TO CAMBER A TRUSS. Fig. 181 is a truss, specification for which says "frame it so it will have 2-inch camber when tightened up." Now, the question is, how to get the proper location of the holes for the rods, also the lengths of each stick of timber.
The method of cambering such a truss in volves some knowledge of mathematics and drawing. Nearly all engineers are now in the habit of making the necessary calculations and figuring the exact lengths of the braces upon the drawings before they are sent out. The proper angle of the cast-iron shoes is also worked out, and a full-sized drawing of each casting is made in the drafting room.
In theory, when such a truss is cambered, the upper chord becomes longer than the bot tom. The panels will thus be out of square and the braces slightly longer in consequence. A rule for finding the increase of length has been 16' 0' ----- worked out and is as follows: To find increase in length of upper chord, put down: (all in feet or inches). Applying this rule to the problem at hand, we have: The figure 8 in the formula is a constant, and is used in all cases.
The increase found by working this very simple calculation, is divided amongst the panels. If your drawings have not been figured for camber, you will require to make a full sized drawing of one panel of the truss upon a board platform or convenient floor. The draw
ing should show the panel as much wider at the top as your calculations will direct, and the braces can then be cut to length and bevel on your drawing. Of course, the upper chord is not actually lengthened inches, all that is necessary being to cut your braces to fit the full-sized drawing of the distorted panel. In this case the distortion (out of square) is very small, as the truss is shallow. It may be taken as one-half inch for each panel; and the braces may be made five-eighths or three-quarters of an inch longer than they would be if panel were square.
This applies, of course, not to all cases, but only to the case of a simple wooden truss. It must be noted that in steel trusses, where the braces abut against machined surfaces, very exact calculations are necessary for finding the lengths of braces, and angles of bearing sur faces. In large wooden trusses, great care is also taken in this respect, although it is easier to adjust the length and cuts of the braces in this material.
The positions of bolts are obtained by spacing evenly, as shown, and should present no difficulty,