When the span of the floor is so great that timber girders of the required scantlings cannot be economically obtained or are objectionable on account of their bulk or for other reasons, girders of other form and material may with advantage be used.
The difficulty in obtaining sufficiently large timber may be overcome by building up a girder out of smaller pieces (see p. 96), by trussing beams of lighter scantling (pp. 99-102), or strengthening them by the introduction of an iron flitch plate (Fig. 230, p. 98), or rolled joists (Fig. 231), sandwich-fashion.
Rolled iron beams (Fig. 255) may with great advantage be used as a substitute for timber girders or binders, for they are less bulky and more durable.
1 Newland's Practical Carpenter's and Joiner's Assistant.
Figs. 282 to 284 show different cases of the application of rolled beams or joists.
In Fig. 282 a rolled beam B is substituted for the binder in a double floor. In Figs. 283, 284, rolled beams are substituted for the girders in framed floors. In Fig. 283 the beam being too deep to be contained within the floor, projects beneath it and is concealed by plastering, which forms part of the panelled ceiling below.
"When still deeper girders are required they may be built up of plate iron as described in Chap. VII. and applied in the same way as the rolled beams shown above.
- Girders should always be placed so as to have good supports for their extremities.
To ensure this, it is sometimes necessary to lay them obliquely across the room, but an inclined position should be avoided if possible. It is better to provide very strong templates over the openings to carry the girder and throw the weight well upon the piers.
Wall Plates are continuous, or in any case, long pieces of timber built into or upon a wall to support the ends of joists or other bearers.
They distribute the weight thrown upon them by the joists, and give the latter a hold upon the side walls, so that these are tied together.
On the ground floor the wall plates generally rest upon an offset in the wall, as in Fig. 285.
Above also they may rest on an offset if there is a change in the thickness of the wall; or,
They may be built into the wall, as shown in Fig. 286, great care being taken that there is a free circulation of air round the ends of the joists; or,
Fig. 285. Joist on Wall Plate on Offset.
Fig. 286. Joist notched on Wall Plate built in.
Fig. 287. Wall Plates supported on Corbels.
They may rest on corbels provided for the purpose, as in Fig. 287, or upon a corbel-course, thus preventing all danger of decay by contact with the masonry and want of air.
The joists are either simply nailed on the wall plates, or "notched" (Fig. 286) or "cogged" (Fig. 287) upon them.
If the joists are of unequal depths, the notches are varied in depth also, so as to keep the upper surfaces of the joists in the same plane.
Cogging gives the joists a good hold upon the wall plates, so as to tie the walls in, but it is seldom done.
Wall plates are sometimes dovetailed into each other where they meet at the angles of a building, but there are great objections to dovetails (see p. 64), and it is better that they should be halved and bolted.
Wall plates should be in as long pieces as possible, and when two or more pieces are required to extend along the length of the wall they should be scarfed together (p. 63).
Boiled Iron Wall Plates with a raised rib running along their centre line are sometimes used, and are free from many of the drawbacks of wooden wall plates.
For a 20-feet bearing, 4 1/2 inches by 3 inches.
Stone templates are often used instead of wall plates, and have the great advantage of being indestructible by fire or decay. The joists cannot, however, be economically fixed to them, which is a disadvantage.
They should be of hard stone, and in lengths of at least 2 or 3 feet, so as to distribute the weight of the joist and its load over a wide bearing.
Bridging Joists or "Common Joists." - These are generally laid about 12 inches apart "in the clear" (i.e. between the side of one joist and that of the joist next to it), or sufficiently near to prevent the deflection of the floor boards. In the best work, however, the joists are laid 12 inches from centre to centre as shown in Fig. 288.