17 Thomas A. Ostrom Steven L. MellonChiefOfce of Structures-NorthCalifornia Department of TransportationSacramento, CaliforniaSenior Bridge Design EngineerQuincy Engineering, Inc.Sacramento, CaliforniaBRIDGE ENGINEERINGBridge engineering covers the planning,design, construction, operation, andmaintenance of structures that carryfacilities for movement of humans, ani-mals, or materials over natural or created obstacles.Most of the diagrams used in this section weretaken from the Manual of Bridge Design Practice,State of California Department of Transportationand Standard Specications for HighwayBridges, American Association of State Highwayand Transportation Ofcials. The authors expresstheir appreciation for permission to use theseillustrations from this comprehensive and author-itative publication.General Design Considerations17.1 Bridge TypesBridges are of two general types: xed and mov-able. They also can be grouped according to thefollowing characteristics:Supported facilities: Highway or railway bridgesand viaducts, canal bridges and aqueducts, pedes-trian or cattle crossings, material-handling bridges,pipeline bridges.Bridge-over facilities or natural features: Bridgesover highways and over railways; river bridges;bay, lake, slough and valley crossings.Basic geometry: In planstraight or curved,square or skewed bridges; in elevationlow-levelbridges, including causeways and trestles, or high-level bridges.Structural systems: Single-span or continuous-beam bridges, single- or multiple-arch bridges,suspension bridges, frame-type bridges.Construction materials: Timber, masonry, con-crete, and steel bridges.17.2 Design SpecicationsDesigns of highway and railway bridges of con-crete or steel often are based on the latest editionsof the Standard Specications for HighwayBridges or the Load and Resistance FactorDesign Specications (LRFD) of the AmericanAssociation of State Highway and TransportationOfcials (AASHTO) and the Manual for Rail-way Engineering of the American Railway En-gineering and Maintenance-of-Way Association(AREMA). Also useful are standard plans issuedby various highway administrations and railwaycompanies.Length, width, elevation, alignment, and angleof intersection of a bridge must satisfy thefunctional requirements of the supported facilitiesand the geometric or hydraulic requirements ofthe bridged-over facilities or natural features.Figure 17.1 shows typical highway clearancediagrams.Selection of the structural system and of theconstruction material and detail dimensions isgoverned by requirements of structural safety;economy of fabrication, erection, operation, andmaintenance; and aesthetic considerations.Highway bridge decks should offer comfort-able, well-drained riding surfaces. Longitudinalgrades and cross sections are subject to standardssimilar to those for open highways (Sec. 16).Provisions for roadway lighting and emergencyservices should be made on long bridges.Barrier railings should keep vehicles within theroadways and, if necessary, separate vehicularDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.Source: Standard Handbook for Civil Engineerslanes from pedestrians and bicyclists. Utilitiescarried on or under bridges should be adequatelyprotected and equipped to accommodate expan-sion or contraction of the structures.Most railroads require that the ballast bed becontinuous across bridges to facilitate vertical trackadjustments. Long bridges should be equippedwith service walkways.17.3 Design Loads for BridgesBridges must support the following loads withoutexceeding permissible stresses and deections:Dead load D, including permanent utilitiesLive load L and impact ILongitudinal forces due to acceleration or decel-eration LF and friction FCentrifugal forces CFWind pressure acting on the structure W and themoving load WLEarthquake forces EQEarth E, water and ice pressure ICE, stream ow SF,and uplift B acting on the substructureForces resulting from elastic deformations, includ-ing rib shortening RForces resulting from thermal deformations T,including shrinkage S, and secondary prestressingeffects17.3.1 Highway Bridge LoadsVehicular live load of highway bridges is expressedin terms of design lanes and lane loadings. Thenumber of design lanes depends on the width ofthe roadway.Fig. 17.1 Minimum clearances for highway structures. (a) Elevation of a highway bridge showingminimum vertical clearances below it. (b) Typical bridge cross sections indicating minimum horizontalclearances. Long-span bridges may have different details and requirements.17.2 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGIn the standard specications, each lane load isrepresented by a standard truck with trailer(Fig. 17.3) or, alternatively, as a 10-ft-wide uniformload in combination with a concentrated load(Fig. 17.2). As indicated in Fig. 17.3, there are twoclasses of loading: HS20 and HS15, which representa truck and trailer with three loaded axles. Theseloading designations are followed by a 44, whichindicates that the loading standard was adopted in1944. The LRFD HL-93 vehicular live load consistsof a combination of the HS20-44 design truckdepicted in Fig. 17.3, or the LRFD design tandem,and the LRFD live load. The LRFD design tandemis dened as a pair of 25 kip axles spaced 4.0 ftapart. The LRFD live load consists of 0.64 k/lfapplied uniformly in the longitudinal and trans-verse direction.When proportioning any member, all lane loadsshould be assumed to occupy, within their res-pective lanes, the positions that produce maximumstress in that member. Table 17.1 gives maximummoments, shears, and reactions for one loadedlane. Effects resulting from the simultaneousloading of more than two lanes may be reducedby a loading factor, which is 0.90 for three lanes and0.75 for four lanes.In design of steel grid and timber oors for HS20loading, one axle load of 24 kips or two axle loadsof 16 kips each, spaced 4 ft apart, may be used,whichever produces the greater stress, instead ofthe 32-kip axle shown in Fig. 17.3. For slab design,the centerline of the wheel should be assumed to be1 ft from the face of the curb.Wind forces generally are considered as mov-ing loads that may act horizontally in any direc-tion. They apply pressure to the exposed area of thesuperstructure, as seen in side elevation; to trafcon the bridge, with the center of gravity 6 ft abovethe deck; and to the exposed areas of the sub-structure, as seen in lateral or front elevation. Windloads in Tables 17.2 and 17.3 were taken fromStandard Specications for Highway Bridges,American Association of State Highway andTransportation Ofcials. They are based on 100-mi/h wind velocity. They should be multiplied by(V/100)2for other design velocities except forGroup III loading (Art. 17.4).In investigation of overturning, add to horizon-tal wind forces acting normal to the longitudinalbridge axis an upward force of 20 lb/ft2for thestructure without live load or 6 lb/ft2when thestructure carries live load. This force should beapplied to the deck and sidewalk area in planat the windward quarter point of the transversesuperstructure width.Impact is expressed as a fraction of live-loadstress and determined by the formula:I 50125 l 30% maximum (17:1)where l span, ft; or for truck loads on cantilevers,length from moment center to farthermost axle; orfor shear due to truck load, length of loaded por-tion of span. For negative moments in continuousspans, use the average of two adjacent loadedspans. For cantilever shear, use I 30%. Impact isnot gured for abutments, retaining walls, piers,piles (except for steel and concrete piles aboveground rigidly framed into the superstructure),foundation pressures and footings, and sidewalkloads.Longitudinal forces on highway bridgesshould be assumed at 5% of the lane load plusconcentrated load for moment headed in oneFig. 17.2 HS loadings for simply supportedspans. For maximum negative moment in continu-ous spans, an additional concentrated load of equalweight should be placed in one other span for max-imum effect. For maximum positive moment, onlyone concentrated load should be used per lane, butcombined with as many spans loaded uniformlyas required for maximum effect.Bridge Engineering n 17.3Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGdirection, plus forces resulting from friction inbridge expansion bearings.Centrifugal forces should be computed as apercentage of design live loadC 6:68S2R (17:2)where S design speed, mi/hR radius of curvature, ftThese forces are assumed to act horizontally 6 ftabove deck level and perpendicular to the bridgecenterline.Restraint forces, generated by preventingrotations of deformations, must be considered indesign.Thermal forces, in particular, from restraint,may cause overstress, buckling, or cracking.Provision should be made for expansion andcontraction due to temperature variations, andFig. 