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Nonlinear viscous dampers are rate-dependent passive energy dissipating devices that enhance the seismic performance of building structures. Performance-based design (PBD) procedures for structures with such damping devices need to be experimentally evaluated to advance the PBD procedures. This paper presents the experimental validation of a newly developed PBD procedure for seismic design of steel frames with nonlinear viscous dampers using real-time hybrid simulation (RTHS). The prototype building in the study is a 3-story office building located on a stiff soil site in Pomona, California, an area of high seismicity in the United States. The steel frame structure is designed for the code-based (ASCE7-10) design base shear, with supplemental nonlinear viscous dampers to control drift and thereby control damage to the building under various earthquake intensity levels. A PBD procedure with specified performance objectives is used to perform an integrated design of the steel moment resisting frame (MRF) and the damping system (damped brace frame-DBF). Large-scale real-time hybrid simulations (RTHS) on the 3-story steel frame building are conducted at the NEES Real-time Multi-directional (RTMD) Earthquake Simulation Facility (NEES@Lehigh equipment site) at Lehigh University. This paper summarizes the implementation of the real-time hybrid simulation, including the hydraulic actuators and power supply, the control architecture for the NEES RTMD facility, and the robust integration algorithm, and the actuator delay compensation. In the RTHS the complete structure is represented as an experimental substructure and an analytical substructure. The experimental substructure consists of a large-scale 3-story steel frame with nonlinear viscous dampers and diagonal braces supporting the dampers, and the analytical substructure is comprised of the remaining parts of building, including steel MRFs, gravity load frames, seismic mass, and the inherent damping of the building. The analytical substructure has a total of 296 DOFs, and includes nonlinear fiber elements and panel zone elements to model the members and panel zones, respectively, of the MRF and gravity load frames. A series of RTHS utilizing an ensemble of ground motion records selected from the PEER NGA database were conducted. Hazard levels chosen for this study are the design basis earthquake (DBE) with an approximate return period of about 500 years and the maximum considered earthquake (MCE) with an approximate return period of about 2500 years. Results from the RTHS are evaluated and compared with expectations from the PBD procedure as well as numerical simulation results. The experimental results show that the structure with nonlinear viscous dampers achieves the specified performance objectives. The results show that RTHS is a practical technique to experimentally evaluate performance under simulated earthquake loading and to validate performance-based design procedures for structures with rate-dependent damping devices.

Implementation of high performance damping devices can increase cost-effectiveness of structural systems for mitigation of natural hazards. However, the applications of these damping systems are limited, due to a lack of 1) mechanical robustness; 2) electrical reliability; and 3) large resisting force capability. To broaden the implementation of modern damping systems, a novel semi-active damping device is proposed. The device, termed Modified Friction Device (MFD), has enhanced applicability compared to other proposed damping systems due to its cost-effectiveness, high damping performance, mechanical robustness, and technological simplicity. Its mechanical principle is based on a duo-servo drum brake, which results in a high amplification of the input force while enabling a variable control force. It is also possible to attach the MFD in parallel with a stiffness element and a viscous damper, which produces a dynamic similar to the magnetorheological damper. Here, we present the MFD, and experimentally demonstrate its principle. The hysteresis of the friction force is characterized for low velocities and small displacements. Results show that the MFD is a promising semi-active device for natural hazard mitigation.

Steel special concentrically braced frames (SCBFs) are efficient and economical seismic lateral force resisting systems. However, limited ductility capacity and structural damage as a result of brace buckling at low drift ratios (0.3% ~ 0.5%) is a drawback of SCBFs. The steel self-centering (i.e., post-tensioned, rocking) concentrically-braced frame (SC-CBF) is a new seismic lateral force resisting system that reduces or eliminates the structural damage and residual drift of SCBFs under design basis earthquakes (DBEs) while maintaining a sufficient margin against collapse under very rare maximum considered earthquakes (MCEs). In order to establish equivalent safety against collapse in an earthquake for SC-CBFs comparable to the inherent safety against collapse intended by current seismic codes, global seismic performance factors need to be evaluated. Nonlinear static pushover analysis and incremental dynamic analyses (IDA) are used as tools to assess median collapse capacity and to determine the collapse margin ratio (CMR) according to the recommended methodology by FEMA P695. The CMR and the adjusted CMR (ACMR) are calculated and reported this paper for the SCBF and SC-CBF systems studied. Various definitions of collapse for both SCBF and SC-CBF systems are also explored and compared. The appropriateness of each definition is discussed for both the SCBF and SC-CBF systems and the sensitivity of the CMR to changes in the definition of collapse is investigated. Finally, effect of inherent damping models on collapse capacity and probability of collapse is discussed.

