the reaction of the portion of the truss on one side of the section AB, acting upon the portion on the other side along the lines of the different members, which holds the truss in equilibrium. If therefore the por tion of the truss to the right of AB is considered as taken away, and if, along the lines of the top and bottom chords and the diagonal, forces are applied of the same intensity as the forces which resulted from the reaction of the portion on the right and which held the truss in equilib rium, then these forces can for the time being be considered as ex ternal forces, and the intensity of them will be such as to fulfill the three conditions of equilibrium as regards the external forces. This condition is indicated in Fig. 273. It will be seen that these forces acting along the lines of the members of the truss cut by the section are the actual stress in these members necessary to maintain the truss in equilibrium. The stresses produced in the members of a structure by the action of the loads, are called the "internal" or "inner" forces, in distinction from the "external" forces or "loads."
Any section, such as AB, cutting three members, gives three stresses to be determined. The top and bottom chord stresses are determined by using the condition that the algebraic sum of the mo ments about any point is zero. For the top chord, the point chosen is the intersection of the bottom chord and the diagonal. The moment of the stress in these two members about this point, is therefore zero, and this leaves only the moment of the top chord stress, which must then be equal to the moment of the loads about this point.
In a similar manner, taking moments about the intersection of the top chord and the diagonal, leaves only the moment of the bottom chord stress to be determined, which must equal the sum of the mo ments of the loads about this point.
In Fig. 272 these top and bottom chord stresses are determined by taking sections through the truss at the left of each panel point. These top chord stresses will be worked out below.