Engineering and Construction

The more important difference between wind and earthquake resistance lies in their basic design approaches. For wind load resistance, the structure is designed so that under the action of maximum wind loads, it remains elastic—that is, the structure is designed to suffer no permanent deformation under the worst expected storm.

The design approach is different for earthquake load resistance. The loads on a building produced by the worst expected earthquake for the location are so large that if the building were designed to remain elastic after the earthquake, it would be prohibitive in cost. Therefore, the earth-quake loads for which a building is designed may be smaller than the maximum earthquake loads expected on it.

The underlying design philosophy is that the building should remain elastic when resisting minor earthquakes. In the event of an intense earth-quake, part of the earthquake’s energy may be dissipated in permanently deforming the building, but the building must otherwise remain intact to provide complete safety to its occupants.

Permanent deformations in a structure are possible only if the structure has the ability to sustain such deformations. Materials that can sustain permanent deformation prior to failure are called   ductile  materials. Ductility is an essential property for buildings located in seismic zones but is not a requirement for resisting wind loads.

A building must not deform permanently even in the most severe windstorm. Additionally, it must be rigid enough to reduce the deflections caused by strong winds. The swaying of the upper floor of tall buildings under wind loads must be controlled to remain within acceptable limits of human tolerance.

The reverse is true for earthquake loads. Buildings must be able to deform, even permanently, to absorb the energy delivered to them by the earthquake. If brittle materials are used, they will fail under earthquake forces. Several reinforced concrete frame buildings with (brittle) unreinforced masonry infill walls between frames have collapsed or suffered serious damage under earthquake forces,  Figure 3.26   . Such buildings would generally be unharmed by violent storms.

FIGURE 3.26 A typical pattern of damage to unreinforced masonry infill walls in buildings with a reinforced concrete frame structure. If the infill walls shown in this illustration had been reinforced horizontally and vertically, they would have suffered minor (or no) damage depending on the earthquake’s intensity.