An Assessment Of Design Criteria For Continuous-Welded .
Transportation Research Recor¿l107119An Assessment of Design Criteria for Continuous-WeldedRail on Elevated Transit StructuresDONALD R. AHLBECK, ÄNDREW KISH' and .ANDREW SLUZABSTRACTFor a safe, econornlcal design. three bAsic problens must be addressed in theuse of continuous-trelded rail (CWR) on aerial structures: (a) the control ofsÈresses in the rail caused by differential longitudinal move¡nents belween therail and superstructure (deck, girder) attrÍbuted to temperature changes orother causes, (b) the control of rail-break gap size and the resulting loadsinto the superstructure caused by a pull-apart, and (c) the transfer of loadsand noments from the suPerstructure into the substructure (i.e., colunns andpiers). These are conflicting design goaIs, however, and the ialeal solutíon toone may worsen the problern with another. Design conpromises âre neceÊsAry toattaín acceptâbLe 1oads, component stresses, and deflections under all expectedconditions. In this papêr, the existing ddsign criterÍa for use of CWR onelevateil transit Ëtructures are reviewed and evaluated. Available literature(e.g', reports, codes, and specifications) is reviewedi and visits to five ofthe newer U.S. transit properties are described, iliscussing design PhllosoPhies'past experience, and current maintenance practices. Simple iterative compufermodels were developed to provide esti¡nates of thermally induced loads into therail and structure, anil rail-gap size and loads caused by a pull-apart, as afunction of nonlinear fastener charäcteristics and structure configuration.Results of analyses of typical transit structures employing these models aredescribed in the paper.As part of the technical support that the Transportation systems center (Tsc) has provided to UMTAunder the Urban Rait Rehabilítation, Construction¡investígatíonsand Maintenance Program (UM-476)have been conducted to develop safe' and cost-effective means to inprove the perfornance of elevatedtransit structures. The primary objectives of theseinvestigations v¡ere to develop methods and tech-niques for assessing the structural integrity oftransit track and elevate¿l structures, and to Cleter¡nine if current design criteria, specifications rrehabilitation requirenents, and maintenance practíces are adeguate to ensure structural integrity.One of thesê investigations (-U addressed thetechnlcal and econonic factors in the use of continuous-welded rail (ClfR) on elevateil structures.The replacenent of boJ-ted-joint rail (BJR) with cwRon elevated transit structures would reduce thenaintenânce and noise problems caused by wheel/railírnpact loads at the rail joints. However, largevariations ín tenperatures over daily or seasonalcycles can generate large lateral and longitudinalIoads in the track because of thermal expansion orcontraction. In BJR, the rail joints provide forsJ.ippage that will reduce these forces. with CWR' onthe other hand, these locked-in thernal loads candamage the supporting structure if not properly handled. The older aeríal- track structures were notdesigned for CWR user hoh'ever, and the current practice of rail replacenent on these older elevatedstructures is to continue the use of BJR. On newereLevated transit structuresr cwR is nore connonlyD.R. Ahlbeck, Battelle colunbus Division, 505 KingAvenue, Columbus, Ohio 43201-. A. Kish and A. Sluz,Transportation Systerns Center. U.S. Department ofTransportation, Kendâll . Square , Cambr idge r Mass .02142,used to ensureoverall structural integríty are variedr and thereis a need to evaluate these criteria to deter¡nine ifthe resultíng designs are ailequate.used. However. the deslgn criteriaBACKGROUNDA general review of design criteria and standardsfor designing elevated structures for urban railtransÍt