Special Issue on NEES 1: Advances in Earthquake Engineering

Five papers focus on seismic protective systems for buildings, and two companion papers introduce the concept and development of a new structural response modification approach using adaptive negative stiffness. The concept, presented by Pasala et al., is that a negative stiffness device (NSD) can be added to a structural system that allows the system to weaken as if it has yielded without actually damaging the structure. Sarlis et al. present the design of the NSD itself, which consists of a highly compressed spring contained in a gap mechanism to allow the structural system to undergo the desired deformation before the negative stiffness engages. Sanchez et al. examine the reduced area formulation commonly used for prediction of the critical (buckling) load on elastomeric bearings through single bearing tests and shake table tests of a rigid frame on four bearings. Their study results in new recommendations for parameter definitions in the reduced area formulation, thereby improving its accuracy. Chae et al. use real-time hybrid simulation to investigate five different magnetorheological (MR) damper control strategies, ranging from passive to semiactive control. The authors consider a three-story steel frame, which was modeled analytically, with two MR dampers installed to control the story drift. The dampers were tested physically and integrated with the frame model using a hybrid simulation technique. They conclude that the passive controller and semiactive controllers perform similarly. Details of the real-time hybrid simulation method are presented in the paper. The same test method and setup were used by Cha et al. to investigate the effectiveness of other control strategies for MR dampers. They conclude that the clipped-optimal controller is better in reducing absolute acceleration, whereas the decentralized output feedback optimal controller is more effective in reducing story drifts.

On the topic of hybrid simulation, Mahmoud et al. report on system-level hybrid testing of a complete steel frame where an experimental substructure of a full-scale, partially restrained, beam-to-column connection was used in conjunction with a numerical model of the remaining frame. Key features of the authors’ work are their ability to apply realistic boundary conditions to the experimental substructure and their ability to investigate the effect of the connection behavior on the system response. Li et al. examine the use of hybrid simulation to update fragility functions based on Bayes’ theorem, essentially performing the fragility development as a two-stage procedure. They apply their methodology to an existing bridge and use four hypothetical levels of available data, concluding that hybrid simulation provides an economic way of validating and improving fragility functions. Phillips and Spencer introduce a new, model-based, feedforward-feedback actuator control approach for real-time hybrid simulation and demonstrate the accuracy and efficiency using both a single-degree-of-freedom system and a 9-story steel building with 18 large MR dampers serving as the physical substructure at the first story. Lin et al. present the results of their large-scale experimental study of a two-bay, 4-story, self-centering moment-resisting frame that utilizes posttensioned strands to close gaps between the beam and column flanges, thereby recentering the system. They also employ a beam web friction device for energy dissipation while keeping the main framing members essentially elastic. The hybrid simulations demonstrate the ability of the combination of these advanced technologies to provide immediate occupancy at the design earthquake level.

Aaleti et al. examine the cyclic response of RC walls with different anchorage details. The impetus for their study is that RC walls are typically tested without splices in the longitudinal reinforcement above the wall-foundation interface in the laboratory, but that these splices are present in virtually all multistory construction. They conclude that existing design provisions lead to good behavior of walls, and that no quantifiable performance difference exists between the use of the spliced in-situ walls and laboratory specimens without splices. Perea et al. recognize that although a number of test results are available on slender, concrete-filled tubes (CFT), limited test results can be applied to justify structural system response factors and equations in the current design specifications. The authors seek to calibrate an advanced computational model using test data to assess the accuracy of the design equations. The CFT specimens were subjected to complex load protocols that included pure compression, uniaxial and biaxial bending combined with compression, pure torsion, and torsion combined with compression. Fleischman et al. summarize a comprehensive experimental program that had the goal of developing a new seismic design methodology for precast concrete diaphragms. The research ranged from connection testing to system-level testing. The authors highlight the entire project and offer seminal new knowledge about these systems, and, with the help of their industry advisory team, they use technology transfer to arrive at a viable design framework. Henkhaus et al. discuss a number of full-scale RC columns with tie spacing exceeding one-half the depth and having 90-degree hooks. The authors show that the test columns had little drift capacity beyond shear failure, and that the drift capacity present decreases for load histories with more cycles.

In closure, as the NEES Special Issue editors, we would like to express our gratitude to the authors for the quality of their manuscripts, making the editorial work a very enjoyable experience. The papers being published in the two special issues reflect a broad range of structural engineering research and discoveries made possible by NEES. Thank you to all the reviewers who donated their time and expertise to maintaining the high quality of these issues of the Journal of Structural Engineering. Finally, a warm thank you from the three of us to Professor Sherif El-Tawil, editor of the Journal of Structural Engineering, for suggesting a NEES Special Issue during the Associate Editor meeting in 2011 and for his guidance in organizing this special issue.