This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The framing of the joists into the girders may be accomplished in several ways, according to the position of the girder. The placing of the girder is quite an important point. The top of the floor, on which rest the sole-pieces of the cross-partitions, must remain always true and level, that is, the outside ends of the joists must be at the same level as the inside ends. Otherwise the doors in the cross-partitions will not fit their frames, and can not be opened or shut and the plastering is almost sure to crack.
Both ends of the joists will sink somewhat, on account of the shrinkage of the timber in seasoning, and the only way to make sure that the shrinkage at the two ends will be the same is to see that there is the same amount of horizontal timber at each end between the top of the floor and the solid masonry. This is because timber shrinks very much across the grain, but almost not at all along the grain. If the joists are framed properly into the sill, so that they are flush on the bottom with the sill, we have at the outer end of the joist a depth of horizontal timber equal to the depth of the joist itself, as shown in Fig. 138; and in order to have the same depth of timber at the inside, the bottom of the joist must be flush with the bottom of the girder, which usually rests on brick piers. Of course the top of the girder must not in any case come above the top of the floor joists; therefore, in general, the girder must be equal in depth to the floor joists and flush with these joists on top and bottom, as shown in Fig. 150. This method is not always followed, however, in spite of its evident superiority; and the girder is often sunk several inches below the tops of the floor joists, as shown in Fig. 138, or even in some cases very much below, as shown in Fig. 151. Both of these methods cause an unsightly projection below the ceiling of the cellar. Where the joists are brought flush with the girder top and bottom, they may be framed into it with a tenon-and-tusk joint, as are the girders, as shown in Fig. 139, and a hole bored through the tenon to receive a pin to hold the joist in place.
Fig. 148. Crack in Joist Due to Bad Construction.
Fig. 149. Another Bad Joist and Sill Construction.
Fig. 150. Superior Floor Joist and Sill Construction.
Fig. 151. Joist Sized Down on Girder.
Fig. 152. Fastening Joists by Iron Strap.
Fig. 153. Fastening Joists by Dog.
Other methods of framing tenon-and-tusk joints are shown in Figs. 47, 48, 49, and also a double-tenon joint in Fig. 50, which might be used in this case, although it is much inferior to the tenon-and-tusk joint. Two joists framing into a girder from opposite sides should be fastened strongly together on top either by an iron strap passing over the top of the girder and secured to each joist, as shown in Fig. 152, or by means of a "dog" of round bar iron, which is bent at the ends and sharpened so that it may be driven down into the abutting ends of the joists, as shown in Fig. 153. These bars should be used at every fifth or sixth joist, to form a series of continuous lines across the building from sill to sill.
If the girder is sunk a little below the tops of the joists these may be gained into it in the same way as they are gained into the sill.
Fig. 154. Joist Gained into Girder.
Fig. 155. Joist Fastened to Girder by Tenon Joint and Dowel.
In this case joists should be arranged as shown in Fig. 154, so that they will not conflict with one another; and the two adjacent joists may be spiked together, thus giving additional stiffness to the floor. If the tenon-and-tusk connection is used, the joists may be arranged exactly opposite each other, provided that the girder is sufficiently wide, but it is always much better to arrange them as shown in Fig. 155, even in this case. The tenon may then be carried clear through the girder and fastened by a dowel as shown. Very rarely a simple double-tenon joint, such as that shown in Fig. 50, might be used, but it is much inferior to either the gaining or the tenon-and-tusk joint. If the girder is sunk very much below the tops of the joists, as in Fig. 151, these will usually rest on top of it and be fastened by spikes only, or will be "sized down" upon it about 1 inch, as shown. There is no mortising of the girder in either case. Joists are also thus sized down upon the girts and partition caps, and are notched over the ledger boards as shown in Fig. 105. In cutting the joists for sizing and notching, the measurements should be taken in every case from the top of the joists, since they may not be all of exactly the same depth, and the tops must be all on a level after they are in place. This is really the only reason why the joists should be sized down at all, because otherwise they might simply rest upon the top of the girder, or girt, and be fastened by nailing.
Fig. 156. Joist Supported by Brick Wall.
Fig. 157. Anchored Joist.