WIND VERSUS EARTHQUAKE RESISTANCE OF BUILDINGS

As discussed in Section 3.4, many of the structural strategies used for wind load resistance and earthquake resistance are essentially similar. Buildings are, therefore, generally designed to resist either earthquake loads or wind loads, whichever causes the worst effect (greater stresses). This is based on the assumption that there is a negligible probability of maximum wind speeds occurring at the same time as an intense earthquake. In regions of low seismic activity, wind loads govern the design of the lateral resistance of buildings; in regions of intense seismic activity, the reverse is the case.

The design of lightweight envelope components, such as glass curtain walls and roof membranes, is governed by wind loads even in highly seismic regions. (Remember that earthquake loads are influenced by the weight of the component.)  Despite the similarities between the provisions for wind load and earthquake load resistance, there are several differences in the details. An important difference is that as an earth-quake shakes the ground on which the building rests, it affects all components of the building—exterior as well as interior components. Damage to the building’s contents and injury to occupants may occur by falling interior contents,  Figure 3.24   . Thus, all compo-nents, equipment, and fixtures in a building located in a seismically active zone must be adequately anchored to remain intact during an earthquake.

FIGURE 3.24 Effect of earthquake shaking (1989 Loma Prieta earth-quake) on the contents of a building’s interior. Note that the building envelope is intact.
FIGURE 3.24 Effect of earthquake shaking (1989 Loma Prieta earth-quake) on the contents of a building’s interior. Note that the building envelope is intact.
Wind, on the other hand, acts on the building envelope. It is through the envelope that wind loads are transmitted to the building structure. If the envelope remains intact, the building will generally retain its overall structural integrity in a storm. Thus, the wind damage in a building usually begins with the damage to envelope components, such as the roof, exterior walls, exterior doors, windows, and other cladding elements. Once the building changes from fully enclosed to partially enclosed (see  Figure 3.15), the wind loads on the building increase due to the ballooning effect, which may result in additional damage or collapse of the structure, damage to interior contents, and injuries to occupants, Figure 3.25.

FIGURE 3.25 Damage to the glass curtain wall of the 37-story Bank One Building, Fort Worth, Texas, by a forceful tornado on March 28, 2000. As soon as the glass in the curtain wall shattered, substantial damage to the building’s interior (ceiling, drywall, electrical fixtures, etc.)
FIGURE 3.25 Damage to the glass curtain wall of the 37-story Bank One Building, Fort Worth, Texas, by a forceful tornado on March 28, 2000. As soon as the glass in the curtain wall shattered, substantial damage to the building’s interior (ceiling, drywall, electrical fixtures, etc.)

FIGURE 3.15 Ballooning of a building with a relatively large opening in one wall. Such a building is referred to as a partially enclosed building. A building that is not partially enclosed is called an enclosed building. Most buildings belong to the enclosed building category.
FIGURE 3.15 Ballooning of a building with a relatively large opening in one wall. Such a building is referred to as a partially enclosed building. A building that is not partially enclosed is called an enclosed building. Most buildings belong to the enclosed building category. 

0 comentarios:

Post a Comment