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GIRDERS

Girders are classified as bearing and nonbearing according to the amount and type of load supported. Bearing girders must support a wall framed directly above, as well as the live load and dead load of the floor. Nonbearing girders support just the dead and live loads of the floor system directly above. The dead load is the weight of the material used for the floor unit itself. The live load is the weight created by people, furniture, appliances, and so forth.

Wood

Wood girders may be a single piece of timber, or they may be laminated (that is, built up) of more than one plank. The built-up girder in figure 1-13, for example, consists of three 2- by 12-inch planks. The joints between the planks are staggered. In framing, a built-up girder is placed so that the joints on the outside of the girder fall directly over a post. Three 16-penny (16d) nails are driven at the ends of the planks, and other nails are staggered 32 inches OC. As shown in figure 1-13, the top of the girder is flush with the top sill plate.

Figure 1-13.—Built-up girder.

Bridging Between Joists

Floor plans or specifications usually call for bridging between joists. Bridging holds the joists in line and helps distribute the load carried by the floor unit. It is usually required when the joist spans are more than 8 feet. Joists spanning between 8 and 15 feet need one row of bridging at the center of the span. For longer spans, two rows of bridging spaced 6 feet apart are required.

CROSS BRIDGING.— Also known as herringbone bridging, cross bridging usually consists of 1- by 3-inch or 2- by 3-inch wood. It is installed as shown in figure 1-26. Cross bridging is toenailed at each end with 6d or 8d nails. Pieces are usually precut on a radial-arm saw. Nails are started at each end before the cross bridging is placed between the joists. The usual procedure is to fasten only the top end of the cross bridging. The nails at the bottom end are not driven in until the subfloor has been placed. Otherwise the joist could be pushed out of line when the bridging is nailed in.

Figure 1-26.—Wood cross bridging.

An efficient method for initial placement of cross bridging is shown in figure 1-26. In step 1, snap a chalk line where the bridging is to be nailed between the joists. In step 2, moving in one direction, stagger and nail the tops of the bridging. Instep 3, reverse direction and nail tops of the opposite pieces into place.

Another approved system of cross bridging uses metal pieces instead of wood and requires no nails. The pieces are available for 12-, 16-, and 24-inch joist spacing (fig. 1-27, view A). You can see how to install this type of cross bridging in views B, C, and D. In view B, strike the flat end of the lower flange, driving the flange close to the top of the joist. In view C, push the lower end of the bridging against the opposite joist. In view D, drive the lower flange into the joist.

Figure 1-27.—Metal cross bridging.

SOLID BRIDGING.— Also known as solid blocking, solid bridging (fig. 1-28) serves the same purpose as cross bridging. This method is preferred by many Builders to cross bridging. The pieces are cut from lumber the same width as the joist material. They can be installed in a straight line by toenailing or staggering. If staggered the blocks can be nailed from both ends, resulting in a faster nailing operation. Straight lines of blocking may be required every 4 feet OC to provide a nailing base for a plywood subfloor.

Figure 1-28.—Solid bridging.

Placing Floor Joists

Before floor joists are placed, the sill plates and girders must be marked to show where the joists are to be nailed. As we mentioned earlier, floor joists are usually placed 16 inches OC.

For joists resting directly on foundation walls, layout marks may be placed on the sill plates or the header joists. Lines must also be marked on top of the girders or walls over which the joists lap. If framed walls are below the floor unit, the joists are laid out on top of the double plate. The floor layout should also show where any joists are to be doubled. Double joists are required where partitions resting on the floor run in the same direction as the floor joists. Floor openings for stairwells must also be marked.

Joists should be laid out so that the edges of standard-size subfloor panels break over the centers of the joists (see insert, fig. 1-29). This layout eliminates additional cutting of panels when they are being fitted and nailed into place. One method of laying out joists this way is to mark the first joists 15 1/4 inches from the edge of the building. From then on, the layout is 16 inches OC. A layout for the entire floor is shown in figure 1-30.

Figure 1-29.—Floor joists layout.

Figure 1-30.—Comp1ete layout for floor joists.