Fig. 163.

Butt joint with cover-plates.

Two cover-plates best.

5 each 7/8 inch rivet of 5/f and if f= 5 (or a factor-of-safety of 5 were used) it would add just one ton to the calculated strength of a 7/8 inch rivet. This increase, however, is a doubtful one, and would probably be lost in time by a gradual wearing of the plates due to this very friction, or possibly from slight rusting or other causes; no allowance should therefore be made for extra strength due to friction, but it certainly is a great advantage in making joints ; and the above facts may account largely for the discrepancies in experiments on riveted joints, where no allowance is made for friction.

Cain by Friction girders and riveted joints in trusses the rule is not to make the pitch less than 2 1/2 diameters for punehed-work, nor more than sixteen times the thickness of the least plate at the joint, or : p = 16.t

Fig. 166..

Fig, 167.

The pitch of rivets is, of course, governed by circumstances. The rule is to try to arrange the rivets, so that the strength of the plate between them shall equal the actual strength of the rivet.

In boiler-work, however, they must be located not only for strength, but must be placed close enough to make the joint steam-tight. For this reason, too, boilerplates are always lapped, the joint being more easily caulked and made tight.

In constructional work, however, there will be a great loss and waste of material, if the rivets are placed too closely. In plate

Boiler Riveting.

(107)

Where p = the greatest pitch, in inches, for rivets of plate-girders or riveted trusses. Where t = the thickness of the thinnest plate, in inches. The pitch is measured from centre of hole to centre of hole on a direct (straight") line.

Greatest Pitch.

p2 = 2 1/2.d

(108)

Where p1 = the least pitch, in inches, for rivets of plate-girders or riveted trusses. Where d = the diameter of rivet-holes, in inches. The exact pitch must be between these two limits ; and is, of course, calculated.

Different writers have attempted to lay down exact rules for the size of rivets to be used, using for a basis for the formulae the thickness of thinnest plate to be riveted. Such rules, however, generally do not agree with good practice, as they either make the rivets too small for thin plates, or too large for thick ones, or vice versa.

As a rule the local circumstances must control the selection of the size of rivet; the following, however, may serve as a general guide:

Least Pitch.

Diameter of rivets.

 For plates from 1/4" to 7/16" thick, use rivet-holes 5/8" diameter. For plates from 1/2" to 5/8" thick, use rivet-holes 3/4" diameter. For plates from 11/16 to 13/16" thick, use rivet-holes 7/8" diameter. For plates from 7/8" to 1" thick, use rivet-holes 1" diameter.

Of course, larger or smaller rivets can be used, but as a rule 5/8 inch, 3/4 inch, and 7/8 inch are most desirable.

Figure 167 shows the different ways in which riveted-work will yield. This Figure is made from a photograph of an actual specimen, tested and torn apart at the Watertown arsenal.

It is evident that the plate began yielding by all of the rivets compressing or crushing the plates, and finally yielded completely by tearing apart from 2 to I and to left edge and the same from 1 to 5 and the right edge. while rivet 3 tore its way completely out, shearing off a piece of the plate, and rivets 2 and 4 partially The iron plates tested were 15 inches wide. 1/4 inch thick, with two iron cover-plates 15 inches x 3/16 inch each. The rivets were 15/16 inch of iron and filled 1 inch drilled holes, pitch 3 inches.

How plate yields.

The gross area of plate was 3,765 square inches, the net area

2,510. The total bearing-surfaces of rivets on the plates aggregated

1,255 square inches, and their aggreoate shearing areas, (being in double shear) was 7,854 square inches. At 116715 pounds strain the edges contracted and scale on the specimen began to start and the plate yielded as shown at 167200 pounds strain. This was equal to 44410 pounds tension per square inch on the uncut plate, or 66610 pounds per square inch on a line at the rivet-holes (or net area).

The compression from the rivets was 133230 pounds per square inch, while the shearing was only 21290 pounds per square inch.

This example shows clearly how the plate yields. Besides this the joint might yield by breaking or shearing off the rivets. We have then the following six manners in which a riveted-joint might yield.

1. By crushing either the rivet or the rivet crushing the plate.

2. By shearing off the rivet - in single shear.

3. By shearing off the rivet - in double shear.

4. By bending or cross-breaking of the rivet.

5. By tearing the plate apart or crushing it between rivet-holes.

6. By the rivets shearing out the part of the plate between them and the edge.

In Figures 168, 169 and 170 are shown three kinds of joints, each with a single rivet transferring the whole strain; in Figure 168 directly from plate A to plate B; in Figure 169 transferring strain from plate A to cover-plate and thence to plate B; and in Figure

Fig. 168.

Fig 169.

Fig. 170.

How Riveted joint yields.

170 transferring one-half of the strain A to each cover-plate and thence each half is transferred back again to plate B.

It should be remarked here that cover-plates (as in Figure 169) should be at least the full width and thickness of the original plates. In practice they are made a trifle thicker (about 1/16 inch or more).