mo. Kinds of Joints. A lap joint is one in which the plates or bars joined overlap each other, as in Fig. 58, a. A butt joint is one in which the plates or bars that are joined butt against each other, as in Fig. 58, b. The thin side plates on butt joints are called the thickness of each is always made not less than one-half the thickness of the main plates, that is, the plates or bars that are joined. Sometimes butt joints are made with only one cover-plate; in such a case the thickness of the cover-plate is made not less than that of the main plate.
When wide bars or plates are riveted together, the rivets are placed in rows, always parallel to the " seam " and sometimes also perpendicular to the seam; but when we speak of a row of rivets, we mean a row parallel to the seam. A lap joint with a single row of rivets is said to be ; and one with two rows of rivets is said to be A butt joint with two rows of rivets (one on each side of the joint) is called "single-riveted," and one with four rows (two on each side) is said to be "double riveted." The distance between the centers of -consecutive holes in a row of rivets is called pitch.
[oz. Shearing Strength, or Shearing Value, of a Rivet. When a lap joint is subjected to tension (that is, when P, Fig. 58, a, is a pull), and when the joint is subjected to compression (when P is a push), there is a tendency to cut or shear each rivet along the surface between the two plates. In butt joints with two cover plates, there is a tendency to cut or shear each rivet on two sur faces (see Fig. 58, b). Therefore the rivets in the lap joint are said to be in single shear ; and those in the butt joint (two covers) are said to be in double shear.
The "shearing value" of a rivet means the resistance which it can safely offer to forces tending to shear it on its cross-section. This value depends on the area of the cross-section and on the work ing strength of the material. Let d denote the diameter of the cross-section, and S, the shearing working strength. Then, since
the area of the cross-section equals 0.7854 the shearing strength of one rivet is : For single shear, 0.7854 S, .
For double shear, 1.5708 5, .
102. Bearing Strength, or Bearing Value, of a Plate. When a joint is subjected to tension or compression, each rivet presses against a part of the sides of the holes through which it passes. By " bearing value" of a plate (in this co.inection) is meant the pressure, exerted by a rivet against the side of a hole in the plate, which the plate can safely stand. This value depends on the thickness of the plate, oil the diameter of the rivet, and on the compressive working strength of the plate. Exactly how it depends on these three qualities is not known; but the bearing value is always computed from the expression t d wherein t denotes the thickness of the plate; d, the diameter of the rivet or hole; and the working strength of the plate.
103. Frictional Strength of a Joint. When a joint is sub jected to tension or compression, there is a tendency to slippage . between the faces of the plates of the joint. This tendency is overcome wholly or in part by frictional resistance between the plates. The frictional resistance in a well-made joint may be very large, for rivets are put into a joint hot, and are headed or capped before being cooled. In cooling they contract, drawing the plates of the joint tightly against each other, and producing a great pressure between them, which gives the joint a correspond ingly large frictional strength. It is the opinion of some that all well-made joints perform their service by means of their frictional strength; that is to say, the rivets act only by pressing the plates together and are not under shearing stress, nor are the plates under compression at the sides of their holes. The " frictional strength " of a joint, however, is usually regarded as uncertain, and generally no allowance is made for friction in com putations on the strength of riveted joints.