This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Brace Joint is met with notably at the junction of cross-bracing with screw piles, columns, etc, in wind-bracing to roofs, etc, and may be either riveted, keyed, or bolted according to circumstances. The bracing between the booms of main girders, which are usually of wrought iron, is generally attached by riveting, though pinned joints are not uncommon.
This will be found described under the Coppersmiths' Joints.
In riveted and bolted structures broken joints are supposed to impart additional strength.
In building up girders, etc, a broken joint consists in allowing no two plates or angles to approach coincidence in any cross section. When the butts of all the members occur in the same section the butt is called a full butt, but there is a difference of opinion as to the propriety of such a joint, yet its advantage is obvious in enabling heavy girders to be completed in detached portions. Though broken joints give, as a rule, an augmentation of rigidity, it does not necessarily follow that, because the joint is the weakest part, the weakness is diminished by transferring part of the joint to another cross section taken at random. As a curious instance of broken joints, it may be mentioned that the tie-bolts of cofferdams, instead of traversing the whole thickness, have been made to break joint by being bolted to an intermediate row of piling in order to lessen the orifices by which water invariably and annoyingly finds its way to the interior.
This term is applied in riveted iron work when the ends of plates, etc, are placed against each other and fished with a cover either on one or both sides to keep them in position. If the joint is in tension and the cover plates are on both sides the rivets will be in double shear, and they need not be so numerous nor the plates so long as when only a single plate is used. In girder work the rivets of cover plates of joints in tension may be advantageously reduced in number towards the ends and centres of the covers. The weakest cross section of any prism is of course that which is most perforated, and in case of a main or cover plate tearing away along this section, if there be on each side of it other rivets presenting sufficient shearing resistance, and so situated by a gradual reduction in the number per row as not to equally weaken any other section, there will not only be a considerable margin of strength left, but the weakest part of the joint will not be through the weakest section of either main or cover plate.
Much skill and nice calculation are, however, required in spacing and distributing the rivets in a butt joint in tension, whereas superior workmanship is the chief desideratum when compressive stress has to be borne, in order that the pressure may pass evenly along from one main plate to the other without the slightest break, either at the rivet holes or ends. The latter of these must be planed square to the direction of the pressure and fair and true to gauge, and not too short or they will be open. As this Utopian perfection, however, is well-nigh unattainable, some engineers elect to design joints in compression as if they were going, in fact, to be racked by tension. Further notice is taken of this joint under the heads of Caulked, and Double Riveted, and Single Riveted, whilst the term full butts has been explained under Broken Joint. Fig. 76 represents a butt angle joint, secured by a bent bolt riveted to one plate, and screwed up with a nut to the other. A butt joint in tubing requires hard solder (explained under Brazed Joint) to be durable.
Cast Iron Joint, as its name implies, occurs between cast iron elements, but the term, though useful enough, is not often met with. As a rule, the surfaces forming it wherever transmitted pressure passes through them should be faced truly parallel by planing or chipping and filing, to insure equal bearing throughout. Broken castings can in many instances be repaired by soldering, as elsewhere explained.