systems is provided in a reporÈ by Harríngton(2). A rnajor concJ-usion of this report r¿as that thevarious criteria are similar enough that a uniformset of industry-wide standards is feasíble. Hovrevertthese design criteria do not explicitly ãddress theuse of CïR on elevated structures. There are nospecific guidelines for rail restraintr nor established lirnits on the size of a rail gap that woul¿lresult fro¡n a thermally induced rail break.current guideway design criteria also have beenreviewed anil conpared by Dorton and Grouni (3'pp.134144). Design rnethods nore specific to the applicationof ClilR to aerial transit structures are found infeasibility and design studies for the newer transitproperties (å-7). Systens described in these reportsrange from Californiars Bay Area Rapid Transit(BART), the oldest of the modern U.S. transit systems (4), to the vancouver, British columbia' Advanced Light Rapid Transit (ALRT) system (:). Aconference paPer by Fox (9) discusses the designphilosophy used in the !'lashington (D.C.) Metropolitan Area Transportation Authority (Y'¡MATA) steelbridge structures.A recent paPer prepared for the Anerican PublicTransit Association (APTA) Track construction andMaintenance Subcommittee by Robert E. Clemons (elsewhere in this Record) specificalty addresses theprobtens of cwR on aerial structures. In this paper(continuous l.¡elded Rait on BART Aerlaf structures) '
Transportation Research Record20in the rail and loads ínto the structure, yet linitthe rail-gap size Ín the event of a rail break' thedif,ferent transit Properties have used cor¡binationsof low- and high-restrainÈ fasteners (WMATA), mediumrestraint fasteners (I{ARTA' MDTA MetrorâiI, MTÀBaltimore), and high-restraint fasteners (BART).trouble with the highBART has had litt1erestraint fasteners, and no eviclence of excessiveloads ínto the aerial structures. Rail gaps fron thefew rail breaks have been controlle¿l to less than 1in. WMATA, on the other hand, has experienced probIems vrith its fasteners ( 'fg) i and a few rail breakshave generated gap rvidths in excess of 6 in. tÍmitedservice experíence has been accunulated on the newestsyste¡ns that utiLize the mediu¡n-resÈraint fastenerstbut, these fasteners have performed well to date.the concepts of direct fixation of rail- to structure,the types of fasteners usedr anil the rail,/structureinteractions are discussecl in detail. The three basi'cmethods for accomnodating thermally induced differential novements between rail and structure are alescribed in this Paper as followss (a) elastic fasteners with nonsliP raíI cla¡nps and raiI,/girder¡notion wíthin the shear deftectlon of the fastener(used by BART) i (b) elastic fasteners with controlled-slip rait clips providing elastic alefor¡nation to the longitudinal force limit of the clipsIused by the Metropolitan Atlanta Rapid TransitAuthoríty (lttARTA) the Þletro Dade Transportation'and the Maryland Mass TransitAdninistration (MDTA),Administration (MTA-Baltinore) I t and (c) elasticfasteners wíth nonslíp (or high slip-lirnit) railclanps near the fixed end of each girder, andcontroJ.led-s1ip (low sliP-Limit) rail clips on therest of the girder, where greater relative movenentis expected (used by WI{ATA).rn sutünaryr these reviews of current publisheddesign criteria and standards shovted that the problems in the applícation of cwR to elevated transitstrucÈures are not specifically addressed. A reviewof the technical reports and design studies shov¡edthat ilifferent organizations have taken substantiallydifferent engineering approaches in handling theseEVAI,UATION OF CIVR DESIGN CRITERIAThree basic problens ¡nust be addressed in the designof aerial sÈructures for use with CWR track:1. The control of stresses in the rail attributedto differential Iongitudinal notions betvreen therail