The relative strength of joint to solid plate expressed in the form of a percentage will be equal to pitch - diameter of hole / pitch X 100.
And using the example for 1/2 in. steel plates, already calculated, the strength of the joint will be -
2 - 9 / 2 x 100 = 55 per cent.
The strengths of all kinds of joints can be multiplied out in a similar manner. Generally, drilled joints, on calculation, show up about 5 per cent. stronger than punched plates; but, practically, this percentage does not represent the difference in value between the joints, as with drilling there is no need for drifting holes, and consequently no local stresses are set up in the plates.
Butt-joints, with double straps, are the strongest form of joint, the strength of a treble-riveted joint of this description being about 90 per cent. of the solid plate. For longitudinal seams in a boiler, this class of joint also has the advantage of the plates pulling directly on the rivets and not obliquely as with a lap-joint. On account of the uncertainty of the stresses that are set up in a lap-joint, it is questionable whether it ought ever to be used in the longitudinal seams of a boiler.