It must be lowered 5 feet (1.5 meters) onto the bearings. Waterway clearance is decreased.

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1 CHAPTER 12 DECK-TYPE BRIDGES Deck-type panel bridges are normally twolane, class 50 or higher bridges assembled to replace single-lane bridges. A deck-type panel bridge has the following advantages over a through-type bridge Roadway can be wider for passage of extra-wide vehicles. Deck-type assembly allows greater side overhang of vehicles. A lighter decking system can be used when the roadway is supported by trusses. With some sloping banks, the span between abutments is shorter than in a through-type bridge, because bearings are set 5 feet (1.5 meters) below road level. Demolished piers need not be built Up to the level of the roadway. There are no overhead restrictions. A deck-type panel bridge has the following disadvantages: Excavation at abutments may be necessary because bearings are 5 feet (1.5 meters) below the roadway. It is more difficult to launch. It must be lowered 5 feet (1.5 meters) onto the bearings. Waterway clearance is decreased. RECOMMENDED BRIDGE DESIGNS Use the following guidance in designing deck-type bridges: Group the trusses into three-truss girders, and space girders evenly under the roadway. The trusses may be single-or doublestory assembly, as shown in Figure Use bracing frames staggered at opposite ends of each bay (see Figure 12-1) to tie the trusses of each girder together. Every two bays are cross braced by angles welded diagonally across the bottom chords of all trusses. The decking system serves as top lateral bracing. Make the decking system from standard panel-bridge parts (transom, stringers, and chess) or timber. End posts attached to top-story panels may be rested on standard panel-bridge bearings. In multistory assembly, omit the end panels of the lower stories to allow room for the abutment. If end posts are not used, rest the trusses on timber blocking or a rocker bearing under the joint between the first and second bay from each end. If the spans are broken at the pier, fit the two ends with end posts. If the spans are continuous, use a distributing beam and rocker bearing (see Chapter 16). CLASS The capacity of the standard two-lane decktype panel bridge varies with the span and the number of traffic lanes loaded. The bridges are given two class ratings, one for one-way traffic and the other for two-way traffic. Each of these ratings may be either a single or a dual classification. For maximum spans and classes of standard design twolane deck-type bridges, see Table 12-1 (page 140). STANDARD DESIGNS Standard design deck-type panel bridges are illustrated in Figure Material requirements of the standard-design deck-type panel bridge can be found in Table A-11, Appendix A. ASSEMBLY The most practical load distribution is obtained by spacing the trusses uniformly under a relatively stiff deck. Use five three-truss girders (15 trusses) under the bridge deck. Space trusses in each girder 1 foot 6 inches (44.1 centimeters) apart and tie together with bracing frames. 138

2 Bracing Use bracing frames as much as possible at panel junctions to space the trusses and to provide lateral stability in each three-truss girder. To brace and tie the five three-truss girders together, weld 3-by 3-inch (7.4 by 7.4 centimeters) angles diagonally across the bottom chords of each two bays. Welding must be done carefully so the properties of the high tensile steel in the panel-bridge parts are not changed. Use mild steel bracing members, and weld them in place before any loads are applied to the bridge. Decking Before the timber decking is laid, weld 3-inch (7.4 centimeters) angles transversely to the top chords of the trusses at 5-foot (1.5 meters) centers. These angles tie the trusses together and provide a brace for clamping the ribband bolts. Laminate the timber decking or lay it in two layers. Laminated decking (Figure 12-2, page 141) is better than layered decking because the nails cannot work out under traffic vibration. This reduces maintenance. Lay timbers on edge perpendicular to the long axis of the bridge and nail together horizontally. For ease of assembly, 2½-foot (73.5 centimeters) sections of laminated deck can be prefabricated before-hand and then two sections laid between each pair of angles. Notch the end timber of each section to fit over the horizontal legs of the angles. Then nail timber wear treads to the deck. 139

3 For layered decking (Figure 12-3, page 142), lay 3- by 12-inch (7.4 by 29.4 centimeters) planks across the trusses between the angles. Notch every fifth timber to fit over the horizontal legs of the angles. Then nail timber wear treads to the deck. Bearings When end posts are used (Figure 12-4, page 143), place them at both ends of each truss and seat them on standard bearings. Cutoff the top lugs of the end posts flush with the trusses so they do not interfere with the decking. When end posts are not used (Figure 12-4), support the span on timber blocking at the first panel junction from each end. The timber blocking must extend at least 1 foot (29.4 centimeters) on each side of the joint. An alternative method is to use a distributing beam on a rocker bearing similar to the support over immediate piers. With this type of bearing, the effective bridge length is 20 feet (15.2 meters) greater than the gap between bearings. Also add timber blocking under the cantilevered end of the panel to eliminate a reversal of stress in panels near the end of the bridge as a vehicle moves onto the bridge. Over intermediate piers, the trusses can be continuous or broken. If they are continuous, provide a rocker bearing (Chapter 16). If they are broken, attach end posts to the ends of the trusses and seat two ends on separate bearings. If timber decking is used, the gap between the ends of the spans may require an intermediate trestle to support the decking (Figure 12-5, page 143). 140 With panel-bridge decking, the gap between the ends of spans can be bridged by expedient timber or steel stringers and chess (Chapter 16). EXPEDIENT ASSEMBLY For ease in launching, group trusses into twoor three-truss girders tied together by bracing frames. (Space these girders uniformly under the deck.) If other spacings of the trusses are used, expedient braces must be welded to the end verticals of the panels in place of bracing frames. Cross bracing must also be welded across the bottom chords. Examples of expedient assembly are given in Table A-12, Appendix A.

4 LAUNCHING Use the following guidelines when launching a deck-type panel bridge: Each three-truss girder may be launched separately, or the entire bridge may be launched as a unit by welding added bracing to tie the girders together. Launch individual girders of a singlestory bridge by pushing or pulling the girder and launching nose out over the gap, by launching from a high line, by launching with derrick and preventer tackle, or by lifting directly into place with one or two cranes. Over a water gap, girders may be placed on rafts and floated out into the gap and then lifted into place by a crane on a raft. See Chapter 19 for details of these launching methods. A single-story bridge may also be launched as a unit by pushing or pulling it on rollers out over the gap. Use the following guidelines when launching a double-story bridge as a unit Tie the girders together by transverse channels welded across the tops of the bottom and intermediate chords. The entire unit may be launched with a launching nose and then jacked down onto the bearings. If a temporary pier can be built in the middle of the gap to support the cantilevered end, the bridge can be launched as 141

5 a single-story platform just below the near-bank seat. This method reduces the jacking height. It is similar to the method for launching triple-story bridges with the underslung bottom story described in Chapter 8. LOWERING TO BEARINGS A crane at each end of the bridge can be used to lower the girders to the bearings. Jacks can be used as an expedient, although the 5-foot (1.5 meters) drop requires several lifts. During jacking, blocking must be used under the trusses to take the load in case the jacks fail. EXPEDIENT DESIGN BRIDGES Table A-12, Appendix A lists several typical World War II deck-type panel bridges built in the European theater of operations (ETO). 142

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