Selection of a Suite of Ground Motions for Boston and its Application to Unreinforced Masonry Structures

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1 Selection of a Suite of Ground Motions for Boston and its Application to Unreinforced Masonry Structures Dr. Eric Hines, P.E., LeMessurier Consultants / Tufts University / Michigan State University REPORT UNREINFORCED MASONRY Past performance of unreinforced masonry (URM) structures during earthquakes shows that the primary failure mechanism is out-of-plane failures of walls, precipitated by in-plane damage under cyclic loads, accompanied by the failure of non-structural elements. Out-of-plane failure, as seen is Figure 1, is extremely difficult to calculate, and although some reliable methods exist, their application is limited. A common method is the equal energy method, which assumes that the masonry wall should be able to withstand accelerations as if it were remaining elastic throughout the loading process. Elastic behavior would allow the wall to withstand much higher acceleration than would be expected if inelastic behavior occurred. While this method has been verified by laboratory testing, it only provides reasonable results for structures with fundamental periods between about 0.3 and 0.5 seconds and is a conservative method for structures with periods greater than about 0.5 seconds. The following two properties of URM structures make it difficult to create accurate computer models of URM structures: (1) URM structures exhibit brittle failures and (2) ground motions are amplified by the walls and floor diaphragms as they move through the building. Commercially available software such as SAP and ETABS cannot model such behavior; however, programs such as RUAUMOKO can capture the behavior of URM structures. To analyze existing unreinforced masonry structures in Boston under expected earthquake loads, appropriate ground motions for the area must be applied to the structural models. GROUND MOTIONS FOR BOSTON, MASSACHUSETTS Background An appropriate suite of ground motions for the Boston area, which did not previously exist, has been identified from records of past earthquakes. Because earthquake records for large earthquakes in the northeastern United States are scarce, records from earthquakes that occurred in other areas were used. In the past, suites have been developed using simulated motions or by scaling records from past earthquakes. Figure 2 shows the suite of ground motions containing both simulated motions and records from past earthquakes that was developed for the Boston area during the SAC project [5]. However, the ground motions were scaled to match the target spectrum using scale factors that were up to two times the maximum recommended scale factor [3]. Because changes in the frequency content of a record occur and are not accurately captured when large scale factors are used, the ground motions that resulted from the SAC project were not representative of an expected earthquake in Boston. Boston Society of Architects Design Research Grants, 2004 Page 1

2 Expected Boston Earthquake Table 1 lists the parameters of an expected Boston earthquake, which were defined by deaggregating the seismic hazard for the area. Source to site distance was not considered as a factor because seismic waves travel further in the Eastern United States (EUS) than in areas of high seismicity, where most of the records used for this study were recorded. It was determined that if the peak ground acceleration (PGA), magnitude and duration for a particular record were within specified bounds and the frequency content of the resulting response spectrum was similar to that of the target spectrum, then the energy content of the ground motion would be appropriate for an expected Boston earthquake. Short duration events, long duration events and events that exhibited near-field effects were all captured in the final suite. Table 1: Parameters for an Expected Boston Earthquake Magnitude Peak Ground Acceleration (PGA) (g) Distance N/A Duration* <20 seconds for rock sites <10 seconds for soil sites Frequency See target spectrum *Note the duration was only used to justify the use of earthquakes with a magnitude larger than 7.0 The target spectrum used was the Uniform Hazard Spectrum for a 2% in 50-year earthquake as defined by the USGS. The ground motions used for this study were taken from the database at [1], which contains ground motions from 50 different earthquakes, most of which took place in California. The ground motions were available as Western United States (WUS) motions and as WUS motions adjusted so they were appropriate for use in the EUS. Suite Selection Any record that matched all of the criteria listed in Table 1 was considered for the final suite. Records were eliminated based on their effect on the error between the target spectrum and the average spectrum of the considered records. The resulting suite, shown in Figure 3, contains records from 7 different earthquakes that occurred in areas of high seismicity; however half of the records were adjusted for use in the EUS. Future work A database containing the time histories and response spectra for the earthquake records that matched the criteria listed in Table 1 will be created. Two additional suites will be developed for comparison with the suite already developed: one suite will contain only records that were adjusted for the EUS and the other suite will contain only unadjusted WUS records. The same three suites will be developed for a 10% in 50 year earthquake using the same method. A single degree of freedom system will be modeled using a computer program that can capture the nonlinear behavior of the masonry. The ground motions from the final selected suite will be applied to the structure. The structural response of the system will be compared to the expected response of the system as predicted by the equal energy method. Boston Society of Architects Design Research Grants, 2004 Page 2

3 LIST OF REFERENCES 1. CCA Dynamic Solutions,LLC, Department of Public Works, Artery (I-93)/Third Harbor Tunnel (I-90) Project - Seismic Exposure Evaluation, Commonwealth of Massachusetts, Kramer, Steven, Geotechnical Earthquake Engineering, Pearson Education, R.K. McGuire and Silva, Walter and Constantino, Carl, Technical Basis for Revision of Regulatory Guidance on Design Ground Motions, Prepared for the U.S. Nuclear Regulatory Commission 5. Somerville, Paul and Smith, Nancy, et al., Development of Ground Motion Time Histories for Phase 2 of the FEMA/SAC Steel Project, Cement and Concrete Research, SAC Joint Venture, SAC/BD-97/04, Figure. 1: Out-of-plane failures in the top story of a URM structure during the 1989 Loma Prieta Earthquake. Boston Society of Architects Design Research Grants, 2004 Page 3

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6 Selection of a Suite of Ground Motions for Boston and its Application to Unreinforced Masonry Structures Dr. Eric Hines, P.E., LeMessurier Consultants / Tufts University / Michigan State University ADDENDUM 01 CONTACT INFORMATION Dr. Eric Hines, P.E. LeMessurier Consultants 675 Massachusetts Avenue Cambridge MA fax ehines@lemessurier.com Boston Society of Architects Design Research Grants, 2004 Page 6