaced at in the parembankmlevels set be set outvertical alaunch. Tconstructwhich wetemporaryand work

risk of floislands wthe maximPhase 2 -the Phasemost steeand attachlaunched.the rear oThe wholthen launlaunch thand tail stThe launcstrand jacattached t

t l h

Figure 3 - Views of the completed bridge adjacent to the existing A74 metal bridge


6/164


Figure 5 - Cross section of the deck

The nose bottom flanges needto be curved to compensate forthe cantilever deflection of the

leading span and to prevent thesteelwork clashing with the piers andabutments as it passes over them.

The nose profile should be detailedto allow sufficient tolerance to ensurethe nose travels past the support

bearings a sufficient amount beforetouch-down. 1m was specified andthe vertical clearance at the tip was165mm before touch-down afterconsidering deflections. The maximumcantilever deflection was 1112mm.

The use of a cdifferent apprcommonly usincrementallybridges in whhas a hydraul

tip to take up to touch downThe tail needeto take accounspan droppinbases and bridLand bases arin the steel lauwhere the stee

A curved profthe tail to gracantilever defthe steelworkdrop as it movAt each stagetouch-downnose were calwere required

contractor tin the nose whdown first andthe nose flang

Figure 6 - View of the launch showing signicant leading span cantileverdeection and temporary cofferdam/island around an intermediate pier


7/164

51 The launching of

Launch b

The steelwtemporary351mm hand then permanenare usuallPTFE padthe underSkates capaint to thflange ancoefficienlaunch fochosen foDeflectioThe softwused to anbeams weand tail dduring thea supportsupport reremoved and the mthe correcto accurat

Figure 9 - Phase 2 launch complete


8/164


9/164


10/164


The contractor had concerns thatthe bridge was not adequatelybraced between the group ofthree beams and the pair and thatthe two sections could deviateapart. Launching 5 beams is nota usual scenario as beams wouldnormally be launched in pairs.They proposed continuous bracing

at all bracing locations should beused. This made the whole bracingsystem effectively fully rigid andso prescribing large deflectionsgenerated large forces resulting inthe need for excessively large bracingmembers and bolted connections.By removing the top member inbay 2-3 the bracing system becamemore flexible as the group of 2beams (beams 1 and 2) could rotateor flex relative to the group ofthree beams (beams 3, 4 and 5).

The bracing would be difficult toremove, being over the river, andwith the EMJ formwork alreadyin place. Therefore, from a healthand safety and CDM perspective itwas decided to leave the bracingin place. The bracing was alsochecked for the in-service case.Web patch load checks on

unstiffened sections of webIn the in-service condition thesteelwork at supports is stiffenedwith web bearing stiffeners to allowthe web to carry the vertical supportreactions. However, for most ofthe time during the launch, theposition of web bearing stiffenersor intermediate transverse web

stiffeners would not coincidewith the position of the supports.Therefore the unstiffened parts ofthe web were checked for carryingthe vertical support reactions.

The vertical sthe launch coweight, formwleading span athe deck reinfreactions werin-service supthe web thickslightly thickefor the in-servaccommodatebearing suppoPart 31 clauseweb patch loperform this csome further design wouldusing the morpatch loading

Splices desigresist haulingSplices were horizontal hauin span bendi

Plan bracing

Figure 16 - Arrangement of bracing

BEAM 1 BEAM 2 BEAM 3 BEAM 4

LATERAL GUIDES ATSUPPORTS TO PREVENTLATERAL MOVEMENT

BOTTOM TIE LIFTED TO CLEARGUIDES IN BAY 2-3

TOP TIE MEMBER ORIGINALLYPROPOSED


11/164

The analopushed arturning thfriction oThis is a gsteeringactually hsteelworkcurve natuThe haulifriction toThe largecan be imlongitudinmovemenat that supcoefficien

The horizlateral gudriving acconsideraequilibriubeam mobe used tosystem toand laterafriction at

as a load of slidingfound by the haulinparallel tolateral guas the horacting perassumptiofriction oand can breasonablare of relacomparedThe hauliforces nee

The bracing, which would eventuallyform the support bracing at thepiers and abutments on completionof the launch, was also checkedas intermediate restraints for the

launch phase when positionedaway from a support.Launch load effects due to plancurvature of the superstructureAt first glance it might appear thatthe lateral launch forces on thePTFE lateral guide bearings wouldbe equal to the force required topush the steelwork sideways over

the support bearings i.e. the totalvertical load at the support multipliedby the coefficient of friction.

The plan bracing provided effectivediscrete lateral restraints wherethe effective length was limited tothe distance between intermediatebracings and enabled an efficient

use of steel plate. The plan bracingwas provided between at least twobeams near the compression flange.Plan bracing was also requiredbetween all 5 beams in the end bayof the leading end of the permanentsteelwork i.e. just behind the nosesplice. This plan bracing was requiredfor erection purposes and duringlaunching to prevent adjacent beamsmoving longitudinally relative toeach other, known as leading.

51 The launching of

Figure 18 - Idealisation of symmetrical constant radius curve launch


12/164

When three oconnected togbracing or topbracing, then This is termedwhere, as the load, some vethrough trans

The participamodelled to dload effects inLubricant coeof friction valA lubricant wthe bottom fladuring the laufriction. The l

a soap normagrease and oilGreen Gel (lDuring the apthe AIP the HSSR requesteassumed coefAs mentionedgirders and su

for additionallaunching of tloads are sum

Wind(a)Loadi(b)verticspeciLater(c)exerte

launccurvethe decoeffidata uschemLong(d)

Launch load effects onabutments and piersThe substructures were checkedfor the load effects from thelaunch hauling load, lateral guideforces and friction effects of thesteelwork while being launched.The steelwork was also being

launched uphill by a gradient 0.3%and this gradient was considered.The lateral and longitudinal forceson the piers and abutments due tomoving the steelwork were consideredin the design of the abutments andpiers. The coefficient of frictionin the longitudinal and transversedirections was taken as 11% (0.11)and 5% (0.05) respectively.The north abutment was designedto resist the full hauling load andthe hauling frame was anchoredinto the north abutment stem.The hauling load effectively pulledthe abutment into the fill and sopassive resistance could be utilised.Participating bracing

The optimum number of main beamswas investigated at the preliminarydesign and value engineering stagesof the design. Cost comparisonswere made between 4, 5 or 6 beams.With an even number of beamsthe bracing is usually arranged indiscrete unconnected pairs i.e. for 6beams - three braced pairs. However,for an odd number of beams thebracing is arranged so that threebeams are connected together. Forthe five beam case on the Esk there isa braced pair and a group of three.

Taking all moments aboutpoint A summed to zero:

PF 3=

Summing all loads in planalong the y axis to zero:

The above assumptions are correctfor a deck on a constant radiushorizontal curve. For the case wherethe deck is on a varying radiushorizontal curve, or a combinationof constant curves and transitioncurves, then additional lateral guideforces need to be considered.These additional forces result fromchanging the plan curvature of thedeck from the initial constructedshape that is forced at any instant bythe guides. It is worth noting that ifthese forces were too large then theassumption of ignoring friction onthe guides would not be correct.The additional lateral guide forcescan be determined by modelling theeffects of the forced deflection ofthe deck in a simple grillage modelor by using simple beam formulae.An impact factor of 1.5 wasapplied to the calculated lateralguide forces and these forces wereconsidered in the bracing design.



13/164

Furthermmargin ofthan that conservatpermanenallow for A load faboth deaddead loadtogether wstated aboshowed thsubstructuconditionthose genresulting during lauoverturnin

main girdThe frictiduring theto use andThe mainin biaxialof major ato the laubending d

Geometrdetails an

The steelwhave prefplates to bbottom flweld and However,

for bolt lowere weldObviouslyonly be adwas launcsnag withA steel ad

If unexpectedly high loads wereindicated then the launch was to behalted and the cause investigatedand resolved to continue theoperation. The monitoring systemprovided a permanent digitalrecord of the hauling loads.

Prisms were attached to eachpier, at the top, middle andbottom levels in order to checkfor displacement and rotation ofthe substructure. Measurementswere taken continuously duringthe launching operation bysurveyors using EDM equipment.

testing in order to determine the mostsuitable friction coefficients specificallyused for the scheme. The frictioncoefficients stated above were basedon data taken from continuouslymonitoring the load effects on thestructure during the Swale Crossing

launch. The same monitoringsystem was adopted for the RiverEsk scheme described as follows:Strand jack loads were monitoredduring the launch operation -calibrated pressure transmitterswere installed in the hydraulic linesto the strand jacks and connectedto a laptop computer to monitorstrand jack launch loads. Thisprovided a digital and graphicaldisplay of the load being appliedby the two jacks every five secondsthat were continuously monitoredby an engineer who was in radiocontact with the Launch Controller.