17.3 Standard truck loading. HS trucks: W combined weight on the rst two axles, which is thesame weight as for H trucks. V indicates a variable spacing from 14 to 30 ft that should be selected toproduce maximum stress.17.4 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGTable 17.1 Maximum Moments, Shears, and Reactions for Truck Loads on One Lane, Simple Spans*H15 H20 HS15 HS20Span, ft MomentEnd shearand endreactionMomentEnd shearand endreactionMomentEnd shearand endreactionMomentEnd shearand endreaction10 60.024.080.032.060.024.080.032.020 120.025.8160.034.4120.032.2160.041.630 185.027.2246.636.3211.637.2282.149.640 259.529.1 346.038.8 337.441.4449.855.250 334.231.5 445.642.0 470.943.9627.958.560 418.5 33.9 558.0 45.2 604.945.6806.560.870 530.3 36.3 707.0 48.4 739.246.8985.662.480 654.0 38.7 872.0 51.6 873.747.71164.963.690 789.8 41.1 1053.0 54.8 1008.348.41344.464.5100 937.5 43.5 1250.0 58.0 1143.049.01524.065.3110 1097.3 45.9 1463.0 61.2 1277.749.41703.665.9120 1269.0 48.3 1692.0 64.4 1412.549.81883.366.4130 1452.8 50.7 1937.0 67.6 1547.350.7 2063.167.6140 1648.5 53.1 2198.0 70.8 1682.153.1 2242.870.8150 1856.3 55.5 2475.0 74.0 1856.3 55.5 2475.1 74.0160 2076.0 57.9 2768.0 77.2 2076.0 57.9 2768.0 77.2170 2307.8 60.3 3077.0 80.4 2307.8 60.3 3077.0 80.4180 2551.5 62.7 3402.0 83.6 2551.5 62.7 3402.0 83.6190 2807.3 65.1 3743.0 86.8 2807.3 65.1 3743.0 86.8200 3075.0 67.5 4100.0 90.0 3075.0 67.5 4100.0 90.0220 3646.5 72.3 4862.0 96.4 3646.5 72.3 4862.0 96.4240 4266.0 77.1 5688.0 102.8 4266.0 77.1 5688.0 102.8260 4933.5 81.9 6578.0 109.2 4933.5 81.9 6578.0 109.2280 5649.0 86.7 7532.0 115.6 5649.0 86.7 7532.0 115.6300 6412.5 91.5 8550.0 122.0 6412.5 91.5 8550.0 122.0* Based on Standard Specications for Highway Bridges, American Association of State Highway and Transportation Ofcials.Impact not included.Moments in thousands of ft-lb (ft-kips).Shear and reaction in kips. Concentrated load is considered placed at the support. Loads used are those stipulated for shear.Maximum value determined by standard truck loading. Otherwise, standard lane loading governs.Table 17.2 Wind Loads for Superstructure DesignTrusses and arches Beams and girders Live LoadWind load 75 lb/ft250 lb/ft2100 lb/lin ftMinimums:On loaded chord 300 lb/lin ftOn unloaded chord 150 lb/lin ftOn girders 300 lb/lin ftBridge Engineering n 17.5Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGon concrete structures, also for shrinkage. For thecontinental United States, Table 17.4 covers tem-perature ranges of most locations and includes theeffect of shrinkage on ordinary beam-type concretestructures. The coefcient of thermal expansion forboth concrete and steel per 8Fahrenheit is 0.0000065(approximately 1150,000). The shrinkage coefcientfor concrete arches and rigid frames should beassumed as 0.002, equivalent to a temperature dropof 31 8F.Stream-ow pressure on a pier should be com-puted fromP KV2(17:3)where P pressure, lb/ft2V velocity of water, ft/sK 43for square ends, 12for angle endswhen angle is 308 or less, and 23forcircular piersIce pressure should be assumed as 400 psi. Thedesign thickness should be determined locally.Earth pressure on piers and abutments shouldbe computed by recognized soil-mechanics for-mulas, but the equivalent uid pressure should beat least 36 lb/ft3when it increases stresses and notmore than 27 lb/ft3when it decreases stresses.Sidewalks and their direct supports shouldbe designed for a uniform live load of 85 lb/ft2.Table 17.3 Wind Loads for Substructure Designa. Loads transmitted by superstructure to substructure slab and girder bridges (up to 125-ft span)Transverse LongitudinalWind on superstructure when not carrying live load, lb/ft250 12Wind on superstructure when carrying live load, lb/ft215 4Wind on live load, lb/lin ft* 100 40Major and unusual structuresNo live load on bridge Live load on bridgeSkewWind ontrusses, lb/ft2Wind ongirders, lb/ft2Wind ontrusses, lb/ft2Wind ongirders, lb/ft2Wind onlive load,lb/lin ft*angle, orwind,degLat-eralloadLongi-tudinalloadLat-eralloadLongi-tudinalloadLat-eralloadLongi-tudinalloadLat-eralloadLongi-tudinalloadLat-eralloadLongi-tudinalload0 75 0 50 0 22.5 0 15 0 100 015 70 12 44 6 21 3.6 13.2 1.8 88 1230 65 28 41 12 19.5 8.4 12.3 3.6 82 2445 47 41 33 16 14.1 12.3 9.9 4.8 66 3260 25 50 17 19 7.5 15 5.1 5.7 34 38b. Loads from wind acting directly on the substructureHorizontal windno live load on bridge, lb/ft240Horizontal windlive load on bridge, lb/ft212* Acting 6 ft above deck.Resolve wind forces acting at a skew into components perpendicular to side and front elevations of the substructure and apply atcenters of gravity of exposed areas. These loads act simultaneously with wind loads from superstructure.17.6 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGThe effect of sidewalk live loading on main bridgemembers should be computed fromP 30 3000l 55 w50 60 lb=ft2(17:4)where P sidewalk live load, lb/ft2l loaded length of sidewalk, ftw sidewalk width, ftCurbs should resist a force of 500 lb/lin ft acting10 in above the oor. For design loads for railings,see Fig. 17.4.17.3.2 Railway Bridge LoadsLive load is specied by axle-load diagrams or bythe E number of a Coopers train, consisting oftwo locomotives and an indenite number offreight cars. Figure 17.5 shows the typical axlespacing and axle loads for E80 loading.Members receiving load from more than onetrack should be assumed to be carrying the fol-lowing proportions of live load: For two tracks, fulllive load; for three tracks, full live load from twotracks and half from the third track; for four tracks,full live load from two, half from one, and one-fourth from the remaining one.Impact loads, as a percentage of railroad liveloads, may be computed from Table 17.5.Longitudinal forces should be computed forbraking and traction and centrifugal forces shouldbe computed corresponding to each axle. See theAREMA Manual for Railway Engineering formore information (www.arema.org).17.3.3 Proportioning of BridgeMembers and SectionsThe following groups represent various combi-nations of loads and forces to which a structuremay be subjected. Each component of the structure,or the foundation on which it rests, should beproportioned to withstand safely all group combi-nations of these forces that are applicable to theparticular site or type. Group loading combinationsfor service load design and load factor design aregiven byGroup (N) g[bDDbL(L I) bCCFbEE bBB bSSF bWWbWLWL bLLFbR(R S T) bEQEQbICEICE](17:5)Table 17.4 Expansion and Contraction of Structures*Steel ConcreteAir temp rangeTemp riseand fall, 8FMovement perunit lengthTemp riseand fall, 8FMovement perunit lengthExtreme:120 8F, certain mountain 60 0.00039 40 0.00024and desert locationsModerate:100 8F, interior valleys and 50 0.00033 35 0.00021most mountain locationsMild:80 8F, coastal areas, Los 40 0.00026 30 0.00018Angeles, and San FranciscoBay area* This table was developed for California. For other parts of the United States, the temperature limits given by AASHTO StandardSpecications for Highway Bridges should be used.Includes shrinkage.Bridge Engineering n 17.7Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGFig. 17.4 Service loads for railings: P 10 kips, L post spacing, w 50 lb/ft. Rail loads are shown onthe left, post loads on the right. (Rail shapes are for illustrative purposes only.)17.8 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGwhere N group number, or number assigned to aspecic combination of loadsg capacity reduction factor to provide forsmall adverse variations in materials,workmanship, and dimensions withinacceptable tolerancesb load factor (subscript indicates appli-cable type of load)See Table 17.6 for appropriate coefcients. See alsoArt. 