Page:Cassells' Carpentry and Joinery.djvu/100

82 of warehouses, etc., if a wrought-iron bar or tube is passed through each joist a little above its centre. The bar must have a thread and nut at each end working against an iron plate, so that the struts and joists may be tightened perfectly close to each other. A view of this arrangement is given at Fig. 360.

Joists are now often supported direct by the brickwork or masonry, or they may take their bearing on a tar and sanded or galvanised iron bar. Figs. 361 to 364 show four general methods of bedding plates for joists in or upon the walls. Fig. 364 shows the plate supported by iron corbels built in the walls. So that the plate may not project below the ceiling, sometimes the joists are notched down to bring their lower edges level with the under side of the plate; but, of course, this weakens the joists.



Common joists are spaced 12 in. apart, with herringbone strutting every 4 ft. Dimensions for common joists are as follow:

The nearest available size should be used, and 2-in. ceiling joists should be ½ in. deep per foot span. The trimming joist is made ⅛ in. thicker for every common joist carried by the trimmer. A rough rule used some years ago was to fix the depth of the joists at one-sixteenth of the clear span, or ¾ in. to each foot between the bearings. The Ecclesiastical Commissioners prescribe the size of joists to be 9 in. by 2¼ in. for 12-ft. spans and 12 in. by 3 in. for 18-ft. spans. A metropolitan authority has fixed upon 8½ in. by 2½ in., and 11½ in. by 2½ in. for the



same respective bearings. By the rough rule of one-sixteenth the distance between the bearings, the depth for an 18-ft. span should be:—

If, however, the thickness of the joist is taken to be 3 in., the strength of the joist will allow for

central breaking load, or

central safe load, which is considerably more than is required (see the calculation given below).

The weight on ordinary joists of, say, 18-ft. span, 12 in. deep, and 3 in. thick, and 1 ft. 3 in. centres, may be taken to be as follows:—The superficial space carried on the joist is 18 ft. by 1 ft. 3 in. = 22.5 sq. ft., and this covered with people at, say, 84 lb. per square foot amounts to 22.5 ft.