Figure 21 - Adaptor plate arrangement

51 The launching of

Plategirder

Tapered bearing plate

Bearingbolted toadaptor plate

Adaptor plate30mm thickwelded tobase plate

Cast in baseplate withshear


14/164

At the piers suavailable to jafrom under ththe main beam1500mm widnorth and soubearing shelvand so no rooplace the jackflanges. At thpermanent beto be providedmade more diproblem, the bthese locationhorizontal I se

jacking pointseach main be

ConclusioThis paper dethe design andlaunched bridmore complicof a comparabconstructed umethods. Theload cases, loto consider. Hwhere access traffic managconventional difficult, bridan economicasummarises srelating to lau

The first phassuccessfully c2007 and the June 2007. Thto traffic in D

Acknowle

This also applied to the launch noseand tail. In the final design of thepermanent steelwork all bottomflanges were 900mm wide andall top flanges 600mm wide. Theflange sizes varied in the nose andtail for economy except in bay 2-3where the same outstand as forthe main steelwork was required.Web to flange welds were prepared ina T & I machine and were not fitted.Therefore for web patch load checksfrom the support reactions the weldwas checked for resisting the directpatch load since the the patch loadwould not be transferred from theflange to the web in direct bearingbut via the web to flange welds.A safe maximum launch wind speedwas specified on the drawings.The wind speed corresponded toa 10 year return period with a 3second gust speed of 18m/s.The permanent formwork and aproportion of deck reinforcement wasadded to the steelwork prior to thelaunch on all but the leading spanto minimise construction operationsworking at height and also mitigateaccess problems. A cost comparisonwas made between EMJ and Omniaformwork. EMJ was chosen as itis half the weight of Omnia.At the integral south abutment thewall reinforcement was required torun vertically up into the diaphragm

and lap horizontally into the deckslab. In normal construction starterbars are usually left protruding wherethe diaphragm is to be cast bothin the front and back face of theabutment stem/diaphragm. Becauseof the launch operation these bars

The temporary launch bearingscomprised two 300 long x 200 widePTFE bearings in a line (600 long).The whole rocker bearing assemblieswere typically 780mm x 370mm.During the launch at least twopoints of lateral restraint wererequired at all times to prevent the

steelwork shifting out of line.During launching the steelwork was351mm higher than its final in-serviceposition. At each support a pair oflateral guides was provided betweenbeams 2 and 3. The bracing in thisbay was lifted higher than in adjacentbays in order to clear the guides. Adistance of 350mm from undersideof bracing to underside of bottomflange was required to clear theguides. Bay 2-3, being a push-pullbracing bay, was easier to modifythan an adjacent cross-bracing bay.Once the launch was complete thestructural steelwork was jackeddown into its final position. Initiallythe preferred sequence was for thesteelwork to be jacked down fully, onesupport at a time. However, deflectioncalculations were carried out todetermine what bending momentswould be induced in the steelworkby this imposed deflection (351mm).It was found that lift-off from the

jacks would occur at some supportlocations before reaching the requiredlevel. The jacking sequence was

revised to allow only jacking in stagesat some supports to prevent lift-off.Bottom flange splice cover platescomprised two separate plates350mm apart rather than a singleplate. This was to ensure the spliceplates ran either side of the launch



15/164


16/164

Abstract


Rail

Group Engineer

Director

Jessica Sandberg

Navil K Shetty


Head of Bridge Designand Technology

Chris R Hendy

Recommendations for assessmentEurocodes for bridges

IntroductionTh St t l E d ff t f t gth ld b i it t t

The new Structural Eurocodes offer increased economy in most existing codes of practice. This is achieved through comore advanced calculation methods, such as non-linear ana

through codified rules which give increased economy in mcompared to the old British Standard rules because they aretesting and numerous non-linear finite element parametric This presents great opportunities for improving sustainabilithe assessment of existing structures is specifically outside Eurocodes, so many of the benefits these new codes bring capplied when reviewing old structures. Increased assessmebe highly beneficial in demonstrating that old structures wior small amounts of deterioration were still adequate to con

without costly refurbishment, modification or reconstructiolend themselves to adaptation for assessment of structures bare generally based around the application of first principlescope for determining the true ultimate strength of a structuincorporation of structure specific information on loads andin a rigorous manner. The assessment criteria can also be dito take account of differences in consequences of failure ofThis paper investigates the areas where the steel and concregive increased resistances compared to existing codes and mrecommendations for sections which could be directly applassessment of existing structures. Areas where the Eurocodapplied directly to existing structures are also identified togthe reasons why, such as reliance on modern material and especifications. Recommendations are then made for how thdesign rules could be modified for assessment situations, inof measured strengths and imperfections in calculation. Acformat for combining actions, are also investigated and recmade for modifications to these aspects for use in assessingFinally, an overall assessment of the scope of work requiredto produce an assessment suite of Eurocodes is made.


17/164

52 RecommendationEuro

of modern steels that are producedto the specification EN 100251.Older steels, or indeed wrought andcast irons, may not comply with the

characteristics. An additional factoris that the Eurocodes do not directlydeal with reduction in strength due todeterioration arising from unexpected

HA standbecause rtend to beavoidancepotential will seek Eurocodedeparturethus creatversion othe controand conseif an apprassembleThe remainvestigatconcrete Eresistancecodes andfor sectioapplied tostructuresrequired tcould notRecommethe modifcalculatio

actions w

Key feaassessmof a EuThe procecrucial imbridges inconditionis to evaluexisting ba minimuthe assessneed to beand judic

Figure 1 - BSALL is used for the operation of the Forth Road Bridge

Figure 2 - Imperfections in longitudinal stiffeners following an impact


18/164


19/164

The load models LM3 (abnormalloads) and LM4 (crowd loading)given in the UK National Annexto BS EN 1991-2 can be used forassessment of existing bridges withoutfurther modification, although thederivation of bridge specific crowdloading models is also possible.

Where the structural dimensions andthe thickness of road surfacing aremeasured for an existing bridge, themeasured dimensions, together withmeasured values of material density,can be used to calculate more realisticvalues of self weight and surfacingload for the bridge. For surfacing,this will allow the tolerances on

range of surfacing depth given in theEurocodes to be reduced, providedcontrols are in place to limit changesto surfacing depths in the future.Depending on the level of variabilityin measured values and the controlsapplied, the partial factors for selfweight, super-imposed dead loadand surfacing load given in theUK National Annex to BS EN 1990Annex A2 could be reduced further.

Bridge specific resistanceparameters

Where material properties, such asyield strength of steel or cylinder

Section D7.2 of BS EN 19906. Furtherdetails of this methodology are givenin reference 7. The calculation ofcharacteristic strength values takesinto account the number of testresults, variability in test results andprior knowledge about the statisticaldistribution of material properties. The

in situ characteristic strength valuescan then be used together with thematerial partial factors given in theUK National Annexes of materialEurocodes EN 1992, EN 1993 andEN 1994 for bridges. Where theultimate resistance of a componentor product is established throughtesting, the design resistance canbe calculated using the proceduresgiven in Annex D of BS EN 1990.In some cases, construction tolerances(imperfections) or impact distortionsmay also affect the structuresstrength and will therefore needto be considered explicitly if theyare greater than those assumed inthe Eurocode design rules Figure2 shows an extreme case of post-impact deformation. Greaterdiscussion on this is included below.

Consequences of failure

Consequence of failure reflects boththe economic impact of the structural

medium (Consequfor a highbe considgreater thculverts othan 10mon a minobe regardstructures1). The adefined ina low conbe taken aused for astructure.factors onby a facto

Class 1 as1990, Ana reassessof failure

Reliabili(Level 5

The Eurouse of pro

reliabilitydesign ofthat their assessmenthey provfor assessreferred tUK8. In adbridge spparameterof failureassessmento be takeexposure heavy loaloads in e

i h )


(i) Accumulated stresses determinedon gross cross section

(ii) Accumulated stresses used todetermine an effective steelcross section for re-calculationof accumulated stresses

Figure 4 - Illustrative procedure for determining effective cross section in Class 4 composite beams


20/164

Overview of theeconomic benefitsof Eurocodes 3 and 4and the modificationsrequired to produce asteel assessment code

The following discussions are basedaround Eurocodes EN 1993-29and EN 1994-210 and the otherEurocodes they cross reference andidentify, in outline, the modificationsrequired to produce an assessmentstandard. The various sections of thecodes are reviewed one by one.General

The scope of EN 1993-2 in itssection 1 would need fairly extensivemodification because of the numberof cross references to modern productand execution standards, but thiswould mostly involve deletions.The terms, definitions and symbolsdefined in the section howeverwould mostly be retained. Guidancewould have to be provided also oninspection for assessment and theinformation on material properties,imperfections and condition thatshould be obtained and recorded. Thiscould largely be taken from existingdocumentation such as BD 5611.

