After the floor joists are leveled and secured in place, and before the floor boards are laid, they should be bridged in the centre for spans between 8 and 16 feet, and twice for spans from 18 to 24 feet, by rows of cross bridging, as shown in Fig. 32. For dwelling house floors 1x3-inch bridging is sufficient; for 14-inch beams 2x3-inch stuff should be used. The pieces should be cut on a mitre and the

Fig. 33 exact length, and each end of each piece nailed with two ten-penny nails. Both ends of the bridging should be nailed at the same time and before the joists are loaded in any way; the bridging should also be continuous and in straight lines across the room.

It should not be supposed that bridging enables a floor to cany a greater distributed load than it would support without bridging, for such is not the case, except in so far as it prevents the joists from twisting or buckling sideways, The principal benefit derived from bridging is in case of a concentrated load, such as the leg of a heavy piece of furniture, and also from suddenly applied loads, as jumping, moving of heavy articles, etc.

In such cases the beam immediately beneath the weight is materially assisted through the bridging by the beams on each side of it

Fig. 33.

50 Bridging 20022

Fig. 34.

Mr. R. G. Hatfield found by testing a model floor, constructed one-eighth full size, that it required three times the load to produce the same deflection in a bridge beam than it did when not bridged.