192Earthquakes Their Cause and Effect

As discussed in Chapter 17, displacement of a fault results in the propagation of time-varying displacements throughout the earth — this vibratory ground motion is what is termed an earthquake. From a structural viewpoint, the essence of earthquakes is the dynamic displacement of the ground supporting a building, resulting in lateral and vertical forces on the building. In this chapter, emphasis will be on the lateral forces on a building due to shaking — vertical forces due to shaking are usually of somewhat less significance, but not always, and building codes and good practice require that vertical forces also be considered in the seismic design of buildings. While earthquakes also result in other effects on buildings,

Diaphragm

Diaphragm

Undeformed building shape

In-plane forces

Ground acceleration "Z"

Lateral force resisting system (shear wall, braced frame, moment frame)

FIGURE 19.1 Effect of ground displacements on a building.

Undeformed building shape

In-plane forces

Ground acceleration "Z"

Lateral force resisting system (shear wall, braced frame, moment frame)

FIGURE 19.1 Effect of ground displacements on a building.

such as partial or complete ground failure, fire, and tsunami, design for these effects is more specialized and will not be covered here.

For a structure connected to the earth (and very few buildings are not connected to the earth through their base or foundation), as the base moves or displaces, the inertia and flexibility of the structure result in a time lag before the rest of the structure can displace in response to its base's displacement. The interaction of these displacements and the response of the structure result in time-varying displacements and strains within the structure, which the structure must be designed to sustain. For a structure responding to a moving base there is an equivalent system, in which the base is fixed and the structure is acted upon by forces (called inertia forces) that cause the same displacements as are occurring in the moving base system. In seismic design it is customary to visualize the structure as a fixed base system acted upon by inertia forces. Figure 19.1 shows such a system, in which the base has moved (due to the ground accelerations) and the roof is deflected. The roof's deflection is relative to its base, and is due to it and its supporting walls not moving or displacing as quickly as the base has displaced underneath it. This is exactly equivalent as if a force had been applied to the roof, which deflected the roof and its supporting walls the same amount, relative to its base.

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