Alternate Design Philosophy

Although earthquake performance objectives are implicit in building codes, significant questions linger. Is the philosophy of inferring the behavior adequate to define the expected earthquake performance? Can the performance be actually delivered? Should the earthquake response objectives be explicitly stated in building codes? Is it feasible to make an existing nonductile building conform to current detailing and ductility provisions? If not, what level of upgrade will provide for minimum life safety? How much more strengthening is required to achieves an "immediate occupancy rating"?

Explicit answers to these and similar questions cannot be found in current building codes. Although a set of minimum design loads are prescribed, the loads may not be appropriate for seismic performance verification and upgrade design because

1. The code provisions do not provide a dependable or established method to evaluate the performance of noncode compliant structures.

2. They are not readily adaptable to a modified criterion, such as one that attempts to limit damage.

3. Since the primary purpose is protection of life safety, the code does not address some building owners' business concerns such as protection of property, the environment, or business operations.

To overcome these shortcomings, a procedure that uses a two-phase design and analysis approach has been in use for some time. The technique explicitly requires verification of serviceability and survival limit states by using two distinct design earthquakes; one that defines the threshold of damage and the other that defines collapse. The serviceability level earthquake is normally characterized as an earthquake that has a maximum likelihood of occurring once during the life of the structure. The collapse threshold is typically associated with the maximum earthquake that can occur at the building site in the presently known tectonic framework. This characterization can vary, however, to suit the specifics of the project, such as the nature of the facility, associated risk levels, and the threshold of damageability.

The principle behind the two-phase approach may be explained by recalling the primary goal in seismic design, which is to provide capacity for displacement beyond the elastic range. Any combination of elastic and inelastic deformations is possible to attain this goal. For example, we could design a structural system that would remain elastic throughout the displacement range. This system would have a high elastic strength but low ductility. Conversely, it is entirely possible to have a system with relatively low elastic strength but high ductility, meeting the same design objective of remaining stable. It may be easier to understand the methodology if it is recognized that a specific earthquake excitation causes about the same displacement in a structure whether it responds elastically or with any degree of inelasticity.

Figure 6.1 shows the behavior of an idealized structure subjected to three levels of earthquake forces FL, Fv, and FC corresponding to lower-level, upper-level, and collapse-level earthquakes. Also shown is an earthquake force FE experienced by the structure if it were to remain completely elastic. The structure designed using the lower-level earthquake

Figure 6.1. Idealized earthquake force-displacement relationships.

force FL deforms elastically from 0 to E and inelastically from E to U. The same structure designed using the force Fu needs to deform 0 to U, responding elastically all through the displacement range. Both systems are capable of attaining the anticipated deformation of Au. However, a building designed using the force FL will require a more ductile system than a building designed for the fully elastic force FE. More important, it will suffer heavier damage should the postulated event occur. Nevertheless, both systems achieve the primary goal: Both remain stable without collapse under the expected deformation Au. Therefore, it is possible to design the structure using any level of force between FL and Fe with the understanding that a corresponding ductility is developed by the detailing of the system. For example, a structure designed for the force level Fu requires a higher strength but less ductility than if it were designed for force level FL. Hence, it is a matter of choice as to how much strength can be traded off for ductility and, conversely, ductility traded for strength. Expressed another way, structural systems of limited ductility may be considered valid, provided they are capable of resisting correspondingly higher seismic forces.

This is the approach used in the seismic retrofit design of existing buildings. Since buildings of pre-1970 vintage do not have the required ductile detailing, the purpose is to establish the strength levels that can be traded off in part for lack of required ductility.

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