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Applying recentresearch to thedesign of short spansteel bridges canlower their cost andmaintenance.GEERHARD HAAIJER

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ADVANCES IN

SHORTSPANSTEELBRlDGESB etter short span steelbridges at lower costoRe-search results are deliver-ing that promise withimproved limit state cri-teria, more uniform lateral loaddistribution, wider spacing ofbeams and girders, prefabricated::omposite units, improved bear-ings and elimination of joints.Since its introduction in 1969,Load Factor Design (LFO)has grad-ually replaced the Working StressDesign (wso) method in moststates because it provides a greaterconsistency in the maximum live10ad carrying capacitiesof steelbridges. Changes in limit state cri-teria are now being proposed forLFD.The LFOmethod of the cur-rent American Association ofState Highway and Transporta-tion Officials (AASHTO)Specifica-tion considers three levels of load-ing-service load overlad andmaximum loado The proposal toimprove limit state criteria coversthese different load levels:

Service loads. The primary per-formance requirement for a struc-ture subjected to service loads is0360-0556/85/0011-

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AutOltress deslgn was used lar the Whltechuck RlYerBrfdge InWashington State. The two-glrder pan earrles extremely heavy ForestServlceIoads on IUsingle1aM. .(PSRL)is taken as half the corre-sponding constant amplitudestress range. If the calculated de-sign 'Stress range (FSR.}or a detaildoes not exceed this maximumalIowable stress tange, che fatiguelife is intinite and no funherfatigue check. is required.Otherwise, the estimated mini'mum estimated Hfe of the detailmust be calculated. First, the esti-mated minimum number of load-ing cycles to failure is determinedfrom formulas or from SN curvescorresponding to the detail cate-gory. Next, the !ife in years is cal-culated from: L '"'" N/(365*T*P).(The FORTRANympol . denotesmultiplication.) Here L is life inyears, N is life in cycles, T is aver-age daily truck traffic, and P isstress cycl~s per passage. Designvalues of the daily truck traffic forurban and rural highways are giv'en for use when better data ar~not available. Usually, the numberof stress cycles per truck passage is1 for longitudinal members and 3

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FIGURE2.AUTOMOMENTS

Jb. ~ ~I--L ,1, L--f Auto/arcesl.fR'. ~2R . fRAutomoments.!. AlCt V~ -~Automoments, self equilibratingresidual forees and moments,may be consid8r$d as. a mocJ.iflcationof dead Ioad moments.

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for cransverse members.Thenew mechod can be tai-lored tthe specitied values of keyparameters, such as the lateral dis-tribution factor and the designtraftic volumc. Current researchmay lead tO less conservative fa-tigue design curves that better re-flecc the. efecc of stress cyclesbelow the fatigue limitoThe structural performance re-quirement for limited concretedeck cracking is not explicidy re-quired in the LFDmethod, but acontrol on maximum crack widthis desirable. Here, the AASHTOequation 8-61 can ensure a limiton stress as a function of rebar dis.tribution. GeneralIy, many smalIbars pegoit a higher allowablestress than do a few large bars.om-tOad. The overload struc-tural performance requirement,control of permanent deforma-tions, dealS only with serviceabili-ty. The LFDmethod controls peromanent deformadons by limitingflangestress due to positive and

FIGURE.MOMENTRESISTINGDIAPIIRAGM1'4" 46'8" 4 '4' '~ I.-44' roadway . I. I 9"/."slab I~~.~12x35 . ~150 C12x207HaIfsection HaIfsectiona tal end InteriordiaphragmStudies are underway to increasediEphragmeffectiveness by usingmomentconnectiOns to !hebeams.

negative momento A propo&edd~-~{'sign method-Autostress Design'~..u(AsD)-places no restriction on'the maximum stress due to nega-tive momentoInstead, it permits a continuousspan bridge to shake down -toundergo small plastic deforma-tions at a pier chat will stabilizeafter a few cydes (Fig. 1). Theseplastic deformations normally oc-cur only in the Range outer tibers,and do not create a plastic hinge.After a member has shaken down,it will respond elastically to allsubsequent loads not exceedingthe overload.In the process of shaking down,a continuous structure develops aset of self equilibrating residualforces and moments, called auto'moments. They ensure that thestructure remains elastic whensubjected to overload and may be. considered as a modification ofthe dead load moments. They donot aifect the stress ranges. relatedto fatigue designoAutostressing of a continuoussteel beam requires no tendonsbecause the automoments are au-tomatically developed-hence thename Autostress Design. Al.though the ASOmethod may con'sider automomencs tO meet theoverload performance requir~mencs, a bridge member may nev-er experience automomencs unlessthe actual loading is sufficientlyheavy. Such behavior is consid.ered acceptable.The deflections caused by auto-moments can be treated as deadload deflections that are negated ,by cambering the. structUre. To -;f.keep the permanent deReccions jcaused by positive bending within ,1acceptable limits, the limit statecriteria of the current LFDmethodare applied without. change. Thepositive moment must, of COUqe,be adjusted for the efect of theautomoment.Finally, to limit cracking of checoncrete deck in negative bend.ing, the stress in the rebars atoverload should not exceed theiryield point.The width of the,- cracks should be controlled as for 1serviceloads.!Maximum Load.The maximum.'load structural performance re--

.jquirement deals only with loadresistance; serviceability is notconsidered. The LFDmethod re-quires the maximum load moment