17.3.1 and Secs. 8 and 9.AASHTO LRFD associates load combinationswith various limit states according to design ob-jectives. The sum of the factored loads must be lessthan the sum of the factored resistance:higiQi wRn (17:6)where hi loadmodier relatingtoductility, redun-dancy, and operational importancegi load factor, a statistically based multi-plier reecting certainty in the value forforce effectQi force effect iw resistance factor, a statistically basedmultiplier reecting certainty in valuefor particular material propertySee Table 17.7 and 17.8 for design objectives, limitstate load combinations and load factors. Resist-ance factors vary according to material and char-acteristic such as bending, shear, bearing, torsion,etc., and are not shown. In LRFD, both the gs andws have been calibrated to achieve a uniform levelof safety throughout the structure.17.4 Seismic DesignSeismic forces are an important loading consider-ation that often controls the design of bridges inseismically active regions. All bridges should bedesigned to insure life safety under the demandsimparted by the Maximum Considered Earthquake(MCE). Higher levels of performance may berequired by the bridge owner to provide postearthquake access to emergency facilities or whenthe time required to restore service after anearthquake would create a major economic impact.Fig. 17.5 Axle Spacing and Axle Loads for E80 loadingBridge Engineering n 17.9Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGAll bridges should have a clearly identiablesystem to resist forces and deformations imposedby seismic events. Experimental research and pastperformance has demonstrated that simple bridgefeatures lead to more predictable seismic response.Irregular features lead to complex and lesspredictable seismic response and should beavoided in high seismic region whenever possible(See Table 17.9). Every effort should be made tobalance the effective lateral stiffness betweenadjacent bents within a frame, adjacent columnswithin a bent, and adjacent frames. If irregularfeatures or signicant variations in lateral stiffnessare unavoidable, they should be assessed withmore rigorous analysis and designed for a higherlevel of seismic performance.Seismic effects for box culverts and buriedstructures need not be considered, except whenthey cross active faults.17.4.1 Seismic DesignApproachOrdinarily bridges are not designed to remainelastic during the MCE because of economic con-straints and the uncertainties in predicting seismicdemands. Design codes permit the designer to takeadvantage of ductility and post elastic strength aslong as the expected deformations do not exceedthe bridges lateral displacement capacity. DuctileTable 17.5 Railroad Impact FactorsStructure type Impact percent*Prestressed concrete:L , 60 35 L250060 , L , 135 800L 214L ! 135 20%Reinforced concrete: 100LLLL DL(80% max. for steam engines)(60% max. for diesel engines)Steel:**Non-hammerblow engine equipmentL , 80 RE 40 3L21600L ! 80 RE 16 600L 30Steam engine equipment with hammerblowL , 100 RE 60 L2500L ! 100 RE 10 1800L 40Truss spans RE 15 4000L 25* For ballasted decks use 90% of calculated impact (steel bridges only)L span, ft; S longitudinal beam spacing, ft; DL applicable dead load; LL applicable live load.RE the rocking effect consisting of the percentage of downward on one rail and upward on the otherrail, increasing and decreasing, respectively, the loads otherwise specied. RE shall be expressed as apercentage; either 10% of the axle load or 20% of the wheel load.** Impact is reduced for L . 175 ft or when load is received from more than two tracks.17.10 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGTable 17.6 Capacity-Reduction and Load FactorsLoad factor b for*Group g D (L I)n (L I)P CF E B SF W WL LF R S T EQ ICE % of basicunit stressesService-Load DesignI 1.0 1 1 0 1 bE 1 1 0 0 0 0 0 0 100IA 1.0 1 2 0 0 0 0 0 0 0 0 0 0 0 150IB 1.0 1 0 1 1 bE 1 1 0 0 0 0 0 0 II 1.0 1 0 0 0 1 1 1 1 0 0 0 0 0 125III 1.0 1 1 0 1 bE 1 1 0.3 1 1 0 0 0 125IV 1.0 1 1 0 1 bE 1 1 0 0 0 1 0 0 125V 1.0 1 0 0 0 1 1 1 1 0 0 1 0 0 140VI 1.0 1 1 0 1 bE 1 1 0.3 1 1 1 0 0 140VII 1.0 1 0 0 0 1 1 1 0 0 0 0 1 0 133VIII 1.0 1 1 0 1 1 1 1 0 0 0 0 0 1 140IX 1.0 1 0 0 0 1 1 1 1 0 0 0 0 1 150Load-Factor DesignI 1.3 bD 1.67}0 1.0 bE 1 1 0 0 0 0 0 0IA 1.3 bD 2.20 0 0 0 0 0 0 0 0 0 0 0IB 1.3 bD 0 1 1.0 bE 1 1 0 0 0 0 0 0II 1.3 bD 0 0 0 bE 1 1 1 0 0 0 0 0III 1.3 bD 1 0 1 bE 1 1 0.3 1 1 0 0 0IV 1.3 bD 1 0 1 bE 1 1 0 0 0 1 0 0V 1.25 bD 0 0 0 bE 1 1 1 0 0 1 0 0VI 1.25 bD 1 0 1 bE 1 1 0.3 1 1 1 0 0VII 1.3 bD 0 0 0 bE 1 1 0 0 0 0 1 0VIII 1.3 bD 1 0 1 bE 1 1 0 0 0 0 0 1IX 1.20 bD 0 0 0 bE 1 1 1 0 0 0 0 1* D dead load LF longitudinal force from live load T temperatureL live load (L I)n live load plus impact for AASHTO EQ earthquakeI live-load impact highway loading SF stream ow pressureE earth pressure CF centrifugal force ICE ice pressureB buoyancy F longitudinal force due to friction (L I)P live load plus impact consistentW wind load on structure R rib shortening with the overload criteria of theWL wind load on live load S shrinkage operating agencyFor service-load design: No increase in allowable unit stresses is permitted for members or connections carrying wind loads only.bE 1.0 for lateral loads on rigid frames subjected to full earth pressure 0.5 when positive moment in beams and slabs is reduced by half the earth-pressure momentCheck both loadings to see which one governs.% Maximum unit stress(operating rating)Allowable basic unit stress100For load factor design:bE 1.3 for lateral earth pressure for rigid frames excluding rigid culverts 0.5 for lateral earth pressure when checking positive moments in rigid frames 1.0 for vertical earth pressurebD 0.75 when checking member for minimum axial load and maximum moment or maximum eccentricity and column design 1.0 when checking member for maximum axial load and minimum moment and column design 1.0 for exural and tension members}bD 1.25 for design of outer roadway beam for combination of sidewalk and roadway live load plus impact, if it governs the design,but section capacity should be at least that required for bD 1.67 for roadway live load alone 1.00 for deck-slab design for D L IBridge Engineering n 17.11Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGTable17.7AASHTOLRFDLoadCombinationsandLoadFactorDesignobjectiveLoadcombinationLimitstateDCDDDWEHEVESELLLIMCEBRPLLSWAWSWLFRTUCRSHTGSEUseoneoftheseatatimeEQICCTCVTohavestructuralintegrityforallstatisticallysignicantloadsSTRENGTHIgP1.751.001.000.50/1.20gTGgSESTRENGTHIIgP1.351.001.000.50/1.20gTGgSESTRENGTHIIIgP1.001.401.000.50/1.20gTGgSESTRENGTHIVgP1.001.000.50/1.20STRENGTHVgP1.351.000.401.01.000.50/1.20gTGgSETosurvivewithoutcollapsingduringaood,collision,orearthquakeEXTREMEEVENTIgPgEQ1.001.001.00EXTREMEEVENTIIgP0.501.001.001.001.001.00Tolast75yearsSERVICE-I1.001.001.000.301.01.001.00/1.20gTGgSESERVICE-II1.001.301.001.001.00/1.20SERVICE-III1.000.801.001.001.00/1.20gTGgSETowithstandcyclicloading,especiallyatconnectionsFATIGUE0.7517.12 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGresponse in bridge systems is typically achievedthrough sustained hysteric force-deformationcycles that dissipate energy. This dissipation occursinternally, within the structural members, by theformation of exural plastic hinges, or externallywith isolation bearings or external dampers.Inelastic behavior should be limited to pre-determined locations within the bridge that can beeasily inspected and repaired following an earth-quake. Preferable locations for inelastic behavioron most bridges include columns, pier walls, andabutment backwalls and wingwalls. Inelastic res-Table 17.8 Load Factors for Permanent Loads gpLoad factorType of load Maximum MinimumDC: Component and Attachments 1.25 0.90DD: Downdrag 1.80 0.45DW: Wearing Surfaces and Utilities 1.50 0.65EH: Horizontal Earth Pressure Active 1.50 0.90 At-Rest 1.35 0.90EL: Locked in Erection Stresses 1.0 1.0EV: Vertical Earth Pressure Retaining Walls and Abutments 1.35 1.00 Rigid Buried Structure 1.30 0.90 Rigid Frames 1.35 0.90 Flexible Buried Structures other than Metal Box Culverts 1.95 0.90 Flexible Metal Box Culverts 1.50 0.90ES: Earth Surcharge 1.50 0.75Table 17.9 Examples of Irregular Bridge FeaturesGeometry Multiple superstructure levels Variable width, bifurcating, or highly curved superstructures Signicant in-plane curvature Highly skewed supportsFraming Outrigger or C-bent supports Unbalanced mass and/or stiffness distribution Multiple superstructure typesGeologic Conditions Soft soil Moderate to high liquefaction potential Proximity to an earthquake faultBridge Engineering n 17.13Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGponse in the superstructure is not desirable becauseit is difcult to inspect and repair and may preventthe bridge from being restored to a serviceablecondition.Members not participating in the primaryenergy dissipating system (i.e. column shear, joints,cap beams and foundations) should be capacityprotected. This is achieved by ensuring that themaximum moment and shear from plastic hinges,isolation bearings and dampers can be dependablyresisted by the adjoining elements.17.4.2 Seismic DemandsThe uniform load method can be used to determinethe seismic loading for bridges that will respondprincipally in their fundamental mode of vibration.Equivalent static earthquake loads are calculatedby multiplying the tributary permanent load by aresponse spectra coefcient:PeCsmWL (17:7)where pe Equivalent uniform static seismic loadper unit length of bridgeCsm Elastic response coefcient seeequation 17.8W Dead load of the bridge superstructureand tributary substructureL Total length of bridge in ftCsm1:2AST2=3m 2:5A (17:8)where Tm Period of vibration of the mth mode(seconds)A Acceleration coefcient from nationalground motion maps.S Site coefcient specied in Table 17.10Tm 2pWgK

