ABSTRACT
The calculation of buttresses, retaining walls, arches, girders, trusses, and such constructions which resolve themselves into statical problems, may be accomplished in two ways: By pure mathematics; by graphic statics. The graphic method gives results varying according to the accuracy and sensibility of the mathematical drawing instruments used, together with the carefulness and skill of the operator; and provided these conditions are absolutely satisfied, correct results would be obtained in a very short time. The bars of a system are probably most surely and satisfactorily determined to be in tension or compression by taking each of the reciprocal figures of the nuclei of the frame polygon, separately from the stress diagram, making the arrows on the forces the same direction about the figure. Roof and other trusses must be designed to support and resist dead and live loads. Usually the weight of the covering and the truss constitutes the former, and the pressure of the wind the latter.
