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FIG. 8. BENDING MOMENTS IN FLOORBEAMS AND STRINGERS FOR 20-TON AUTO TRUCK.

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FIG. 9.

14 16 18 20 22 24
Panel Length in Feet.

BENDING MOMENTS IN FLOORBEAMS AND STRINGERS FOR 15-TON AUTO TRUCK.

(30 PER CENT IMPACT). CONCRETE FLOOR.

2 3 4
Spacing in Feet.

5

FIG. 10.

DESIGN OF FLOORBEAMS.-The floor loads may be carried to the floorbeams by means of stringers or joists, or the loads may be carried to the floorbeams directly by the floor slabs. The loads carried by the floorbeams consist of (1) the dead load which is the weight of the floor system; (2) a uniform live load or a concentrated moving load. The uniform live loads are the same as the uniform live loads used in designing the floor slabs and stringers, but the distribution of the concentrated moving load is not the same as for either the floor slabs or the stringers. The distribution of the moving concentrated load to floorbeams as specified by different highway commissions and others, and by the author have been given in Chapter IX.

Steel I-Beam Floorbeams.-The sizes of steel I-beams required for floorbeams for panel lengths of 10 ft. to 24 ft. and widths center to center of trusses or girders of 15 ft. to 26 ft., to carry a dead load of 100 lb. per sq. ft., and a 20-ton auto truck with 30 per cent impact, or a uniform live load of 125 lb. per sq. ft. with 30 per cent impact are given in Fig. 8; while the floorbeams required to carry a 15-ton auto truck with 30 per cent impact, or a uniform live load of 100 lb. per sq. ft. with 30 per cent impact are given in Fig. 9. It will be noted that the uniform live load controls for wide roadways or for long panels.

For a bridge 17 ft. center of trusses and 18 ft. panels, from Fig. 8 the required floorbeam is a 24 in. I @ 80 lb., while from Fig. 9 the required floorbeam is a 20 in. I @ 70 lb.

The sizes of steel I-beams required for floorbeams for panel lengths of 10 ft. to 24 ft., and widths center to center of trusses or girders of 15 ft. to 26 ft., to carry a dead load of 100 lb. per sq. ft. and a 15-ton auto truck without impact, or a uniform live load of 100 lb. per sq. ft. without impact are given in Fig. 10. These are practically the floorbeams required by the specifications of the Illinois, Iowa, and Wisconsin Highway Commissions. Steel stringers for the same loading are given in Fig. 10.

The bending moments for the design of built-up floorbeams may be obtained from Fig. 8, Fig. 9, or Fig. 10.

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BENDING MOMENTS IN FLOORBEAMS AND STRINGERS FOR 15-TON AUTO TRUCK.

(NO IMPACT). CONCRETE FLOOR.

2 3 4
Spacing in Feet.

CHAPTER XI.

DESIGN OF BEAM HIGHWAY BRIDGES.

Introduction.-Beam bridges are made by placing steel beams side by side with the ends resting on the abutments. The roadway floor may have a concrete sub-floor with an earth fill wearing surface, a timber block wearing surface, an asphalt or bituminous wearing surface, or a concrete wearing surface; or a plank floor may be used. The spacing of the beams depends upon the load to be carried and upon the type of floor. An old rule for the thickness of oak floor planks was that the floor should have at least one inch in thickness for each foot of spacing of joists or stringers. With the modern auto truck a better rule is that the floor planks shall have three inches in depth for each two feet of spacing of joists or stringers, with a minimum thickness of 3 inches. Joists or stringers with plank floors should not have a greater spacing than 3 feet. The thickness of concrete slabs required to carry a given load is practically a constant from 2 ft. to 4 ft. or 5 ft., and it is therefore commonly economical to use a larger spacing for joists or stringers when used to support a concrete sub-floor than when used to support a plank floor. The outside beams should be the same size as the intermediate beams. It is commonly specified that rolled beams when used for stringers shall have a depth not less than one-thirtieth of the span.

Standard steel beam bridges, as designed by the American Bridge Company, are shown in Fig. 1 and Fig. 2. The details of both bridges are the same with the exception of the fence. Angle cross-braces are used on both bridges. The beams rest directly on the bridge seat of the abutment and not on wall channels as is a more common practice, although a channel is sometimes laid on the bridge seat with the legs turned down to carry the beams. The gas pipe rail in Fig. I is much cheaper than the lattice rail in Fig. 2.

In the place of the spiking strips on the tops of the beams, as shown in Figs. 1 and 2, spiking strips are sometimes bolted on the sides of the channels and the center I beam, or two channels are used for the center beam with the spiking strip bolted between them. The floor planks are spiked to these spiking strips, and are fastened to the other beams by clinching spikes, which have been driven through the planks, around the top flanges of the beams.

The maximum span for beam bridges is usually given as 40 feet. A better limit for beam spans is 32 feet. Riveted truss bridges should be used for spans of 32 feet and upwards for country bridges, and plate girders for heavy city bridges. Riveted bridges for spans of, say, 40 feet are more economical than beam bridges and will give fully as great a length of service. The ends of beam bridges should always be supported on masonry abutments.

Details of a standard beam bridge with a concrete floor as adopted by the Wisconsin Highway Commission are shown in Fig. 3. The beams are fastened to the wall plate channels by means of U-bolts. Drains through the floor slab at the gutter are provided at distances of 10 ft. or 12 ft. The reinforced concrete slab is 6 in. thick at the center and 5 in. thick at the curbs, reinforced transversely within. square twisted bars spaced 6 in. centers, with center of bars I in. from bottom of slab, and the longitudinal bars are in. square twisted, spaced two between adjacent joists. The posts of the rail are anchored to the concrete slab floor. Channels are used for the outside stringers in the place of I-beams, which is the better practice. The sizes of I-beams used by the Wisconsin Highway Commission for different spans are given in Table I. These beam spans are designed for a 15-ton road roller, not considering impact.

Details of a beam bridge with concrete floor and fence as designed by the Iowa Highway Commission are given in Fig. 4. The sizes of I-beams, quantities of material and other data for

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beam spans from 16 ft. to 32 ft. are given in the cut. It will be noted that I-beams are used for the outside stringers in this design. The outside stringers are wrapped with wire mesh and are encased in concrete. Weep holes 2 in. in diameter spaced 4 ft. apart are provided on each side of the bridge.

Details of a beam bridge with concrete floor and an angle rail, as designed by the Iowa Highway Commission are given in Fig. 5. The sizes of I-beams and channels for spans of 16 ft. to 32

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FIG. 3. STANDARD BEAM BRIDGE. WISCONSIN HIGHWAY COMMISSION.

ft. are given in the cut. This bridge is also constructed with a gas pipe fence in place of the angle fence. Estimated quantities of steel and concrete in beam spans of this type are given in Table II.

The depths of I-beams for different spans for beam spans with concrete floors designed to carry a 20-ton, a 15-ton, or a 10-ton auto truck with 30 per cent impact are given in Table III. The minimum weights of I-beams are to be used in each case. These beam spans were designed for a dead load of 100 lb. per sq. ft. in addition to the concentrated live loads. The outside beams are to be the same as the intermediate beams. The thickness of the slabs for the different loadings

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