If the joists are flush with the girder on top, and are framed into the sill in the ordinary way, as shown in Fig. 137, the girder can not be flush on top with the sill; while, on the other hand, if the girder is flush with the sill on top, it can not at the same time be flush with the joists on top. If joist hangers arc used on the girder to support the joists, they will probably be used on the sill as well, as explained in connection with the sill; and in this case the girder can be made flush with the sill on top and the joists hung from both girder and sill with hangers, thus bringing both ends of a joist to the same level, as shown in Fig. 13S. If the girder were framed into the sill at all, it would almost always be made flush with the sill on top, and by the proper adjustment of the hangers the joists would be arranged so as to be level.
For framing the girder into the sill, a tenon-and-tusk joint, as shown in Fig. 139, would be used if the girder is to be flush with the sill on top. Since the girder would in most cases be deeper than the sill, the latter having a depth of only 6 inches the wall would neces sarily have to be cut away in order to make a place for the girder.
This condition is clearly shown in Fig. 140. The girder itself should not be cut over the wall, as shown in Fig. 141, because this greatly weakens the girder.
If this method is used, the joists should be framed into the girder in the same way as they are framed into the sill, a mortise being cut in the girder, and a tenon on the joist. This is called "gaining" and is shown in Fig. 137. The top of the girder thus comes several inches below the top of the floor.
Another method is to make the top of the girder flush with the top of the joists. The joists are then framed into the girder with a tenon-and-tusk joint, as shown in Fig. 139, and the girder is "gained" into the sill, as shown in Fig.
Still another method in common use is simply to "size down" the joists on the girder about 1 inch, as shown in Fig. 142. In this case, of course, the girder is much lower than the sill, usually so low that it can not be framed into the sill at all, but must be supported by the walls independently. Holes are left in the wall where the girders come, the latter being run into the holes, and their ends resting directly on the wall, inde pendent of the sill. This is not very good construction, however, because the floor is not tied together as it is when the girder frames into the sill. The first method is the best and is the one in most common use.
The girders serve to sup port the partitions as well as to support the floors, and should, therefore, be designed to come under the partitions whenever this is possible. When the distance between the outside walls is too great to be spanned by the girder, it is supported on brick piers or posts of hard wood or east iron in the cellar. Such piers or posts should always be placed wherever girders running in different directions intersect.
Girders are also often supported on brick cellar partitions.
Joists. Joists are the light pieces which make up the body of the floor frame and to which the flooring is nailed. They are almost always made of spruce, although other woods may be used, and may be found more economical in some localities. They arc usually 2 or 3 inches thick, but the depth is varied to suit the conditions. Joists as small as 2X6 inches are sometimes used in very light buildings, but these are too small for any floor. They may sometimes be used for a ceiling where there are no rooms above, and, therefore, no weight on the floor. A very common size for joists is 2XS inches, and these are probably large enough for any ordinary construction, but joists 2X10 inches make a stiffer floor, and are used in all the best work. Occasionally joists as large as 2X12 inches are used, especially in large city houses, and they make a very stiff floor, but this size is un usual. If a joist deeper than 12 inches is used, the thickness should be increased to 21 or 3 inches, in order to prevent it from failing by buckling, as ex plained for girders, P. 95. The size of the joists depends in general upon the span and the spacing.
The usual spacing is 16 or 20 inches between centers, and 16 inches makes a better spacing than 20 inches, because the joists can then be placed close against the studding in the outside walls and spiked to this studding. It is generally better to use light joists spaced 16 inches on centers than to use heavier ones spaced 20 inches on centers. The spacing is seldom less than 16 inches and should never be more than 20 inches.
Supports and Partitions. In certain parts of the floor frame it may be necessary to double the joists or place two of them close together in order to support some very, heavy concentrated load. This is the case whenever a partition runs parallel with the floor joists, unless there is another partition under it. Such partitions may be supported in several different ways. A very heavy joist, or two joists spiked together, may be placed under the partition, as shown at A in Fig. 143, C being the sole, B the under or rough flooring, and DDD the studding. This method is objectionable for two reasons: It is often found convenient to run pipes up in the partition, and if the single joist is placed directly under the partition, this can not be done except by cutting the joist and thus weakening it. Moreover, if the single joist is used, there is no solid nailing for the finished upper flooring, unless the joist is large enough to project beyond the partition stud ding on each side. The joist is seldom, if ever, large enough for this, and the finished flooring must, therefore, be nailed only to the under flooring at the end where it butts against the partition, so that a weak, insecure piece of work is the result. This may be seen by referring to the figure.