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.
In girder work, webs ought to be made to abut fairly against flanges. Where cast iron arched ribs are cast in segments, the abutting flanges of the joints should be truly planed, or chipped and filed and scraped to bear throughout the whole surface thus rendered fair and true, and which, of course, ought to be strictly at right angles to the curve of the rib or line of maximum pressure. In some instances the surfaces are payed over with asphalte previous to bolting together, and the lower arris chamfered off to obviate bearing on the edge. Sometimes the segments of cast iron ribs abut upon intervening plates or cross girders, which pass through the abutting joints of the parallel ribs lying in the same plane. In such instances, and in others where struts occur between the segments of the top boom or compressive flange of a girder, all the abutting surfaces of contact should be accurately and truly planed, or turned if of circular section. Cross plates, as well as the bed plates of piers and abutments, may have fillets cast on them to hold keys or wedges to secure by their careful adjustment an even bearing between the segments acting as voussoirs, but a correctly radiating and true and parallel facing to their abutting ends is a more essential expedient, and dispenses with such an artifice.
Adjustable Joint is a contrivance or union of parts used for adjusting the lengths of ties and struts after the trusses they belong to have been fully weighted, etc. A cottered joint, explained further on, is one variety. Another kind consists in connecting screwed ends by means of a nut, Fig. 67, having right and left hand threads cut in it at opposite ends, and sufficiently long to receive the extremities of the parts of the braces to be united, which have screw threads cut on them to fit the nut, the turning of which draws together or separates them as desired. Another great advantage of screwed ends is that they form excellent substitutes for welds, which unfortunately can never be relied upon. The kind of screw-shackle just described is called a box nut, but other forms are made use of. Where a compressive force only has to be encountered, the length of the strut may be adjusted by making it in two parts, one of which is terminated by a long screw carrying a stout nut, as in Fig. 68, whilst the other part has one end bored out to a sufficient depth to receive the threaded end as far as the nut, whose position can be altered at will, allows it to enter. Another variety is described under Double Nut Joint.
This is obtained between the bearing plate and bed plate of a girder, or arched rib, or ponderous iron principal, by inserting between them in a cylindric seating a cast iron or steel roller at right angles to the longitudinal axis, as shown in Fig. 69, the arrangement for expansion being supposed to be at the other end. The same motion can be equally well obtained by making the pin and either plate in one casting, as represented in Fig. 70. The joint thus formed provides for the tilting action set up at the bearings through the deflection of the girder, etc, under its load. The friction or bearing rollers in this figure give a sliding motion in addition to the springing motion arising from the hinging, and though it illustrates the application of the joint to a girder, the same principle is quite applicable to a roof truss.
The bed plate under the rollers must be fixed with care to insure an even bearing, and firmly secured to the stone landings or other capping of the pier, or template on the wall, whilst the bearing plate that rests on the rollers requires to be ground or otherwise rendered smooth and truly horizontal.