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Analysis of Trusses

truss, loads, weight, wind, roof and joints

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ANALYSIS OF TRUSSES; " rIETHOD OF JOINTS." 18. Trusses. A truss is a frame work used principally to support loads as in roofs and bridges. Fig. 16, 25, 26 and 27 represent several forms of trusses. The separate bars or rods, 12, 23, etc. (Fig. 16) are called members of the truss and all the parts immediately concerned with the connection of a number of mem bers at one place constitute a joint. A "pin joint" is shown in Fig. 15 (a) and a " riveted joint " in 15 (b).

19. Truss Loads. The loads which trusses sustain may be classified into fixed, or dead, and moving or live loads. A fixed, Or dead load, is one whose place of application is fixed with refer ence to the truss, while a moving or live load is one whose place of application moves about on the truss.

Roof truss loads are usually fixed, and consist of the weight of the truss, roof covering, the snow, and the wind pressure, if any. Bridge truss loads are fixed and moving, the first consisting of the weights of the truss, the floor or track, the snow, and the wind press ure, and the second of the weight of the passing trains or wagons.

In this paper we shall deal only with trusses sustaining fixed loads, trusses sustaining moving loads being discussed later.

Weight of Roof Trusses. Before we can design a truss, it is necessary to make an estimate of its own weight; the actual weight can be determined only after the truss is designed. There are a number of formulas for computing the probable weight of a truss, all derived from the actual weights of existing trusses. If W denotes the weight of the truss, 1 the span or distance between supports and a the distance between adjacent trusses, then for steel trusses and the weight of a wooden truss is somewhat Roof Covering. The beams extending between adjacent trusses to support the roof are called purlins. On these there are sometimes placed lighter beams called rafters which in turn sup port roof boards or " a/teat/a/7w " and the other covering. Some times the purlins are spaced closely, no rafters being used.

The following are weights of roof materials in pounds per square foot of roof surface: Sheathing: Boards, 3 to 5.

Shingling: Tin, 1; wood shingles, 2 to 3; iron, 1 to 3; slate, 10; tiles, 12 to 25.

Rafters: 1.5 to 3.

Purlins: Wood, 1 to 3; iron, 2 to 4.

Snow Loads. The weight of the snow load that may have to be borne depends, of course, on location. It is usually taken from 10 to 30 pounds per square foot of area covered by the roof. Wind Pressure. Wind pressure per square foot depends on the velocity of the wind and the inclination of the surface on which it blows to the direction of the wind. A horizontal wind blowing at 90 miles per hour produces a pressure of about 40 pounds per square foot on a surface perpendicular to the wind, while on surfaces inclined, the pressures are as follows: 10° to the horizontal, 15 pounds per square foot, 200 44 44 44 , 24 CC 44 44 44 300 44 44 44 , 32 44 44 (4 ,t 400 44 CC , 36 a 44 44 44 500_300 44 44 40 CC 44 41 The wind pressure on an inclined surface is practically per pendicular to the surface.

20. Computation of " Apex Loads." The weight of the roof covering including rafters and purlins comes upon the trusses at the points where they support the purlins; likewise the pressure due to wind and snow. Sometimes all the purlins are supported at joints; in such cases the loads mentioned act upon the truss at its joints. However, the roof, snow, and wind loads are always assumed to be applied to the truss at the upper joints of the trusses. This assumption is equivalent to neglecting the bend ing effect due to the pressure of those purling which are not sup ported at joints. This bending effect can be computed separately.

The weight of the truss itself is assumed to come upon the truss at its upper joints; this, of course, is not exactly correct. Most of the weight does come upon the upper joints for the upper members are much heavier than the lower and the assumption is in most cases sufficiently correct.

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