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(7- T — 3 X X X 12,000 = 31,500 pounds; 3 6 x 0.7854 x (4 X 7,500 = 19,880 pounds ; 1 3 T 6 X x X 15,000 = 33,750 pounds.

Since is the least of these three values, the strength of the joint depends on the shearing value of its rivets, and it equals 19,880 pounds.

2. Suppose that the plates described in the preceding example are joined by means of a butt joint (two cover-plates), and 12 rivets are used, being spaced as before. What is the safe tension which the joint can bear ? Here n, = 3, = 12, and n, = 6; hence, as in the preced ing example, = 31,500; and = 33,750 pounds; but 3 = 12 X 0.7854 X X 7,500 = 39,760 pounds. The strength equals 31,500 pounds, and the joint is stronger than the first.

3. Suppose that in the preceding example the rivets are arranged in rows of two. What is the tensile strength of the joint ? Here n, = 2, = 12, and = 6; hence, as in the preced ing example, = 39,760; and = 33,750 pounds; but 3 1 Pt = (7 1 X x 12,000 = 36,000 pounds.

The strength equals 33,750 pounds, and this joint is stronger than either of the first two.

Note. Use working strengths as in example 1, above.

= 12,000, S. = 7,500, and = 15,000 pounds per square inch.

1. Two half-inch plates 5 inches wide are connected by a lap joint, with two finch rivets in a row. What is the safe strength of the joint ? Ans. 6,625 pounds.

2. Solve the preceding example supposing that four rivets are used, in two rows.

Ans. 13,250 pounds.

3. Solve example 1 supposing that three 1-inch rivets are used, placed in a row lengthwise of the joint.

Ans. 17,670 pounds.

4. Two half-inch plates 5 inches wide are connected by a butt joint (two cover-plates), and four /-inch rivets are used, in two rows. What is the strength of the joint ? Ans. 11,250 pounds.

lo6. Efficiency of a Joint. The ratio of the strength of a joint to that of the solid plate is called the "efficiency of the joint." If ultimate strengths are used in computing the ratio, then the efficiency is called ultimate efficiency; and if working strengths are used, then it is called working efficiency. In the following, we refer to the latter. An efficiency is sometimes ex pressed as a per cent. To express it thus, multiply the ratio strength of joint strength of solid plate, by 100.

Example. It is required to compute the efficiencies of the joints described in the examples worked out in the preceding article.

In each case the plate is inch thick and 7i inches wide; hence the tensile working strength of the solid plate is 1 1 7 7 X 2 X 1.2,000 = 45,000 pounds.