Figure 10-5.-Locating seats for trestle bents.less the thickness of a footing, will be the height of thefirst bent.Next, stretch the tape from the location of the firstbent, level it as before, and again drop a plumb bob fromthe 12-foot mark. The position of the plumb bob will bethe location of the section bent. The vertical distancefrom the location of the bob to the horizontal tape, plusthe height of the first bent, less the thickness of thefooting, will be the height of the second bent.Finally, stretch the tape from the location of thesecond bent and proceed as before. The verticaldistance from the location of the bob to the horizontaltape, plus the height of the second bent, less thethickness of a footing, will be the height of the thirdbent, and so on.CONSTRUCTING A TRESTLE BENTWhen a trestle bent is laid out and constructed, thelength of the posts is equal to the height of the bent, lessthe combined depths of the cap and sill. In a four-postbent, the centers- of the two outside posts are locatedfrom 1 to 2 1/2 feet inboard of the ends of the sill, andthe centers of the two inner posts are spaced equallydistant between the other two.Sills, caps, and posts are commonly made of stockthat ranges in size from 12 by 12s to 14 by 16s. If asill or cap is not square in a cross section, the largerdimension should be placed against the ends of theposts. The usual length for a sill or cap is 2 feet morethan the width of the roadway on the trestle. Theminimum width for a single-lane trestle is 14 feet; fora two-lane trestle, 18 feet.LayoutPart of the terrain at an assembly site may be gradedflat and used as a framing yard, or a low platform maybe constructed for use as a framing platform. To assem-ble a bent, lay the posts out parallel and properly spaced,and set the cap and sill in position against the ends. Borethe holes for the driftpins through the cap and the sillinto the ends of the posts, and drive in the driftpins. Cuta pair of 2- by 8- by 18-inch scabs for each joint andthen spike, lag-screw, or bolt the scabs to the joints.Finally, measure the diagonals to determine thelengths of the transverse diagonal braces. Cut thebraces to length and spike, lag-screw, or bolt them tothe sills, caps, and posts. Transverse diagonal bracingis usually made of 2 by 8 stock.Trestle Bent ErectionAfter assembly, the trestle bent is moved to theabutment, and set in place on the footings at the seat.Carefully plumb the bent and temporarily brace it withtimbers running from the top of the bent to stakes drivenat the abutment. Lay the superstructure (girders anddecking) from the abutment out to the top of the firstbent. The second bent is then brought out to the end ofthe superstructure and set in place. Plumb the secondbent and measure the diagonals to determine the lengthsof the longitudinal diagonal braces between the first andsecond bents. Then, cut the braces and spike, lag-screw,or bolt them in place.The superstructure is then earned out to thesecond bent, after which the third bent is brought tothe end of the superstructure. This procedure isrepeated, usually by parties working out from bothabutments, until the entire span is completed.SUPERSTRUCTURETimber girders are usually 10 by 16s, 14 feet long,spaced 3 feet 3 1/2 inches on center (OC). Variousmethods of fastening timber stringers to timber capsare shown in figure 10-6, view A. Various methods offastening steel girders to timber caps are shown in viewB. This view also shows three ways of fastening aFigure 10-6.-Methods of fastening timber stringers and steelgirders to timber caps.10-3
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