Page:Cassells' Carpentry and Joinery.djvu/101

Rh by 84 lb. = 1,890 lb.

The sound-boarding and pugging may be taken at 100 lb. per yd. super., and the lath, plaster, etc., at 80 lb., giving a total weight of 180 lb. per yd., or per ft. super. 180/9 = 20 lb. This multiplied into the area gives 22.5 x 20 lb. ( = 450)

The floorboards will be 18 ft. by 15 in. by 1½ in. = 2•81 ft. cube the joists 18 ft. by 1 ft. by 3 in. = 4.50 /7.31 ft. cube

and the total weight of timber will be 7•30 ft. by 35 lb. ( = 257)

Thus the total distributed weight is 2,597

This is equal to $2597⁄2$ = 1,299 lb. central load, or 11•6 cwt. The strength of the joists under this load will be, by the formula already given, $122 × 3 × 2•5⁄18$ = 60 cwt. breaking load, or $60⁄6$ = 10 cwt. safe load.

Floors should be estimated for according to the nature of the building and the probable load. A crowd of persons is variously estimated to weigh from 41 lb. to 147•4 lb. per square foot of the surface covered. Probably a safe average would be 1 cwt. per ft. super, considered as a live load. Dwelling houses are usually designed for a dead load of 1¼ cwt. per ft. super., churches and public buildings 1½ cwt., and warehouses 2½ cwt. The weight of the structure must be allowed for in addition to the above loads, and this is most important to bear in mind in connection with fireproof floors. For dwelling houses the 1⅓cwt. is usually made to include the weight of the floor itself.

Fig. 362.—Joists supported by Wall Plate bedded on Set-off.

Fig. 363.—Wall Corbelled Out to carry Wall Plate.

Fig. 364.—Plate carried by Wrought-iron Corbels built in Wall.

Let it be required to determine the size of a bridging joist suitable for a span of 18 ft. and capable of carrying a load of 1 cwt. per ft. super., the joists fixed 12 in. centre to center. The preliminary calculation will be as follows: (1) The total weight on one joist is equal to the load on the half space on either side of the joist—that is, 6 in. on each side. Then the total load = 18 × 1 ft. × 1 cwt. = 18 cwt. (2) The load