Conventional steel concentrically braced frames (CBFs) are an efficient, stiff lateral force resisting system but have limited ductility capacity due to brace buckling. A new, robust building frame system known as a Self-Centering CBF (SC-CBF) that has large ductility capacity and minimal residual drift under earthquake loading is being developed and has been extensively validated using hybrid earthquake simulation at Lehigh University's NEES Equipment Site. The ductility capacity of the SC-CBF system is increased by allowing the frame to rock on its base. High ductility vertical post-tensioning (PT) bars act to self-center the SC-CBF after an earthquake. Results from laboratory hybrid simulations show that this system is very resilient under intense ground motion at the design basis earthquake (DBE) and the maximum considered earthquake (MCE) levels. The SC-CBF is designed using a performance-based seismic design procedure that limits yielding of the PT bars under the DBE but permits significant yielding of the PT bars under the MCE. Since the PT bars may yield and some of the initial prestress force may be lost during intense MCE ground motions, knowledge of SC-CBF system performance in aftershocks is important. Experimental results indicate good performance of the SC-CBF with yielded PT bars and a loss of prestress force when the SC-CBF system is subjected to a strong post-MCE aftershock. This paper presents and discusses results from laboratory hybrid simulations of an SC-CBF subjected to DBE, MCE, and strong aftershock ground motions.

A self-centering moment resisting frame (SC-MRF) is a viable alternative to a conventional steel SMRF with welded beam-column connections for seismic resistant steel frame buildings. An SC-MRF is characterized by gap opening and closing at the beam-column interface under earthquake loading. The beams are post-tensioned to columns by high strength post-tensioning (PT) strands oriented horizontally to provide self-centering forces when gap opening occurs. The SC-MRF is typically designed to meet several seismic performance objectives in order to perform in a resilient manner. For instance, no damage and no residual drift under the Design Basis Earthquake (DBE) leads to immediate occupancy performance following the DBE. In addition, under the Maximum Considered Earthquake (MCE) the structure is designed to have minimal damage and minimal residual drift. Recent analytical and experimental research has shown that a SC-MRF can achieve these performance objectives. In order to establish adequate safety against collapse in an earthquake the global seismic performance factors (SPFs) such as the response modification factor (R), the system overstrength factor (Ω0), and deflection amplification factor (Cd) need to be evaluated. Since an SC-MRF system is a new concept, little is known about its collapse resistance under seismic ground motions. For an SC-MRF to be accepted in practice, the global SPFs need to be evaluated in accordance with FEMA P695 to establish whether the collapse margin ratio for such a structural system under extreme ground motions is adequate. In this paper a series of incremental dynamic analyses are performed on several archetype SC-MRFs and the results utilized to determine the probabilistic collapse resistance of a SC-MRF system. The model of the SC-MRF system combines stress-resultant elements with continuum elements in order to enable the complete structural system to be modeled while capturing the important limit states that can occur, including gap opening at the beam-column interface, yielding and fracture of the PT strands, second order (P-Δ) effects due to gravity loads imposed on the gravity load frames of the building, panel zone elastic and inelastic deformations, column inelastic deformations, and cyclic local buckling and strength degradation in the beam plastic hinge regions of the SC-MRF. The modeling procedure for the SC-MRF is presented, along with the incremental dynamic analysis procedure and results. The results show that a properly designed SC-MRF has a margin against collapse under the MCE that is comparable, or better, than a conventional steel SMRF. The probability of collapse under the MCE was found to be less than 4%, which is within the limit of 10% established in the FEMA P695 - Quantification of Building Seismic Performance Factors Guidelines, and therefore indicates that and the building system performance and response parameters (R, Cd, Ω0) used to design an SC-MRF system appear to be rational. Therefore, an SC-MRF system can perform in a resilient manner under the DBE while having a satisfactory margin against collapse.

Passive damping systems can improve the seismic performance of buildings by reducing drift and inelastic deformation demands on the primary lateral load resisting system, in addition to reducing the velocity and acceleration demands on non-structural components. Recent research by the authors shows that adding passive damper systems to steel moment resisting frames (MRFs) enables significant reductions in the steel weight of the MRFs, while enhancing the seismic performance of the structure. A large scale elastomeric damper was constructed by pre-compressing a high damping elastomeric material into steel tubes. Characterization tests of the damper were performed and the damper characteristics were derived for various amplitudes, temperatures, and excitation frequencies. A rate dependent hysteretic model for the damper was calibrated and incorporated into OpenSEES for use in seismic response analyses of steel MRF buildings with compressed elastomeric dampers. This paper investigates the collapse potential of steel MRFs with passive dampers. A simplified design procedure (SDP) proposed by Lee et al (2003) was used to design steel MRFs with elastomeric dampers. The potential for collapse of these structures is determined from the results of incremental dynamic analysis (IDA) involving a suite of 44 ground motions proposed by FEMA P695. Nonlinearities in the dampers, P-delta affect, and strength and stiffness cyclic deterioration are accounted for in models of the structures developed in OpenSees. Collapse potential of steel MRFs with elastomeric dampers are compared with collapse potential of conventional MRFs.