Page:Cassells' Carpentry and Joinery.djvu/99

Rh are sometimes strengthened by trussing. Various methods are adopted. Two ways of trussing by wrought-iron rods are shown by Figs. 352 to 358. In the case of Fig. 352 the beam is sawn down the middle, ends reversed, and bolted together with blocks



between, so as to allow of the iron rod passing through the iron heel plate at each end (Fig. 354), so that it can be tightened. Figs. 356, 357, and 358 illustrate a very strong form of trussing by using a solid beam and a tension rod on each side.

Herringbone Strutting.—Cross-pieces of wood, about 2 in. by 1½ in., or 2 in. by 2 in., are frequently fixed between joists, as already shown by Figs. 315, 317, and 332, with



the view of strengthening and increasing the rigidity of the whole floor. To prevent splitting at the ends by boring, it is usual to make a saw kerf at each end of the struts (see Fig. 317) for the insertion of the nails. A great advantage in this form of strutting is that, although the joists may shrink in thickness and depth, the strutting remains firm owing to the greatest shrinkage taking place in depth. This will be made clear by Fig. 359. Let a, b, c, d represent the original position of the strutting; then upon shrinkage taking place, the struts move about



their centre O, and tend to the positions indicated by the dotted lines a′ b′ and c′ d′, the greatest movement being produced by the depth shrinkage; thus the greater this is the more the compression on the struts, which would produce greater distances between the joists, were it not for the floor boards being nailed to the joists.

Solid Strutting.—When pieces of board are cut and simply driven in tightly between the joists and nailed, they often become loose some months after the floor is completed, owing to the shrinkage of the joists



in thickness, and thus they are of very little use for the purpose for which they were intended. Solid strutting is a most valuable form for stiffening and strengthening floors