Earthquake Engineering & Structural Dynamics最新文献

This paper demonstrates the efficacy of a new reinforced concrete shear wall seismic retrofit method through a series of nonlinear static and incremental dynamic analyses. Unlike traditional retrofit methods, the method investigated aims to convert conventional walls into self-centering walls whose behavior is governed by rocking and flexure. The retrofit involves creating a cold joint at the foundation–wall interface, cutting some reinforcing bars to allow rocking, adding external post-tensioning to enable self-centering, and externally confining wall toes to prevent concrete crushing. The retrofit was applied to two building archetypes, each with two different shear wall designs. The four walls were retrofitted by varying retrofit parameters (portion of the vertical reinforcement bars cut, and external post-tensioning amount). Nonlinear static and nonlinear response history analyses were performed using experimentally validated, computationally efficient models that simulate walls with fiber-based beam–column elements. Incremental dynamic analysis was used to create collapse fragility functions for pre- and post-retrofit walls. The results show that the retrofit is effective when some vertical reinforcement bars are left uncut across the foundation–wall interface. The retrofit is more effective for walls with vertical reinforcement distributed across cross-section as compared to walls with reinforcement concentrated near boundary elements and for walls with structurally efficient amounts of reinforcement as compared to walls with higher amounts of reinforcement. This is attributed to the larger amount of reinforcement bars cut in walls with concentrated reinforcement layouts or heavy reinforcement amounts, leading to a larger loss of strength, recovery of which requires larger amounts of post-tensioning.

Advancing the seismic resilience of building systems is an active area of research in earthquake engineering. Ensuring safe egress in and out of buildings during extreme events, such as an earthquake, is essential to supporting this effort. To this end, understanding the seismic response of stairs facilitates the robust design of egress systems to ensure they can remain operable after an earthquake. From prior earthquake events and physical experiments, it is understood that stairs with a flight to landing fixed connection at multiple levels within a building are prone to damage. In addition, the stair system with flight to landing fixed attachments may affect the dynamic behavior of the building. To accommodate seismic inter-story drifts, a kinematically free connection between the stairs and landing has been proposed. Herein this connection is referred to as a drift-compatible stair connection. To investigate and aid in the design of such a connection, a unique set of shake table experiments were conducted at the University of Nevada, Reno. In this paper, an overview of these tests is presented, and a high-fidelity finite element model of the tested stair system is used to predict the responses measured during these experiments. Developed in Abaqus, the robustness of the modeled stair unit is investigated considering a variety of contrasting connections, namely, drift-compatible connections, fixed ends and one end fixed and the other free. Results from these numerical simulations offer guidance towards development of simplified models of multi-level stair subsystems. Such models are needed when investigating seismic resilience of building systems across a wider range of hazard levels. Furthermore, best practices observed utilizing the models developed and evaluated herein against experimental data will be useful for subsequent analysis of larger stair tower models, such as the 10-story stair system implemented in the NHERI Tall Wood mass timber building with post-tensioned rocking walls, conducted in 2023 at the UC San Diego Large High-Performance Outdoor Shake Table.

Corrugated steel plate shear walls (CSPSWs) can be applied to high-rise building structures to serve as lateral load-bearing and energy-dissipating members. Arranging stiffeners on corrugated steel plates shows substantial potential to further improve the seismic performance of ordinary CSPSWs. This study experimentally and numerically investigated the hysteretic behavior of unstiffened and stiffened CSPSWs. Cyclic quasi-static tests were conducted on four, 1/2-scale, two-story, single-bay specimens with different corrugation orientations, including two specimens with unstiffened corrugated steel plates and two specimens installed with stiffened corrugated steel plates equipped with two pairs of stiffeners. According to the hysteretic curves of different specimens, the effects of corrugation orientations and arrangement of stiffeners were revealed in terms of the skeleton curves, energy-dissipating capacity, and stiffness degradation. Then, finite element (FE) models, which were applied to gain a deeper understanding of the experimental results, were established, and validated against the test results. This study demonstrated that the arrangement of stiffeners was effective in improving the seismic performance of CSPSWs regarding the ultimate shear strength and energy-dissipating capacity, while the influence of corrugation orientations on the performance of the specimens involved in this study could be ignored.

