Framed floors possess, in a still greater degree, the advantages and disadvantages attributed above to Double Floors.
The girders may be of any form or material selected after duly considering all the requirements of the case. (See Part IV.)
If the girders are simple balks of timber, the binders are framed into them by double tusk tenons. They should be kept as far as possible from the centre of the length of the girders, in order not to weaken them at the points where the strain is the greatest.
The girders and binders should be as deep as possible, so that the floor may be stiff, not liable to shake or crack the ceiling below. Tredgold recommends that the distance apart of the girders should not exceed 10 feet; their position depends, however, on the plan of the building.
Figs. 279, 280 represent a framed floor in plan and section.
Fig. 280. (Double the scale of Plan.)
Plan and Section of Framed Floor.
N.B. In Fig. 280 the graining of the binders shown in section is omitted for the sake of clearness.
The girders rest upon templates, and the binders are framed into them as above described. The end of one of the binders, which is close to a flue in the wall at A, is protected against fire by a cast-iron shoe C.
Another way of effecting this would be to allow the end of the binder to rest upon a corbel projecting from the wall.
One end of the floor is supported by a half-girder, in order that it may not rest upon the wall containing flues; if it were not for these the ends of the binders would rest upon the wall. On the upper side is shown the trimming necessary for a lift.
A great portion of the boarding is broken away to show the timbers below, and the ceiling joists are, as before, omitted in plan to avoid confusion.
When binders are tenoned into a girder they cut into and weaken it considerably, especially when, as is generally the case, the binders are opposite to one another; to avoid this, iron stirrups (Fig. 281) are sometimes used to carry the ends of the binders, and so to leave the girder intact.
Fig. 281. Stirrup for carrying end of Binder.
In framed floors, especially in Scotland,1 the binders are sometimes omitted, and the girders are of slighter scantling, placed closer together. The ceiling joists are suspended by straps of wood (see Fig. 308, p. 143).
This makes a strong, stiff, and economical floor, but if the bridging joists are simply notched (as they should be) it occupies a considerable depth.