Concentration of Seismic Stress due to Complex Plans

By their nature, structural facilities tend to be large and complex, which often causes their configuration to be quite complex as well. One of the greatest causes of damage to buildings has been the use of improper architectural-structural configurations.

Generally speaking, it may be said that a departure from simple structural forms and layouts tends to be severely punished by earthquakes. Concentration of stress arises in buildings with complex floor plans. A complex plan is defined as that in which the line joining any two sufficiently distant points lies largely outside of the plan. This occurs when wings of significant size are oriented in different directions, for instance in H, U, or L shapes (see figure 1).

Figure 1. Complex plan

In irregularly shaped floor plans, the wings may be likened to a cantilever built into the remaining body of the building, a point that would suffer smaller lateral distortions than in the rest of the wing. Large concentrations of stress appear in such transition areas, frequently producing damage to the nonstructural elements, the vertical structure, and even the diaphragms (that is, the horizontal resistant elements of a structure such as floors and roofs).In such a case, the solution currently used is to introduce seismic expansion joints like those mentioned in the case of long buildings. These joints allow each block to move without being tied to the rest of the building, which interrupts the cantilever effect of each wing. The joints, obviously, must be wide enough to permit the movement of each block without striking adjacent blocks.