Journal of Structural Engineering

This paper describes the cyberinfrastructure (CI) of the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) and examines the evidence that this infrastructure is facilitating earthquake engineering research. Among the key features of the CI are the NEES Project Warehouse (PW), which is a data repository for earthquake engineering, an environment that supports the use of tools for web-based data analysis and simulation, and tools that support research collaboration. The value that such CI offers to the user community is discussed. The CI also gathers a myriad of usage statistics, some of which are presented in this paper. Among them are the number of users, pageviews, recorded NEES projects, and other stored resources. This information demonstrates that the CI is used significantly and increasingly so.

Yielding can be emulated in a structural system by adding an adaptive negative stiffness device (NSD) and shifting the yielding away from the main structural system, leading to the new idea of apparent weakening that occurs, ensuring structural stability at all displacement amplitudes. This is achieved through an adaptive negative stiffness system (ANSS), a combination of NSD and a viscous damper. By engaging the NSD at an appropriate displacement (apparent yield displacement that is well below the actual yield displacement of the structural system) the composite structure-device assembly behaves like a yielding structure. The combined NSD-structure system presented in this study has a recentering mechanism that avoids permanent deformation in the composite structure-device assembly unless the main structure itself yields. Essentially, a yielding-structure is mimicked with no, or with minimal, permanent deformation or yielding in the main structure. As a result, the main structural system suffers less acceleration, less displacement, and less base shear, while the ANSS absorbs these effects. This paper presents comprehensive details on development and study of the ANSS/NSD. Through numerical simulations, the effectiveness and the superior performance of the ANSS/NSD as compared with a structural system with supplemental passive dampers is presented. A companion paper presents the NSD and its mechanics in detail.

Structural weakening and addition of damping is an approach previously proposed for the reduction of seismic forces and drifts in the retrofit of structures. It is also used in the design of new buildings with damping systems. While this approach is efficient, it does not significantly reduce and may even amplify inelastic excursions and permanent deformations of the structural system during a seismic event. This paper describes a negative stiffness device (NSD) that can emulate weakening of the structural system without inelastic excursions and permanent deformations. The NSD simulates yielding by engaging at a prescribed displacement and by applying a force at its installation level that opposes the structural restoring force. The NSD consists of (a) a self-contained highly compressed spring in a double negative stiffness magnification mechanism; and (b) a gap spring assembly (GSA) mechanism which delays the engagement of negative stiffness until the structural system undergoes a prescribed displacement. The NSD employs double chevron braces that self-contain the large vertical forces needed for the development of the horizontal negative stiffness without transferring these forces to the structure. This paper reports the development and operation of the NSD and presents analytical and computational tools that describe the behavior of the device. The principles of global control of structures using the NSD are presented in a companion paper. Additional papers present the results of testing of the device, and the results of analytical and experimental studies on the application of the device in a three-story conventional structure and a three-story seismically isolated structure.

The behavior of semirigid partial-strength connections has been investigated through either experimental component testing or detailed three-dimensional (3D) finite-element (FE) models of beam-column subassemblies. Previous experiments on semirigid partial-strength connections were conducted under idealized loads and boundary conditions, which do not represent real situations. In addition, the developed 3D FE models are computationally expensive and have primarily been used under monotonic loadings. Evaluating the full potential of any connection requires a system-level investigation, whereby the effect of the local behavior of the connection on the global response of the structural system is considered. Moreover, the connection should be tested under realistic load and boundary conditions and/or analyzed using an accurate yet computationally affordable analytical model. This paper represents a new system-level hybrid simulation application aimed at investigating the seismic performance of semirigid partial-strength steel frames with top and seat angles with double web-angle connections. The analytical component of the simulation comprises a detailed two-dimensional nonlinear FE model. The experimental component of the simulation consists of a full-scale beam-column subassembly with loading and boundary conditions that are in full interaction with the rest of the frame. The paper provides an overview of the hybrid simulation application and highlights the major results. The simulations were conducted at the Multi-Axial Full-Scale Sub-Structured Testing and Simulation Facility at the University of Illinois, which is part of the National Science Foundation Network for Earthquake Engineering Simulation.