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..~t any section in a bridge memberto be below either the yield or eheplastic momentoIn the LFDmeehod, a simplespan bridge member with a com-pace seceion reaches its maximumcarrying capaciey when a plastichinge forms near the midspan.This hinge, together with the tWotroe hinges at the supports, formsa mechanism and prevents addi-tional loading. Plastic design is,eherefore, permitted by AASlITOnthe LFD procedure for simplespans.The plastic design concept ofmechanism formation permittedby LFDfr a simple span bridgemember can be extended to con-tinuous spans. Simple plastic de-sign, as defined in the AISCspecifi-cation, requires a compact sectionat the pier so that the hinge canrotate inelaseical1y at the plasticmomento The recently proposedconcept of an effective plastic mo-ment makes it possible to use plas-eic design for continuous bridgemembers ehat are not compace.OTHERCOST SAVERS

Other recently developed de-sign features can also improve eheeconomy of steel beam and girderbridges.

MOTe uniform lateral distribution.Current provisions specify thefraceion of a wheelload carried byeach interior girder. These ac-count for the effects of the slab,but not the cross frames, whichrecent finite element studies haveshown can be substantial. Liveload moments can be reduced 30%or more by properly accountingfor ehe effects of conventionalcross frames spaced at ehe usualdiseance of 25 feoThe live loadmoments in exeerior girders, how-ever, usually cannot be signifi-cancIy reduced below ehe AASHTOprovisionsoAe present, a special analysiswould be required eo justify theuse of these lessconservative later-al distribution factors on any par-ticular bridge. Research, includingstudies sponsored by NCHRPand

METRICSConversion faccors for mea-suremencs in chis article are:1 kip .. 4.4kN; 1 ino = 25mm;1 fc - 0.3m.

AISI,is being applied to this prob-lem, and may eventually result inspecification changes.Diaphragms usual1ydo not con-tribute substantially to lateral dis-tribution because of their flexibleconnections to the longieudinalbeams, eheir relatively low stiff-ness compared with those of thebeams artd slab, and their relative-ly wide spacings compared withehe span length. lemay be possibleto increase the effectiveness of thediaphragms by employing mo-ment connections to the beams.Thus, ehe diaphragms would func-tion as a continuous beam fromone exterior longitudinal beam tothe other (Fig. 3).To provide a satis{actory mo-ment connection, the stiffeners orgusset plates to which the dia-phragms are attached should bewelded to both beam flangesoThey should also extend beyondthe flange edges so that an ade-quate number of high strengthbolcs can be instal1ed.

Fewer beams or girders. Many re-cent cost studies have shown thatincreasing the distance betWeengirders, thus using fewer, reducessteel costs and more than offseesehe addieional costs of ehickerdeck slabs. Er~ction and shippingcoscs are less, and there are fewercross frames and bearings eo behandled.

Larger overhangs should beused wieh the wider girder spac-ings to balance loads ninteriorand exterior girders. The smallerdistance between exeerior girdersreduces the required pier widthand lowers substructure costs ac-cordingly.Even small increases in girder'spacing, which can be done withtraditional reinforced concretedecks, can provide significant costsavings. For example, increasingthe girder spacing from 7 ft 6 in.to 10 ft 8 in. would eliminate threegirders from a multilane, 11 girderbridge, saving 8 to 10% in the costof the fabricated girders. Even ifthe deck thickness is increased,9 to 10ft should be considered anapproximate minimum spacingfor economy with most traditionaldesigns.

Prefabricated composite unirs. An-other way of lowering the cost ofshort span steel bridges is to useshop assembled composiee unitssmall enough to be readily

.. shipped to the site. Such unitsusually consist of an 8 to 16 ftwide deck of composite concretesupported by tWo or morerolledbeams or welded girders. Crossframes and guard railings can alsobe installed in the shopoTwo or more units are placedside by side, often on simple pre-cast abutments. Erection usuallytakes one or two days. The highconcreee strength available withplane casting contributes eo theeconomy, as does the fact that asmall gap can be left betWeen unitsrather than installing a special fit-ting or grouting along ehe longi-tudinal joints.Improoed bearings.Elastomericbearing pads provide an alterna-cive to conventional steel rockeror roller assemblies. They can ac-commodate both rotation andtranslation, and are now widelyaccepeed because they prevent ehe"frozen" joints ehat sometimes oc-cur wieh steel sliding bearings orsmall rollers.Elastomeric pads can be com-bined wieh Tef/on sliding surfacesto permit large translations incombination with roeations. Stain-lesssteel plates are often used withTef/onto provide such sliding sur-faces.THEFUTURE

AASHTOs presently consideringa guide specification that includesa number of these advanceso Ifapproved, short span steel bridgesshould cost significancIy less.Research on: ',' improved limiestate criteria for'plaee girders isunder way at the FHWATurner-Fairbanks Laboratory as part ofAISIProject 330. In addieion tocomponent tests, chis projece in-dudes eests of a scale model of atwo-span, three-girder bridge withprecast preseressed concrete deckpanels. The advancages achievedthrough simpler design of beambridges will thus be extended eoplate girders. (JGeerhard Haaijer, MASCE,is vice presi-dent and director of engineering at eheAmerican lnstitute of Stul Conscruction.He is also a member of the lnternacionalAssociation for Bridge and Struceural En-gineering and the Royal (Netherlands)lnstitute of Engineerso

This artide is adapted from his presen-tation at ehe 1985lnternational Engineer-ing Symposium on Structural Seul held inChicago iri May.

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