This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Trusses subjected to wind, acting perpendicular to the gable, should be braced with diagonal braces connecting the several trusses. If, however, the roof sheathing is of planking secured directly to the trusses, and especially if run diagonally, other bracing may not be required. If stone gables protect the roof in a longitudinal direction, lateral bracing may in many cases be omitted.
The vertical and wind loads of a truss may be figured from Tables I, VII, pages 62, 68. The weight of the truss is the most uncertain factor, being practically unascertainable until the design is made; hence, it is well, after designing the truss, to calculate and compare its weight with that assumed.
The stresses being determined, the members must be proportioned to sustain them. Roof trusses differ from bridge trusses in that the loads are generally statical and not suddenly applied; consequently, a smaller factor of safety may be employed. It is the practice, in ordinary construction, to employ for timber members in a roof truss a factor of safety of from 4 to 6; and for structural steel and wrought iron, 3 to 4; cast iron is seldom used in roof trusses, and is never used with a factor of safety less than 10, unless the load creates compressive stress only, in which case a somewhat smaller factor may be used. The factor of safety is much a matter of judgment, and may be altered as the designer's experience dictates.
Tension members are liable to fail at the least cross-section; therefore, the screw ends of long rods and bolts should be enlarged, so that the cross-section at the root of the threads will be at least as large as elsewhere. Allowance
Fig. 49. (a)
Fig. 49. (6) must be made for diminution of cross-section by bolt or rivet holes. To determine the net section required in any tension member, divide the stress upon the member by the allowable tensile stress of the material.
In proportioning compression members, the length of which is not over from 8 to 10 times the diameter or least side, the cross-section maybe figured by dividing the stress by the allowable direct unit compressive stress. If, however, the length exceeds these dimensions, the members must be regarded as columns, and figured as such. If a member is subjected alternately to compression and tension, its section should be somewhat increased over that required to sustain either stress.
Where the rafter members of a truss support purlins between the panel points, the members are subjected to bending, as well as to direct compressive stress. In designing them, it is customary, with wooden members, to propor-tion them by first determining the proper size of beams to sustain the transverse stress, and adding sufficient sectional area, by increasing the width, to sustain the direct compres-sive or tensile stress, as the case may be. Where the direct stress is compressive, it should be also checked by applying the column formulas. Structural-steel or wrought-iron members are usually proportioned for the bending stress, the direct stresses being provided for by increasing the thickness of the rolled section, care being taken that the sum of the extreme transverse-fiber stress and direct-fiber stress is not more than the safe stress of the material.
Where members are connected by bolts or pins, the pins are usually proportioned to sustain the transverse stresses, and, if so proportioned, generally have sufficient resistance to shearing. Strength of pins is treated on pages 133-137, and the information given there will be found sufficient. Since the direct stresses in members connected at a common joint by a pin creates bending stresses upon the pin, the members should be packed closely, and those members having opposite stresses brought in juxtaposition if possible.
In designing any structure, and especially roof trusses, the following points should be carefully observed: (a) Proportion all parts of a joint so that the maximum strength will be realized throughout, in order that one part will not be likely to yield before another. (b) Weaken as little as possible the pieces connected at a splice, (c) Give sufficient bearing surface to bring the compression on the surface well within the safe limits, so that there will be no danger of crippling plates or crushing the ends of members before their maximum strength is realized, (d) Distribute rivets and bolts so as to give the greatest resistance with the least cutting away of other parts. (e) See that the central axis of every member coincides as nearly as possible with the line of action of the stress. (f) Examine all sections and parts for tensile, compressive, transverse, and shearing stresses, (g) Finally, bear constantly in mind that the strength of a structure depends on the strength of its weakest part, and that the failure of a single joint may be as fatal to the life of a structure as though a member were insufficient.
A close study of tne details of the trusses shown in Figs. 47, 48 and 49, in connection with the foregoing explanations of principles, will give a general idea of how such parts are designed.