112b STEEL HIGHWAY BRIDGES. CHAP. III. 



ILLINOIS HIGHWAY COMMISSION. Steel highway bridges with reinforced concrete 

 floor. Reinforced concrete floor slabs 4 in. thick with a wearing surface assumed to weigh not 

 less than 50 Ib. per sq. ft. Live load for floor and its supports a 1 5-ton traction engine, supported 

 on two axles spaced 10 ft. apart, with two thirds of the load on the rear axle; or a uniform live load 

 of 125 Ib. per sq. ft. The trusses designed for the loads given in Table I. No allowance is made 

 for impact. 



Let W = weight of steel in Ib. per lineal foot of span, L = span of bridge in feet, b = width of 

 roadway in feet (without sidewalks). 



1. Steel Low Truss Spans, with Stringers. The weight of the structural steel in low truss 

 steel highway bridges with spans of 50 ft. to 85 ft., not including weight of the fence or the steel 

 stringers, is given approximately by the formula. For a i6-ft. roadway, b = 16 ft. 



W = 2 35 + 2.35!,. (21) 



and for an i8-ft. roadway, b = 18 ft. 



W 240 + 2.4-L. (22) 



2. Steel High Truss Spans, with Stringers. The weight of structural steel in high truss steel 

 highway bridges with spans of 90 ft. to 160 ft., not including the weight of fence or the steel string- 

 ers, is given approximately by the formula. For a i6-ft. span, b = 16 ft. 



W = 140 + \L. (23) 



and for an l8-ft. span, b = 18 ft. 



W = 1 80 + 4.5 L. (24) 



The weights given by formulas (21) to (24) are for bridges with concrete floors weighing 

 loo Ib. per sq. ft. Calculations by Mr. Clifford Older, Bridge Engineer, Illinois Highway Com- 

 mission, show that a variation of the weight of the floor of 10 Ib. per sq. ft. makes a similar variation 

 in the weight of the structural steel, including the joists, of 4.35 per cent for a 50-ft. span, of 3.75 

 per cent for a i6o-ft. span, and proportional for intermediate spans. For the structural steel, not 

 including the joists, an average value of 4 per cent may be used for each decrease of 10 Ib. per sq. 

 ft. of floor surface. 



BOSTON BRIDGE WORKS STANDARDS.* The weights of steel highway bridges 

 designed by the Boston Bridge Works are as follows: 



Through truss highway bridges without sidewalks designed for a live load of 80 Ib. per sq. ft. 

 for the trusses, 100 Ib. per sq. ft. and a 6-ton wagon for the floor The weight, w, of steel in Ib. 

 per sq. ft. of area covered by the floor, not including joist or fence, for a span of L ft., is 



w = 5 + L/g.s (25) 



The weight of through truss highway bridges with two sidewalks is 



w = 2.8 + L/ii.3 (26) 



The sidewalks were 5 or 6 ft. wide, and the clear roadways were 1 6 to 20 ft. The total area 

 covered by the roadway and sidewalk floors is to be used in calculating the weight of steel. 



Weights of Steel Highway Plate Girder Bridges. The weights of highway plate girder 

 bridges as designed by the Boston Bridge Works for the live loads shown are as follows. 



Deck plate girder highway bridges without sidewalks designed for a live load of 100 Ib. per 

 sq. ft. for girders, 100 Ib. per sq. ft. and a 6-ton wagon for the floor. The weight, w, of steel in 

 Ib. per sq. ft. of area covered by the floor, not including joist or fence, for a span of L ft., is 



w = 2.5 + L/34 (27) 



* Published by permission of John C. Moses, Chief Engineer. 