and suPerstructure because of tenperåturechanges or other causes,2. The control of rail-break gap size and resulting loads into the suPerstructure attributed toaerial Pu1l-apart. and3. The t.ransfer of superstructure loads and monenEs into the substructure (e.g.' píers and bents)'problens.TBANSIT PROPERTY SITE VISITSFive transit properties erere vísited during thecourse of this stualy (1) to (a) proviile a firsthandl-ook at the track and aerial structuresi (b) interview system design engineers and track maintenancepersonneJ.; and (c) gather available material' on design criteriaf standards, and methods.- These Properties were as foLlows¡A solution to one of these problems may conflictwith the ideal solution to anotheri therefore, ilesigncompronises rnust be made thât wíll result in acceptable levels of conponent l-oad, stress, and defLectionunder all expectedl conditions.Longitualinal loads are developed betv¡een the cwRand the suPerstruct.ure (i.e., deck and girders) ofan aerial transit structure by differential ¡novementand shear of the fasteners. Reaction loads arê carried into the substructure (i.e., colunns, piersr'and bents) through fixed bearings and by shear orfriction through expansion bearings. On curved tracktlateral components of the longítudinal loads mustalso be reacted by Èhe structure.lfhen the rail tenperature droPs nany degrees lov¡erthan the rail neutral (stress-free) te¡nperature 'high-tensíle, Iocked-in loads are developed. If arail breaks, this tensile load is reLeaseil. The loadis distributed through raÍl fasteners in each direction from the point of break to points where the. MARTAT. MTA-Baltimore,. MDTA Metrorail'. BART' and. WMATA.(Note that summaries of trackr fastenerr and aerialstructure characteristics for these five systens aregiven in Tâble 1.)The newer transiÈ systens have followed dlifferentdesí9n philosophies in their uEilization of CWR anddirect-fixation (DF) fasteners on aeriaÌ structures.In an effort to control therrnally induced stressesTABLEI1071Track and Aerial Structure Characteristics for Representative North Àmerican Transit SystemsElastomericSystemRail SizeMARTA115REREMDTA Metrorail 115 RE119 CF&IB.A.RTll5 tenerTypical FastenerTypeHixson H-l0, Landis/Pandrol, Hixson H-15trackwo¡kNoneNoneNoneHixson II-l 5ALandis/PandrolLandis/Pandrol, HixsonLandis/Pandrol, l ClipVertical30314Bolted clampSpring clipSpring clipt30-3004 32-3630s/8(mod)SpecialFastener StiffnessPadLateral Longitudinal80 3880-120 20-30-36Spring clipSpring clipBolted clampi20-l31431430314Bolted clamp80-i10408-s0lO-c2504008 24-33l8-363244124't30(spans lied logether) ard lloating BE/E E besringNote: pcc precast concrete and clpc cast-in-place concrete; for span lyper slmple single span as opposed to continuousa¡d slip intentionâl (eapected) tall/fastetre¡ moveñent'd¡angement (se Figu¡e lB,.lghlhand span); and for faslener type: nonslt; no expetted rail/fa;tener movementasome stlffening with agÊ l assumedcNo maxlmum stlffnes speclfled.dFo¡ ¡ail stros¡ colculatlons,
Ahlbeck et al.2Lten. The third configuration (C) is an asymmetricalarrangement cornmonly used on railroad and highwaybridges.SUPERSTRUCTURE(oecr,c¡RoEns)Rail StressesSUBSTRUCTURECONFIGURATIOII A(ptEns, cor-u¡ns)stresses ¡nust include contributions of vehicular and structural loads, as weII asthe thernally induced stresses. Current transit trackdesign practice uses the factors reco¡nmended by theAssociation of A¡nerican Railroads (AAR) when raiLbending stress is calcr¡Iated (ÀL). Starting with thenaxirnu¡n bending stress under the peak exPected dynamic wheel load, these factors account for contributions attributed to lateral bending' track conditionr rail wear and corrosion, and unbalancedsuperelevation. Additional components can includeAn analysis of railCONFIGURAÏION BFASTENERS. stress