Highway Bridges

The tru es of highway bridges are proportioned to carry safely a live load of 60 lbs. per sq. foot of floor surface in country districts, 80 lbs. in towns, and 100 in cities where traffic is heavy. In addition, the floor stringers and beams are proportioned for the loads of heavy wagons and of electric cars. The weight of the heavier cla of highway bridges may be approximately found from Merriman's formula: W=(140+12b+0.2b1-0.41)1, in which I is the span in feet, b the width in feet, including sidewalks, and W the total weight in pounds. In the case of several spans, the most economic arrangement is usually that where the cost of the piers equals the cost of the superstructure. The types of tru es most frequently used are the Warren, the Pratt, and the bowstring. See BRIDGES and TRUSSES.