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.
This should be carefully hard soldered, and as few joints as possible used.
Longitudinal Joint is one running lengthwise. It sometimes occurs as a sort of butt joint in wide booms made up of two or three widths of plate instead of one only, the advantage of such a disposition being that this plates are longer and fewer covers required. In such situations the tensile or compressive strength of the boom is not affected by the longitudinal joint. In the manufacture of wrought iron piping, a perfect longitudinal joint, as well as a perfectly true pipe, is obtained by passing plates of iron at welding heat through properly prepared rollers. So sound is the joint thus made that the pipes required for steam purposes are often tested up to a pressure of 3,000 lbs. per square inch.
An example of this kind of joint is noticeable at the junctions of the cross-bars of Harris's patent wrought iron windows.
Mortise and Tenon Joint is similar in principle to that made by the joiner. A caulking answers for the tenon, whilst the mortise is cut by the mason. In some localities the projection, or stub, or caulking left on the end of a casting for letting into masonry is called a tenon. Holes may likewise be punched in a piece of sheet-iron, and corresponding tenons cut on the edge of another and inserted therein, to be fixed with little rivets or keys. The framework of the panels of ornamental ironwork was usually connected together by means of the mortise and tenon joint during the sixteenth century.
Pin Joint occurs between tension bars or links. The diameter given to the pin, which is ascertained by formula, will depend upon whether it will be in double or quadruple shear, & c, and it must be such as to make it as strong in shearing resistance as the least section of the bar is in tensile resistance. The width of the metal round the pin-hole, and the distance of the centre of the latter from the end of the bar; have also to be calculated so as to obtain the necessary strength with the least possible weight at the junction. In some instances the pin is retained in place by a split key passing through a slot in the tail. Bars and booms of iron girders are connected by tight-fitting turned steel pins, sometimes as much as 7 in. in diameter, inserted through accurately-bored or drilled holes, or eyes. The pins are coated with grease and forced with considerable pressure into the holes, being kept therein by means of iron castings screwed on to both ends. The advantage claimed for this form of joint in girder work over a riveted one is, that the former admits of the girder being set up at the site exactly as when temporarily united at the works. It is alleged, however, and apparently with much reason, that pins are untrustworthy, since the long members of a bridge truss in compression jointed with them are circumstanced similarly to columns with rounded ends and susceptible of motion, or, in fact, are only one-third as strong as if their joints were made rigid by riveting. Interties between girders in fireproof floors are sometimes secured by surrounding the girder with a wrought iron band, saddle, or strap, and inserting the end of the intertie into a slot made in it at a point just above the lower flange, and securing it therein with a pin clinched with a split key.