In the previous examples, we have studied the behavior of bearing walls of unreinforced masonry, idealized as a series of vertical strips, simply supported at the level of the floor slab, and at the level of the roof. Lets see how this changes in the case of bearing walls with openings.
In Fig. 5.17, load applied above the window and door openings clearly cannot be resisted by vertical strips, because those vertical strips have only one point of lateral support (at the roof level). For that reason, the wall must be idealized as horizontal strips above and below the openings, supported by vertical strips on both sides of the openings, as shown in Fig. 5.18. Each set of horizontal strips, idealized as simply supported, must be supported by the adjacent vertical strips. For example, the horizontal strips above the door are supported by Strip A and Strip B. The window and door are considered to transfer loads applied to them, via horizontal strips, to the vertical strips on either side of the openings. Therefore, Strip A has to support, spanning vertically, the out-of-plane loads acting directly on it, plus the out-of-plane loads acting on the left half of the horizontal strips above the door. In other words, Strip A has to resist the out-of-plane loads acting on what might be termed a tributary width, which extends from the left-hand edge of Strip A itself, to the midspan of the horizontal strips above the door. In the same way, Strips B and C have to resist the loads corresponding to Tributary Widths B and C, respectively. For example, if Strip B has to resist the loads acting over Tributary Width B, this represents an increase in the design loads on Strip B. That strip must resist the loads that normally would be applied to it (if no openings had existed), multiplied by the ratio of Tributary Width B, divided by Width B:
The same applies to vertical loads, because these also must be transferred from horizontal to vertical strips. In any event, the presence of openings can be considered to increase the initial actions in the vertical strips adjacent to the openings. Aside from this increase, the design of those elements proceeds exactly as before.