Basis of designThe material on the principles oflimit state design and design assistedby testing would require very littleamendment for use in assessment;indeed design assisted by testingwould be a useful assessment tool.MaterialsThe section on materials would

This will require either modificationsto the strength equations andprocedures or the use of non-linearanalysis using known materialproperties for the structure. Otherrequirements for minimum toughnessto guard against brittle fracturewould need less modification, but theCharpy energies for use in calculationwould again need to be determinedvia the means above. The sectionson brittle fracture referenced inEN 1993-1-1012 permit the useof fracture mechanics methodsto check adequacy, so actually

lend themselves to assessment.The corresponding section in anassessment code could usefully givekey material characteristics suchas yield strength, ultimate tensilestrength, ultimate strain, toughnessand modulus of elasticity for historic

Structural anaMuch of whaanalysis woulamendment aare to model tway that reflethis applies eqassessment. Esubsequent mand codified blittle adaptatiowould be neebuckling curvallow use of m

Figure 6 - Typical interaction diagram for shear and moment to EN 19



21/164

stresses aon the resThe proceto first caon the grothe constrto determthe compthis stressaccumulausing the section atThis is illClass 4 bestiffenersway as bestiffenerswhere a c

to calcula56, indiviare checkwere deteusing grothan for fallowancedue to locis therefobetween coverstressthe designIn EN 199are again of web anunstiffenethe same stiffenersallows loa

the varioucombinedused. Thchange frand can gincreasedresistance

This aims to fix material factorssuch that a base level of reliability isobtained when using the assessmentrules, based on the true resistancesobtained in tests. The rules in EN 1993utilise relatively few values of M forsimplicity. Background papers 13 are,however, available in a number ofcases to support the use of reducedvalues where adequate reliabilitywould still be obtained and advantageof this could be taken in assessment.The Eurocode cross section resistancesmake some allowance for strainhardening in a number of areas(for example in determining thereduction of strength due to holesand in the shear-moment interaction

curves) so it would be necessary toset out what to do if the ductilitylimits were not met. The simplestsituation in this case would be torevert back to the more conservativeprovisions of BD 56, adapted tomake them fit the general Eurocodeterminology in terms of notation.A similar exercise has already beendone in the drafting of reference14 for salvaging non-contradictorymaterial from BS 5400 Part 315 tobe used in design with Eurocode 3.Bending cross section resistanceFor Class 1 to 3 cross sections, thecalculation procedure and results forbending resistance are very similarto that for previous practice. One

small area of economy for Class 3cross section design is that where thebending resistance is based on firstyield at an extreme fibre, EN 1993defines an extreme fibre as the mid-thickness of the flange, rather thanthe outer surface as was previous UK

Non-linear analysis, particularly ofslender structures, will often give themost accurate and realistic pictureof true structural strength and isthus a very effective assessment tool.The model in Figure 3, for example,predicted the physical test failure loadto within 4%. Non-linear analysismethods are, however, influencedby imperfections (residual stressesand tolerances on straightness andflatness) and those given in Eurocode3 may not be appropriate for anexisting structure. Requirementsfor non-linear analysis wouldtherefore require either magnitudesof equivalent imperfections to beprovided for old structures or for

imperfections to be measured inthe existing structure and residualstresses determined by calculationbased on, for example, weld details.Often, doing the latter will in anycase be beneficial compared withthe alternative of using the designcode values. The rules would need toprovide guidance on the application ofimperfection patterns to the structure.Guidance would also be neededon the material properties to use inanalysis where the material behaviourdoes not conform to the assumptionsand limitations in the design code,but non-linear analysis is, in principle,the best form of analysis providedthat the material characteristics areknown and can be modelled.

Plastic analysis would also bemore readily justifiable for usein assessment since in-serviceperformance can be visuallyassessed. The Eurocodes set out therequirements for plastic analysis soonly a relaxation in the conditions



22/164

One importanlongitudinal ssusceptible tothis method isadequate ductstiffeners are of buckling, ato that in BD required. EN10 provides srules in EN 1susceptibilitythemselves a ignore restraito which theycould easily bthis restraint aimperfections

conservative buckling strenof the rules inLateral torsioand distortionWhilst BD56guidance on lbuckling, ENtheoretical ap

as a general aan expression

where M cr is buckling momis given on thwhich tends ttowards perfoelastic criticaits determinatthis in an asseit usually prodprediction of reduction than

Even if the cross section is inClass 3 or 4 (so that the bendingresistance is limited to first yield),the interaction is performed usingthe plastic bending resistance.The interaction is truncated bythe requirement to limit themoment to the elastic moment.This has the effect of permittingalmost full web shear resistancewith full bending resistance, asshown in Figure 6, which reflectsthe findings of recent non-linearparametric finite element studies.

The rules do rely on ductilebehaviour and an amount of strainhardening being available. Ifrelevant limits for these were not

met, the simplest resolution in thiscase would be to revert back to themore conservative provisions of BD56, adapted to make them fit thegeneral Eurocode terminology.Longitudinally stiffened cross sectionsare treated in essentially the sameway in EN 1993-1-5 as for unstiffenedcross sections, so the same economic

benefits can be obtained. To BD56, the check of the cross sectionwould have to be performed on apanel by panel basis in such a waythat any shear stress at all has theeffect of reducing bending strength.

The implications of the differenttheories are, however, moresignificant in the design of transversestiffeners as Hglunds theory (usedin the Eurocode) places less demandon their strength. This is reflectedin EN 1993-1-5, which allows lightertransverse shear stiffeners to bedesigned than would be permittedto BS 5400 Part 3. The rules in EN1993-1-5 have still been shown tobe conservative and could be refinedfurther in the assessment Eurocode,as suggested in Reference 18.Shear - moment interactionEN 1993 produces a moreeconomic check of shear andmoment interaction than does BD56 and consequently has beenused in assessment to justify notstrengthening existing bridges. It ismore economic for three reasons:

Shear does not interact withlateral torsional bucklingresistance. It only interactswith cross section resistance The interaction diagram isa continuous curve, ratherthan a series of straightlines as was the case in BD56, as shown in Figure 6


R d i


23/164

added to tcharacterias discussas-built resection onwould alsas usuallycompositifor these available such casegive recocommon used at thFor reinfocomplianstandardsand the pr

validity oas ductilitreinforcinEN 1992,those fouto be usedshould be1992 ruleremovinghardeningcheck whbrittle. Gincluded round barEN 1992 bond chaof commosteels cousection. A

Annex wothis inform

Durabili

As for strthe design

FatigueThe approach to calculation offatigue life is unlikely to be differentbetween design and assessment,but it may be appropriate to usea different vehicle spectrum forassessment than for design basedon measured traffic conditions. EN1993-1-9 already allows reducedfactors of safety to be used wheredamage tolerant conditions exist (i.e.the structures details are regularlyinspected for fatigue cracks and thereis some redundancy available in theevent of a component failure) andthis approach is particularly suitablefor assessment where monitoringof the structure is possible. It would

also be possible to incorporatemodifications to the S-N curves wherefracture toughness properties weremeasured directly in the structure.

Overview of the economicbenefits of Eurocode 2and the modifications

required to produce aconcrete assessment codeThe following discussions arebased around Eurocodes EN 1992-220 (and its cross-references toEN 1992-1-121) and identify, inoutline, the modifications requiredto produce an assessment standard.The various sections of the codeare reviewed one by one.General and basis of designThe modifications required for thegeneral and the basis of designsection would be similar to thosediscussed for steel above.

