56 MOLDING CONCRETE ROOFS to replace the roof-sheathing in monolithic roof construction may be safely estimated as follows: No. 5 wire weighs .1143 pounds to the linear foot, or 8' 9" to the pound; No. 8 wire weighs .0700 pounds to the linear foot, or 14' 4" to the pound; " steel bars weigh .375 pounds to the linear foot, while "steel bars weigh .668 pounds to the linear foot. In planning the reinforcement for the roof the allowable stress for structural steel is sixteen thousand pounds to the square inch, and is the maximum fiber stress universally employed; the ultimate strength is usually estimated at sixty thousand pounds to the square inch, thus giving a factor of safety of 3.75. The sectional area in square inches of No. 5 wire is .033654; of No. 8 wire is .020612; of "steel bars is .1104, and of " steel rods or bars is .1963; " rods is .3070. The above data covers the field in estimating the necessary reinforcement for the average roof after the worker has determined the weight of the material or dead load, plus the snow and wind-loading. The reinforcing demanded may be estimated by using 16,000 pounds per square inch of sectional area as the allowable unit stress. As the weight of the reinforcement cannot be secured until this has been determined, it will be necessary to revise the estimate to include same. While the average roof is not required to carry a live load or weight in addition to the weight of materials, wind, and snow-pressure, yet when this is required it must be considered in addi tion to the dead loading. The average live loading for roofs is taken at thirty pounds to the square foot, although for some purposes this is increased to fifty pounds to the square foot of surface. INDEX Adjustable chimney forms.. Adjustable molds for concrete chimney.. PAGE 31-36 30-31 25 25 14-56 19-20 31-32 Batter to chimney wall, how secured.... Blocks, inter-locking, for small chimneys.. for large chimneys.. 35 21-22 23-29 Bracket for concrete roof construction.. 41-43 Coating molds to prevent sticking.. 19-30 Comparative weights of clay-tile, slate, and concrete slate 46 Core for small chimney, construction of. 17-19 41 48 41 slate or tile. Curing a monolithic concrete roof.. Dead load of roofs..... Dividing plates for block molds.. 55 29 5343 |