which might occur from the plate being of inferior metal to the rivets, or from the rivets being too closely spaced. If the rivets were to shear, and the plate remain intact, it would indicate insufficient strength in the rivets, which might occur from faulty material in the rivets, from smallness of rivet diameter, or from the rivets being too widely spaced. The object then, in designing a rivetted joint, is to have its resistance

to tearing and shearing proportionately equal, whatever form of joint be employed, and by proportionately equal is meant equal, taking into account the before mentioned fact that the rivet is liable to shear, if the joint works at all, and an extra allowance of rivet diameter is necessary to meet this contingency.

If it were necessary to increase the percentage of the strength of the rivetted seam to that of the solid plate, and obtain a higher calculated bursting pressure in the example we have been considering, we should have to increase the pitch of the rivets, leaving a larger percentage of plate between the rivets as compared with that cut away by the drill, and to correspondingly increase the percentage of rivet section by employing a second row of rivets, or in other words, employ a double rivetted joint, such as shown in Figs. 13 and 14, which represent respectively, a double rivetted joint zigzag rivetted, and a double rivetted joint chain rivetted.

By double rivetting the joint it is made stiffer, and the pitch may be increased, thus giving a larger plate percentage, while at the same time

the rivet section is obviously increased, hence the whole joint is strengthened.

Suppose, for instance, that we pitch the rivets at 2 from centre to centre, and use rivets 1 inch in diameter instead of, and with the same inch plate double rivetted we shall find, by calculation, that we get a plate section between the rivet holes of 70 per cent. of the solid plate, and a rivet section of 75 per cent., and have therefore increased the calculated bursting pressure by 20 per cent. by employing a second row of rivets and an increased pitch.

There is, however, a limit to the distance apart at which we may pitch the rivets, viz., that at which we can efficiently caulk the joint without a possibility of the plate springing, which of course is governed by the thickness of the plate.

When dealing with thin plates there is no difficulty in the way of obtaining a sufficiently high percentage of joint strength with the ordinary double rivetted lap joint, but when dealing with thick plates, and particularly from one inch upwards, a more costly joint must be employed, in order to obtain the required percentage of strength at the joint, hence the ordinary double rivetted joint is replaced by various other forms, as follows:

1st. A treble rivetted lap joint, such as in Fig. 15, the third row of rivets enabling the rivet pitch to be increased, which increases the percentage of plate left between the rivet holes, while at the same time the percentage of rivet section is increased.

2nd. By employing butt joints with butt straps either double or treble rivetted.

A double rivetted joint with double butt straps is shown in Fig. 16, and a treble with double butt straps in Fig. 17.

3rd. By various arrangements of the rivets in conjunction with butt

joints and double butt straps, with which it is not necessary, at this point, to deal.

о

--p

E

О

Fig. 16.

One of the great advantages obtained by the use of the double butt strap is that of bringing the rivet into double shear (or in other words, the rivet must shear on each side of the plate, or in two places, instead

of between the plates only, before the joint can give way by shearing), and thus obtaining an increased calculated strength of 12 times the or dinary or single shear, the rule being to find the rivet strength in the ordinary way (as before explained), and then multiply the result by 1.75.

The Board of Trade rules* for spacing the rivet of these joints, are as follows:

THE SPACING OF ORDINARY

CHAIN AND ZIGZAG RIVETTED JOINTS.

Iron plates and iron rivets, or steel plates and steel rivets.

To find dimension E multiply the diameter of the rivet by 3 and divide by 2, thus: 3 x d

2

E

*See appendix to Board of Trade Instructions, issue of 1886.

To find the distance V between the rows of rivets in chain. rivetted joints. This distance must not be less than twice the rivet diameter, and a more desirable rule is four times the rivet diameter plus 1 divided by 2, thus:

4 d + 1

2

= V

To find the distance between the rows of zigzag rivetted joints:

or multiply 11 times the pitch plus 4 times the rivet diameter, by the pitch plus 4 times the rivet diameter, then extract the square root and divide by 10.

To find diagonal pitch pd multiply the pitch p by 6, then add 4 and divide by 10, thus: 6p + 4 d