(17:9)where g Acceleration of gravityK Bridge lateral stiffnessSingle span bridges do not require seismicanalysis. The minimum design force at the con-nections between the superstructure and substruc-ture shall not be less than the product of the sitecoefcient S, the acceleration coefcient A, and thetributary permanent load.The multimode spectral mode analysis methodshould be used if coupling between the longitudi-nal, transverse and/or vertical response is expec-ted. A three dimensional linear dynamic modelshould be used to represent the bridge. The elasticseismic forces and displacement generated frommultiple mode shapes are combined using accep-table methods such as the root-mean-squaremethod or the complete quadratic combinationmethod. The number of modes in the model shouldbe at least three times the number of spans beingmodeled. Site-specic response spectra are oftendeveloped for multi-modal analysis that incorpor-ates the seismic source, ground attenuation, andnear fault phenomena.When response spectra analysis is used, amaximum single seismic force is calculated bycombiningtwohorizontal orthogonal groundmotioncomponents. These components are applied along aTable 17.10 Soil CoefcientsSoil proletypeSitecoefcient S DescriptionI 1.0 Rock of any description (shale-like or crystalline) or stiff soils (sands,gravels, stiff clays) less than 200 ft in depth overlying rockII 1.2 Stiff cohesive or deep cohesionless soils more than 200 ft in depthoverlying rockIII 1.5 Soft to medium-stiff clays and sands characterized by 30 ft or more ofclay with or without intervening layers of sand.IV 2.0 Soft clays or silts greater than 40 ft of depth.17.14 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGlongitudinal axis dened by a chord intersecting thecenterline of the bridge at the abutments and anormal transverse axis (See Fig. 17.7).It is uneconomical to design bridges to resistlarge earthquakes elastically. Columns are assumedto deform inelastically where seismic forces exceeddesign levels established by dividing the elasticallycomputed moments by the appropriate responsemodication factors, R. (See Tables 17.11 and 17.12)The AASHTO Bridge Design Specicationdenes three levels of response modicationfactors for critical bridges, essential bridgesand other bridges. The bridge owner mustdetermine the performance level required consi-dering social/survival and security/defenserequirements.More rigorous analysis such as inelastic timehistory analysis should be used on geometricallycomplex bridges, critical bridges and bridges withinclose proximity of earthquake faults. The nonlinearanalysis provides forces and deformations as afunction of time for a specied earthquake motion.Fig. 17.6 Equivalent static earthquake loads.Fig. 17.7 Orthogonal bridge axis denition.Bridge Engineering n 17.15Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGA minimum of three ground motions representingthe design event should be used.Nonlinear static analysis, commonly known aspushover analysis has recently been adopted byCaltrans. The inelastic displacement capacity of thepiers is compared to the displacements from anelastic demand analysis that considers the bridgescracked exural stiffness. The inelastic deformationcapacity of the earthquake resisting members iscalculated using moment curvature analysis utiliz-ing expected material properties and dependablematerials strain limits. Displacement capacities arealso limited by degradation of strength and P-Deffects that occur under large inelastic defor-mations. If the P-D moments are less than 20% ofthe plastic moment capacity of the member, theyare typically ignored.17.4.3 Seismic Design ofConcrete BridgeColumnsCross sectional column dimensions should belimited to the depth of the superstructures or bentcap to reduce the potential for inelastic damagemigrating into the superstructure. The longitudinalreinforcement for compression members shouldnot exceed 4% of the columns gross cross sectionalarea to insure adequate ductility, avoid congestionand to permit adequate anchorage of the longi-tudinal reinforcement. Conversely not less than 1%of the columns gross cross sectional area to insurea reasonable level of strength. In the column po-tential plastic hinge regions, the transverse rein-forcement rs for circular columns shall not be lessthan:rs 0:12f0cfy(17:10)f0c specied compressive strength of con-crete at 28 days (ksi)fy yield strength of reinforcing bars (ksi)Table 17.11 AASHTO Substructure Response Modication FactorsSubstructureImportance CategoryCritical Essential otherWall-type pierslarger dimension 1.5 1.5 2.0Reinforced concrete pile bents Vertical piles only 1.5 2.0 3.0 With batter piles 1.5 1.5 2.0Single Columns 1.5 2.0 3.0Steel or composite steel and concrete pile bents Vertical pile only 1.5 3.5 5.0 With batter piles 1.5 2.0 3.0Multiple column bents 1.5 3.5 5.0Table 17.12 AASHTO Connection ResponseModication FactorsConnection All ImportanceCategoriesSuperstructure to abutment 0.8Expansion joints within aspan of the superstructure0.8Column, piers, or piles bent tocap beam or superstructure1.0Column or piers tofoundations1.017.16 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGFor rectangular sections the total gross sectionalarea of rectangular hoop reinforcement shall not beless than either:Ash 0:30shcf0cfyAgAc1 or Ash 0:12shcf0cfy(17:11)where:s vertical spacing of hoops not to exceed4.0 inches (in)Ac area of column core (in2)Ag gross area of column (in2)Ash total cross sectional area of tie reinforce-ment, including supplementary cross tieshaving a vertical spacing s and crosssection having core diameter of hc (in2)hc core dimension of tied column in thedirection under consideration (in2)The potential plastic hinge region is dened asthe larger of 1.5 times the cross sectional dimensionin the direction of bending or the region of columnwhere the moment exceeds 75% of the maximumplastic moment.The column design shear force should becalculated considering the exural overstrengthdeveloped at the most probable location within thecolumn with a rational combination of the mostadverse end moments. The shear resisting mech-anism is provided by a combination of truss action(Vs), concrete tensile contribution (Vc) and arch orstrut action (Vp).Vu , VsVpVc (17:12)The concrete contribution is signicantly dimin-ished under high ductilities and cyclic loading andis often ignored in the plastic moment regions. Theexural reinforcement in continuous or cantilevermembers needs to detailed to provide continuity ofreinforcement at intersections with other membersto develop nominal moment resistance of the jointcan be developed to resist the shear depicted in Fig.17.8. Several shear design models dening theshear resisting mechanisms for columns and jointscan be found in the AASHTO Design Specicationsor the Caltrans Seismic Design Criteria.The unseating of girders and abutments must beavoided in all circumstances. The seat width needsto accommodate thermal movement, prestressshortening, creep, shrinkage and anticipated earth-quake displacements. The seat width should not beless then 1.5 times the elastic seismic displacementFig. 17.8 Joint shear stresses in T-joints.Bridge Engineering n 17.17Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGof the superstructure at the seat or:N (8 0:02L 0:08H) 1 S28000 (17:13)N Support width normal to the centerline ofbearingL Length of the bridge deck to the adjacentexpansionjoint, or the endof the bridge (ft)H Average height of the columns support-ing the bridge deck to the next expansionjoint (ft) (H 0 for single span bridges)S Skew of the support measured from linenormal to span (deg)(MCEER, Recommended LRFD Guidelines for theSeismic Design of Highway Bridges, CaltransSeismic Design Criteria, vol. 1.1 (www.dot.ca.gov);AASHTO LRFD Bridge Design Specication(www.aashto.org).)Steel BridgesSteel is competitive as a construction material formedium and long-span bridges for the followingreasons: It has high strength in tension and com-pression. It behaves as a nearly perfect elasticmaterial within the usual working ranges. It hasstrength reserves beyond the yield point. The highstandards of the fabricating industry guaranteeusers uniformity of the controlling propertieswithin narrow tolerances. Connection methods arereliable, and workers skilled in their applicationare available.The principal disadvantage of steel in bridgeconstruction, its susceptibility to corrosion, is beingincreasingly overcome by chemical additives orimproved protective coatings.17.5 Systems Used for SteelBridgesThe following are typical components of steelbridges. Each may be applied to any of thefunctional types and structural systems listed inArt. 17.1.Main support: Rolled beams, plate girders, boxgirders, or trusses.Connections: (See also Art. 17.7.) High-strength-bolted, welded, or combinations.Materials for trafc-carrying deck: Timber string-ers and planking, reinforced concrete slab or pre-stressed concrete slab, stiffened steel plate(orthotropic deck), or steel grid.Timber decks are restricted to bridges on roadsof minor importance. Plates of corrosion resistantsteel should be used as ballast supports on throughplate-girder bridges for railways. For roadwaydecks of stiffened steel plates, see Art. 17.13.Deck framing: Deck resting directly on mainmembers or supported by grids of stringers andoor beams.Location of deck: On top of main members: deckspans (Fig. 17.9a); between main members, theunderside of the deck framing being ush with thatof the main members: through spans (Fig. 17.9b).17.6 Grades and DesignCriteria for Steel forBridgesPreferred steel grades, permissible stresses, andstandards of details, materials, and quality of workfor steel bridges are covered in the AREMA andAASHTO specications. Properties of the variousgrades of steel and the testing methods to be usedto control them are regulated by specications ofASTM. Properties of the structural steels presentlypreferred in bridge construction are Tabulated inTable 17.13.Dimensions and geometric properties of com-mercially available rolled plates and shapes aretabulated in the Steel Construction Manual, forallowable stress design and for load-and-resistance-factor design of the American Institute of SteelConstruction (AISC), and in manuals issued by themajor steel producers.All members, connections, and parts of steelbridges should be designed by the load-factordesign method, and then checked for fatigue atservice-level loads. The fatigue check should assurethat all connections are within allowable stressranges (FSR).The design strength of a beam or girder is basedon the dimensional properties of the section andthe spacing of compression ange bracing. Thethree types of member sections are (1) compact, (2)braced noncompact, and (3) partially braced. TheAASHTO Flexural design formulas for the threetypes of I-Girder sections are shown in Table 17.14.17.18 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGDesign Limitations on Depth Ratios,Slenderness Ratios, Deections nAASHTOspecications restrict the depth-to-span ratios D/Lof bridge structures and the slenderness ratios l/rof individual truss or bracing members to thevalues in Table 17.15.where D depth of construction, ftL span, ft, c to c bearings for simple spansor distance between points of contra-exure for continuous spansl unsupported length of member, inr radius of gyration, inThese are minimum values; preferred values arehigher.Fig. 17.9 Two-lane deck-girder highway bridgeBridge Engineering n 17.19Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGBoth specications limit the elastic deections ofbridges under design live load plus impact to 1800ofthe span, measured c to c bearings, except that11000may be used for bridges used by pedestrians;1300of the length of cantilever arms. Deectioncalculations should be based on the gross sectionsof girders or truss members. Anticipated dead-loaddeections must be compensated by adequatecamber in the fabrication of steel structures.Splices nShop, assembly yard, or erectionsplices must be provided for units whoseoverall length exceeds available rolled lengths ofplates and shapes or the clearances of availableshipping facilities. Splices also must be providedwhen total weight exceeds the capacity of availableerection equipment.Accessibility nAll parts should be accessibleand adequately spaced for fabrication, assembly,and maintenance. Closed box girders and box-typesections should be equipped with handholes ormanholes.On long and high bridges, installation of per-manent maintenance travelers may be justied.17.7 Steel Connections inBridgesConnections of steel members to other steelmembers are usually made with high-strengthbolts, welds, or pins. In composite construction,steel beams are joined to concrete decks by steelstuds or channels welded to the top ange of thebeams.17.7.1 Connections withHigh-Strength BoltsThe parts may be clamped together by bolts ofquenched and tempered steel, ASTM A325. Thenuts are tightened to 70% of their specied tensilestrength.Details and quality of work are covered by theSpecications for Structural Joints Using ASTMA325 and A490 Bolts, approved by the ResearchCouncil on Structural Connections of the Engin-eering Foundation. Maximum stresses for bearingtype connections are given in Table 17.11.Tensioned ASTM A325 bolts are the preferredbolt for all steel bridge connections. The nuts onTable 17.13 Minimum Mechanical Properties of Structural SteelProperty StructuralsteelHigh-strengthlow-alloysteelQuenchedand temperedlow-alloysteelHigh-yield-strength, quenchedand temperedalloy steelAASHTOdesignationM270Grade 36M270Grade 50M270Grade 50WM270Grade 70WM270 Grades100/100WEquivalentASTMdesignationsA709Grade 36A709Grade 50A709Grade 50WA709Grade 70WA709 Grades100/100WThickness ofplates, inUp to 4incl.Up to 4incl.Up to 4incl.Up to 4incl.Up to 2.5incl.Over 2.5to 4 incl.Shapes AllgroupsAllgroupsAllgroupsNotapplicableNotapplicableNotapplicableMinimumtensilestrength, Fu, ksi 58 65 70 90 110 100Minimum yieldpoint or yieldstrength, Fy, ksi 36 50 50 70 100 9017.20 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGTable 17.14 ASSHTO Flexural Design Formulas for I-GirdersSectiondescriptionCompressionangeslendernessWebslendernessLateral Bracingof compressionangeMuCompact bt

