SPECIFICATIONS FOR DESIGN OF CONCRETE BRIDGES Bureau of Surveys, Department of Public Works, Philadelphia Data for Calculation. The structure shall be propor tioned to carry the dead load—consisting of the weight of the structural material, driveway and sidewalk forma tion, rails, curbs, and railings—and one of the following classes of live load: A uniformly distributed load over the entire surface, including driveway and sidewalks, as follows: For spans up to 75 feet, 100 pounds per square foot. For spans over 75 feet and not over 150 feet, 90 pounds per square foot.
For spans over 150 feet, 80 pounds per square foot. Sidewalk brackets, joists, and floor to be proportioned for a local loading of 110 pounds per square foot.
A concentrated moving load as follows: For bridges with permanent pavement on metal deck, a load of 40 tons equally distributed on four wheels spaced 20 feet between axles, and 6 feet gauge.
The following weights are to be used in computing the dead load: Concrete, 140 pounds per cubic foot.
Binder coat, 135 pounds per cubic foot.
Granolithic pavement, 140 pounds per cubic foot. Asphalt wearing surface, 87 pounds per cubic foot. Granite block paving, 6 inches in depth, 80 pounds per square foot.
Vitrified brick paving, 4 inches in depth, 50 pounds per square foot.
'Woodwork, 4 pounds per foot board measure.
Concrete-Steel. When concrete-steel is used, it shall be so designed that the stresses in the concrete and the steel shall not exceed the following: The tensile strength of concrete shall not be consid ered. The ratio of the moduli of elasticity of concrete and steel shall be taken as 1-12.
The steel rods must be of a design that will provide a mechanical lock with the concrete, and not depend upon the adhesiveness of the cement.
In the determination of bending moments, beams and girders shall be considered as simply supported at the ends, no allowance being made for continuous construc tion over supports. Floor-plates, when constructed con tinuous and provided with reinforcement at top of plates over supports, may be treated as continuous beams, the bending moment for uniformly distributed loads being taken not less than the bending moment, in the case of continuous square floor-plates reinforced in both direc tions at top and bottom and supported on all sides, may be taken at The floor-plate to the extent of three times the width of beam or girder, may be taken as part of that beam or girder in computing its moment of re sistance.
Concrete-steel may be used for columns in which the ratio of length to least side or diameter does not exceed 12. The reinforcing rods must be tied together at inter vals not greater than the least side or diameter of column.
Cement Cement concrete, un less otherwise specified, shall be composed of 1 part ce ment; 3 parts coarse sand, or gravel, or stone screenings; and 6 parts crushed stone. These proportions shall be determined by measurement, a barrel of cement being taken to contain 3.6 cu. ft., and a bag of cement .9 cu. ft.
Sand or Gravel. The sand or gravel or screening& shall consist of grains graded from coarse to fine, contain ing not more than 5 per cent by weight of loam, clay, dirt, and other impurities, and be equal in quality to the best New Jersey bank sand or gravel.
Crushed Stone. The stone shall be clean, hard, cubical, crushed natural stone or crushed pebbles to be approved by the Chief Engineer, comprising the whole run of the crusher from 1/4 inch to inch in size when used for bedding on metal decks, or for balustrades or parapets, or for any concrete in which large stone or steel rein forcement is to be embedded; and from 1/4 inch to 2 inches for other purposes, screened of dust and particles less than 1/4 inch. Care shall be taken that each batch as used is mixed as to size. No limestone shall be used iu arches over steam railroads.
Storage piles of material shall be on a platform or other hard, clean surface, to avoid admixture of dirt.