The crescent truss shown in Fig. 60 may be considered as a special type of the bowstring truss (see Section 38, Chapter II (Foundations On Compressible Soils).). It is not as frequently used, and it is not as economical a type for large spans.
This truss can be made of wood, the chords being built up of planks bent to the curve and firmly bolted together. The radials should be of wrought iron or steel rods.
26. Fig. 61 shows another type of arched truss that may be used for wooden construction, when the span does not exceed 100 feet. This truss is built on the principle of the quadrangular truss explained in Section 36, Chapter II (Foundations On Compressible Soils)., the direction of the diagonals being reversed, so that they will be in compression and the radials in tension. The lower chord must, of course, be in tension, but it can be built up of planks, bent to the curve and bolted together. It should be remembered that the strain in the lower chord terminates at the points X, X, and is transmitted through the rods to the ends of the rafters at A and B, the bracing below the points X, X being merely stay bracing. Counter braces should also be inserted as shown by the dotted lines.
♦Described in the "Engineering Record," of January 9, 1897.