4100Fy

Dtw

19,230Fy

Lbry

3:6 2:2 MlMu Fy

106FyZWhen b=t and D=tw exceeds 75% of the abovelimits, the following interaction equation shallapplyDtw4:68 bt

33,650Fyf

BracedNon-Compactbt 24 With transverse stiffeners only:Dtw

36,500Fy

With transverse stiffeners and onelongitudinal stiffeners:Dtw

73,000Fy

Lb

20,000,000Fyd Lessor of:Fy Sxt orFcr Sxc RbPartiallyBracedNo Max.RequirementsNo Max.RequirementsNo Max.RequirementsSee AASHTOStandardSpecicationsfor HighwayBridgesAf ange area (sq in2)b compression ange widthD clear distance between angesd depth of beam or girderf the lesser of ( fb/Rb) Fy (psi)fb factored bending stress in the compression ange (psi)fcr critical stress of the compression ange (psi)Fy specied minimum eld strength of the steel being used (psi)Fyf specied minimum eld strength of the compression ange (psi)Lb distance between points of bracing of the compression ange (in)Ml smaller moment at the end of the unbraced length of the member (lb-in)Mu design strength equal Fy*Z at the other end of the unbraced length:(Ml/Mu) is positive when moments cause single curvature between brace points(Ml/Mu) is negative when moments cause reverse curvature between brace points (lb-in)Rb Bending Capacity Reduction Factor, See AASHTO Standard Specications for Highway Bridgesry radius of gyration of the steel section with respect to the Y-Y axis (in)Sxc elastic section modulas with respect to compression ange (in3)Sxt elastic section modulas with respect to tension ange (in3)t ange thickness (in)tw web thickness (in)Z plastic section modulas (in3)Bridge Engineering n 17.21Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGtensioned A325 and A490 bolts will not loosenunder vibrations associated with bridge loadings.If ASTM A307 bolts or non tensioned high strengthbolts are used, provisions should be made toprevent nut loosing by the use of thread lockingadhesive, self-locking nuts, or double nuts. Boltedconnections subject to tension, or combined tensionand shear, or stress reversal, or severe vibration, orheavy impact loads, or any other condition wherejoint slippage would be detrimental, shall use ten-sioned High-Strength bolts and be designed as aslip-critical connection.Slip-critical connections are designed to preventslip at a specied overload condition in addition tomeeting the strength requirements in bearing. Theoverload condition at which the connection isrequired work as a friction connection (no slip) isequal to Dead load 1.67(Live load Impact). Theslip strength of the connection is based on thenumber of slip plains, the friction coefcient of thecontact surfaces, the type of hole, andthe bolt tensionstress. The AASHTO specications provide equa-tions for determining the slip strength of connec-tions.17.7.2 Welded ConnectionsIn welding, the parts to be connected are fused athigh temperatures, usually with addition of suit-able metallic material. The Structural WeldingCode, AWS D1.5, American Welding Society,regulates application of the various types and sizesof welds, permissible stresses in the weld andparent metal, permissible edge congurations,kinds and sizes of electrodes, details of quality ofwork, and qualication of welding procedures andwelders. (For Maximum welding stresses, seeTable 17.16.)Many designers favor the combination of shopweldingwithhigh-strength-boltedeldconnections.17.7.3 Pin ConnectionsHinges between members subject to relativerotation are usually formed with pins, machinedsteel cylinders. They are held in either semicircularmachined recesses or smoothly tting holes in theconnected members.For xation of the direction of the pin axis, pinsupto 10-indiameter have threadedends for recessednuts, which bear against the connected members.Pins over 10-in diameter are held by recessed caps.These in turn are heldby either tap bolts or a rodthatruns axially through a hole in the pin itself and isthreaded and secured by nuts at its ends.Pins are designed for bending and shear andfor bearing against the connected members. (Forstresses, see Art. 9.6.)17.8 Rolled-Beam BridgesThe simplest steel bridges consist of rolled wide-ange beams and a trafc-carrying deck. Rolledbeams serve also as oor beams and stringers fordecks of plate-girder and truss bridges.Reductions in steel weight may be obtained, butwith greater labor costs, by adding cover plates inTable 17.15 Dimensional Limitations for BridgeMembersAASHTOMin depth-span ratios:For noncomposite beams or girders 1/25For simple span composite girders* 1/22For continuous composite girders* 1/25For trusses 1/10Max slenderness ratios:For main members in compression 120For bracing members in compression 140For main members in tension 200For bracing members in tension 240* For composite girders the depth shall include the concrete slab.17.22 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGthe area of maximum moments, by providingcontinuity over several spans, by utilizing the deckin composite action, or by a combination of thesemeasures. The principles of design and detailsare essentially identical with those of plate girders(Art. 17.9).17.9 Plate-Girder BridgesThe term plate girder applies to structural elementsof I-shaped cross section that are welded fromplates. Plate girders are used as primary support-ing elements in many structural systems: as simplebeams on abutments or, with overhanging ends, onpiers; as continuous or hinged multispan beams;as stiffening girders of arches and suspensionbridges, and in frame-type bridges. They also serveas oor beams and stringers on these other bridgesystems.Their prevalent application on highway andrailway bridges is in the form of deck-plate girdersin combination with concrete decks (Fig. 17.9). (Fordesign of concrete deck slabs, see Art. 17.20. ForTable 17.16 Design Strength of ConnectorsType of fastener Strength (fF)Groove welda1.00FyFillet weldb0.45FuLow-carbon steel bolts ASTM A307Tensiong30 ksiShear on Bolt with threads in shear plane 18 ksiPower-driven rivets ASTM A502ShearGrade 1 25 ksiShearGrade 2 30 ksiHigh-Strength BoltsAASHTO M 164 (ASTM A325)Applied Static Tensionc,g68 ksiShear on bolt with threads in shear planec,d,e35 ksiAASHTO M 253 (ASTM A490)Applied Static Tensiong85 ksiShear on bolt with threads in shear planed,e43 ksiBolt bearing on connected material f0.9LctFu 1.8dtFuaFy yield point of connected materialbFu minimum strength of the welding rod metal but not greater than the tensile strength of theconnected parts.cThe tensile strength of M 164 (A 325) bolts decreases for diameters greater than 1 inch.The design values listed are for bolts up to 1 inch diameter. The design values shall be multipliedby 0.875 for diameters greater than 1 inch.dTabulated values shall be reduced by 20 percent in bearing-type connections whose lengthbetween extreme fasteners in each of the spliced parts measured parallel to the line of axial forceexceeds 50 inches.eIf material thickness or joint details preclude threads in the shear plane, multiply values by 1.25fBearing on connected material in standard oversized short slotted holes loaded in any direction orlong slotted holes parallel to the applied bearing force. For long slotted holes perpendicular to theapplied bearing force calculated values shall be reduced 20 percent. Lc is clear distance between theholes or between the hole and the edge of the material in the direction of the applied bearing force(in); d is the diameter of the bolt (in), t is thickness of connected material (in) and Fu is the speciedminimum tensile strength of the connected part given in Table 17.9.gFor combined tension and shear when fv/Fv. 0.33 the design tensile strength, Ft (in table) shall bereduced to F0t where:F0t Ft1 ( fv=Fy)2

