Box-type structures resist lateral loads as shown in Fig. 5.19.
This resistance mechanism involves three steps:
Walls oriented perpendicular to the direction of lateral load transfer those loads to the level of the foundation and the levels of the horizontal diaphragms. The walls are idealized and designed as vertically oriented strips.
The roof and floors act as horizontal diaphragms, transferring their forces to walls oriented parallel to the direction of lateral load.
Walls oriented parallel to the direction of applied load must transfer loads from the horizontal diaphragms to the foundation. In other words, they act as shear walls.
As noted previously in the sections dealing with unreinforced bearing walls, this overall mechanism demands that the horizontal roof diaphragm have sufficient strength and stiffness to transfer the required loads. This is discussed again in a later section dealing with horizontal diaphragms.
The rest of this section addresses the design of shear walls. We shall see that in almost all cases, the design itself is very simple, because the cross-sectional areas of the masonry walls are so large that nominal stresses are quite low.