Flexural buckling of strutsFlexural buckling slenderness issimilarly written in terms of acritical buckling force N cr so that,

Once again, this allows the assessorto use a computer elastic criticalbuckling analysis which usuallyproduces a less conservativeprediction of slenderness and strengthreduction than does a simplifiedhand calculation of slenderness,particularly in cases where otherwisesimplified effective lengths wouldneed to be used. A tapered archwith hangers and transversebracing, see Figure 8, provides agood example of a case where aneffective length would be difficultto determine without a computeranalysis. The reduction factor-slenderness curves themselves wouldagain need to be adapted to allowuse of measured imperfections in the

assessment code, but this would bestraightforward to do and could beadapted from provisions in BD 56.ServiceabilityMost of the design requirementsinvolving serviceability would beretained in an assessment version ofthe standard, but inspections woulddetermine to what extent the design

had succeeded in that regard.Fasteners and weldsMost of the provisions for boltsand welds would be able to remainin an assessment standard butgreater coverage of the resistancef th t h i t


R d ti f t


24/164

ShearThe variable aprovided in E10, provides sreinforced cousing this mowill provide aresistance mosolution that ato be tailored provided. Whstresses are noresistances causing a flat trushear stressescompared to Bgive improvefor beams wit

TorsionAs for shear, tprovided in Every little mobe directly adOnce again, thmodel gives ttool to tailor tto the reinforc

longitudinallyStrut and TieThe strut and directly adoptFigure 11. Sinis a special caonce again thto be developactual arrange

provided. SomEurocode for however, be uin struts whertension is curin cases wherare small and

Plastic analysis would also be morereadily justifiable for use in assessmentsince in-service performance canbe visually assessed. The Eurocodesset out the requirements for plasticanalysis so only a relaxation in theconditions of use is likely to be neededin the assessment version, providedthat requirements for rotationcapacity are met. To that extent, it

would be necessary to give guidanceon rotation capacity for reinforcingsteels not covered by EN 1992.

Ultimate limit states

BendingThe rules for flexural resistancewould require little modificationbut reinforcing steels would need

to be classified for ductility; thereis a small amount of benefit thatcould be gained from these rulesbecause of the consideration ofreinforcement strength beyond yield,though this would depend on theductility of the reinforcement and

Structural analysis

As for structural steel, the structuralanalysis section would requirelittle modification as many of theassumptions would remain valid forassessment. For non-linear analysishowever, measured imperfectionswould need to be used if the actualconstruction tolerances used werenot known. Non-linear analysis is aparticularly appropriate assessment

tool because it produces the mostrealistic prediction of structuralbehaviour and resistance. However,the practical application of thetechnique is more limited for concretestructures than for steel ones becauseof the added complexity. Formembers where the predicted failureis a flexural one, such as for beamsand slender piers, see Figure 9, theanalysis is relatively straightforward.For more complicated cases whereconcrete failure under a multi-axial stress field is concerned, theanalysis can be much more timeconsuming, iterative and difficult

i h h k b f

Figure 9 - Non-linear analysis of slender piers (deected shapes shown for various load cases)


52 Recommendation


25/164

DetailingSection 8with detamodified non-comprules tendto ensure or to guarassumptio

Detailingworkmanas poor waccountedThis wouminimumnon-compto poorly this shoulfrom exama core samDetailingapplicabilto be maiprovided complian

ServiceabilityMost of the design requirementsinvolving serviceability could beretained in an assessment versionof the standard, but inspections aregenerally more useful in determiningserviceability performance.This again would be consistentwith previous UK practice.

FatigueChecks of fatigue in concrete andsteel were not covered to anysignificant degree by previous UKassessment standards, but fatiguecan be important in some structureswhere the details are unusual. Detailsthat may need assessment includebars connected with threadedcouplers and bent bars used atlocations where the bars are highlystressed. Both these situations canlead to fatigue problems 24,25 . EN1992 provides a realistic assessmentof fatigue in both these situationsand would require minimal

Membrane rules andsandwich modelsThe membrane rules in EN 1992-2Clause 6.109 would be useful forassessment where shell element arerequired in analysis due, perhaps, tothe complexity of structure geometryand the lack of applicability of othercodified assessment rules. These rulesallow stresses to be used directlyfrom an FE analysis model for theassessment calculations. However, thesignificant limitation of the approachis that design is done element byelement and hence no redistributionbetween elements is possible. Thismakes the use of the membranerules conservative compared to othermember design rules (such as those

for shear and torsion) and thesemember rules should therefore alwaysbe used where they are applicable.Within an element, however, thereis some scope to tailor the analysisto suit the reinforcement directions.

Figure 11 - Simple application of strut and tie rules for bridge pier


Recommendations for assessment


26/164

ConclusionThe new Structural Eurocodeslend themselves to adaptation forassessment of structures because therules are generally based around theapplication of first principles. Thisnot only means that the rules canbe adapted relatively easily but they

also provide scope for determiningan accurate prediction of the trueultimate strength of a structure basedon structure specific informationon loads and material properties ina rigorous manner. This paper hasidentified areas where the Eurocodescould be directly applied to theassessment of existing structures andthose which would need modification.It has also identified areas where theEurocodes give increased resistancescompared to existing codes.The amount of work to produce anassessment version of the Eurocodeswould not seem to be prohibitive,even given a modest budget, ifthe scope was limited to adaptingthe existing rules to assessmentwithout refining them further foreconomy. This should be the priorityfor the work because the Eurocodesthemselves provide a better estimateof strength than many of the UKassessment standards and the lackof a Eurocode assessment standardis likely to lead to inappropriateapplication of the design Eurocodes

to assessment. The secondary taskof updating the rules and the partialfactors would be considerably moretime consuming and would mostlybe equally applicable to design.It is therefore unlikely that thisrefinement would be undertaken


52 Recommendation


27/164

ReferencesBS EN 10025 (2004): Hot rolled products of structural steels. British Standards Institution, L1.BS EN 1090-2 (2008): Execution of steel structures and aluminium structures - Part 2:2.Technical requirements for steel structures. British Standards Institution, London.BD 50/92: Technical Requirements for the Assessment and Strengthening Programme for Hi3.Structures Stage 3 - Long Span Bridges, Design Manual for Roads and Bridges, Highways ABS EN 1991-2 (2003): Eurocode 1: Actions on structures - Part 2: Traffic4.

loads on bridges. British Standards Institution, London.BD 21/01: The Assessment of Highway Bridges and Structures, Highways Agency (2001)5.BS EN 1990 (2002): Eurocode - Basis of structural design. British Standards Institution, Lon6.Shetty, N. and Chubb, M. (2001), Probabilistic Methods for Improved Bridge7.Assessment, International Symposium on Bridge Management, Singapore.Shetty, N. Chubb, M.S. and Manzocchi, G.M.E. (1998), Advanced methods for the assessm8.bridges, Int. Symposium on Management of Bridges, Institution of Civil Engineers, LondonBS EN 1993-2 (2006): Design of Steel Structures. Part 2: Steel bridges. British Standards In9.

BS EN 1994-2 (2005): Design of Composite Steel and Concrete Structures. Part 2:10. General rules and rules for bridges. British Standards Institution, London.BD 56/96: The assessment of steel highway bridges and structures, Highways Agency (199611.BS EN 1993-1-10 (2005): Design of Steel Structures. Part 1.10: Material toughness12.and through-thickness properties. British Standards Institution, London.B. Johansson, R. Maquoi, G. Sedlacek, C. Mller, D. Beg (2007), Commentary and worked 13.to EN 1993-1-5 "Plated structural elements", JRC Scientific and Technical Reports, ItalyPD 6695-2:2008 Recommendations for the design of structures to BS EN 1993-2:2006, BSI 14.

BS 5400:Part 3 (2000): Design of steel bridges. British Standards Institution, London.15.BD 61/96: The Assessment of Composite Highway Bridges and Structures, Highways Agen16.BS EN 1993-1-5 (2006): Design of Steel Structures. Part 1.5: Plated17.structural elements. British Standards Institution, London.Presta, Hendy et al Numerical validation of simplified theories for design rules of transverse18.stiffened plate girders, The Structural Engineer, Volume 86, Number 21 pp 37 - 46 (4/11/200BA 56/96: The assessment of steel highway bridges and structures, Highways Agency (199619.BS EN 1992-2 (2005): Design of Concrete Structures. Part 2: Concrete20.Bridges. British Standards Institution, London.BS EN 1992-1-1 (2004): Design of Concrete Structures. Part 1.1: General rules21.and rules for buildings. British Standards Institution, London.BD 44/95: The assessment of concrete highway bridges and structures, Highways Agency (122.fib bulletin 45 (2009), Practitioners' guide to finite element modelling of23.reinforced concrete structures, fib, Lausanne, Switzerland


Limit equilibrium assessment of


28/164

Abstract


Graduate Engineer

Chris Mundell

Clarke Bond

Project EngineerClaire Bailey

University of Bath

Senior Lecturer

Andrew Heath

University of Bath

Senior Lecturer

Peter Walker

University of Bath

Senior Lecturer

Paul McCombie

Most construction in the UK datesto the 19th and 20th centuries.Th h l d ll

Such figures hfor the means

d

Limit equilibrium assessment ofdrystone retaining structures

A limit equilibrium analysis program has been developed ainvestigation into the stability of drystone retaining structuof the programmes function was in relation to field trials c

