Component Behavior

Nonstructural components can be classified as deformation-or acceleration-sensitive. If the performance of a component is controlled by the supporting structure's deformation, such as the interstory drift, it is deformation-sensitive. Curtain walls and piping systems running floor-to-floor are some examples of deformation-sensitive components. These components spanning from floor-to-floor are often rigidly connected to the structure. They are thus deformation-sensitive and are susceptible to damage due to a building's interstory drifts.

When a component is not vulnerable to damage from the interstory displacements, it is generally acceleration-sensitive. A mechanical unit anchored to the floor or a roof of a building is a good example. Acceleration-sensitive components are vulnerable to shifting and overturning and as such their anchorage or bracing is of prime concern. The force provisions of building codes generally predicate design forces high enough to prevent sliding, toppling, or collapse of acceleration-sensitive components. Many components are both deformation-and acceleration-sensitive, although a primary mode of behavior can generally be identified. For example, the exterior skin of a building such as anchored veneer or prefabricated panels are both deformation-and acceleration-sensitive. However, their design is primarily controlled by deformation.

Acceleration-sensitive components should be anchored or braced to the structure to limit their movement during seismic events. However, these components should not be anchored in such a way as to inadvertently affect the seismic behavior of the structural system. For example, if the base of a component is anchored to the floor with its top rigidly braced to the floor above, it can have the unintended effect of altering the response of the structural system. An example is a nonstructural masonry partition rigidly connected at the top and bottom to the building floors. The wall acts as a shear wall, leading to an

Figure 2.62. Slip joint in nonstructural masonry partition; connection at top provides out-of-plane support without restricting in-plane movement of wall. (a) Wall perpendicular to metal deck span; (b) wall parallel to metal deck span.

Figure 2.62. Slip joint in nonstructural masonry partition; connection at top provides out-of-plane support without restricting in-plane movement of wall. (a) Wall perpendicular to metal deck span; (b) wall parallel to metal deck span.

unintended redistribution of lateral load. A solution to prevent this condition is to provide isolation joints between the masonry wall and the structural columns wide enough to prevent interaction between the two elements. A sliding connection at the top of the wall should be designed to provide out-of-plane support allowing in-plane movement of the wall (see Fig. 2.62a and b).

The dynamic behavior of components mounted at or below grade is similar to that of buildings. On the other hand, the behavior of components attached to the upper floors of buildings is quite complicated. Its response not only depends on the mass and stiffness of the component, and the characteristics of the ground motion, but also on the dynamic characteristics of the structure itself.

Mechanical components are often fitted with vibration isolation mounts to prevent transmission of vibrations to the structure. By increasing their flexibility, the vibration isolation mounts can alter the dynamic properties of the components, resulting in a dramatic increase in seismic inertial forces. Isolation mounts must be specifically designed to resist these increased seismic effects. For example, 1997 UBC requires the design forces for equipment mounted on a vibration isolator to be based on a dynamic amplification factor, ap = 2.5, and a component response modification factor, Rp = 1.5. Comparable values for a rigid equipment with supports fabricated of ductile materials attached to rigid mounts are ap = 1.0 and Rp = 3.0. Since the seismic design force is a function of the ratio ap /Rp, all other things being equal, the design force for an equipment with vibration isolation mounts would be five times larger than the design force when it is mounted on rigid supports.

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