Seismic Performance Of Structural Systems Research Articles

Improving seismic performance of structural systems via reinforcing column bases

Improving seismic performance of structural systems via reinforcing column bases

Computational and experimental study of structural fuses optimized to resist buckling

The concept of structural fuses, i.e., replaceable hysteretic dampers that limit the forces that can be transferred to the surrounding structure, has been used in research and practice to improve the seismic performance of structural systems. A recent study used topology optimization to develop new structural fuse shapes that discourage buckling and promote stable yielding of shear-acting structural fuses. The objective of the current study is to further validate and develop the optimized shapes to allow the design and application of these optimized structural fuse shapes in practice. First, the pixelated optimized topology was interpreted as a collection of geometric shapes, parameterized to make it accessible for design, and plastic mechanism analysis was used to develop a generalized equation for shear strength. An experimental study was conducted on three specimens that were 711 mm by 762 mm by 25 mm thick, to evaluate strength, stiffness and hysteretic behavior. Finite element models were then validated against the experimental results, and used to conduct a computational parametric study to evaluate the effect of varying key design parameters on strength, stiffness, out-of-plane buckling, effective plastic strain, and energy dissipation. The experimental study showed that one of the optimized geometries possessed a good balance between buckling resistance and fracture resistance reaching approximately 11% shear angle without either. Design guidance is provided for this topology including a parameterized method for reproducing the shape, design equations, and recommendations for proportioning.

Multi-level simulation studies on seismic performance evaluation of steel-sheathed cold-formed steel framing shear walls

Cold-formed steel (CFS) framing shear walls has been widely used as the lateral force resisting system of light-weight steel structures. While the behaviors of walls have been numerically studied, there is a need to develop more convenient and efficient modeling strategies. In this paper, a simulation framework consisting of component-level to system-level analysis for the seismic evaluation of steel-sheathed CFS walls was proposed by using OpenSees software. Based on the improved fastener-based method, refined numerical models were developed and validated against tests of steel sheathed CFS shear walls. Self-drilling screw connection tests were conducted to characterize the nonlinear properties of the member-to-sheathing connections as inputs in the models. Then, a set of structural archetypes were systematically analyzed by nonlinear static and incremental dynamic analysis in accordance with FEMA P695. The results showed that the proposed simulation framework could well capture the response of CFS walls and evaluate the seismic performance of structural system. The post-peak behavior of wall had a significant impact on the collapse resistance of the structure. The seismic response modification coefficient value was suggested (R = 4.25) for the high-performance steel-sheathed CFS framing shear walls studied in this paper.

Highlighting the need for multiple loading protocols in bi-directional testing

Major earthquakes, such as the Canterbury and Kaikoura events recorded in New Zealand in 2010-2011 and 2016 respectively, highlighted that floor systems can be heavily damaged. Quasi-static cyclic experimental tests of structural sub-assemblies can help to establish the seismic performance of structural systems. However, the experimental performance obtained with such tests is likely to be dependent on the loading protocol adopted. This paper provides an overview of the loading protocols which have been assumed in previous experimental activities, with emphasis on those adopted for testing floor systems. The paper also describes the procedure used to define the loading protocol applied in the testing of a large precast concrete floor diaphragm as part of the ReCast floor project jointly conducted by the University of Canterbury, the University of Auckland and BRANZ. Subsequently the limitations of current loading protocols for bi-directional testing are discussed. The relevance of local seismicity on bidirectional demand is demonstrated by examining a large dataset of records from the RESORCE database. It is concluded that bi-directional experimental testing be undertaken using at least two loading protocols that impose different ratios of demand in orthogonal directions.

Hysteretic Behavior of Composite Vertical Connection Structures used in Prefabricated Shear Wall Systems