1834 by Lieut-General Burgoyne, which have been the maito date for checking numerical modelling of drystone retainParametric studies and investigations of bulging mechanismand analysed. Program predictions have been compared wiresults from new small scale and full scale drystone retainin

IntroductionDrystone technology is an ancientform of construction, suitable for

li i i f i l

Limit equilibri53


29/164

Limit equilibridrystone ret

Element Cdetails regthe potentgiven suffmodellingseveral hostudies ofa lengthy Neither oroutine ustherefore efficient adesign whby hand cof simpleThe progr2-dimensappraisal

with the adeformatioccur. In abeing utilthe mechadeformatias well asaffect this

Objectives and scopeAs part of an ongoing investigation,several unmortared retainingstructures of both large and smallscale have been built and testedto failure. These experiments arecarefully monitored and both thestress changes and deformations at

critical locations are recorded, andthen used to determine the underlyingmechanisms behind the failures.In parallel with these studies, theauthors are developing alternativemeans of analysing drystonestructures which are then verifiedwith the gathered physical test data.Current analysis techniques for

drystone walls are either simplisticby considering the static equilibriumof the wall as a monolithic structure,or too complicated, using timeconsuming numerical packages tomodel each element within the walland backfill. Numerical packagessuch as UDEC (Universal Distinct

There is in any case an importantphilosophical difference betweenassessing an existing structure anddesigning a new one, in that manyof the uncertainties that are to becovered by factors of safety in designhave been resolved by the fact thatthe wall has been standing andserviceable. While this fact gives noassurance that the structure has everexperienced a full design appliedloading, there remains the importantfact that the assessor is mostconcerned by possible changes fromthe status quo, in which the factorof safety must at least exceed oneunder permanent loading conditions.Inappropriate interventions such as

pointing become of greatest concern,because while they may increase thecompressive strength and stability atthe face of the structure, they canlead to catastrophic changes in thepore water pressure regime. It istherefore very desirable to be able toassess the possible impact of changesin geometry and loading on thestructural stability of an existing wall,especially given that old structuresoften appear to have departed fromtheir originally constructed geometry.It is also important to understand theextent to which structural stabilityis dependent upon precision ingeometry and quality of construction.Standards for modern drystoneretaining wall construction are very

high, with good practice resultingin very strong structures with a highdegree of integrity. However, suchconstruction is time consuming andexpensive. A proper understanding ofdrystone retaining wall stability couldlead to narrower structures requiring

53



30/164

qdrystone retaining structures

Based on thesnumerical stubeen conductused by variotest both the vmodes of anafurther the vawork within dAlthough higinvestigationstime consumiseveral hours of analysis. Wbeing carried with this projdimensional mtests describe

were built to various standards by theproject masons within timber boxes.As the density of the wall materialis known, together with the overallvolume of the sample, a stone/voidratio can be easily determined. A verycarefully constructed tightly packeddouble faced wall with almost idealCotswold limestone has around20% voidage, while over 40% ispossible within poorly built walls. Theconsequences of a high void ratioare more extensive than just reducedweight of the structure: a looselypacked wall gives the blocks within ita much greater opportunity to rotateand slide, facilitating bulging andother deformation or even collapse.

Previous workTo date, despite its widespreaduse, only a limited number ofinvestigations into drystone behaviourhave been conducted. Until recently,the only physical test data for fullscale drystone walls dated back over170 years to work conducted by

Lieut-General Burgoyne in 18346

.Burgoyne built four full scale granitewalls, up to 6.1m tall, 6.1m long andof varying thickness, in an attemptto quantify the effect that the wallprofile has upon stability. These wallswere then gradually backfilled untileither full retention was achieved orcollapse occurred. Movements andgeneral observations were recordedupon the placement of each layer offill but only reported posthumouslyin 1853 from Burgoynes notes.

Drystone constructionAlthough many differences existbetween the various drystoneconstruction styles, several commonfeatures are usually exhibited. Typicaldrystone walls are built in horizontallayers or courses, with each courseideally consisting of stones of a

'uniform' thickness, retaining astraight and level appearance. Thecross section of the wall usuallyconsists of a well made, tightly-packed outer face, with a core ofsmaller random material packedbehind. Some drystone retainingwalls follow this core material directlywith the retained backfill material,whilst others have a second innerface, usually less well finished thanthe outer face. Tie-stones spanfrom the outer to the inner face,binding the wall together. Wherethere is no inner face, tie-stones areoften used to anchor the outer facefurther back into the packing fill.Coping stones can act in a similarmanner, spanning the entire width of

the wall at the crest, see Figure 1.Each block within the wall shouldideally be in contact with several otherstones, and pressure upon any partof a freshly placed stone should notcause any rocking or lifting at theopposite corner. In practice it is usuallynecessary to wedge in small piecesof rock known as pins to preventrocking. The unavoidable presenceof these pins presents a weakness forall drystone structures, especially asweathering of these smaller elementshas a substantial effect much morequickly than for the larger stones.Pins are often used to allow a more

Limit equilibri53


31/164

bulges. Oand the wuser may to any ponew co-ocursor at face. Rec

of the newinstantanea manner,can be decommonlpatterns, tupon overexisting wmay be quprogram t

of the masonry were zero, a thrustline in front of the middle thirdwould simply result in the stoneat the back of the structure beingprogressively unloaded, which neednot have any immediate seriousconsequences. As the thrust line

moves further forwards, the areacarrying the vertical load reduces, soincreasing the stress. The compressivestrength of most masonry, includingdrystone, is usually relatively highcompared with the stresses acting.Therefore, compression failure ofthe main stones is very unlikely, buta concentrated thrust may causelocalised crushing of weakened pinsor a flexural fracture of some stonesleading to further deformation. Inaddition, foundation settlement mightgive rise to significant deformations.Given sufficiently strong masonry andfoundation, failure would only occur

Program operationBy analysing the stabilising forceswithin the wall and using Coulombsearth pressure coefficients todetermine the horizontal and verticalstresses acting at each level up theback of the wall 11 , the magnitudeand direction of the overall thrusts

are determined, see Figure 2. Theinitial wall geometry is enteredalong with the material propertiesof both the wall and the backfill(mass, friction angles, etc) andthe eccentricity ( ) is calculated ata number of levels to generate athrust line as shown in Figure 3.In addition to forces arising fromthe self-weight of the backfill, patchsurcharging may also be applied.Additional pressure is then applied tothe backfill, spreading out by a ratioof 1h:2v. This is clearly a simplification,as used for example in BS8006 12,compared with the more rigorousapproach of Bolton 13 as suggested inBS8002 4, but for the present purposesthis approximation allows the

combination of rapid calculation andreasonable accuracy required here.Further justification of this approachwas given by Corte 14. It is currentlyassumed that the surcharge willhave no effect upon the calculatedthrust line until the expanding areaover which it is distributed crossesthe boundary of the wall. Althoughthe analysis is two-dimensional,three-dimensional load dissipationcan be assessed as spreading in bothhorizontal dimensions. The mostproblematic loading is wheel loadingfrom a heavy vehicle, so it is importantto model the three-dimensional

qdrystone ret

Figure 3 - Program operation screen



32/164

the backfill. Aacts against thand stabiliseswhich would interface allowit is not alwayfull friction a

One of the modifficult to asthe density ofNon-destructisuch as grounor horizontal give some indprofile and evthe density ofgreatly, its agestyle, and the will all affect and hence thewithin. Whilslittle impact ochanged by a may vary by mas mentionedreduces the w

both sliding amost criticallyallows easier of the individthe flexibilityamount of bu

Bulging inBulging is cousually occurhalf the heigha distinctive see Figure 5. of the effects deformation,

Parametric analysisDue to the nature of the program, aparametric analysis of any structureis a rapid process. This has a two-fold application; firstly it allows usersto quickly grasp which parametershave the greatest impact uponwall stability and secondly it allows

engineers in the field a greaterflexibility when assessing existingwalls. Once the cross section ofa wall has been recreated withinthe program, each variable may bealtered to examine the safety factorsat the worst possible conditions.From the program it is apparentthat for any given geometry of wall,several parameters are dominant forstability. For example, the assumptionof a 1h:2v load spread fromsurcharges means that the loads mustbe close to a wall to have an effect,but it is also found that loads mustbe relatively large, corresponding towheel loads from the heaviest trucks.This corresponds with anecdotalevidence, confirmed by numerical

modelling studies 9, that there is arelationship between increasinglyheavy traffic and failures of wallswhich had been safe for many years.The friction angle of the backfillmaterial is critical to wall stability.This angle determines the coefficientof active pressure ( k a), which in turndetermines the magnitude of the

horizontal forces upon the retainingwall. A stiff, tightly-packed backfillmaterial might have a high density,but its consequently high angle offriction is likely to result in a lowerhorizontal pressure than a muchlooser yet less dense material