caused by acceleration or braking of. stress in rail caused by bending of super-vehicles,structureoôEXPANSI0N BEARINGFIXÊD BEARINGCONFIGURATIONFIGUREI.ing,rail,.tionBearing configurationsrnaximumexpected temperature-induced stress is subtracte¿lfrom the yield stress of the rail. This difference,for elevated structurereduced by a suitable factor of safety, wÍll estab-lish the maxímum ilesign stress resulting from differential novement between rail and superstructure.From tvro recent desígn studies (6'7) the following'rail stress contributions were expected:. Bendíng stress (aI1 sources, nultiplied bythe factor of safety): 37 to 40 percent of yieldthermal Èensile load is agaín sustained in equilibr ium.Part of the load fro¡n a broken rail is transferredto the unbroken rait(s) of the guideway, and part istransferred to the substructure through fixed andexpansion bearings. The exact nagnitudes of theseloads depend strongJ.y on the substructure (bent orcolunn) longitudinal stiffness, the structure-bearingconfiguration, and the rail fastener restraint character istics.The effects of column and girder bearing stiffnesses depend on the specific bearing configurationused in t.he structure. Three conmon conflgurationsused v¡ith aerial transit structures âre shown inFigure t. The first of these (A) is a symmetricalarrangernent co¡ilnon to noclern transit systems such asBART. The second configuration (B), also sym¡netrical'is used on the level track sections of the MDTA sys-Longitudinal Restraint Load(max., aerial, lb)AedalSt¡uctureTotal Lengthby thernally induced movenentof the superstructure (girders or deck). One methodassunes a constant fastener restraint per unitlength over the total span length. This is conservative (i.e., the calculated load is higher than theactual load), and not too inaccurate for lovr- toMin.Max.Girdert30Steel2,000-3,0004,658 (fÐ70701,200-r,600l0,5oo (ft)20.8 (mi)9 (mi)8580'70653,090 (fÐ8063250-750Several methods are used to calculate the loadgenerated in the railTypical10,000o,ooo-t s,ooo. ThernâI stress: I7 to 18 percent of yieldstress; and. Rail or structure differential tnovementr orboÈh: 26 to 29 percent of yield stress.(ft)SIipt,540-2,400stres s iSpan LengthNonslip7,000-l 0,000andstress caused by rail or structure interacforce (or both) through fasteners.The total of these stresses plus theCgirders.ttAxiaL stress causetl by conposite-beam bendthus inducing shear in fasteners and load into40Rail-LayingTemperatureAerial Structure95ll0100107PCCPCCPCCDeckunitunitunitSteel CIPCSpan SimPle55-75Expected Temperature Range (oF)Structure!70160130160 50-9od
)tTransportåtion Research Recordnedium-restraint fasteners. Another method assumes aJ-inearly decreasing fastener load over the J.ength ofthe span, based on fastener shear stiffness and the¡naxi¡num ther¡na1 novement at. the expansion end of thespan. This nethod assumes that the fastener shearforce does not exceed the slip limit force of thefastener, and is therefore accurate only for highrestraint fasteners. A nuch better solution is ¡nadepossible by including the slip-limit force, up tothe point where the linearly decreasing shear forcedrops beLow thís force ]i¡nit.Over the span length of a gírder, the iail is arelatively flexible element when compared v¡ith thegirder itself. A sma1l computer progran was set upduring this study to calculate fastener loads intothe rail. assuning the rail to be a nu¡nber of finiteflexible elements betseen individual- fasteners,rather than the rigid rail of the previously citedmethods. An iÈerative solution is necessary, but thesolution converges quickty (5 or 6 iterations), andthe prograrn can be run on a desk-top computer. Totalloads bethreen the rail and superstructure over one80-ft span were calculate¿l for three representativecases: a high-restraint