TABLE I.

STEEL I-BEAM BRIDGES. WISCONSIN HIGHWAY COMMISSION.
Channels on outside. Weight includes railing.

18 Ft. Roadway.

Steel, Lb.

ESTIMATED QUANTITIES FOR STANDARD BEAM SPANS. IOWA HIGHWAY COMMISSION.

Rails 8 2" X 21′′ × 1′′ × 5′-9′′. Top of rail 3′-2′′ above grade. Post ≤8 3′′ × 3′′ × 1′′

are given in Table I and Table II, Chapter X. The depths of I-beams for different spans for beam spans with plank 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 IV. The minimum weights of I-beams are to be used in each

FIG. 4. STANDARD BEAM BRIDGE. IOWA HIGHWAY COMMISSION.

case. In the calculations it was assumed that one stringer carried that fraction of the concentrated load equal to the fraction found by dividing the stringer spacing by 6 feet for concrete floors, and by dividing the stringer spacing by 4 feet for plank floors. It will be noted that heavier beams are required for short spans with plank floors than with concrete floors.

TABLE III.

DEPTH IN INCHES OF I-BEAMs for Different Spacings and Spans Required to Carry 20-TON, 15-TON AND IO-TON AUTO TRUCKS AND 30 per cent Impact. DEAD LOAD 100 LB. MINIMUM Weights of I-BEAMS ARE USED.

PER SQ. FT.

The proportion of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists divided by six feet when reinforced concrete floor is used. The outside beams to be the same as the intermediate beams.

TABLE IV.

DEPTH IN INCHES OF I-BEAMs for Different SPACINGS And Spans REQUIRED TO CARRY 20TON, 15-TON AND 10-TON AUTO TRUCKS AND 30 PER CENT IMPACT.

MINIMUM

The proportion of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists divided by four feet when timber floor is used. The outside beams to be the same as the intermediate beams.

Details of standard beam spans designed by the Michigan State Highway Department are given in Fig. 6. The reinforced concrete floor has a bituminous wearing surface. Data and details are given in the cut.

The details of a beam bridge with a concrete floor has designed to comply with the author's specifications are given in Fig. 7.

Leg Bridges.-Beam and truss bridges are sometimes supported on steel legs in the place of abutments. A steel leg beam bridge is shown in Fig. 7, Chapter VIII. The legs are composed of I-beams supported on a timber or a steel channel mudsill. The backing is steel plate, or plank,

FIG. 5. STANDARD BEAM BRIDGE. IOWA HIGHWAY COMMISSION.

or stone. The legs should be designed to carry the thrust of the filling in addition to the live and dead load on one-half of the span. (For methods of calculation of thrust due to the earth filling, see Chapter XIX, and also the author's "The Design of Walls, Bins and Grain Elevators.") Truss leg bridges should be built with stiff lower chords designed to take the thrust due to the filling. Leg bridges, unless very carefully designed and constructed, are not to be recommended.

Note:

Section A-A
Elevation Pipe Rail

(With Pipe Railing) Typical Cross Section (With R. Concrete Railing)

Outside Elev. Concrete Rail

FIG 6. STANDARD BEAM BRIDGE. MICHIGAN STATE HIGHWAY DEPARTMENT.