LIGHT TRUSS FOR 100-FOOT SPAN. In design ing trusses of wide span such as those used to support temporary roofs of light construction, the same principle of triangulation is carried out as for very small trusses. Carpenters some times overlook this; and, in their amazement at the width of span, trying to arrange framing adequate to the task, lay out a series of mem bers not a truss at all, but a number of quadri lateral panels which offer little resistance to change of form, and which would easily be racked by an extra force acting on one side, such as a gale of wind.
Fig. 175 shows a good form for a long-span truss. For the bottom chord (tie-beam), two pieces of 9-inch by 4-inch, at least, will be neces sary. The splicing of the pieces to obtain length enough for the span will reduce thQ sectional area considerably, and must be allowed for. The
system of tension rods and struts must be car ried out carefully as to joints. The blades or rafters (top chords) should be of two pieces of 8-inch by 3-inch, blocked and bolted together at frequent intervals. The sizes of the other members are shown in the diagram. It is very much better to use iron rods for all the tension members as shown. If wood is used, straps and bolts at top and bottom of each member will be required, to hold up the weight of tie-beam and thrust of strut. If the rods are upset at the ends, and a plus thread cut upon them, slightly smaller iron can be used.
It is usual to allow on trusses of this descrip tion a camber of half an inch for every ten feet of span. Five inches may seem a lot for this truss, but is none too much when the number of joints is considered.