fastener (BART), a rnediunrestraÍnt fastener (MARTA, MDTA-Uetrorail), and alow-restraint fastener (Ì tlATA). Loads computed for aflexible rail are conpared in T able 2 with loadscalculated frorn the three cited rigid-rail methods.An inportant point in considering the rail flexible,however, is that the total load into the raíI isdistributed so that less than 70 percent of thisIoad is reacted at the expansion end of the span atthe highest-stressed point in the rai1. This peakTABLE 2 Comparison of Methods for Estimating TotalLongitudinal Load Between Rail and SuperstructureLongitudinal Loads for Diffe¡ent Methods (lb)FastenerRestraint Constant(slip,lb)Restraint(10,000) 320,000(2,500) 80,000Low ar LoadPlus166,000166,000166,000ComputedDecreasing 60oThe rail-break gap size is generally eÊtirnated byan equation in the form:6 2 (Xcl YcZ-Y,cS)(1)wherexCt Pfns/Kfr themaximum longitudinaldeflection of the "nonslipn fastener;Xg2 oATLsr the noninal rail contractioniXç3 (n P¡" nr"P¡¡g)Is/2\Ep the reductionin rail contraction caused by fastenerconstraint¡q coefficient of expansion, 6.5(10)-6in.,/in.-oF for steel¡ temperature change, oFi T length of span (fixed to expânsionLspoint) ;Pfns minirnu¡n longitudinal restraint force,nonslip fasteneriP¡s mininum longitudinal restraint forcetcontrolled-slíp fastener ¡Kf fastener longitudína1 stiffness (Ib,/in. ) Inrr" number of nonslip fasteners in span;n" number of controlled-slip fasteners inspaniAr cross-sectional area of rail (I1.25 in.2for lI5 lbrzyd RE rai1, the most com¡nonlyused size in U.S. systems) t andEr rail ¡nodulus of elasticÍty, 30(10)6Lb/ín.2.A sinplifieil form of this vras used in the MDTAl,tetrorail design, based on a Length, L, n.eitherside of the break over which full rail anchorage isprovided.rn so thatG (c T)2 AxF.t/Rf(2',,where Rf is the longitudinal restraint per inch ofrail in pounds per ínch.To check the validity of these âpproxinate rnethods, a finite-element model of the track structurewas used with sone typical syste¡n paraneters givenin Table 3. This nodel, ca1ledTBTRÀCK,hasbeen23,000Note: qringconfiguration,1071F lempe¡ature change, and an 8o-ft span length.rail stress ls a function of fastener shear stiffness: the lo}rer the stiffness, the lower the percentage of total load (down to 50 percent) reactedby the rail at the expansion end.TABLE1135A raiL break or pulJ.-apart will occur when the thernalJ.y induced tensile force in the rail attributedto a large drop in temperature exceeds the ultinatetensile strength of the rail. À pull-apart will ¡nostprobably occur at or near an expansion joint ln thesuperstructure (deck or girders), but the actualIocation of the break wiII be at a bad welcl, railf1aw, or other weak spot.Controllíng a rail break presentÊ two dÍstlnctproblems denandlng a sonewhat opposite solution: (a)for safety reasons, the length of the rail-break gapnust be minimized to reduce the possibilíty of derailment if train wheels pass over the break, and(b) the forces and tnoments lnto the superstructureattributed to the release of the locked-in thermalload must be ¡nÍninized. The first requires higherfastener longitudinal restraint limits, and thesecond requires lower restraint limits.Test Casee Rrm in Parameter Variation StudyStiffnessCase4Control of a Rail Break3678(lb/in.)30,00030,000I 0,00030,000I ription2,5002,50010,000Spring clip (MARTA, MDTA)Spring clip, wornSpring clip, wornBolted clamp (BART)Stiff pad, spring clipLow-slip, loose clampMedium slip, loose clampNominal slip (\YMATA)2,5001,2501,2501 0,0002,500250750500used for several years to investigate the råil buckLing phenonenon (I1). The preceding equatlons forestirnating the rail-break gap size aEsu¡ne Iinearload distributions. Results fron the finite-elementmodel, however¿ shoyr the fastener load distributionsto be nonlinear. Using the para¡neter values of Table3, raiJ--break gap size for several cases as a function of tenperature drops fron the zero-stress(neutral) point are plotted in Flgure 2.The finite-element ¡nodel is sonelrhat awkward andcostly to run. Instead of completing the pararnetricstudy with TBTRÀCK, a simply iÈerative solutionsi¡nflar to that described in the previous sectionwas developed. This model, called TRKTHRM, considerseach rail elenent betneen fasteners as a sprLngr and