This paper investigates the effects of transverse shear walls (TSWs), out-of-plane bending stiffness of diaphragms (FDIA), and axial loading (AXL) on the lateral response of strong wood-frame shear walls (SWs) used for multistory light frame timber buildings (LFTBs) located in highly active seismic zones. Experimental tests were conducted to understand the requirements for SW-to-TSW connections to achieve desirable TSW effects in non-planar SWs and to characterize the lateral cyclic response of T-shaped SW assemblies with and without diaphragms and axial load. Both slotted and screwed connections were evaluated as SW-to-TSW connections, and both showed sufficient stiffness and strength to achieve TSW effects. However, the slotted connection is preferred because it has a more ductile failure mode. Tests on T-shaped SW assemblies with and without diaphragms and axial load revealed that TSWs significantly enhance the lateral stiffness and strength but reduce the deformation capacity with respect to that of planar SWs. FDIA and AXL effects further influence the stiffness and strength, overcoming the limitation of smaller deformation capacity in T-shaped SWs without diaphragms. Diaphragms also make the T-shaped SW response more symmetrical and improve the evolution of the secant stiffness, the cumulative dissipated energy, and the equivalent viscous damping over increasing levels of lateral drift. Numerical analyses of a theoretical building model with T-shaped SWs show significant reductions in lateral drift (up to 46%) and uplift (up to 100%) compared to the case with planar SWs only, emphasizing the importance of considering system effects in the seismic design of LFTBs.

This paper investigates the possibility of using self-centering walls (SCWs) as tuned mass dampers (TMDs) to control the seismic response of structures. The basic characteristics of this system are investigated using a two-degree-of-freedom (2-DOF) model, representing the main structure as a single-degree-of-freedom (SDOF) oscillator interconnected with an SCW through viscous and elastic devices. The nonlinear equations of motion for the proposed coupled system are derived and then utilized to determine the displacement amplification response. The coupling parameters are optimized to minimize the dynamic response of the oscillator using a numerical method. A simulation study is conducted to investigate the response of the proposed system by considering six recorded earthquakes. Several parameters are studied, including the mass ratio, the influence of prestressing, and the slenderness angle of the wall. The displacement spectra are generated using the optimized parameters and compared to those of the uncoupled system and the traditional coupled system, which features a rigid connection between the structure and the wall. The results reveal that the proposed system effectively suppresses both maximum and root mean square responses of structures, outperforming the traditional coupled system in most cases. Improved performance for the proposed system can be achieved by increasing the wall's mass ratio and decreasing the slenderness angle. Moreover, prestressing has an adverse impact on the system's displacement response. Finally, the influence of wall flexibility is examined using a finite element model, revealing a minimal effect on the system's response.

Large-scale multiaxial testing facilities mainly serve to experimentally examine the horizontal behavior of full-scale critical structural members, such as columns and seismic isolation bearings, on which a large vertical compression load is exerted as they simultaneously undergo horizontal deformation. The system friction and inertia force play an important role in obtaining sufficiently reliable test results, and it is not easy to comprehensively grasp all the issues involved. To understand the system friction and inertia force of the Bi-Axial Dynamic Testing System (BATS) at the National Center for Research on Earthquake Engineering Tainan laboratory and to avoid complexity caused by specimens as much as possible, a lubricated flat sliding bearing is chosen as the specimen to be tested under horizontal triangular and sinusoidal reversed loading together with a constant vertical compression load. When no specimens are installed, that is, without vertical compression loading, the system friction of BATS generated by the various sliding surfaces can be identified and mathematically characterized using the horizontal triangular reversed loading test results; then, the effective mass of BATS can be estimated using the horizontal sinusoidal reversal loading test results to solve the inertia force problem. When applying a vertical compression load, it is assumed that the system friction of BATS and the shear force of the specimen are simply related to the applied total normal force (or vertical compression load) and horizontal excitation rate. An iteration methodology is proposed to identify and mathematically describe the dependency of the friction performance of BATS and the specimen on total normal forces (or vertical compression loads) and horizontal excitation rates by iterating the horizontal triangular and sinusoidal reversed loading test results. Finally, a lead-rubber bearing and a direct force measurement system are connected in series such that the measurement system precludes the system friction and inertia force and a series of tests are conducted. The reliability of the proposed mathematical model for BATS and the feasibility of the proposed direct force measurement strategy are further demonstrated by comparing the calibrated force response with the directly measured response.