fv calculated bolt stress in shearFv design shear strength of bolt (in table)Bridge Engineering n 17.23Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGgirders with steel decks (orthotropic decks), see Art17.13. Girders with track ties mounted directly onthe top anges, open-deck girders, are used onbranch railways and industrial spurs. Throughplate girders (Fig. 17.9b) are now practicallyrestricted to railway bridges where allowablestructure depth is limited.The two or more girders supporting each spanmust be braced against each other to providestability against overturning and ange buckling,to resist transverse forces (wind, earthquake,centrifugal), and to distribute concentrated heavyloads. On deck girders, this is done by transversebracing in vertical planes. Transverse bracingshould be installed over each bearing and atintermediate locations not over 25 ft apart. Thisbracing may consist either of full-depth crossframes or of solid diaphragms with depth at leasthalf the web depth for rolled beams and preferablythree-quarters the web depth for plate girders.End cross frames or diaphragms should be pro-portioned to transfer fully all vertical and lateralloads to the bearings. On through-girder spans,since top lateral and transverse bracing systemscannot be installed, the top anges of the girdersmust be braced against the oor system. For thepurpose, heavy gusset plates or knee braces may beused (Fig. 17.9b).The most commonly used type of steel bridgegirder is the welded plate girder. It is typicallylaterally braced, noncompact, and unsymmetrical,with top and bottom anges of different sizes.Figure 17.10b shows a typical welded plate girder.Variations in moment resistance are obtained byusing ange plates of different thicknesses, widths,or steel grades, butt-welded to each other in suc-cession. Web thickness too may be varied. Girderwebs should be protected against buckling bytransverse and, in the case of deep webs, longitu-dinal stiffeners. Transverse bearing stiffeners arerequired to transfer end reactions from the web intothe bearings and to introduce concentrated loadsinto the web. Intermediate and longitudinal stiff-eners are required if the girder depth-to-thicknessratios exceed critical values (see Art. 9.13.4).Stiffeners may be plain plates, angles, or Tsections. Transverse stiffeners can be in pairs orsingle elements. The AASHTO Specications con-tain restrictions on width-to-thickness ratios andminimum widths of plate stiffeners (Art. 9.13.4).Web-to-ange connections should be capable ofcarrying the stress ow from web to ange at everysection of the girder. At an unloaded point, the stressow equals the horizontal shear per linear inch.Where a wheel load may act, for example, at upperange-to-web connections of deck girders, the stressow is the vectorial sum of the horizontal shear perinch and the wheel load (assumeddistributedover aweb length equal to twice the deck thickness). Weldsconnecting bearing stiffeners to the web must bedesigned for the full bearing reaction.Space restrictions in the shop, clearance restric-tions in transportation, and erection considerationsmay require dividing long girders into shortersections, which are then joined (spliced) in theeld. Individual segments, plates or angles, mustbe spliced either in the shop or in the eld if theyexceed in length the sizes produced by the rollingmills or if shapes are changed in thickness to meetstress requirements.Specications require splices to be designed forthe average between the stress due to design loadsand the capacity of the unspliced segment, but fornot less than 75% of the latter. In bolted design,material may have to be added at each splice tosatisfythis requirement. Each splice element must beconnected by a sufcient number of bolts to developits full strength. Whenever it is possible to do so,splices of individual segments should be staggered.No splices should be located in the vicinity of thehighest-stressed parts of the girder, for example, atmidspan of simple-beam spans, or over the bearingson continuous beams. Figure 17.10a presents adesign ow chart for welded plate girders.(F. S. Merritt and R. L. Brockenbrough, Struc-tural Steel Designers Handbook, 2nded., McGraw-Hill Inc., New York (books-mcgraw-hill.com).)17.10 Composite-GirderBridgesInstallation of appropriately designed shear con-nectors between the top ange of girders or beamsand the concrete deck allows use of the deck as partof the top ange (equivalent cover plate). Theresulting increase in effective depth of the totalsection and possible reductions of the top-angesteel usually allow some savings in steel comparedwith the noncomposite steel section. The overalleconomy depends on the cost of the shear connec-tors andany other additions to the girder or the deckthat may be required and on possible limitations ineffectiveness of the composite section as such.17.24 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGIn areas of negative moment, composite effectmay be assumed only if the calculated tensilestresses in the deck are either taken up fully byreinforcing steel or compensated by prestressing.The latter method requires special precautions toassure slipping of the deck on the girder during theprestressing operation but rigidity of connectionafter completion.If the steel girder is not shored up while thedeck concrete is placed, computation of dead-load stresses must be based on the steel sectionalone.Fig. 17.10 Welded plate girder. (a) Flow chart gives steps in load-factor design. (b) Typical plategirderstiffened, braced, and noncompact.Bridge Engineering n 17.25Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGThe effective ange width of the concrete slabthat is used as a T-beam ange of a compositegirder is the lesser of the following:1. One-fourth of the span length of the girder2. The center to center distance between adjacentgirders3. Twelve times the least thickness of the slabShear connectors should be capable of resistingall forces tending to separate the abutting concreteand steel surfaces, both horizontally and vertically.Connectors should not obstruct placement andthorough compaction of the concrete. Their instal-lation should not harm the structural steel.The types of shear connectors presently pre-ferred are channels, or welded studs. Channelsshould be placed on beam anges normal to theweb and with the channel anges pointing towardthe girder bearings.The modular ratio recommended for stressanalysis of composite girders under live loads isgiven in Table 17.17.For composite action under dead loads, theconcrete section may be assumed to be subjectedto constant compressive stress. This will cause theconcrete to undergo plastic ow and thus willreduce its capacity to resist stress. This is taken intoaccount in design of a composite girder for deadloads by multiplying by 3 the modular ratio n givenin Table 17.17. Most composite girders, however,are designed for composite action only for liveloads and dead loads (usually, curbs, railings, andutilities) that are added after the concrete deck hasattained sufcient strength to support them.ExampleStress Calculations for a CompositeGirder: The following illustrates the procedure fordetermining exural stresses in a composite weldedgirder for factored loads. The girder is assumed tobe fabricated of M270, Grade 50, steel, with yieldpoint Fy 50 ksi. It will not be shored duringplacement of the concrete deck. For the concrete,the 28-day compressive strength is assumed to bef0c 3:25 ksi, n 10 for live loads, n 30 for deadloads. Dimensions, section properties, and bendingmoments are given in the following:The section properties of the steel girder aloneare determined rst. For the purpose, the momentof inertia I1-1 of the steel section (Fig. 17.11a) iscalculated with respect to the bottom of the girder.Then, the moment of inertia INAwith respect to theneutral axis is computed. Next, the section prop-erties of the composite section (Fig. 17.11b) arecalculated. Stresses in the concrete are small, sincethe steel girder carries the weight of the deck.Table 17.17 Modular Ratio for CompositeGirders with Live LoadsSpecied minimumcompressive strengthof concrete deck f0c, psiModularratio n Es=Ec20002400 1525002900 1230003900 1040004900 85000 or more 6*Es elastic modulus of the steelEc elastic modulus of the concreteFig. 17.11 Sections of composite plate girder:(a) steel section alone; (b) composite section.17.26 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERING17.11 Fatigue Design ofBridge MembersAll members and connections should be designedso that maximum stresses induced by loads are lessthan allowable stresses and also so that the range ofstresses induced by variations in service loads isless than the allowable fatigue stress range. If amember is entirely in compression and never issubjected to tensile stresses, a fatigue check is notrequired.Fatigue is an important consideration in designof all bridge components but may be especiallycritical for welded girders. Welding leaves residualstresses in welded regions due to heat input duringthe welding process and subsequent differentialcooling.The types of connections that are most com-monly used with welded plate girders and thatshould be checked for fatigue are illustrated inFig. 