SHOES AND PEDESTALS. 135 



The details of a riveted truss highway bridge for light country traffic designed by Mr. H. S. 

 Crocker, Consulting Engineer, Denver, Colo., are given in Fig. 25 and Fig. 26. The details of a 



Pin-connected truss highway bridge designed for country traffic are given in Fig. 27, Fig. 28 and 

 ig. 29. Both of these bridges represent standard practice in the design of steel highway bridges 

 for light country traffic. For additional examples of steel highway bridges, see the author's 

 "The Design of Highway Bridges." 



Economic Depth and Panel Length of Trusses. The economic depth and panel length of 

 trusses is not capable of mathematical calculation. The minimum depth is determined by the 

 required clear head room, which varies from \2\ to 15 ft. Short panel lengths give heavy trusses 

 and light floor systems; while long panels give light trusses and heavy floor systems. For ordinary 

 conditions it is not economical to use panel lengths less than 15 ft. for short spans nor more than 

 25 ft. for long spans. The minimum depth for through spans is about 16 feet where the floor- 

 beams are placed below the lower chords. To make a stiff structure, the depth should be suffi- 

 cient to permit the placing of the floorbeams above the lower chords and to permit of efficient portal 

 and sway bracing. Experience has shown that the most economical conditions occur when the 

 angle 9, the tangent of which is the panel length divided by the depth, is about 40 degrees. The 

 top chord points of bridges with inclined chords should be approximately on a parabola passing 

 through the pin at the hip. 



Depth and Panel Length of High Trusses. The depths and number of panels in Iowa High- 

 way Commission high truss riveted bridges are as follows: Pratt, riveted trusses, go-ft. span, 5 

 panels, 20 ft. deep; loo-ft. and no-ft. spans, 6 panels, 20 ft. deep; i2O-ft. span, 7 panels, 20 ft. 

 deep; i4O-ft. span, 8 panels, 21 ft. deep. The depths and number of panels in Wisconsin Highway 

 Commission high truss riveted bridges are as follows: go-ft. and 96-ft. span, 6 panels, 18 ft. deep; 

 ico-ft. span, 6 panels, 20 ft. deep; io5-ft. span, 7 panels, 20 ft. deep; i2O-ft. span, 8 panels, 20 ft. 

 jp; 128-ft. span, 8 panels, 21 ft. deep; i4O-ft. span, 8 panels, 20 ft. deep at hip and 27 ft. deep at 

 iter; i5O-ft. span, 8 panels, 20 ft. deep at hip and 28 ft. deep at center. 



The depths and number of panels in American Bridge Company's high truss bridges are as 

 allows: Riveted and pin-connected trusses with parallel chords, 8o-ft. to go-ft. span, 5 panels, 

 pth equal to panel length; 90- to i2O-ft. span, 6 panels, depth equal to panel length; i2O-ft. span 

 i4O-ft. span, 7 panels, depth equal to panel length, I2o-ft. to i68-ft. span, 8 panels, ratio of 

 :pth to panel length 1. 1. For bridges with inclined chords with spans of 162 ft. to 180 ft., 9 

 inels, and ratios of depth to panel length of l.o, 1.16, 1.25 and 1.29; i^o-ft. to 22O-ft. span, 9 

 .nels, and ratios of depth to panel length of l.o, 1.24, 1.28 and 1.43. For Petit trusses, 24O-ft. 

 276-ft. span, 12 panels, and ratios of depths to panel length of l.o, 1.4, 1.6 and 1.7; 294-1!. to 

 22-ft. span, 14 panels, and ratios of depth to panel length of i.o, 1.36, 1.60, 1.8 and 2.0. 



SHOES AND PEDESTALS. The bridge rests on shoes or pedestals, the loads being trans- 

 red to the shoes in pin-connected bridges by means of pins, and through the riveted joints in 

 iveted bridges. The shoes at the expansion ends of the bridge are placed on smooth sliding plates 

 bridges of less than, say, 65-ft. span, and on nests of rollers or rockers for spans of greater 

 igth. The action of the rollers under the expansion ends of riveted bridges will be much more 

 itisfactory if the shoes are pin-connected to the truss the same as for pin-connected trusses, 

 lollers should be made with as large diameters as practicable in order to reduce the pressure on 

 ic base plate and also to reduce the resistance to movement. Experience shows that even for 

 jht bridges rollers smaller than 3 in. diameter are practically worthless. To economize space, 

 jmental rollers, as shown in Fig. 35, Chapter IV, are often used for heavy spans. 



It is usual to specify that a movement produced by a variation of 150 degrees Fahr. be pro- 

 dded for. The coefficient of expansion of steel is approximately 0.0000067 per degree Fahr., 

 lich makes it necessary to provide for approximately one inch of movement for each 80 ft. of 

 ridge span. 



Where both bridge seats are of ihe same height, the fixed end is carried on cast iron pedestal 

 blocks. The blocks are usually made with recesses (honeycombed) to reduce the weight. 



The Illinois, Iowa and Wisconsin Highway Commissions use rockers in the place of rollers 

 for highway bridges. Detail* of rockers are shown in Fig. 17, Fig. 18, Fig. 23, and Fig. 24. The 

 specifications of the Illinois Highway Commission contain the provision that rockers shall be made 

 of cast iron as specified. They shall have a thickness of not less than 2\ in. for spans of 45 ft. or 

 less, and a thickness of 3 in. for spans exceeding 45 ft. in length, but in no case shall the unit com- 

 pressive stress exceed 9,000-40 l/r Ib. per sq. in. All rockers shall have bearing surfaces turned to 

 a uniform radius and smooth surface and shall be provided with two 2-in. holes through the web to 

 facilitate handling. 





