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1-5. Load Area

Both the foundation walls and the girder carry the load area of a building. Because the ends of each joist rest on the girder, there is more weight on the girder than on either of the wall.

Example 1. Before considering the load on the girder, consider the weight of a single joist. Suppose that a 10-ft board weighing 5 pounds per foot is led by two men. If the men are at opposite ends of the plank, they would each be supporting 25 pounds (see Figure 1-10 ).

Figure 1-10. Example of weight on a single joist

Example 2. Now assume that one of these men lifts the end of another 10-ft board with the same weight as the first one, and a third man lifts the opposite end. The two men on the outside are each supporting half the weight of one plank, or 25 pounds apiece, but the man in the center is supporting one half of each of the two boards, or a total of 50 pounds (see Figure 1-11 ).

Figure 1-11. Example of weight on a girder

The two men on the outside represent the foundation walls. The center man represents the girder. The girder carries half of the weigh and the other half is equally divided between the outside walls. However the girder may not always be located halfway between the outer walls.

Example 3. Imagine the same three men lifting two planks that weigh 5 pounds per foot. One of the planks is 8 feet long and the other is 12 feet long. The total length of these two planks is the same as before. The weight per foot is the same, so the total weight in both cases is 100 pounds.

One of the outside men is supporting half of the 8-foot plank, or 20 pounds. The man on the opposite outside end is supporting half of the 12-foot plank, or 30 pounds. The man in the center is supporting one half of each plank, or a total of 50 pounds. This is the same total weight he was lifting before. It is important to remember that a girder carries the weight of the floor on each side to the midpoint of the joists which rest upon it.

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