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.
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 studding 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.
Fig. 142. "Sizing Up" Joist on Girders.
A much better way is to use two joists far enough apart to project a little on each side of the partition, as shown at A A in Fig. 144, and thus afford a nailing for the finished flooring. These joists must be large enough to support the weight of the partition without sagging any more than do the other joists of the floor, and, therefore, joists 3 or even 4 inches thick should be used. They should be placed about 6 or 7 inches apart on centers, and plank bridging should be cut in between them at intervals of from 14 to 20 inches, as shown at E in Fig. 144, in order to stiffen them and make them act together. This plank bridging should be made of pieces of joist 2 inches thick and of the same depth as the floor joists, and should be so placed that the grain will in every case be horizontal.
Fig. 143. Objectionable Construction for Partition Support.
Fig. 144. Approved Construction for Partition Support.
A partition, supported as described above, is bound to settle somewhat as the 10 or more inches of joist beneath it shrinks in seasoning, and the settlement may cause cracks in the plastering at the corner between the partition and an outside wall. In order to prevent this settlement, partitions running parallel with the floor joists are often supported on strips which are secured to the under side of the floor joists, as shown at A in Fig. 145. These strips can not be allowed to project into the room below, and so they must be made as thin as is consistent with safety. Strips of iron plate about 1/2 inch thick and wide enough to support the partition studs are, therefore, used for this purpose, and are fastened to the joists by means of bolts or lag screws. Partitions which run at right angles to the floor joists can also be supported in this way. If a partition runs at right angles to the joists near the center of their span, the tendency for the joists to sag under it will be very great, and they must be strengthened either by using larger joists, or by placing them closer together. If the span of the floor joists is large and the partition is a heavy one, it may be necessary to put in a girder running at right angles to the joists to carry the partition. In this case the partition stud will set directly on the girder, which may be a large timber, or in some cases, a steel I-beam.
Fig. 145. Partition Supported by Strips Secured to Under Side of Joist.
Fig. 146. Header and Trimmer Construction.