Report - PEER - University of California, Berkeley

There are many questions to be answered, with a few of them addressed ingeneral next, and a few more addressed specifically in the subsequent example.What is an appropriate EDP? The choice of the EDP is driven by the sensitivityof the loss to variation in a basic design parameter. As discussed in Section 2.1, forthe NSDSS subsystem, story drift is an appropriate choice. The best choice isbelieved to be the average of the maximum drifts in each story, which is a measure ofthe “expected” maximum drift. It is larger than the global drift, because maximumstory drifts occur at different times (Medina 2003).What are the S a -EDP curves for design alternatives? They are relationships, fora specific design solution, between the mean value of a relevant EDPs and anappropriate intensity measure, IM. Customarily, the spectral acceleration at the firstmode period of the structure is selected as an IM. The curves represent mean IDA(incremental dynamic analysis) curves obtained by subjecting design alternatives to astatistically representative set of ground motions. The design variables could be baseshear strength V y , stiffness or strength variation over the height, or first mode period(if T is varied, the hazard curve changes accordingly). For regular frame structuresmany such S a -EDP curves are documented in Medina (2003), and more are underdevelopment. Similar curves are being developed for wall structures.Why bother with MDOF S a -EDP curves rather than use approximate SDOFrepresentations? In the writers’ opinion the usefulness of SDOF representations inseismic design is overestimated. Their use invites approximations that are justified insome cases but not in others. Why not avoid their use if appropriate MDOFinformation is available? This, in fact, is a necessity when the EDP is flooracceleration, which is an EDP that correlates poorly with any SDOF parameter.3.1.1 Example of Design for Acceptable LossesAn example of design decisions based on acceptable losses is illustrated in Figure 3.The example addresses a 9-story frame structure, located in Southern California at asite for which the spectral acceleration hazard curves for the periods of 0.9 sec. and1.8 sec. are as shown in the upper left portion of the figure. The expected loss-EDPcurves for the three subsystems are as shown in the lower portion. The NSDSS and SSsubsystems account for 60% and 20% of the replacement cost, respectively, and theNSASS subsystem accounts for the remaining 20%. The EDP is the average of themaximum interstory drifts (IDR) for the SS and NSDSS subsystems, and the averageof maximum floor accelerations (FA) for the NSASS subsystem. At this time there islittle hard data behind these loss-EDP curves; they are based on judgment.Based on relative value of the subsystems, in this example “designing foracceptable losses” needs to focus on the NSDSS subsystem. An owner could targetacceptable losses in the NSDSS subsystem of 5% of total replacement cost at the50/50 hazard level. Thus, design targets are created by entering the figure with theEDP associated with the expected loss of 5% and with the 50/50 mean S a hazards forappropriate periods. For illustration, periods of 0.9 sec. and 1.8 sec. are selected. Theupper right portion of the figure contains mean S a -EDP curves for various design511