"On the arch strip... we have certain forces acting; usually... forces of gravity... vertical loading. ...the dead weight of the arch and superstructure together with the useful or live load. Now consider the archstrip (Fig 1) divided into separate segments or s by joint-planes or sections 1, 2, 3, ..., with the end planes A and B resting against fixed abutments. With the... assumption of no shears in the head planes, each voussoir, e.g., 1-2, is subjected to the action of three forces, namely, the given external load P2 and the two resultant pressures R1 and R2 in the abutting joints. These forces must hold each other in equilibrium and may therefore be represented by a force triangle 1-2-O; in this way either joint-pressure may be determined when the other is known. Consequently all of the joint-pressures may be found as soon as any one of these forces is given in its magnitude, direction, and point of application. The forces acting at the end planes A and B are the pressures, and their opposing forces are the abutment reactions K1 and K2. The application of these reactions takes the place of the abutments, so that the arch may be considered an independent system in equilibrium under the forces \sum P, K1 and K2."