For ordinary conditions and for spans under 100 feet some one of the types shown by Figs. 64 to 75, will generally meet the requirements of strength and economy.
For a narrow shed or shop the shape of truss shown by Fig. 64 is the most economical, the truss proper being that portion enclosed within the points A, B, C. This truss is practically the same as that shown by Fig. 65.
For spans of from 24 to 48 feet, and with an inclination not exceeding 6" to the foot, types 66 and 67 are the most suitable.
The truss type represented by these two figures has received the name of "Fan truss." The truss shown by Fig. 65 is known as a "simple Fink truss." The truss shown by Fig. 67 differs from that in 66, principally in the inclination of the braces. The braces A, B, in Fig. 67 being inserted to brace the truss from the column to prevent racking under wind pressure. Fig. 67 should be used when the truss is supported by columns, and Fig. 66 when the truss rests on brick walls. When the roof construction demands three purlins on each side of the truss, one of the forms shown by Figs. 68, 69, 70 or 71 should be used.
Fig. 66. - Simple Fink Truss; Span, 20 to 36 ft.
Fig. 66. - Simple Fan Truss.
Fig. 67. - Fan Truss; Span, 40 to 50 ft.
The names given to these trusses are often confounded by different writers; many engineers class the French and Fan trusses with the Fink truss. The term "French" appears to be generally given to those trusses in which the tie-beam is raised in the centre. The truss shown by Fig. 71, appears to have no generally recognized name. One writer refers to it as an "English" truss. This truss is not as economical as the Fink truss, except when the inclination of the rafter is less than one-fourth pitch, on account of the great length of the inner struts.
Fig. 68. - Compound Fink Truss.
Fig. 69. - Eight-Panel French Truss.
Although Fig. 71 somewhat resembles the Queen truss, Fig. 12, it will be seen that the diagonals run in the opposite direction, the diagonals in Fig. 71 being in tension, and the verticals in compression, the reverse of the Queen truss.
Much of the economy of Fink and Fan trusses lies in the fact that most of the members are in tension and the struts are short. Comparing Figs. 70 and 71, it will be noticed that the inner strut in the former is only 1/3 as long as the strut in the latter. Another advantage of these trusses is that a partial load, as, for instance, a wind or snow load on one side of the truss never causes stresses in excess of those produced by a uniform load of the same intensity over the whole truss. As a general rule, the struts in Fink trusses are placed at right angles to the rafters, as in Figs. 68, 72 and 73, but if there are trussed purlins it is desirable to have vertical members to receive the ends of the purlins. Vertical struts are generally required in hip trusses.
Fig. 70. - Fink Truss with Vertical Struts.