17.12, and the stress category assigned for eachtype is given in Table 17.18a. Table 17.19 gives theallowable stress ranges for various stress categ-ories. Table 17.20 lists allowable stress cycles forvarious types of roads and bridge members.(Economical and Fatigue-Resistant SteelBridge Details, FHWA-HI-90-043, Federal High-way Administration; Guide Specications forFatigue Critical Non-Redundant Steel Bridges,American Association of State Highway andTransportation Ofcials (www.aashto.org).)17.12 Orthotropic-DeckBridgesAn orthotropic deck is, essentially, a continuous,at steel plate, with stiffeners (ribs) welded to itsunderside in a parallel or rectangular pattern. Theterm orthotropic is shortened from orthogonalanisotropic, referring to the mathematical theoryused for the exural analysis of such decks.When used on steel bridges, orthotropic decksare usually joined quasi-monolithically, by weldingor high-strength bolting, to the main girders andoor beams. They then have a dual function asroadway and as structural top ange.The combination of plate or box girders withorthotropic decks allows the design of bridges ofconsiderable slenderness and of nearly twice thespan reached by girders with concrete decks. TheSteel Section for Slab and Girder Loads (Fig. 17.11a)Material Size, in Area y Ay Ay21 Top ange 1258 7.50 95.31 715 68,1302 Web 94516 29.38 48.00 1410 67,6803 Bottom ange 14 1 14.00 0.50 7 4SA 50.88 SAy 2132 135,814Moment of inertia of web 21,629ybSAySA 41:9 I1-1 157,443y2bSA 289,325yt 95.6 2 41.9 53.7 INA 68,118Section for Curb Loads, Railing, Utilities (Composite) (Fig. 17.11b)Material Area y Ay Ay2Steel section 50.88 2132 135,8144 Concrete: A/n, n 30 51.57 103.6 5342 553,463102.45 7474 689,277Moment of inertia of girder web 21,629yb 7474102:45 72:9 I1-1 710,906y2bSA 2544,461yt 95.6 2 72.9 22.7 INA 166,445Section for Live Loads (Composite) (Fig. 17.11b)Material Area y Ay Ay2Steel section 50.88 2132 135,8144 Concrete: A/n, n 10 154.70 103.6 16,027 1,660,389205.58 18,159 1,796,203Moment of inertia of girder web 21,629yb18:159205:58 88:3 I1-1 1,817,832y2bSA 21,603,995yt 95.6 2 88.3 7.3 INA 213,837MomentsFor slab and girder loads 1825kips-ftFor curb loads 347kips-ftFor live loads 6295kips-ftStresses in Steel GirderType of Load Bottom fs Top fsSlab and girderloads18251268,11841:9 13:5 18251268,11853:7 17:3Curb loads 34712166,44572:9 1:8 34712166,44522:7 0:6Live loads 629512213,93788:3 31:2 629512213,9377:3 2:6Bottom fs 46.5kips/in2Top fs 20.5kips/in2Bridge Engineering n 17.27Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGmost widespread application of orthotropic decksis on continuous, two- to ve-span girders on low-level river crossings in metropolitan areas, whereapproaches must be kept short and grades low.This construction has been used for main spans upto 1100 ft in cable-stayed bridges and up to 856 ftwithout cable stays. There also are some spectacu-lar high-level orthotropic girder bridges and somearch and suspension bridges with orthotropic stiff-ening girders. On some of the latter, girders anddeck have been combined in a single lens-shapedbox section that has great stiffness and low aero-dynamic resistance.17.12.1 Box GirdersSingle-web or box girders may be used fororthotropic bridges. Box girders are preferred ifstructure depth is restricted. Their inherent stiff-ness makes it possible to reduce, or to omit,unsightly transverse bracing systems. In crosssection, they usually are rectangular, occasionallytrapezoidal. Minimum dimensions of box girdersare controlled by considerations of accessibilityand ease of fabrication.Wide decks are supported by either single boxgirders or twin boxes. Wide single boxes have beenbuilt with multiple webs or secondary interiortrusses. Overhanging oor beams sometimes aresupported by diagonal struts.17.12.2 Depth-Span RatiosGirder softs are parallel to the deck, tapered, orcurved. Parallel anges, sometimes with taperedside spans, generally are used on unbraced girderswith depth-to-main-span ratios as low as 1: 70.Parallel-ange unbraced girders are practicallyrestricted to high-level structures with unrestrictedclearance. Unbraced low-level girders usually aredesigned with curved softs with minimum depth-to-main-span ratios of about 1: 25 over the mainpiers and 1: 50 at the shallowest section.17.12.3 Cable-SuspendedSystems withOrthotropic DecksMain spans of bridges may have girders suspendedfrom or directly supported by cables that are hungfrom towers, or pylons. The cables are curved if theFig. 17.12 Fatigue stress categories for some commonly used connections (see Table 17.13). In (c),category C applies also to transverse loading.17.28 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGgirders are suspended at each oor beam (suspen-sion bridges); otherwise they are straight (cable-stayed bridges). In cable-stayed bridges, the cablesmay extend from the pylons to the connectionswith the girders in tiers, parallel to each other(harped), or in a bundle pattern (radiating from thepylons). See Fig. 17.24.Each cable stay adds one degree of statical in-determinancy to a system. To make the actual con-ditions conform to design assumptions, the cablelength must be adjustable either at the anchoragesto the girders or at the saddles on the towers. (Seealso Art. 17.16.)17.12.4 Steel GradesThe steel commonly used for orthotropic plates isweldable high-strength, low-alloy structural steelM270, Grade 50. Minimum thickness is seldom lessthan 716in (10 mm), to avoid excessive deectionsunder heavy wheel loads. The maximum thicknessseldom exceeds 34in because of the decrease inpermissible working stresses of high-strength low-alloy steel and the increase of llet- and butt-weldsizes for plates of greater thickness.17.12.5 Floor BeamsIf, as in most practical cases, the deck spanstransversely between main girders, transverse ribsare replaced by the oor beams, which are thenbuilt up of inverted T sections, with the deck plateacting as top ange. Floor-beam spacings arepreferably kept constant on any given structure.They range from less than 5 ft to over 15 ft. Longerspacings have been suggested for greater economy.Table 17.18 Fatigue Stress Categories for Bridge Members(a) Stress Categories for Typical ConnectionsType of connection Figure No. Stress CategoryToe of transverse stiffeners 17.13a Tension or reversal CButt weld at anges 17.13b Tension or reversal BGusset for lateral bracing(assumed groove weld,R ! 24 in)17.13c Tension or reversal BFlange to web 17.13d Shear F(b) Stress Categories for Weld Conditions in Fig. 17.13cWeld condition* CategoryUnequal thickness; reinforcement in place EUnequal thickness; reinforcement removed DEqual thickness; reinforcement in place CEqual thickness; reinforcement removed B(c) Stress Categories for Radii R in Fig. 17.13cCategory for weldsR, inFillet Groove24 or more D BFrom 6 to 24 D CFrom 2 to 6 D D2 or less E E* For transverse loading, check transition radius for possible assignment of lower category.Also applies to transverse loading.Bridge Engineering n 17.29Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERING17.12.6 RibsThese are either open (Fig. 17.13a) or closed(Fig. 17.13b). The spacing of open ribs is seldomless than 12 in or more than 15 in. The lower limitis determined by accessibility for fabrication andmaintenance, the upper by considerations ofdeck-plate stiffness. To reduce deformations ofthe surfacing material under concentrated trafcloads, some specications require the platethickness to be not less than 125of the spacingbetween open ribs or between the weld lines ofclosed ribs.Usually, the longitudinal ribs are made continu-ous through slots or cutouts in the oor-beam websto avoid a multitude of butt welds. Rib splices canthen be coordinated with the transverse decksplices.Closed ribs, because of their greater torsionalrigidity, give better load distribution and, otherthings being equal, require less steel andless weld-ing than open ribs. Disadvantages of closed ribsare their inaccessibility for inspection and main-tenance and more complicated splicing details.There have also been some difculties in deningthe weld between closed ribs and deck plate.Table 17.19 Allowable Fatigue Stress Range FSR*, ksi, for Bridge MembersFor Structures with Redundant Load PathsCategory For 100,000CyclesFor 500,000CyclesFor 2,000,000CyclesFor over2,000,000 CyclesA 63S 37S 24S 24SB 49 29 18 16C 35.5 21 13 1012D 28 16 10 7E 22 13 8 4.5F 16 9.2 5.8 2.6G 15 12 9 8Nonredundant-Load-Path StructuresCategory For 