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CHAPTER 13

R A I L W A Y B R I D G E S

Panel-bridge equipment can be used as anexpedient for the assembly of railway bridges.However, use it only in special conditionsbecause there is much deflection. Spanslonger than 70 feet (21.5 meters) are normallyimpractical because a quadruple-double trussbridge is required (Table 13-1). Usually,panel-bridge railway bridges are assembledas single-track bridges.

Panel-bridge equipment has the followingadvantages for use as railway bridging

Equipment can be transported in trucksto the bridge site. This permits bridgeassembly at the same time repairs arebeing made on the approach tracks.

Either through- or deck-type bridges canbe assembled.

Panel-bridge equipment has the followingdisadvantages for use as railway bridging:

Through-type bridges provide restrictedclearance.

Traffic over bridge must be controlled toeliminate excessive vibration and sidesway.

Pin clearance allows more sag than isfound in a normal bridge.

Bridge requires more maintenance than astandard bridge.

RAILWAY BRIDGE ASSEMBLYRailway panel bridges are either through-type or deck-type. Assemble the through-typerailway bridge the same as the normal panelbridge, but use ties and rails in place of chess.Girders can be single-, double-, triple-, orquadruple-truss and single- or double-story.

The trusses of double-story bridges infringeUS main line and Berne international clear-ance gages but allow passage at slow speeds(Figure 13-1). If decking of double-storytrusses is placed in the top story, the trussesdo not infringe standard clearance gages.

In the deck-type railway bridge, space thetrusses under the ties. The trusses are usuallysingle story. Tie them together laterally by

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bracing frames, tie plates, expedient anglecross bracing, and the ties.

CLASSThe standard designs described will carrystandard or modified Coopers E-72 loading.See Figure 13-2 (page 146) for diagrams ofloadings. Table 13-2 gives the shears andmoments caused by these loadings. Table 13-1 gives the assembly required for 10-to 100-foot (3.1 to 30.1 meters) spans using twostandard designs.

ASSEMBLY OFTHROUGH-TYPE BRIDGE

Single-, double-, and triple-truss assemblycan be used as in normal panel-bridge as-sembly. A quadruple truss can be assembledby inserting a fourth truss between the innerand second truss of a triple-truss assembly.Use bracing frames and tie plates to tie thefour trusses together (Figure 13-3, page 147).Use transom clamps on all panel verticalsexcept the three verticals in each bay coveredby bracing frames. Modify transoms by

cutting a hole in the flange and web at eachend to seat the pintle of the fourth truss. Sincethe fourth truss interferes with the use ofrakers, double-story quadruple-truss bridgesare usually assembled with the decking in thetop story.

Decking systemFor railway loads, always use double tran-soms. Place stringers as in a normal panelbridge. If 8- by 10-inch (19.6 by 24.5 centi-meters) by 14-foot (4.3 meters) ties are used,place them directly on the stringers at 2-inch(4.9 centimeters) spacings and hook-bolted tothe button stringers (Figure 13-4 page 147).To use standard ties (6 by 8 inches by 8 feet 6inches) (14.7 by 19.6 centimeters by 2.6meters), lay chess and ribbands in the normal

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manner and spike the ties to the chess (Figure End bearings13-5). By building up timber treads on each Use end posts and bearings as in a normalside and between the rails, the bridge can be panel bridge. Grillage must be enough toused for rail or highway traffic (Figure 13-6). carry the loads given in Table 13-3. RampTo reduce impact, rail joints on the bridge sections must be level with the bridge deck.should be tight, with no allowance forexpansion.

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ASSEMBLY OF DECK-TYPE BRIDGETwo deck-type assembly designs are de-scribed here. Type I is used for spans up to 90feet (27.7 meters), type II is used for spans upto 100 feet (30.8 meters). Use the followingsteps to assemble type I designs:

1 Arrange trusses side by side and connectthem by bracing frames and tie plates, asshown in Figure 13-7. Bracing is suppliedby the ties, welded sway bracing, andmodified transoms. Seat the modified tran-soms adjacent to the center vertical in thetop and bottom chord of every second andthird bay. To seat the upper transom,invert every other truss. Cut the modifiedtransoms to the desired length and holethem to seat the pintles on the panels.Weld three-inch (7.4 centimeters) anglesway bracing diagonally under the bot-tom chords of every two bays.

2 Use four 6- by 12-inch (14.7 by 29.4centimeters) ties in each bay for the decksystem. Chord bolt every other tie to thetrusses. Drill holes for the chord bolts asshown in Figure 13-8 (page 150). Spike a6-by 6-inch (14.7 by 17.4 centimeters) curbto the ties.

3 Use end posts at each end of each trussand seat them on standard bearings.Grillage under the bearings at each abut-ment must be sufficient to support loadsgiven in Table 13-3. Rocker bearings overintermediate piers can be made similar tothose described in Chapter 16.

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Use the following steps to assemble type IIdesigns:

1 Brace trusses by bracing frames and tieplates into two-, three-, or four-trussgirders suitable for launching separately.Group the girders together to form six-,seven-, eight-, nine-, ten-, twelve-, andsixteen-truss bridges as shown in Figure13-9 (page 150). The two-truss girder ismade from two trusses braced at the endverticals by bracing frames. A 3-foot (92.3centimeters) wide three-truss girder is

made from three trusses braced by bracingframes. A 1-foot (44.1 centimeters) three-truss girder is made by adding a thirdtruss between the trusses of the two-trussgirder and bracing it with tie plates to oneof the outer trusses. A four-truss girder ismade by adding another truss 8 inches(21.6 centimeters) outside a 1-foot (45.7centimeters) three-truss girder andbracing it with tie plates. Tie the girderstogether in the bridge by the ties and twomodified transoms on the bottom chord ofeach bay. Modify transoms by cutting

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holes in the flange to seat the pintles onthe panels. Weld raker lugs to the transomso rakers can be used betweeen thetransom and the outside trusses.

2 Use the same deck system as that used inthe type I bridge.

3 Use bearings of the same type as thoseused in the type I bridge.

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LAUNCHINGLaunch a through-type bridge on rollers inthe same manner as that for a normal panelbridge.

Use the following guidance when launchinga deck-type bridge:

Type I bridges are designed to be launchedcomplete on rollers. They can be pushedor pulled across by a winch line.Launching noses can be used as shown inFigure 13-10 (page 152). During launching,use extra bracing frames and tie plates onthe top chords.

Type II bridges are launched, girder bygirder, by cantilevering out on rollers.Add decking and bracing between girdersafter girders are in place. For othermethods of launching single girders, seeChapter 19.

EXPEDIENTSTable A-13, Appendix A lists panel railwaybridges built in World War II in the Europeantheater of operations (ETO). Figures 13-11through 13-17 (pages 153 and 154) illustrateexpedient bridges. The following expedientsare available:

Welded vertical cross bracing at eachpanel junction can be used instead ofbracing frames and tie plates. Four-inch(10.2 centimeters) channels welded acrossthe panel chords can be used in place oftransoms.

If end posts are not used, the abutmentbearings can be made from a rigid distri-buting beam on timber grillage. The beammust support at least two panel-supportpoints (Figure 13-11).

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Each truss or two-truss girder can belaunched from a highline or lifted directlyinto place by a crane.

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