An innovative composite vertical connecting structure (CVC) with capacity carrying and energy-dissipating ability is proposed in this study, which could be used in prefabricated composite shear wall structural systems to enhance the resilience and seismic performance of structural system. The CVC structure is mainly composed of three parts, including the connecting zone, the capacity bearing zone characterized by high strength and elastic deforming ability, and the energy-dissipating zone assembled by replaceable metal dampers. The low-yield strength steel and high-strength steel are used, respectively, for the metal dampers in the energy-dissipating zone and the concrete-filled high-strength steel tubes in the bearing capacity zone to enhance the energy dissipation and self-centering abilities of CVC structures. The working mechanism is analyzed and validated through finite element models built in ABAQUS. The hysteretic behavior is simulated to evaluate their performance. First, the metal dampers are designed. The theoretical and finite elemental parametric analysis are carried out. According to the simulation results, the “Z-shaped” metal dampers exhibit better energy-dissipating ability than the rectangular shape, in which the “Z-shaped” metal dampers with 45∘show the best performance. Simultaneously, the results of the models calculated by the finite element method and theoretical analysis work very well with each other. Furthermore, seven FE models of shear walls with CVC structures are designed. Monotonic and cyclic loading simulations are conducted. The failure modes and comprehensive mechanical performance are investigated and evaluated according to their calculated force–displacement curves, skeleton curves, and ductility coefficients. The results indicate that the CVC structure delivered preferable lateral-bearing capacity and displacement ductility. Finally, according to available design standards, the lateral stiffness of CVC structures could be conventionally controlled and some practical design recommendations are discussed.

Effect of Outrigger Panels on Seismic Performance of Steel Plate Shear Wall Structural System

Structural systems with steel plate shear wall (SPSW) have been used in many tall buildings in the past decades. This system is particularly suitable for seismic regions because it can dissipate the seismic energy through the inelastic response of steel plate in shear. However, the relatively low lateral stiffness of the SPSW structural system in tall and slender buildings may result in excessive lateral drift and high seismic demands on vertical boundary elements (VBE). Placement of additional SPSW panels in the structure acting as outriggers could significantly reduce the lateral drifts and seismic demand on the VBEs by redistributing some of the overturning forces to adjacent columns. In this paper, a 40-story building is evaluated using steel plate shear wall structural system with and without outriggers. The outrigger panels are placed in 20th, 30th, and 40th story levels and the effectiveness of these elements on the overall seismic behavior is studied. Seven nonlinear time history analyses are carried out and the seismic performance of the structural system with outriggers is compared to the performance of the system without outriggers. The results indicate a significant reduction in lateral drifts and VBE seismic demands when the system is augmented with steel plate outrigger panels.

Ground motions for FEMA P-695 application in subduction zones

Abstract The FEMA P-695 report presents a methodology for the rational quantification of seismic performance factors for use in seismic design. Despite the fact that the methodology is comprehensive and covers a wide range of possible applications, it only provides ground motion sets recorded from shallow crustal earthquakes. Therefore, this paper presents a new set of ground motions aimed at extending the scope of the FEMA P-695 methodology to zones prone to subduction earthquakes. To consider the effect of spectral shapes on collapse capacities, the spectral shape factors SSF were also computed. Four light-frame buildings were analyzed to study the impact of using the new ground motion set. Results showed a decrease in collapse capacities of 12.4%, an increase in peak floor accelerations of 31.7%, and an increase in the dissipated hysteretic energy of 15.7%. Additionally, the number of ground motions and the intensity levels of the set proved to be robust enough to provide the same reliability that can be obtained when employing a much larger ground motion set. The ground motions and the information provided in this paper have been designed to be fully consistent with the FEMA P-695 guidelines. However, their application can be extended to other analyses that evaluate the seismic performance of structural systems in subduction areas. A database that includes the proposed ground motion set is available online.

CONTRIBUTION of GRAVITY FRAMES TO SEISMIC PERFORMANCE OF STEEL MOMENT RESISTING FRAMES

Traditionally, lateral stiffness and strength of the gravity frames in steel buildings are neglected in structural analysis. During the past earthquakes, such as Northridge, USA, 1994 and Kobe, Japan, 1995, unexpected failures were detected at beam-to-column connections of steel moment resisting frames (MRFs). In the aftermath of these earthquakes, extensive research has been carried out to reveal the causes of these failures. Based on the detailed observations, it is likely that the reserve capacity provided by the gravity frames prevented the highly damaged steel buildings from collapsing, since majority of the moment-resisting connections failed prematurely during the Northridge earthquake (1994). Even though the influence of gravity frames (GFs) on structural behavior can be substantial, little attention is paid to evaluate its impact on structural response. With this paper, the contribution of interior GFs in seismic performance of special moment resisting steel frames (SMRFs) is evaluated. For this purpose, 4- and 9-story SMRFs were designed in accordance with the requirements of Draft Turkish Seismic Code (2016). The frames are, then, subjected to incremental dynamic analysis. To evaluate the contribution of the interior GFs on the overall seismic performance of structural system, inelastic behavior of shear tab (simple) connections at beam-to-gravity-only columns were idealized as semi-rigid joints. A general purpose structural analysis software, ETABS, is utilized for the analyses. The results of the study are presented in terms of story drifts, base shear vs. roof displacement.

