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This report assesses the validity of the simplified methods of analysis and design of buildings with damping systems specified in FEMA’s National Earthquake Hazard Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures issued in 2000 and updated for 2003, and the upcoming ASCE-7 Standard for 2005 when the effects of near-field and soft-soil ground motions are taken into account. The procedures outlined in these documents are largely based on studies that excluded these effects. To determine their impact, both single- and multiple-degree-of-freedom structures with linear and nonlinear viscous damping devices were studied using two sets of near-field ground motions and one set of soft-soil ground motions.
The study found that the damping coefficient values are accurate or conservative; the ductility demand for near-field and soft-soil motions are very similar to those previously observed for far-field motions; simplified methods of analysis for single-degree-of-freedom systems produce results on displacement and acceleration that are generally of acceptable accuracy or conservative for near-field or soft-soil motions and are very similar to that previously observed for far-field motions; and their application to steel moment frames with linear and nonlinear viscous damping systems provided conservative estimates of drift and predictions for damper forces and member actions in good overall agreement with the average of results of nonlinear response-history analysis.
This report summarizes the findings from an evaluation of geotechnical reports submitted as part of the compliance reports required by Senate Bill 1953 (SB 1953) for all hospitals in California. The geotechnical reports from 153 of the 470 licensed hospitals in California were reviewed with the cooperation of the California Office of Statewide Health Planning and Development (OSHPD). Review of this data indicates that less than half of the hospital buildings in California in 2001 were considered to be structurally compliant with the requirements of SB 1953. Almost 40 % were determined to be at significant risk for structural collapse and a danger to public safety in the event of a strong earthquake. Over 70 % had basic nonstructural systems essential to life safety and patient care that were inadequately anchored to resist earthquake forces. The survey of the geotechnical evaluations indicated that about 20 % of the hospital sites had a potential for liquefaction based on the SB 1953 design ground motions.
This report describes an energy dissipation system configuration that extends the utility of fluid viscous damping devices to structural systems that are characterized by small interstory drifts and velocities. The geometry of the brace and damper assembly is such that the system resembles a jacking mechanism, and thus the name “scissor-jack-damper energy dissipation system” is adopted. The system is a variant of the toggle-brace-damper system, and offers the advantage of a more compact configuration. A theoretical treatment of the scissor-jack-damper system is presented and its effectiveness is demonstrated through testing of a large-scale steel framed model structure under imposed harmonic displacement on the strong floor, as well as dynamic excitations on the earthquake simulator. Experiments demonstrate that despite its small size, the scissor-jack system provides a significant amount of damping while also substantially reducing the seismic response of the tested structure. Application of the system in a new building in Cyprus is described.
The research presented in this report investigates a seismic retrofit technique for steel truss bridge piers that allows pier rocking by using passive energy dissipation devices implemented at the anchorage locations to control the rocking response. Specially detailed hysteretic energy dissipating elements (buckling-restrained braces) are used to act as easily replaceable, ductile structural “fuses.” The dynamic characteristics of the controlled rocking/energy dissipation system are investigated in order to formulate a capacity design procedure using simplified methods of analysis. Design constraints are established that attempt to satisfy performance objectives and nonlinear time history analyses are used to assess the seismic behavior of the bridge piers retrofitted per this strategy. The retrofit strategy is shown to be more applicable to slender piers. The methods of predicting key response values were found to be conservative in most cases and capacity protection of the existing pier (to the prescribed limits) was achieved in all cases considered.
This report describes the development of a novel uplift-prevention Friction Pendulum isolator called the XY-FP. It presents the principles of operation and mathematical model of the XP-FP isolator, describes its mechanical behavior through testing of a single isolator, and demonstrates its effectiveness through testing of a quarter-scale steel-frame model structure. The computer program 3D-BASIS-ME was modified to include an element representative of the mechanical behavior of the new XY-FP isolator, and the validity and accuracy of analytical methods to predict its behavior is assessed. The study shows that the XY-FP isolator provides effective uplift prevention regardless of the state of displacement in the bearing, allows for decoupling of the bi-directional horizontal motion along two orthogonal directions, and has the capability to provide distinct stiffness and energy dissipation along the principal directions of the bearing. In addition, by encompassing much less structural material, the isolator offers a lighter and more economical alternative to the standard Friction Pendulum bearing.
This report presents a study of irregular structures near collapse and the development of an experimental model to study many types of structural systems in the near collapse state. Many analytical studies have been carried out to evaluate irregular structures, but few experimental works have been done on this subject. This study provides an overview of the accuracy of the analytical methods in predicting the structural response. Equally important in this research was the design of a structural model for study of structural systems near collapse. A versatile reconfigurable structural model was developed to be used and reused with structures undergoing severe damage to sacrificial elements, thus capable of being repaired and further tested without complete collapse.The study shows that a separation of lateral and gravity load resisting systems can produce a stable structure in case of major damage to lateral system, provided that redundancy exists to control lateral deformations. Such a system can be implemented when retrofitting structures, by weakening the connections of gravity columns and providing a redundant external lateral load resisting system.