Page:Highways for the National Defense.pdf/29

28 of the specific classes of tanks diagrammed in Plate 10, on standard H–15 bridges of width equal to two or more traffic lanes:

Light (13½ ton) tanks.—Simultaneous loading of two lanes with these tanks does not produce greater than allowable stresses.

Medium (20 and 25 ton) tanks.—If the movement is controlled so that there will be no more than a single file of such tanks following approximately the center line of the bridge and spaced at least 40 feet apart, no greater than allowable overstresses will result. There will be no necessity to limit the speed of the tanks.

Heavy (55 ton) tanks.—Such tanks will be safely supported if the movement is controlled so that there will be no more than a single file following approximately the center line of the bridge, with separating intervals of not less than 50 feet, and with speed (on the bridge) reduced to not more than 4 miles per hour.

Horizontal clearance.—Existing bridges and underpasses on two-lane roads having a horizontal clearance of less than 18 feet should be replaced or widened as promptly as possible.

New and widened bridges on two-lane roads should have at least the minimum, and if feasible, the preferred horizontal clearances indicated in the following schedule of widths corresponding to various classes of present average 24-hour traffic volume:

The minimum two-lane bridge clearances specified are substantially adequate, and the preferred clearances completely adequate for civil traffic, including the usual small percentage of vehicles of width equal to and slightly exceeding 8 feet. They are similarly adequate for the accommodation of mobile military equipment, the maximum width of which (exclusive of the 25- and 55-ton tanks) is approximately 8½ feet.

Existing two-lane bridges affording horizontal clearance of less than 20 feet, if not widened, should be posted as narrow bridges. Existing bridges on three- and four-lane roads should be replaced or widened as promptly as possible if, in horizontal clearance, they afford less than 30 and 40 feet, respectively.

N ew and widened bridges on three- and four-lane roads should have horizontal clearances of at least 40 and 52 feet, respectively, and preferably horizontal clearances between extreme lateral curbs 6 feet greater than the width between the outer edges of an approach pavement or pavements designed for 12-foot lanes. Bridges on 4-lane roads having a wide central dividing strip may be built as dual bridges, each bridge providing horizontal clearance of at least 26 and preferably 30 feet.

New and widened underpasses on two-lane roads should have at least the minimum, and if feasible the preferred, horizontal clearances