Program validity checkInitially, the program was validatedagainst Burgoynes four test walls.The geometries of each wallwere recreated, and the materialproperties entered from Burgoynestests 6. Backfill heights were thensystematically increased by 300mm

(simulating Burgoynes test procedure)until the thrust line reached the wallface, indicating failure via toppling.The final simulated heights werevery similar to those recorded byBurgoyne, and indeed also similarto previous attempts using othermore sophisticated and complexnumerical packages 8, see Table 1.Both the first and second ofBurgoynes test walls were backfilledto their full height without excessivemovement, and by using the LEprogram it can be demonstratedthat the thrust line lies within theboundaries of the wall. It shouldbe noted that for both these wallsthe eccentricity is outside of themiddle third at the base, indicating

uplift at the heel. The third andfourth walls both fell before fullheight of retention was achieved.For both these wall geometries,the LE program predicted failureat a height similar to that foundby Burgoyne, although it has beendemonstrated that considerationof individual block rotation gavea tighter correlation with actualfailure heights 5. To allow this to beseen in the program, the directionof the resultant force at each levelis also shown at the point at whichit acts. A resultant which pointsin front of the toe of an individual

drystone retaining structures

Limit equilibri53


33/164

Below theforwards,within thegreater efthe wall. Twill be grcomponenso increasportion of

the face hforwards no longerchanges tincreasinghave comsafe for ytype of bueffects. Has excavatoe of defin loadingattributedDue to thwalls, sigtake placegiving visFinal collby topplinusually a

Bulging begins when the loadsbehind the wall cause blocks orentire sections of wall to move,and the resulting movementcauses both the forces acting onthe wall and the equilibrium of itsown mass to change, such that anew equilibrium position is found.Were this not the case, the wall

would continue to move resultingin collapse. This rearrangementusually occurs lower down thewall, and can be due to slips in theretained earth, increased pore waterpressure or an increase in loadingconditions, or the equilibration ofnegative pore pressures within thebackfill. Bulging probably occursmuch more commonly than isappreciated, but is usually on ascale too small to be noticed.

Bulging and movement can alsooccur much higher up a wall usuallycaused by localised surcharging, ordisturbances to the wall itself, suchas growth of vegetation, althoughthis is generally detrimental towall stability and can easily leadto partial or full wall collapse.Once a bulge is formed, the pressuresacting upon the wall must changein response to the new geometry. Asection of a typically bulged wall isshown in Figure 6, highlighting thecommon features. Above the bulge,the wall is leaning back somewhat,having a two-fold effect. Firstly,it stabilises the wall by moving itscentre of gravity away from the

toe of the wall, which is usuallythe overturning point. Secondly, itreduces the magnitude of the forcesapplied to the wall by the backfill.

drystone ret



34/164

Small scale testingA series of small scale tests hasbeen conducted to determine ifobserved drystone behaviour canbe recreated in smaller, simplerexperiments. To house the tests, asteel box was constructed, with thecapacity to hold scale walls 500mm

in height and 500mm in width, seeFigure 7. The box was lined with adouble layer of plastic sheeting, tohelp reduce friction at the edgesand hence minimise end effects 16.As the goal of these experimentswas to reproduce full scale drystonebehaviour, small pellets (2-3mmdiameter) of lead shot were usedas backfill to induce sufficientpressures to cause deformations andfailures. The lead shot used has anuncompacted unit weight of 50kN/m 3 and an internal friction angle of 31 O,allowing the generation of sufficientlateral pressures to overcome thestabilising forces within the test walls.

To overcome three-dimensionaleffects long blocks were used, eachspanning almost the whole widthof the steel box but with gaps ateither end to allow small rotationsof the wall elements. Both timberand concrete block walls were tested

independently; the timber blockswere quickly discarded as theirdensities proved too low for realisticmodelling of drystone behaviour(5.5kN/m 3 as opposed to 24kN/m 3 for the concrete blocks) although thedata proves useful for comparisonwith the LE program results.For each test, the scale walls werefully constructed without anyretained backfill, and then slowlybackfilled. Results from the smallscale tests are shown in Table 2together with the backfill heightspredicted by the LE program.From table 2 it is clear that theprogram is accurately predicting

the collapse hscale tests. It interface frictblocks and thvalue of the bangle. Evidenscale tests sup

although in prto ascertain pthe backfills been mobiliseobviously haswall stability,that ground seand the roughstructures resuangle being m

Large scalIn addition toscale walls hato failure to vand analysis twalls, a bespoconstructed, aof localised osettlement, balocalised surcEach wall waCotswold limAt 2.5m high the test walls representativefound throughare built usingincluding regthrough-stonestones at the pfirst wall vari600mm at its peak, and was


Limit equilibrid

53


35/164

relativelyhelp suppload shedthe first wextent thahad the wplain strashowing ttoe by som

Followingwall was quality thnumber oused to debetween tin positiobelow. Thof load al

and openeto allow tmore easisections. the wall bakin to thprogram, to sectionfail in praare localithan beinfull lengthdimensionthe transfof a wall,a weak semore likesuch suppwhich is n

plain straanalysis imost situawere recofailure, wstable in tgiving ind

Throughout each experimentthe geometry of the wall face isconstantly monitored, allowing thewall profile to be recreated withinthe LE program, so that stabilitycan be assessed as the loads arechanged and the walls deform. Theimages of the thrust lines generatedimmediately prior to failure for thefirst two tests are shown in Figure 11.From Figure 11 it is evident that thestructures are both on the verge ofcollapse, although three-dimensionaleffects may have given added stability,especially in the case of the first testwall. Both walls developed bulgesonly through the central region of thewall adjacent to where the surchargeloading was applied. The high friction

with a sufficient margin of safetyduring installation of remaininginstrumentation and loading plates,whilst being close enough to itsultimate conditions that the proposedmovements and loadings can takethe structure to failure. Both wallscomplied with these criteria; howeverthe second wall was significantlyless stable, with the eccentricityon completion lying in front ofthe walls middle third (fig 11c).The initial phase of each test to

date has involved the raising ofthe platform up to ensure thatthe maximum possible frictionis generated at the wall/backfillinterface. In real walls, full friction islikely to be achieved due to settlementf th b kfill f ll i g t ti

drystone ret

Table 2 - Small scale testing results

Limit equilibrium assessment ofd t t i i t t


36/164

by the LE proloading was dthan load conprogressive dload with fullcollection of dcollapse assocdistortion of tstructure. Botabsolutely stadeformation, loads were rethey were ver

ConclusioThe limit equdescribed in tpotential comanalysis packallows any enknowledge ofand material preliable underinfluencing itneed for the d


Figure 11 - Limit physical tests: (a)

Limit equilibridrystone ret

53


37/164

ReferencesPOWRIE, W., HARKNESS, R. M., ZHANG, X., and BUSH, D. I. Deformation and failure1.modes of drystone retaining walls. Geotechnique, 2002, 52, No. 6, 435-446. O'REILLY, M. P. and PERRY, J. Drystone retaining walls and their modifications2.- condition appraisal and remedial treatment. 2009, RP723,O'REILLY, M. P., BRADY, K. C., and BUSH, D. I. Research on masonry-faced3.retaining walls. 2nd European Road Research Conference, 1999,BRITISH STANDARDS INSTITUTION(BSI) Code of practice for earth retaining structure4.CLAXTON, M., HART, R. A., MCCOMBIE, P., and WALKER, P. J. Rigid block distinct-ele5.modelling of drystone retaining walls in plane strain. ASCE, 2005, 131, No. 3, 381-389. BURGOYNE, J. Revetments or retaining walls. Corps of royal engineers, 1853, 3, 154-1596.DICKENS, J. G. and WALKER, P. J. Use of distinct element model to simulate behaviour7.of drystone walls. Structural engineering review, 1996, 8, No. 2/3, 187-199. HARKNESS, R. M., POWRIE, W., ZHANG, X., BRADY, K. C., and O'REILLY, M. P. Nume8.full-scale tests on drystone masonry retaining walls. Geotechnique, 2000, 50, No. 2, 165-179 BRADY, K. C. and KAVANAGH, J. Analysis of the stability of masonry-9.faced earth retaining walls. 2002, TRL Report TRL550,WALKER, P. J., MCCOMBIE, P., and CLAXTON, M. Plane strain numerical model for10.drystone retaining walls. Geotechnical Engineering, 2006, 159, No. GEI, 1-7. COOPER, M. R. Deflections and failure modes in drystone retaining11.walls. Ground Engineering, 1986, 19, No. 8, 28-33. BRITISH STANDARDS INSTITUTION (BSI) Code of practice for strengthened/ 12.reinforced soils and other fills. 1995, BS 8006,BOLTON, M. D. Geotechnical stress analysis for bridge abutment design.13.

