Before we can show the application of graphic statics to trusses having loads applied at several points, it is necessary to first describe the truss diagram and the notation employed in lettering both the truss diagram and the stress diagram.
After the truss has been drawn out, and the joint loads computed, the truss diagram should be carefully drawn, with the loads and supporting forces indicated, and then lettered as described in § 130.
In the truss diagram the members must meet at the joints, otherwise it will be impossible to draw the stress diagram. Theoretically the truss diagram should coincide with lines drawn longitudinally through the centre of each member, but in practice it is generally impracticable to locate all the members of a wooden truss so that their center lines will meet at the joints, and in such cases, the truss diagram is drawn by taking the centre lines of the principal members and usually of the rods, and. then connecting the joints for the braces.
Thus, to draw the truss diagram for the truss Fig. 249, draw lines through the centre of the tie beam, rafters and rods, and for the braces connect the points where the lines through the centre of the rods intersect the rafters and tie beam, as in Fig. 276, which is the truss diagram for this truss. For the queen rod truss, Fig. 256, draw centre lines for the tie beams, rods and straining beam. Obtain the intersections for the end joints by drawing short lines through centre of rafters, intersecting centre line of tie beam and connect these points with the intersections of centre lines of rods, with centre lines of straining beam. For the truss shown in Fig. 253, draw lines through the centre of the beam, rafters and rods, and connect the points of intersection for the braces.
For Howe Trusses, draw lines through the centre of the chords and rods, and connect the points of intersection for the braces, the end points being obtained as explained above for the queen rod truss.
Sometimes the lines of the truss diagram will vary considerably from the actual centre lines of the truss, as for instance, in Fig. 263. For this truss we would draw centre lines through the tie beam and rafter, and the rod B, but instead of drawing a line through the centre of rod A, it will be better to locate the line for this rod directly under the point where the load is applied. As a rule any reasonable variation between the truss diagram and the actual centre lines will not materially affect the amount of the stresses, but to some extent it produces a bending movement in the rafters or tie beams, which should be avoided as much as possible. For steel trusses, the truss diagram and actual centre lines should practically coincide with each other. After the lines of the truss diagrams are drawn all of the external loads and the supporting forces should be indicated by lines and figures as in the following examples. If the truss is symmetrical and symmetrically loaded, it is only necessary to show a little more than one-half of the truss. If the truss is not symmetrical, or is not symmetrically loaded, then the diagram for the entire truss must be drawn.