This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Figures 213 to 219 are best adapted to wood, or wood and iron construction ; while Figures 220 to 228 are best adapted to iron construction, they being on the principle of a pair of inclined trussed beams tied together at one point and there taking up the horizontal thrust due to the inclination. Sometimes, if more convenient for securing roof beams, etc., the principals are made of wood, the balance of truss usually being of iron.
1 here are, of course, many different truss designs, based on those given in Figures 213 to 228 and they can be similarly analyzed.
Figure 229 shows a " Howe " truss with six bays, it will present no difficulty in analyzing.
The central vertical member is not needed unless the weights are placed along the bottom chord, in which case it will be needed to transfer the load to the top, and all the other verticals will be increased over the strains shown in diagrams by the amount of their respective loads on bottom chord.
This truss is frequently drawn and used upside down from that shown in Figure 229, in which case all the strains will be reversed (see Figure 230), those in compression in Figure 229 becoming tension in Figure 230; and those in tension in Figure 229 becoming compression in Figure 230. It will also be found that the central vertical member is needed, if the loads are placed along the top chord. If the loads (in the reversed truss, Figure 231) are placed along the bottom chord the central vertical will not be needed, and as the loads will be taken up to the top chord by means of the slanting ties, it is better in this case to make a strain diagram with the load arrows in their right place and as shown in Figure 231. The end loads of this and subsequent trusses have been omitted, as they do not affect the strain diagrams, and unnecessarily increase the number of letters used. It will be noticed that the entire strain diagram and each part, is the reverse of what it was in Figure 229. There is, however, no difficulty in analyzing this truss.
In Figures 232 and 233 we have two examples of the "Warren" truss.1 Here, too, it will be seen that the reversing of the truss reverses all the strains. If the loads were along the bottom chords we should have to draw the arrows in their proper places. Figure 235 gives an example. In Figure 234 we have an example of a " lattice "truss. This cannot be analyzed unless we divide it into two reversed Warren trusses as shown in Figures 232 and 233. We analyze each of these separately, and then imagine them laid over each other, adding together the separate strains, where they cover.
If there were vertical members in Figure 234, we should analyze it by dividing it into two reversed Howe trusses, like those in Figures 229 and 230. In this case our widths of panels would be the same as in the original truss, and we should use all the verticals in both trusses, the loads therefore to be assumed on each of the joints of the divided or part trusses should be only one-half of the original loads. The "Whipple" truss is on the Howe truss principle with verticals bisecting the diagonals. This truss can he analyzed same as the Warren latticed truss, by dividing it into two halves, each having every other vertical and every other diagonal, and consequently a full load on each joint. Arched trusses are usually built up of a series of panels formed on the Howe or Warren or latticed truss principles, the only difference being that their top and bottom chords instead of being horizontal,
1 The true "Warren" truss usually baa the diagonals drawn at 60 inclination.
Fig. 232 a.
Fig. 233 a.
are made up (or assumed to be made up) of a series of straight lines at different inclinations. They are analyzed without difficulty, where they do not have horizontal tie-rods or abutments to take up their horizontal thrusts.
Figure 236 is an example of an arched truss built on the Warren principle. It will be noticed that the struts B F and F 0 are shown as if coming to a point. If this were done in practice the truss in all probability would not have sufficient bearing. We should, therefore, enlarge the foot of truss, by means of heavy plates and angles, but our calculation will have to be made as drawn, or we will find the analysis of the truss impossible Figure 237 shows a latticed arched truss, built upon the Howe principle. This we separate into two reversed Howe trusses, Figures 238 and 239 and find no difficulty in analyzing the strains. We must remember, however, to use only half loads at each joint.