Example 3.—llesign the principal members for a bridge of 35 feet clear span, 17 feet wide between girders, to carry roadway and loads as in Example 2. Also to carry sidewalks .5 feet wide, loaded with 100 pounds per square foot.
Solufian.—Assume the spacing of cross-girders at 4 feet e. to c. and the road slab as in Example 2; slab 8 inches thick, d=7 inches, ?-inch square steel 5 inches c. to c. top and bottom.
Cross-beams.—The dead load upon the T-beams, assuming weight of stem at 150 pounds per linear foot will he 1S0 X4+150 =S70 pounds per linear foot. The live load is composed of two 14,000 pounds wheel loads, 6 feet apart. As these are distributed over 6 feet of width, 14000X4/6=9333 will be carried by the 4 feet width of beam.
The effective length of beam is 18 feet, and Assuming that the nearest wheel load may pass 18 inches from the side girder, maximum shear in the cross-beam is Assuming width of stern of T-beam to he 12 inches, we have the required depth, Use six -inch square bars, four in lower row, two in upper.
The total depth of beam is 22 inches, and the weight of the stem is 12X 14X X150;144 = 175 pounds per linear foot, 25 pounds more than assumed.
Using U-shaped stirrups of 1-inch round bars, the spacing at end of beans is Place eight stirrups with this spacing then space 12 inches apart to middle of span.
The sidewalk slab carries 100 pounds per square foot moving load, on 4-foot continuous spans. We will make it 3 inches thick, reinforced with s-inch round bars 6 inches apart. The sidewalk supports are cantilever beams carrying 4 feet of sidewalk with its load and 4 feet of handrail at the end.
Side Girders.—The sidewalk with its load weighs about 800 pounds per foot of girder. One-half the weight of bridge floor and T-beams is 1900 pounds per foot. Assume weight of girder as 1600
pounds per foot, and the total dead load is 4300 pounds per linear foot..
The maximum moving load is the weight of a truck whose nearest wheels are 18 inches from the girder. These loads are Table X shows that nine 1;-inch square bars may be used, or six 1;-inch square bars will answer. These can be spaced four in the lower and two in upper row. The maximum bond stress for the latter is _ V _ 106250 _ .S75X73X33 5001b. in' the live loads to be applied over a length of 2 feet, ? (live) =25900(3G-5.1) 36 = 2-1S00 pounds.
? (impact)=25 per cent of 24800 = 6200 pounds.
? (dead) =-1300X 17.5 = 75250 pounds.
The maximum shear at the middle of the beam occurs when the heavier load is just past the middle point, or and The maximum shear varies from 83 lb. at the support to 9 lb. at the middle of the girder. Reinforcement for diagonal tension i; needed where r is more than 40 lb. If U-shaped stirrups be spaced 12 inches apart, at the abutment, By Table X, b-inch round are needed. The tops of these bars should be turned into hooks to secure ample bond. Seven stirrups will be used spaced 12 inches apart, three spaced 18 inches and two spaced 30 inches, at each end of the girder.
prevent the T-beams breaking loose from the girders, bars passing under the steel in the stem of the T-beam, and extending up into the girder are used to carry the reactions at the ends of the T-beams. These reactions equal the maximum shear upon the T-beams, and the area of steel required is 16000 =1.52 By Table X, we find 1-inch round bars to be needed. These should extend upward a distance sufficient to develop a bond strength equal to the tensile strength of the bars, or at least 50 diameters.