Passive energy dissipation is an emerging technology that enhances the performance of a building by adding damping (and in some cases, stiffness) to the building. The primary use of energy dissipation devices is to reduce earthquake displacement of the structure. Energy dissipation devices will also reduce force in the structure, provided the structure is responding elastically, but would not be expected to reduce force in structure that is responding beyond yield.
For most applications, energy dissipation provides an alternative approach to conventional stiffening and strengthening schemes, and would be expected to achieve comparable performance levels. In general, these devices are expected to be good candidates for projects that have a target building performance level of life safety or perhaps immediate occupancy, but would be expected to have only limited applicability to projects with a target building performance level of collapse prevention. Other objectives may also influence the decision to use energy dissipation devices, since these devices can also be useful for control of building response to small earthquakes and wind loads.
A wide variety of passive energy dissipation devices is available, including fluid viscous dampers, viscoelastic materials, and hysteretic devices. Ideally, energy dissipation devices dampen earthquake excitation of the structure that would otherwise cause higher levels of response and cause damage to components of the building. Under favorable conditions, energy dissipation devices reduce drift of the structure by a factor of about two to three (if no stiffness is added) and by larger factors if the devices also add stiffness to the structure.
Unlike base isolation, passive energy dissipation does not intercept earthquake energy entering the structure. It allows earthquake energy into the building. However, the energy is directed toward energy dissipation devices located within the lateral resisting elements. Earthquake energy is transformed into heat by these devices and dissipated into the structure.
A fluid viscous damper attached to diagonals of a braced frame, shown in Fig. 8.35a, is one such energy dissipation device. It dissipates energy by forcing a fluid through an orifice, similar to the shock absorbers of an automobile (Fig. 8.35b). The fluid used is usually of high viscosity, such as a silicone. The unique feature of these devices is that
their damping characteristics, and hence the amount of energy dissipated, can be made proportional to the velocity. The response of a fluid viscous damper is considered to be out-of-phase with those due to seismic activity. This is because the damping force provided by the device varies inversely with the dynamic lateral displacements of a building. To understand the concept, consider a building shaking laterally back and forth during a seismic event. The stress in a lateral-load-resisting element such as a frame-column is at its maximum when the building deflection is also at maximum. This is also the point at which the building reverses direction to move back in the opposite direction. The damping force of a fluid viscous damper will drop to zero at this point of maximum deflection. This is because the damper stroking velocity goes to zero as the building reverses direction. As the building moves back in the opposite direction, a maximum damper force occurs at the maximum velocity which happens when the building goes through its normal upright position. This is also the point when the stresses in the lateral-load-resisting elements are at a minimum. Therefore, the damping provided by the device
Figure 8.35b. Viscous fluid damper, consisting of a piston in a damping housing filled with a compound of silicone or similar type of oil. The piston contains small orifices through which the fluid passes from one side of the piston to the other. The damper thus dissipates energy through the movement of the piston in a highly viscous fluid.
varies from a maximum to a minimum as the building moves from an at-rest position to its maximum lateral deflection position. This out-of-phase response is considered a desirable feature in seismic designs.
A photograph of a fluid viscous damper installed in an existing building as part of seismic upgrade is shown in Fig. 8.35c.
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