Fig. 240.
The Bowstring Girder.—Only one other form of bridge truss need be mentioned here, and that is the bowstring shown in Fig. 240.
In this type the bow receives the entire compression thrust, and the chords act merely as suspending members.
Fundamental Truss Form.—In every form of truss, whether for building or for bridge work, the principles of the famous A-truss must be employed in some form or other; and the boy who is experimentally inclined will readily evolve means to determine what degree of strength the upper and the lower members must have for a given length of truss to sustain a specified weight.
There are rules for all these problems, some of them very intricate, but all of them intensely interesting. It will be a valuable addition to your knowledge to give this subject earnest study.