100. 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 cover=plates; 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.

Riveted Joints 030090

Fig. 58.

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 single=riveted; and one with two rows of rivets is said to be doubIe=riveted. 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.

ioi. 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 coverplates, there is a tendency to cut or shear each rivet on two surfaces (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 working 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 d2 the shearing strength of one rivet is:

For single shear,

0.7854 d2 Ss .

For double shear,

1.5708 d2 Ss .

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 connection) 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, on 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 Sc, wherein t denotes the thickness of the plate; d, the diameter of the rivet or hole; and Sc, the working strength of the plate.

103. Frictional Strength of a Joint. When a joint is subjected 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 correspondingly 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 computations on the strength of riveted joints.

104. Tensile and Compressive Strength of Riveted Plates. The holes punched or drilled in a plate or bar weaken its tensile strength, and to compute that strength it is necessary to allow for the holes. By net section, in this connection, is meant the smallest cross-section of the plate or bar ; this is always a section along a line of rivet holes.

If. as in the foregoing article, t denotes the thickness of the plates joined ; d, the diameter of the holes; n1, the number of rivets in a row ; and w. the width of the plate or bar; then the net section = (w - n1d) t.

Let St denote the tensile working strength of the plate ; then the strength of the unriveted plate is wtSt , and the reduced tensile strength is {w - n1d) t St.

The compressive strength of a plate is also lessened by the presence of holes ; but when they are again rilled up, as in a joint, the metal is replaced, as it were, and the compressive strength of the plate is restored. No allowance is therefore made for holes in figuring the compressive strength of a plate.

105. Computation of the Strength of a Joint. The strength of a joint is determined by either (1) the shearing value of the rivets ; (2) the bearing value of the plate ; or (3) the tensile strength of the riveted plate if the joint is in tension. Let P8 denote the .strength of the joint as computed from the shearing values of the rivets ; Pc, that computed from the bearing value of the plates ; and Pt, the tensile strength of the riveted plates. Then, as before explained,

Pt

=

(w - n1d) tSt;

(20)

Ps

=

n2 0.7854: d2Ss; and

Pc

=

.n3tdSc ;

n2 denoting the total number of rivets in the joint; and n3 denoting the total number of rivets in a lap joint, and one-half the number of rivets in a butt joint.

Examples. 1. Two half-inch plates 7 inches wide are connected by a single lap joint double-riveted, six rivets in two rows. If the diameter of the rivets is 3/4 inch, and the working strengths are as follows: St-= 12,000, Ss= 7,500, and Sc= 15,000 pounds per square inch, what is the safe tension which the joint can transmit?

Here n1= 3, n2 = 6, and n3= 6 ; hence

Pt= (7 -3 X )xx 12,000 = 31,500 pounds; Ps = 6 X 9.7854 X ()2 X 7,500 = 19,880 pounds; Pc=6 X X X 15,000 = 33,750 pounds.

Since Ps 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 tensio:. which the joint can bear?

Here n1 = 3, n2 = 12, and nz = 6; hence, as in the preceding example,

Pt = 31,500; and Pc = 33,750 pounds; but

Ps = 12 X 0.7854 X ()2 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 n1 = 2. n2 = 12, and n3 = 6; hence, as in the preceding example,

Ps = 39,760; and Pc = 33,750 pounds; but

Pt = (7 -2 X ) X 12,000 = 36,000 pounds.

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

Examples For Practice

Note. Use working strengths as in example 1, above. St = 12,000, Ss = 7,500, and Sc = 15,000 pounds per square inch.

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

Ans. 6,625 pounds.

2. Solve the preceding example supposing that four -inch 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.

106. 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 expressed 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 7 inches wide; hence the tensile working strength of the solid plate is

7 X X 12,000 = 45,000 pounds.

Therefore the efficiencies of the joints are : (1) 19,880/ 45,000 = 0.44, or 44 per cent;

(2) 31,500/45,000 = 0.70, or 70 per cent;

(3) 33,750/45,000 =0.75, or 75 per Cent.

HALF VIEW DOWN CENTER AISLE OF MACHINE SHOP OF WESTINGHOUSE ELECTRIC & MFG. CO., EAST PITTSBURG, PA.

HALF-VIEW DOWN CENTER AISLE OF MACHINE SHOP OF WESTINGHOUSE ELECTRIC & MFG. CO., EAST PITTSBURG, PA.

Length of building, 1,658 ft., in three bays. Crane runways in all bays. Total weight of steel work, 16,840,000 lbs.

Courtesy of American Bridge Company.

GODDARD CHAPEL, TUFTS COLLEGE

GODDARD CHAPEL, TUFTS COLLEGE.

J. P. Rinn, Architect, Boston, Mass.

The Architecture is Chiefly Romanesque; the Tower, Lombardic. Built in 1881. Cost, $44,000.