TRUSSES UNDER DEAD AND LIVE LOADS 39. The Pratt Truss. The Pratt truss is used to perhaps a greater extent than any other form; probably 90 per cent of all simple truss spans are of this kind.
Let it be desired to determine the stresses in the 8-panel 200-foot single-track span shown in Fig. 42, the height being 30 feet, the dead panel load being 30 000 pounds, and the live panel load 62 400 pounds. The secant is ,( 2b _ 30 = 1.302, and the cosine is 0.7685. The dead-load reaction is 31 X 30.0 = 105.0. The dead-load shears are: V, _ +105.0 V,=+75.0 V, = + 45.0 V, — + 15.0 V, = — 15.0 The dead-load chord stresses may be tabulated as follows (see Articles 27 and 29) : • In determining dead-load stresses in web members, it is cus tomary to assume one-third of the dead panel loads as applied at the upper chord points. This, as will be seen, makes no difference in the stresses in the chords or in the diagonals, the stresses in the verticals only being different from what is the case when all the dead load is taken on the lower chord.
The stresses in the diagonals (see Articles 27, 28, and 30) are: points of the upper chord, and two-thirds (or 20.0) is at the lower chord. The stress in the hip vertical U,L, is determined by passing a circular section around T. It is solved thus: In order to find the stress in the remaining verticals, sections 1 — 1 and 2 — 2 are passed, cutting them, and the shears on these sections computed. The shears are: +105.0-2 X20-1 X 10= +55.0 V,_, = + 105.0 - 3 X 20 - 2 X 10 = +25.0 The stress equations are written, remembering that as the verticals make an angle of zero with the vertical, their cosine is equal to unity. These equations are: