For spans of from 34 to 44 feet, queen-rod trusses should be made of the form shown in Fig. 20, with a rod at the centre if the tie-beam is loaded, otherwise it is not needed, unless there are counter-braces.

When one of the points A or B is liable to be much more heavily loaded than the other, counter-braces should be used, as indicated by the dotted lines, and a rod placed at the centre to take up the vertical component of the stress in the counter-braces. If the members d and e, h and k are made and connected so as to resist both tension and compression, the counter-braces may be omitted, but it is generally more practicable to make d and e of rods, and use counter-braces. When the loads are nearly uniform and symmetrically disposed counter-braces are not required.

8 The Howe Or Bridge Truss 30023

Fig. 20.

In this truss the loads at A and B, when uniform and symmetrical, are borne by the frame a b c. The load at C is supported by the braces h and k, and these again by the ties d and e, which transmit the vertical components of the stresses in h and k to the points A and B. The tie t resists the thrust of a and c through its entire length, and the middle portion, in addition, has to resist the horizontal components of the stresses in h and k. When the points A, B and C are symmetrically disposed with regard to the supports and there is no load on the tie-beam, the rods d and e will each be strained by an amount equal to one-half the load at C.

This type of truss may be extended by increasing the number of struts to almost any length, as shown by Figs. 21-24, although 100 feet is about the greatest practical span. The truss may also have either an even or uneven number of panels, although an even number is generally to be preferred.

Trusses of this type, with five or more panels are commonly called "Howe" trusses, although the original Howe truss was designed for bridges.

When adapted to the shape of the roof it is the most economical truss for wooden construction, for spans not exceeding 100 feet, and is a very easy truss to build. The horizontal members in these trusses are commonly called "chords," and the portion of the truss between two adjacent vertical members, a "panel."