Bearings used in the seismic isolation of buildings can be subjected to large horizontal deformations combined with high axial loads from overturning forces during strong shaking. Elastomeric bearing design requires an evaluation of the critical load capacity under this combined loading to ensure stability. To better understand and estimate the capacity of elastomeric bearings, a comprehensive experimental program was carried out to examine bearing stability using two quasi-static and one dynamic loading procedures. The first method to evaluate the bearing critical load follows previous experimental research, where a single elastomeric bearing is held at prespecified horizontal displacements while increasing the axial load until the critical load is achieved. In a second method implemented here, a similar setup is used to apply a constant axial load followed by increased horizontal displacement until the stability limit is observed. This second method is shown to be an accurate and more direct approach to obtaining the critical load directly from the test data. The third method evaluates the dynamic stability of a rigid frame on four elastomeric bearings subjected to earthquake motions on a shake table. These tests are able to quantify the response of elastomeric bearings at and beyond their stability limit under more realistic loading conditions. Results from the three experimental procedures are compared, and the reduced-area formulation commonly used to predict the critical load of elastomeric bearings is evaluated, resulting in new recommendations for defining the parameters used in this formula to improve its accuracy.

The accuracy of fragility functions is critical for regional risk and loss estimations. This paper proposes a two-stage approach to generate improved fragility functions for engineering structures using field measurement and experimental data. In the first stage, the linear and nonlinear parameters of the bridge model are calibrated using measured earthquake responses and cyclic testing data; analytical fragilities are then generated with the calibrated model. In the second stage, a Bayesian updating approach is used to further update the derived fragilities using hybrid (analytical-experimental) simulation results. To illustrate the effectiveness of the proposed approach, fragility functions are generated for the Meloland Road Overcrossing Bridge considering four cases that represent an increasing level of data availability. A comparison of the four sets of fragility functions shows that appropriate calibration of bridge model is critical to the accuracy of the fragilities. In addition, hybrid simulation provides an economic and efficient way of validating and improving the accuracy of fragility functions through Bayesian updating.

Results from tests on eight full-scale RC columns with ties with large spacing ($s>d/2$) and 90° hooks are presented. The test results showed that increasing the number of displacement cycles and applying displacements along more than one axis decreased the maximum drift ratio reached before the columns experienced failure in axial compression. Test columns had little drift capacity beyond shear failure. The mean difference between the maximum drift ratio at axial failure and the drift ratio at shear failure was less than 1%.

Previous tests of structural walls have routinely used continuous reinforcement extending from the foundation to the top of the specimen. This detailing is consistently different from that of multistory walls in the field, which incorporate splices in the wall longitudinal reinforcement above the wall-foundation interface. As a result, the performance of walls incorporating continuous reinforcement in the laboratory may not be representative of walls in the field that use lap splices or mechanical couplers near the wall base. This paper investigates lateral load behavior of three nominally identical structural walls with continuous reinforcement, lap splices, and mechanical couplers in the plastic hinge region, and quantifies the differences in their responses using force-displacement response, lateral deformation components, and energy dissipation estimated using equivalent viscous damping.

A new seismic diaphragm design methodology has been developed for precast concrete floor diaphragms. The knowledge required to create the design methodology was obtained through an integrated analytical and experimental research project using two Network for Earthquake Engineering Simulation (NEES) equipment sites. The activities of the project involved a sequence of integrated research tasks that systematically developed knowledge about diaphragm behavior from the reinforcement detail level to the structural system level. These tasks included the use of state-of-the-art experimental techniques made possible through the NEES facilities including hybrid simulation and large-scale shake table tests. Such techniques were crucial to the research as the complex behavior of precast diaphragms is not well described by simple analytical models or idealized experiments. Valuable industry oversight in the planning, execution, and technology transfer stages of the project guided the research activities, including a design framework established at the outset of the project. This paper describes the research project approach, activities, and outcomes.

Substructure hybrid simulation is a powerful, cost-effective alternative for testing structural systems, closely coupling numerical simulation and experimental testing to obtain the complete response of a structure. In this approach, well-understood components of the structure are modeled numerically, while the components of interest are tested physically. Generally, an arbitrary amount of time may be used to calculate and apply displacements at each step of the hybrid simulation. However, when the rate-dependent behavior of the physical specimen is important, real-time hybrid simulation (RTHS) must be used. Computation, communication, and servohydraulic actuator limitations cause delays and lags that lead to inaccuracies and potential instabilities in RTHS. This paper proposes a new model-based servohydraulic tracking control method including feedforward-feedback links to achieve accurate tracking of a desired displacement in real time. The efficacy of the proposed approach is demonstrated through RTHS for a single-degree-of-freedom system and a 9-story steel building, each using a 200-kN large-scale magnetorheological damper as the rate-dependent physical specimen.

