A lattic truss acts in very much the same way as a beam in supporting a transverse load. The chords resist the bending moment and the bracing transmits the load to the supports, or, in technical language, resists the shearing stress. Half of the braces are in compression and half are in tension. Uprights at the ends are necessary to receive the shear at the top and at the middle, and transmit it to the support below.
The stress in the braces is greatest at the ends and decreases to nothing at the centre, and hence the braces near the centre of the truss may be made smaller than those at the ends.
The stress in the chords, on the contrary, is greatest at the centre and decreases toward the ends, hence the centre planks should be as long as can be obtained.
Rules for Computing the Stress in the Chords and Braces.
I. Under a uniformly distributed load, the maximum stress in the chords may be found by multiplying the total load by the span and dividing by eight times the height, both in feet.
II. The stress in each of the end braces, a, a, a, when the angle of inclination approximates 45 degrees, will be one-sixth of the total load, multiplied by 1.4.
The following table gives the dimensions for lattice trusses, built as shown in Fig. 25, for five different spans, and different spacings and heights, which will cover nearly all of the conditions under which these trusses should be used. In localities where a fall of snow 2 feet in depth is liable to occur these dimensions should be increased.
Referring to Fig. 25, it may be stated that each chord of the truss is built of four 2 x io's in 10 and 20-foot lengths, the braces a, a, a, are 2" x 10", and the other braces 2" x 8". The joints at 1, 2, 3, 4, and 5 have three 1 1/8-inch bolts, the joints between 6 and 8 and 7 and 9 have two 7/8-inch bolts, while the other joints have two 1-inch bolts. There should also be two 7/8-inch bolts in the tie-beam, in each space between the joints, to assist in transmitting the tension from one plank to the other.