1991, Transport and road research laboratory, report 270,CORTE, J. F. Reinforced earth retaining walls under strip load: Discussion.14.Canadian Geotechnical Journal, 1981, 18, 324-326. WONG, H. N. and HO, K. K. S. The 23 July 1994 landslide at Kwun Lung Lau,15.Hong Kong. Canadian Geotechnical Journal, 1997, 34, 825-840. BAILEY, C. Model tests of dry stone retaining walls. MEng Dissertation, University of Bat16.

drystone ret

ITSO smart cards in Welsh public transport


38/164

AbstractIntegrated TransportDivision, WelshAssembly Government

Intelligent TransportSystems


Senior Consultant

Project Support

Joanna Scott

Sarah Warlow

Vivien CollinsThis paper will discuss the progress of the Welsh Assemblyproject to introduce smart card technology to public transpoWales and addresses the achievements of the project to datethe lessons learnt during its initial phases can benefit other card schemes looking to implement within the Integrated TSmartcard Organisation (ITSO) environment and how the fexploration of smart card technology will move forward in

Wales ITSO environmentfor smart cardsWales is one of the four nations thatmake up the United Kingdom and hasa population of just over 2.9 million.The Welsh Assembly Governmentis the devolved government forWales and is responsible for health,

education, economic development,culture, the environment andtransport. Residents aged over 60and disabled passengers of all ageshave been entitled to free travel onlocal bus services across Wales since2002 d th i f

The benefits of ITSOstandards in Wales

The scheme makes use of integratedsystems that follow a common openspecification developed by ITSO 2, anorganisation formed by leaders inthe UK transport industry to establishstandards in smart card technology

with the intention to make nationalmultimodal interoperability a reality. Incontrast to other closed or blackbox smart card schemes that actwithin self-prescribed boundaries, theinteroperable ITSO specification is built

th i i th t lti l h

The ITSO op

After becomisignature of thLicence is thelive transport and forms thea schemes re

scheme owneIt represents tall parties to pthe environmthat cover usethemselves, s

d f Alth

54 ITSO smart cards in Welsh


39/164

P O R T S Y S T E M S

Our recommendation would be tohave early sight of this document atthe project inception stage, identifyany potential issues at the start andto discuss and understand them withITSO at the earliest opportunity. Thismay well help to expedite a potentiallylengthy process, and is very importantas the systems cannot be built

without signature of this document.

Setting up the HOPS

The central hub of the ITSO systemis known as the AMS/HOPS (AssetManagement System / Host OperatorProcessing System - usually knownsimply as HOPS), and acts as apassenger journey database through

which all secure ITSO transactions areprocessed. Setting up a HOPS is oneof the most technically challengingaspects of entering the ITSOenvironment and requires experiencedsuppliers and advisors as it is themission-critical system that must beaddressed before any other. Schemesmust first understand how they wishtheir Operator Identification (OID)structure to work - this is essentially aset of identifiers that allow definitionof the structure of your scheme, toallocate ticketing product ownership,provide the ability to uniquelyencode the ITSO artefacts andconfigurations, and future proof forchanges in that structure. Wales hasan umbrella OID structure, where

the Welsh Assembly Governmentretains the top level (or parent)OID identification, with a series ofother OIDs below. Every schemehas its own requirements, differenttypes of flexibility and differentg l H th id ti

POSTs, ISAMs, PersoPOSTs,and the CMS

The HOPS communicates with thefront office bus depot systems andPoint of Service Terminals (POSTs);these are the Electronic TicketMachines (ETMs) on-board buseswhich allow the flow of data to and

from the ticket machine to the busdepot system to the HOPS on a dailybasis. Inside every POST is a speciallycommissioned ITSO Secure ApplicationModule (ISAM), a literal key tointeroperability, which determineswhich ticketing products and servicesmay be accepted and includes theability to securely add additionalITSO ticketing products and servicesto the smart card on bus. The ISAMalso allows the ITSO transaction to beidentified for operator reimbursement.When the chip is encoded, as wellas containing the unique Walesidentification, it is assigned to apublic transport operator so that itmay be individually managed andtracked throughout the course of

its life, and so that each operatorstransactions can be differentiatedfrom the next - especiallyimportant for reimbursement.The personalised smart card issuanceequipment (Perso-POSTs) in WelshLocal Authorities enable new livesmart cards to be produced andencoded on demand, and forproducts and services on the card tobe added, confiscated, hot-listed ordeleted by the issuer through a directand secure web-based interface withthe centralised CMS. The CMS can beaccessed by all 22 Local Authorities inWales with hierarchy based security

Reimbur

The centrsystem, apartitioneextracted

journey trAuthoritithe amoufor concethis pointto operatoto be idenanalysed or geograthen transvalue usinfactor set Governm

operatorsconcessiothe passen(the boardLocal Auadministrtravel). Tmultiple oboundarieof reimbugrouped aservice proccurs fo

journey) tsystem alto adjust tagainst chprinciplesgeograph

ISMS, exand ITSO

Finally, toarchitectutransactio



40/164

dealing with hot-lists that protectall schemes against fraudulent cardor product use. Hotlisting is describedin further detail later in this paper.To maintain system security andintegrity, each aspect that sits withinthe Wales architecture that requiresreciprocal messaging from the HOPSmust be ITSO compliant, and testedand certified to this standard. Thisis extremely important, not onlyfor the immediate needs of Walesscheme, but also for the integrity ofany operator that sits within the ITSOenvironment. True interoperabilitymeans that all the secure systems areinteracting constantly, so the ITSOcommunity as a whole must remain

confident that any new schemeowner or ITSO Licensed Operatorprotects the security and integrity ofthe entire estate and not merely itsown portion of it. On signature ofthe ITSO Operating Licence, this alsobecomes a contractual obligation, andalthough this certification comes at apremium in terms of contractual andfinancial commitment, the ultimatebenefits of cross-scheme capability forpassengers merits the investment.

Project ImplementationPhase 1: Commissioningsystems and migration

The first major phase of the projecthas been system commissioningand the bulk reissue of bus passesto eligible concessionary travellers.Prior to this project, concessionarytravel pass data were held onlocal databases by each of Wales22 Local Authorities. For a cost-effective implementation of an

The avoidance of a big-bangrollout has been beneficial toWales, especially in the confidencethat now exists with regard to thequality and security of our nationalpassenger database. The abilityto show flexibility in addressing22 different sets of needs in eachlocation has also been essential to

retain enthusiasm, momentum andbuy-in from all the Welsh authorities.Prior to migration, the quality ofthe data held by Local Authoritiesvaried widely and, as part of theirresponsibility as data owners,extensive data cleansing was carriedout by each Authority prior tomigration to the new database.

For the majority, this phase usuallyworked through two cycles over twoyears to ensure maximum benefit andminimum risk. Usually data were sentto a contracted cleansing service andthe remaining records were manually,sometimes painstakingly, searchedand updated in-house based on theup-to-date information available. Therelationships between local registrarsduring this process also both helpedand hindered progress dependingon the strength of informationthat could be obtained at eachLocal Authority. Recommendationshave therefore been taken forwardto further improve internalrelationships and an understandingof the real and sometimes assumed

boundaries around sharing data.In addition to these steps, during themigration process itself authoritieshave been able to use a holdingarea within the CMS for any recordwhich may have had a question

k i lidi Thi h ll d

Mandatory fieand data handwithin the sysdata recordedconsistently aensuring ongoThese extra stdata quality hAssembly Godeal of confidthe data held important givAuthority is gthe number oftravel card ustheir system eaccuracy of thto audit this in

required enabto ensure thatspent approprwhere possiblfairly. It has arequirement fin the additionlarge-scale dabeen avoided issues remaindata quality cmanaged in-hlooking to miuse, ITSO or be endorsed aAs well as renthe quality ofrecords are althe CMS allo

access to the their area, witpartitions beinrequest basis,of records to tAllowing thisl l i h


41/164



42/164

Although at the moment an invalidconcessionary cards shell would behot listed if it was lost or stolen, inthe future it may be that a productsits within an ITSO shell containingmultiple products. If one productbecame invalid - for example, acancelled prepaid product - no otherproduct would be affected and there

would be little point in incurring theadditional cost and time involved incancelling and reissuing a full cardif only one aspect of it is affected.Products could also then be prioritisedon the hotlist based on their value:a season ticket has inherently morevalue than a prepaid day ticket.We plan to use one hotlist thatis broadcast to our OID group ona weekly basis with incrementalupdates of new or prioritised shellsor products, avoiding the additionalmessaging traffic that would incurfrom a point to point approach (HOPSto individual POST). There are alsocertain constraints within this simpleimplementation: the capacity ofmemory on ticket machines dictates

the maximum size of the list; the sizeof warm-list within the HOPS needsto be managed and pruned as thescheme grows; software changeswill require implementation in theback office as well as changes tothe ITSO specification itself; andinteracting with other ITSO schemehotlists in the future will becomemore complex as more operatorsbecome live and implement theirown hotlisting policies. In Wales weare planning to implement our initialhotlisting structure by the end of2009 and are currently likely to bethe first ITSO scheme to do this

The flexibility of the system will alsohave been demonstrated by theintroduction of a commercial smartcard scheme by Cardiff Bus, one ofWales major bus operators. Theexpansion into a commercial schemewill go further towards highlightingthe key benefits of the replacement ofpaper ticketing by smart technology,

such as the invaluable ability tocapture data on passenger travelhabits to better inform marketingpractices and the potentially vastreduction in fraud. Wales remainscommitted to ITSO, interoperability,and the implementation of furtherticketing types within the smartcard environment which willextend across other modes ofpublic transport in the future.

54 ITSO smart cards in Welsh


43/164

P O R T S Y S T E M S

On balance - the see-saws of congestioncharge business cases


44/164

Abstract

Intelligent Transport

Systems


Senior Consultant

Graduate Engineer

Andrew Ferry

Adrian Bourne


Technical Director

Paul Grayston

charge business cases

IntroductionOverview

Many transport practioners see theuse of a congestion charge as apotential part of their armoury to

a referendum when the generalpublic is unable to comprehend whatthe whole package of measures

priming moneproposals. Thhas dampened

The design of urban congestion charge schemes within busproposals to the UK Department of Transports Transport InFund has presented a variety of competing and sometime cconsiderations for designers. This paper looks at how balanto be struck between geographic coverage, revenue, effect technology choice, and the treatment of discounts and exem

55 On balance - the see-sawchar


45/164

P O R T S Y

S T E M S

But, reduof an areaa point wcordon beplaces whpoint wouAnd thenout for cowill raise

have an einstance gpaid workwill be peto champbe severeexemptiocost to thehuman ad

See-saw

For the m justified cDistance for an urbbe implemTIF progrchoice seauthority weighted a cautiousRealisticacase musttechnologwide scalNumber Por a comband tag anas the twoTags havethey havedetection transactiointeropera

The level of charge (for a localcharge) is driven by a need togenerate a certain level of revenue.As well as covering the operatingcosts, this could be to provide fundsfor paying off loans, paying foradditional works, or even generalcontribution to authority funds. Toavoid overloading the see-saw with

unachievable objectives it is essentialto derive an early indication of:(a) the average level of charge

users will be required to pay(b) the transaction cost target.

See-saw 2 - The schemesize and boundary

Schemes will either be charging

for crossing one or more cordons(they could be concentric as theManchester proposals or adjacentzones), or for travel within an area.In considering the size and shapeof a scheme boundary, the extentof the traffic modelling will set thebasic coverage. However, referenceto detailed maps and on the ground

surveying are needed to produce aset of gateways into the scheme.They may be junctions, bridges,or other physical constructions. Itis essential that traffic funnelledtowards the gateways is the trafficthat will be liable for the chargeand can be charged. Similarlycommunities should not be isolated

by the boundary of the scheme.The issue of splitting conurbations bya scheme boundary is a vexed one,bound to cause local resentment.This may result in considerationsfor discount schemes which will

insight into those see-saws andhow to account for them within thebusiness case for charging schemes.The graphical plots used in thispaper have been generated usingcost and volume models developedby Atkins for road pricing schemedesign option analysis and sensitivitytesting. They are used to give a

typical indication of results and donot represent any specific scheme.

The see-saws

See-saw 1 - What to charge forversus the political requirement

The political instruction to investigatethe options for introducing a localroad pricing scheme is almost certainto contain some caveats. They couldbe about: the level of charge, therevenue expectations, the discountedor exempted users, geographicalcoverage and so forth. More oftenthan not (and quite understandably)to the scheme designer this initialback of an envelope concept

will be fraught with complication.Furthermore, at an early stage littlethought is usually given to howthe charge will be presented toend users. Keeping it simple is abyword that is often adhered to, butthe consultation processes for theManchester and Cambridge schemessuggest that a large proportionof people didnt even understandthat the proposals were for peakhour charges only, not all day.The only way to demonstrate robustlyto decision makers what is likely towork and what is not is to carry out

h i t ffi d lli g B t

On balance - the see-saws of congestioncharge business cases


46/164

In the draft proposals for theCambridge scheme (October 2007) 2,it was considered that a tag solutionoffered the most efficient schemeoption. To encourage tag take up,discounted charges would be offered.Different scales of charges anddegree of take up of the tag optionwere analysed using the scheme

cost model. Even small differencesin the charges caused quite large(20%) differences in the overallannual revenue for the scheme.The possible scheme illustrated in thedraft proposals for Cambridge usedthe output of traffic modelling thatsuggested an average charge of 4in the modelled year would providethe benefits in traffic reductionrequired. An objective was for moreefficient, more accurate operationthrough high take up of tags. Ananalysis of the optimum scale ofcharges to give a flat net daily revenueprofile was undertaken see Figure

See-saw 4 - The discountand exemption see-saw

Public opinion expressed duringscheme consultation will often resultin the need to provide discounts,even exemptions to certain types ofuser. These discounts may be quitesubstantial. The to be discontinuedWestern Extension to the Londonscheme gives residents a 90%discount for the whole scheme area.Has this level of discount actuallycontributed to a rise in trafficwithin the zone? We might neverknow, but the level of (discounted)residents charge needs to besufficiently high to persuade at leastsome proportion of the residents

not to use their cars and continueto contribute to the congestion.In Stockholm the Liding exemptionis managed using tags. Tags areuseful for this application, as theyprovide a certainty of registration

The group of a discount caninteraction rein London paof residents dto reduce scheUsing expectethe cost and vcreated can be

various analyof take up of pon the overallillustrates an residents' disca tag might bea general tag registered usewhere automais maximisedmessage), andchannel (e.g. terminal) wheinteraction mIt must be not

Figure 1 - Example of balancing revenue sensitivity to tag take up

55 On balance - the see-sawchar


47/164

P O R T S Y

S T E M S

See-saw and the n

It would bthat transwith highselection been examvolume meffect of vproduces In Figure x and y axscheme exwith a tra0.91 wit

it useof daithe tr50% to be

A range otested (inrelating toand all hawith simiThe three

of each ottogether s(number ousing tagsto adminisensitive effort plothe sensittransactioin detectisome sligthe step inthe numbnumber oa curve thof users c

Figure 2 Typical plot of transaction cost against take up of solutions

Figure 3 Typical plot of revenue against solution assumptions


48/164

On balance - the see-sawchar

55


49/164

P O R T S Y

S T E M S

ReferencesUK DfT Transport Innovation Fund Guidance (2006/7)1.http://www.dft.gov.uk/pgr/regional/tif/ Cambridgeshire CC Outline Proposal for Funding (October 2007)2.http://www.cambridgeshire.gov.uk/transport/strategies/tacklingcongestion/ourproposals/bid.h

Improvements to ramp metering system inEngland: VISSIM modelling of improvements


50/164

Abstract

BackgroundRamp metering was first trialled inEngland in 1986, with a small numberof sites around Birmingham and thenSouthampton. From the outcomeof these trials the Highways Agency(HA) commissioned Atkins to designa system for wider roll-out, and IPL& PEEK to implement it. This systemwas first installed in 2005 and thereare now over 80 sites in England. Theexisting system was not modelledprior to implementation and no siteswere modelled before being installed

These improvdetail in the pRamp MeteriD

Atkins Technical Journal 04

Documents

Transcript of Atkins Technical Journal 04