If the joint is like those shown in Figs. 15, 16, and 17, the rivet is said to be in single bear ing in plate or t. It is also said to be in single bearing in plate Evidently, the thicker the plate, the larger the value of one rivet, and the fewer rivets are required to sustain a given total stress. Therefore it is only necessary to ex amine a joint for the number of rivets required in bearing in the thinnest plate, and this number will evidently be amply sufficient for the thicker plate.
It often happens that a joint is as in Fig. 13 (b), the two side plates being of the thickness and the intermediate one being of a thickness t2. In many cases and are equal. In any case the rivet bears in each plate and in plate also. In such a case the rivet is said to be in single bearing in and in double bearing in t1.
Just as in the case of double shear, the value of a rivet in double bearing is double that of its value in single bearing in the same plate. Also, it is determined in a similar manner. Thus the value of a rivet in double bearing in a plate (the unit bearing stress being taken as 25,000 pounds), is 2X8,210=16,420 pounds. See 4th table (C 195).
In making riveted joints, the hole is made slightly larger than the rivet. The rivet is then heated to a welding heat, and driven by a pow erful machine which holds the heads tightly against the plate until the rivet cools. As the rivet is at a welding temperature when it has the other head formed, it soon cools to the tem perature of the surrounding air, and, in doing so, contracts. If it cooled to 75 degrees the contrac tion would be so great as to cause a tensile stress in the rivet of over 30,000 pounds per square inch; and in the case of a rivet, whose section is 0.613 square inch, the total stress would amount to over 0.613X30,000=18,390 pounds. This pulls the two heads toward each other, and, in doing so, clamps together very tightly the plates which are riveted together. The plates, according to tests made, are indeed clamped together so tightly that it takes a considerable force to start them slip ping over each other, even when the rivets are not large enough to fill the holes.
The design of riveted joints will be taken up later.