Real-time hybrid simulations using large-scale magnetorheological (MR) dampers were conducted to evaluate the performance of various structural control strategies to control the seismic response of a three-story steel-frame building. Magnetorheological dampers were installed in the building to limit the story drift to less than 1.5% under the design-basis earthquake (DBE). The laboratory specimens, referred to as experimental substructures, were two individual MR dampers, with the remainder of the building modeled as a nonlinear analytical substructure. The experimental technique enables an ensemble of ground motions to be applied to the building, resulting in various levels of damage, without the need to repair the experimental substructures because the damage will be within the analytical substructure. Five different damper control algorithms, including passive and semiactive control algorithms, were selected. An ensemble of five ground motions scaled to the DBE was used for the real-time hybrid simulations to obtain statistical responses of the structure for each control. The real-time hybrid simulation results show that the MR dampers can control the drift, enabling the performance objective of 1.5% maximum story drift to be achieved. Although some semiactive controllers show better performance for a specific ground motion, the response statistics from the real-time hybrid simulations show that the overall performance of the semiactive control algorithms with the selected user-defined parameters is similar to that for the passive controller for the three-story building used in this study. A comparison of real-time hybrid simulation results with numerical simulation results using OpenSees was conducted to further gain insight into the performance of the damper control algorithms observed in the real-time hybrid simulations.

This paper presents an experimental study of a 0.6-scale 2-bay 4-story steel self-centering moment-resisting frame (SC-MRF) test structure under maximum considered earthquake (MCE) ground motions. A SC-MRF uses high-strength posttensioning (PT) strands to precompress the beams to the columns and to close the gaps between the beam flanges and column flanges that occur at the beam-column interface under earthquake loading, returning the frame to its initial position (i.e., the frame is self-centering). In this study, a beam web friction device is included in each beam-column connection to dissipate energy under seismic loading. The SC-MRF design objectives are to be without structural damage, creating the potential for immediate occupancy performance under the design basis earthquake, and to suffer only modest damage, leading to collapse prevention (CP) performance under the MCE. The CP performance is achieved by avoiding beam web buckling and PT strand yielding and fracture. A special fuse that prevents PT strands from yielding is described. Experimental results from MCE-level earthquake hybrid simulations and quasi-static pushover tests on the SC-MRF test structure are presented. The experimental results show that the SC-MRF did not collapse under the MCE, and that the fuse is a viable alternative to protect PT strands from yielding.

This paper presents comparisons of the performances of three semiactive control algorithms for use with multiple magnetorheological (MR) dampers. The three controllers are (1) the clipped-optimal controller, (2) the decentralized output feedback polynomial controller, and (3) the simple passive controller. These controllers use different types of inputs to calculate control signals for the MR dampers, based on different control mechanisms. To investigate the advantages of each controller, a three-degree-of-freedom steel moment-resisting frame, designed using a performance-based design methodology, was developed. The performance was investigated by using four different earthquakes utilized during the design of the building frame. Comparisons of the controllers’ performance were carried out in terms of reduction in the maximum interstory drifts, displacements, absolute accelerations, and control forces. Real-time hybrid tests were carried out to validate these comparisons.

Composite steel-concrete columns and beam-columns have been shown to provide superior performance when the intrinsic synergistic characteristics of concrete and steel are properly exploited. However, limited test data are available to justify the structural system response factors and comprehensive design equations in current design specifications. This research, through the testing of 18 full-scale, slender concrete-filled steel tube (CFT) beam-columns, addresses these needs by providing comprehensive data to calibrate advanced computational models and assess 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. The results for the pure compression tests reported in this paper indicate that current AISC design provisions provide an accurate estimation of column capacity for both strength and stiffness. The experimental loading response was also contrasted with column curves obtained from advanced, nonlinear fiber analysis models. The experimental and computational column curves are strongly correlated in the elastic critical load range, but showed some differences in the inelastic buckling load range. These differences are attributed to the higher concrete strength assumed in the computational analysis because of the confinement effect of the steel tube; this strength is underpredicted in current design specifications.