100,000CyclesFor 500,000CyclesFor 2,000,000CyclesFor over2,000,000 CyclesA 50S 29S 24S 24SB 39 23 16 16C 28 16 10 91211D 22 13 8 5Ex 17 10 6 2.3F 12 9 7 6* The range of stress is dened as the algebraic difference between the maximum stress and the minimum stress. Tension stress isconsidered to have the opposite algebraic sign from compression stress.Structure types with multiload paths where a single fracture in a member cannot lead to the collapse. For example, a simplesupported single-span multibeam bridge or a multielement eyebar truss member has redundant load path.For transverse stiffener welds on girder webs or anges.x Partial-length welded cover plates should not be used on anges more than 0.8 in thick for nonredundant-load path structures.S For unpainted weathering steel, A709, the category A allowable FSR values are less then values shown, see AASHTOSpecications.17.30 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERING17.12.7 FabricationOrthotropic decks are fabricated in the shop inpanels as large as transportation and erection faci-lities permit. Deck-plate panels are fabricated bybutt-welding available rolled plates. Ribs and oorbeams are llet-welded to the deck plate in upside-down position. Then, the deck is welded to thegirder webs.It is important to schedule all welding se-quences to minimize distortion and locked-upstresses. The most effective method has been to tup all components of a paneldeck plate, ribs, andoor beamsbefore starting any welding, then toplace the llet welds from rib to rib and from oorbeam to oor beam, starting from the panel centerand uniformly proceeding toward the edges. Sincethis sequence practically requires manual weldingthroughout, American fabricators prefer to join theribs to the deck by automatic llet welding beforeassembly with the oor beams. After slipping theoor-beam webs over the ribs, the fabricators weldmanually only the beam webs to the deck. Thismethod requires careful preevaluation of ribdistortions, wider oor-beam slots, and conse-quently more substantial or only one-sided rib-to-oor-beam welds.17.12.8 AnalysisStresses in orthotropic decks are considered asresulting from a superposition of four staticsystems:System I consists of the deck plate considered asan isotropic plate elastically supported by the ribsTable 17.20 Allowable Stress Cycles for Bridge MembersMain (longitudinal) load-carrying membersType of road Case ADTT* Truck loading Lane loadingFreeways, expressways,major highways,and streetsI 2500 or more 2,000,000500,000Freeways, expressways,major highways,and streetsII less than 2500 500,000 100,000Other highwaysand streets not includedin case I or IIIII 100,000 100,000Transverse members and details subjected to wheel loadsType of road Case ADTT* Truck loadingFreeways, expressways,major highways,and streetsI 2500 or more over 2,000,000Freeways, expressways,major highways,and streetsII less than 2500 2,000,000Other highwaysand streetsIII 500,000* Average daily truck trafc (one direction).Longitudinal members should also be checked for truck loading.Members should also be investigated for fatigue when over 2 million stress cycles are produced by a single truck on the bridge withload distributed to the girders as designated for trafc lane loading.Bridge Engineering n 17.31Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERING(Fig. 17.14a). The deck is subject to bending fromwheel loads between the ribs.System II combines the deck plate, as transverseelement, and the ribs, as longitudinal elements.The ribs are continuous over, and elasticallysupported by, the oor beams (Fig. 17.14b). Theorthotropic analysis furnishes the distribution ofconcentrated (wheel) loads to the ribs, their exuraland torsional stresses, and thereby the axial andtorsional stresses of the deck plate as their topange.System III combines the ribs with the oor beamsand is treated either as an orthotropic system or asa grid (Fig. 17.14c). Analysis of this systemfurnishes the exural stresses of the oor beams,Fig. 17.13 Rib shapes used in orthotropic-platedecks.Fig. 17.14 Orthotropic-plate deck may be considered to consist of four systems: (a) Deck platesupported on ribs; (b) rib-deck T beams spanning between oor beams; (c) oor beam with deck plate astop ange, supported on girders; (d) girder with deck plate as top ange.17.32 n Section SeventeenDownloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDGE ENGINEERINGincluding the stresses the deck plate receives astheir top ange.System IV comprises the main girders with theorthotropic deck as top ange (Fig. 17.14d). Axialstresses in the deck plate and ribs and shearstresses in the deck plate are obtained from theexural and torsional analysis of the main girdersby conventional methods.Theoretically, the deck plate should be designedfor the maximum principal stresses that may resultfrom the simultaneous effect of all four systems.Practically, because of the rare coincidence of themaxima from all systems and in view of the greatinherent strength reserve of the deck as a mem-brane (second-order stresses), a design is generallysatisfactory if the stresses from any one system donot exceed 100% of the ordinarily permissibleworking stresses and 125% from a combination ofany two systems.In the design of long-span girder bridges,special attention must be given to buckling stabilityof deep webs and of the deck. Also, considerationshould be given to conditions that may arise atintermediate stages of construction.17.12.9 Steel-Deck SurfacingAll trafc-carrying steel decks require a covering ofsome nonmetallic material to protect them fromaccidental damage, distribute wheel loads, com-pensate for surface irregularities, and provide anonskid, plane riding surface. To be effective, thesurfacing must adhere rmly to the base and resistwear and distortion from trafc under all con-ditions. Problems arise because of the elastic andthermal properties of the steel plate, its sensitivityto corrosion, the presence of bolted deck splices,and the difculties of replacement or repair undertrafc.The surfacing material usually is asphaltic.Strength is provided by the asphalt itself (mastic-type pavements) or by mineral aggregate (asphalt-concrete pavement). The usefulness of mastic-typepavements is restricted to a limited temperaturerange, below which they become brittle andabove which they may ow. The effectiveness ofthe mineral aggregate of asphalt concrete dependson careful grading and adequate compaction,which on steel decks sometimes is difcult toobtain. Asphalt properties may be improved byadmixtures of highly adhesive or ductile chemicalsof various plastics families.(Design Manual for Orthotropic Steel PlateDeck Bridges, American Institute of SteelConstruction, Chicago, Ill.; F. S. Merritt and R. L.Brockenbrough, Structural Steel Designers Hand-book, 2nd ed., McGraw-Hill, Inc., New York.)17.13 Truss BridgesTrusses are lattices formed of straight membersin triangular patterns. Although truss-type con-struction is applicable to practically every staticsystem, the term is restricted here to beam-typestructures: simple spans and continuous andhinged (cantilever) structures. For typical single-span bridge truss congurations, see Fig. 6.50. Forthe stress analysis of bridge trusses, see Arts. 6.46through 6.50.Truss bridges require more eld labor thancomparable plate girders. Also, trusses are morecostly to maintain because of the more complicatedmakeup of members and poor accessibility of theexposed steel surfaces. For these reasons, and as aresult of changing aesthetic preferences, use oftrusses is increasingly restricted to long-spanbridges for which the relatively low weight andconsequent easier handling of the individualmembers are decisive advantages.The superstructure of a typical truss bridge iscomposed of two main trusses, the oor system, atop lateral system, a bottom lateral system, crossframes, and bearing assemblies.Decks for highway truss bridges are usuallyconcrete slabs on steel framing. On long-spanrailway bridges, the tracks are sometimes mounteddirectly on steel stringers, although continuity ofthe track ballast across the deck is usually pre-ferred. Orthotropic decks are rarely used on trussbridges.Most truss bridges have the deck locatedbetween the main trusses, with the oor beamsframed into the truss posts. As an alternative, thedeck framing may be stacked on top of the topchord. Deck trusses have the deck at or above top-chord level (Fig. 17.15); through trusses, near thebottom chord (Fig. 17.16). Through trusses whosedepth is insufcient for the installation of a toplateral system are referred to as half throughtrusses or pony trusses.Figure 17.16 illustrates a typical cantilever trussbridge. The main span comprises a suspendedBridge Engineering n 17.33Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright 2004 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.BRIDG