23Ahlbeck et 41.RAILELEI4ENTSRAILL.xnO xnl I **r/ xnJ xn(¡¡-l) . xnl. xn(r l)lKnl KnI oR I ¡¡. iI ¡îâÂt.-1R- i¡!1Â/åI l*v\'f l-[ÙoÉ ts ):DISPLACEMENTSz)o èdr- l* ¡ -ò-a, l* - t-,BIxc0ojFIGUREsC,ASE0qis all in the slip zone. Equation J- providesa reasonable estinate of gap size for medium-tohigh-restraint fastenersr but badly underestimatesgap size with low-restraint fasteners. Equatíon 2provides a surprisÍngly accurate esti¡nate in manycases, except where high-restraint fasteners areused.Improved accuracy can be obtained with Equation Iif the term Xç2 is ¡nodified to use the esti¡natedtotal number of fasteners over which the locked-inIoad is distributed so thatIEIIPIRATUlìE DROP, DTGRTE FFIGURE 2 Rail-break gap size predicted by finiteelement computer model,each fastener as a bilinear spring with longitudinalslip (restraint limit). Girder displacements areinputs at one end of the fastener spring, with railmotions calculated for the other end.Progra¡n TRKTHRM assumes that the rail breaks atthe expansion jointr one spacing ahead of Fastener1, and the locked-in Èhermal Ioad must then be dissipated over an unknown number of fasteners. A firstestinate of this number is calculated, just to startthe iteration process. The solution noves ín theappropriate direction to add or subtract fastenersuntil equilibrium with the locked-in load isachieved. The effects of girder contraction, whichdepend on the particulår bearing confíguration thatis used (see Figure l-), are included in this model(see Figure 3).The several methods for predicting rail-break gapsize are compared in Table 4 with results fro¡n thetwo cornputer progransr TBTRACK and TRKTHRM. Note,first of all, that the finite-elenent model TBTRACK,with its linited number of lumped-fastener elements'tends to underestirnate gap size. With medium-to-highfastener restraint, girder contraction increases gapsize from 25 to 72 percent of that predicted if thegirder does not contract. with low-restraint fasteners, girder contraction hãs Iitt1e effect because4Model of broken rail on elevated transit structure.movement(BART)20 40 60 80 100 I20 1rr0 160TAtsLE3G 2(Yrt )¡çz-Y.g:)(3)where 0.5cATnxLsin* P/P¡*¿x PfmaxKr/2P'yK¡iP1 cATA¡E¡, the thernal loadr 1b;Pfmax (nnsPfns n"P¡s),/(nn" * ns) rthe average fastener restraint lirnitt lb¡Kr AtE/Lt, the raíI spring, Ib/in.¡ andKi fastener l-ongitudinal stiffness, Ib/ín.XCZLimited data are available on rail-break gap sizesfor specific fastener systens. Records of gap sizeand rail tenperature are seldom kept. Rail breakswith high-restraint fasteners (e.9. ' BART and MARTA)have produced gaps of less than I in. The large gapsizes experienced by WMÀTA, on the other handr maybe induced by the dynamic stick-slip resPonse of therail in low-restraint fasteners.Load Transfer l4echanismsAs discussed in the previous sections, Iongitudinalloads are developed between the CWR and the suPer-Comparison of Rail-Break Gap Size by Different Formr¡lasRail-Break Gap Size Estimates (in.)TBTRACK TRKTHRM TRKTHRMCase Equation 1Equation 2Equation3 ATg 0I23456780.690.720.890.620.8 3t.231.3 51.5 50.510. 150.570.850.622.29 0.990.682.470.821.4340.68l.2l1.23Atg 0ÀTg a0.550.7 40.8 5r.29o.671.3 I1.38L470.500.662.390.790.632.68t.621.54I .611.44t.20\.140.970.401.77Note: temperâtùre change in the gir
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