Strength and proportions of riveted joints

The subject of riveting is by no means so simple as might at first sight be supposed, and the author, having had much trouble in collecting and arranging the various experiments which have been published on the subject and drawing thence practical conclusions for his own guidance, thinks that other Engineers, who have neither time nor opportunity for traversing the same ground, may, perhaps, find the following paper a useful resume of our present knowledge on the subject of riveting. It was originally read at the Institution of Civil Engineers in Ireland, and the author is indebted to the Council of the Institution for their kind permission to publish the paper in a separate form from the "Transactions,"

Double-riveted Lap joints.

The unit-strength of the plate in a double-riveted is considerably higher than in a single-riveted lap joint, in consequence of there being fewer perforations in each transverse row of rivets, and also in consequence of the double rows keeping the joint from bending through so great an angle as the single-riveted joint, and the average efficiency of double-riveted lap joints, with iron plates not exceeding 5/8 inch in thickness and designed for strength (not for steam-tightness), is probably about 60 per cent, of that of the solid^ plate, as shown by the experiments in Table XVI.

The efficiency of double-riveted lap joints with thick iron plates, however, probably does not exceed 55 per cent, of the strength of the solid plate.


Butt joints Crushing Pressure of Rivets.

Single-covered butt joints, with covers of the same thickness as the plates, are merely modifications of lap joints, and the few experiments on the subject indicate that their efficiency is nearly, but not quite, the same as that of similarly riveted lap joints. If, however, the cover of a single-covered butt joint be made a good deal thicker than the plate, the bending of the joint will be restrained by the stiffness of the cover, and the strength of the joint will be somewhat increased. The Board of Trade rules for marine boilers specify that, "when single butt-straps are used and the rivet holes in them punched, they must be one-eighth thicker than the plates they cover," and that when double butt-straps are used they should be at least 5/8ths the thickness of the plates they cover. Professor Unwin states that in some experiments butt-straps of equal thickness with, the plates proved weaker than the latter, and he recommends that single butt-strips should be made 1 1/8 times the thickness of the plate and double butt-straps, each f ths of the thickness of the plate. Mr. T. Aveling, who has had much experience in boiler-making, states that when the strap for butt joints with single covers was made 25 per cent, thicker than the boiler plate itself, that would be found to be the best method of forming the longitudinal seams, as well as of coupling the rings of the boiler together. In double-covered butt joints the rivets are in double shear, and the bearing pressure of each rivet against the central plate is twice as great as in a lap joint, provided the shearing unit-stress of the rivet be the same in both cases. In lap joints this bearing pressure rarely exceeds 30 tons per square inch when the joint is tested to fracture that is, assuming that the pressure is uniformly distributed over the bearing surface, which in reality is not the case on account of the bending of the joint (see 6), and there seems some reason for supposing that when the bearing pressure of a rivet in single shear much exceeds this, the tenacity of the plate is reduced. There is also reason for supposing that when the bearing pressure of a rivet in double shear that is, its bearing pressure against the middle plate of a double-covered butt joint, exceeds 40 tons at the crisis of fracture, the tenacity of the plate is somewhat reduced, and Professor Kennedy concludes from his experiments that the shearing strength of rivets is also reduced by severe bearing pressure. However this may be, the efficiency of a well proportioned single-riveted butt joint with double covers is, judging from the few experiments on the subject, somewhat less than that of a double riveted lap joint say 55 per cent, of the strength of the solid plate, and that only when the rivets are pitched so that the joint will break indifferently by shearing the rivets or tearing the plate. The efficiency of double-riveted and double-covered butt joints, if well proportioned, may probably be calculated at about 66 per cent, of the strength of the solid plate, unless the plates exceed f inch in thickness, when the efficiency of the joint will be somewhat less. The joints in the tension flanges and tension bracing of girders are generally triple or quadruple-riveted, and the transverse pitch of the rivets generally ranges from 3 to 6 inches, so that the efficiency of the joints varies according to the design; but it can scarcely exceed 80 per cent, of the strength of the solid plate, and hence it happens in girder-work that the increased width of flange in consequence of the rivet holes adds seldom less than 20 per cent., and sometimes much more, to the theoretic weight of a tension flange that is, its weight calculated on the imaginary hypothesis that it is made of solid iron without joints or perforations. As the covers are gene- rally triple or quadruple-riveted, their length is considerable, and their weight forms an important addition, in many cases over 12 per cent., to that of the theoretic solid flange.

Contraction of Rivets and resulting Friction of Plates. Rivets contract in cooling and draw the plates together with such force that the friction produced between their surfaces is generally sufficient to prevent them from sliding over each other so long as the stress lies within limits which are not exceeded in ordinary practice, and in this case the rivets are not subject to shearing stress.