Combining the accurate physical description of high-fidelity mechanical formulations with the practical versatility of low-order discrete models is a fundamental and open-ended topic in structural dynamics. Finding a well-balanced compromise between the opposite requirements of representativeness and synthesis is a delicate and challenging task. The paper systematizes a consistent methodological strategy to identify a physics-based reduced-order model (ROM) preserving the physical accuracy of large-sized models with distributed parameters (REM), without resorting to classical techniques of dimensionality reduction. The leading idea is, first, to select a limited configurational set of representative degrees of freedom contributing significantly to the dynamic response (model reduction) and, second, to address an inverse indeterminate eigenproblem to identify the matrices governing the linear equations of undamped motion (structural identification). The physical representativeness of the identified model is guaranteed by imposing the exact coincidence of a selectable subset of natural frequencies and modes (partial isospectrality). The inverse eigenproblem is solved analytically and parametrically, since its indeterminacy can be circumvented by selecting the lumped mass matrix as the primary unknown and the stiffness matrix as a parameter (or vice versa). Therefore, explicit formulas are provided for the mass matrix of the ROM having the desired low dimension and possessing the selected partial isospectrality with the REM. Minor adjustments are also outlined to remove a posteriori unphysical effects, such as defects in the matrix symmetry, which are intrinsic consequences of the algebraic identification procedure. The direct and inverse eigenproblem solutions are explored through parametric analyses concerning a multistory frame, by adopting a high-fidelity Finite Element model as REM and an Equivalent Frame model as ROM. Before mass matrix identification, modal analysis results indicate a general tendency of ROM to underestimate natural frequencies, with the underestimation strongly depending on the actual mass distribution of the structure. After the identification of the mass matrix and the elimination of unphysical defects, isospectrality is successfully achieved. Finally, extensions to prototypical highly massive masonry buildings are presented. The qualitative and quantitative discussion of the results under variation of the significant mechanical parameters provides useful insights to recognize the validity limits of the approximations affecting low-order models with lumped parameters.

Correlation coefficients for 5%-damped spectral accelerations (SAs) of horizontal ground-motion components have been extensively studied and applied in probabilistic seismic analysis using vector-valued intensity measures (IMs). Such correlations are, however, not sufficient for structures with different damping ratios under multidirectional earthquake shakings. This paper presents a comprehensive study on the correlations between horizontal-horizontal (H-H), horizontal-vertical (H-V), and vertical-vertical (V-V) pairs of SAs for different damping ratios based on the NGA-West2 ground motion database. The correlations of SAs with peak ground acceleration (PGA) and peak ground velocity (PGV) are also investigated. The uncertainty in correlations is measured by integrating the bootstrap method into the logic-tree framework. Comparative results indicate that the correlation coefficients generally increase for IMs of larger damping ratios. The relative difference between the damping-dependent and conventional 5%-damped correlation coefficients can be notable, reaching 100% and 35% in the SA-SA and SA-PGV (or PGA) pairs, respectively. Based on the empirical correlation results, an artificial neural network is utilized to develop parametric models of the correlations and the executive files for implementing these models are provided. The ANN-aided damping-dependent correlation models developed can be considered as a generalization of the conventional 5%-damped correlation models, and may serve as useful tool in applications such as ground-motion selection and vector-valued probabilistic seismic risk assessment for structure systems.

Recent earthquakes have highlighted that aftershocks can considerably increase the structural demand and seismic risk of engineering structures. This study presents a probabilistic approach to assess the seismic risk of reinforced concrete (RC) frame structures subjected to mainshock-aftershock sequences. In this approach, a predictive fragility method is used to evaluate the probabilities of structural damage under sequential excitations. The Bayes theorem is employed to generate posterior distributions of unknown model parameters. Then, a practical seismic hazard assessment method is used to conduct mainshock-aftershock hazard analysis. The Copula technique is employed to develop a joint distribution model of the mainshock and aftershock intensity measures. Finally, the seismic risk is evaluated using the classical risk integration equation with the mainshock-aftershock fragilities and hazard surfaces. Confidence bounds for fragilities and seismic risks are also obtained to account for the uncertainties of model parameters caused by aftershocks. The proposed approach is demonstrated by considering a seismic-designed RC frame building. It can be concluded that aftershocks can significantly increase the seismic risk throughout the entire structural service life. The additional uncertainties caused by aftershocks result in wider confidence bounds for seismic risk.

Nonlinear response history analysis is the primary tool for risk-targeted design and seismic performance evaluation of structures. These analyses require the selection of a set of ground motions that satisfy predetermined conditions such as spectral acceleration. Numerous efforts have been made so far to obtain ground motion records which are expected to represent possible earthquakes. Even though spectral acceleration-based ground motion scaling is a common procedure, recent studies showed that structural response can be better represented through the energy content of the records. To this end, this study aims to develop an energy and acceleration spectra-compatible record selection and scaling methodology to achieve higher efficiency and lower bias in the predicted structural response. The efficiency of the proposed method is evaluated through the standard deviations of the computed story drifts of benchmark structures resulting from the records processed by either the proposed or commonly used methods. The results demonstrated that considering input energy together with spectral acceleration for the selection and scaling of the records can considerably reduce the bias in structural response, especially for structures located on stiff soils.