Even symmetrical buildings must be able to withstand significant lateral forces. Engineers counteract these forces in both the horizontal and vertical structural systems of a building. Diaphragms are a key component of the horizontal structure. They include the floors of a building, as well as its roof. Engineers generally place each diaphragm on its own deck and strengthen it horizontally so it can share sideways forces with the vertical structural members. On the roof, where a strong deck isn't always possible, engineers strengthen the diaphragm with trusses, which are diagonal structural members inserted into the rectangular areas of the frame.
The vertical structural system of a building consists of columns, beams and bracing, and functions to transfer seismic forces to the ground. Engineers have several options when building the vertical structure. They often build walls using braced frames, which rely on trusses to resist sideways motion. Cross-bracing, which uses two diagonal members in an X-shape, is a popular way to build wall trusses. Instead of braced frames or in addition to them, engineers may use shear walls -- vertical walls that stiffen the structural frame of a building and help resist rocking forces. Engineers often place them on walls with no openings, such as those around elevator shafts or stairwells.
Shear walls do, however, limit the flexibility of the building design. To overcome this downfall, some designers opt for moment-resisting frames. In these structures, the columns and beams are allowed to bend, but the joints or connectors between them are rigid. As a result, the whole frame moves in response to a lateral force and yet provides an edifice that's less obstructed internally than shear-wall structures. This gives the designer more flexibility in placing architectural elements, such as exterior walls, partitions and ceilings, as well as building contents, such as furniture and loose equipment.
Of course, the structural members of a building rest on its foundation. On the next page, we'll look at how engineers are improving building foundations to make them more resilient in strong earthquakes.