Seismic risk sensitivity of structures equipped with anti-seismic devices with uncertain properties

Damping and isolation devices are often employed to control and enhance the seismic performance of structural systems. However, the effectiveness of these devices in mitigating the seismic risk may be significantly affected by manufacturing tolerances, and systems equipped with devices whose properties deviate from the nominal ones may exhibit a performance very different than expected.The paper analyzes this problem by proposing a general framework for investigating the sensitivity of the seismic risk of structural systems with respect to system properties varying in a prescribed range. The proposed framework is based on the solution of a reliability-based optimization (RBO) problem, aimed to search for the worst combination of the uncertain anti-seismic device parameters, within the allowed range of variation, that maximizes the seismic demand hazard. A hybrid probabilistic approach is employed to speed up the reliability analyses required for evaluating the objective function at each iteration of the RBO process. This approach combines a conditional method for estimating the seismic demand at a given intensity level, with a simulation approach for representing the seismic hazard.The proposed method is applied to evaluate the influence of the variability of the properties of linear and nonlinear fluid viscous dampers on the seismic risk of a low-rise steel building. The study results show that the various response parameters considered are differently affected by the damper properties and unveil the capability of the proposed approach to evaluate the potentially worst conditions that jeopardize the system reliability.

Reliability-based optimal design of nonlinear viscous dampers for the seismic protection of structural systems

Viscous dampers are widely employed for enhancing the seismic performance of structural systems, and their design is often carried out using simplified approaches to account for the uncertainty in the seismic input. This paper introduces a novel and rigorous approach that allows to explicitly consider the variability of the intensity and characteristics of the seismic input in designing the optimal viscous constant and velocity exponent of the dampers based on performance-based criteria. The optimal solution permits controlling the probability of structural failure, while minimizing the damper cost, related to the sum of the damper forces. The solution to the optimization problem is efficiently sought via the constrained optimization by linear approximation (COBYLA) method, while Subset simulation together with auxiliary response method are employed for the performance assessment at each iteration of the optimization process. A 3-storey steel moment-resisting building frame is considered to illustrate the application of the proposed design methodology and to evaluate and compare the performances that can be achieved with different damper nonlinearity levels. Comparisons are also made with the results obtained by applying simplifying approaches, often employed in design practice, as those aiming to minimize the sum of the viscous damping constant and/or considering a single hazard level for the performance assessment.

A ground motion prediction equation for novel peak ground fractional order response intensity measures

Peak ground fractional order responses (PGRα) as a generalization of conventional seismic intensity measures (IMs) such as peak ground acceleration (PGA) and peak ground velocity (PGV) can better predict the seismic performance of structural systems. This paper proposes the first ground motion prediction equation (GMPE) for PGRα for active shallow crustal regions using a subset of the PEER NGA-West2 ground motion database. The model development database consists of 4491 accelerograms from 82 different earthquakes in California with the magnitude and rupture distance ranges of Mw 4.0–7.9 and RRUP 0–300 km, respectively. PGRα intensity measures are computed from the modified Oustaloup’s recursive approximation to Caputo’s definition of differintegral operator. The main functional form of the predictive model is decided by implementing statistical ground motion data-driven testing methods such as the likelihood approach and Euclidean distance concept. The final functional form of the predictive model accounts for magnitude, distance, style-of-faulting, linear and nonlinear site, hanging wall, basin response, and anelastic distance attenuation effects, and models the aleatory variability with respect to Mw and VS30. The final predictive model produces PGA (α = 0), PGV (α = −1), and peak ground fractional order responses at 19 different α values ranging from −0.05 to −0.95 for the average horizontal component. The proposed predictive model draws estimates of ground motion amplitudes that are consistent with those from the NGA-West2 models for PGA and PGV for sample earthquake scenarios. Moreover, it can offer a basis for predictive modeling of peak ground fractional order response quantities for performance assessment of structures and infrastructures across a region.

A new integration scheme for application to seismic hybrid simulation

Hybrid simulation is a powerful test method for evaluating the seismic performance of structural systems. This method makes it feasible that only critical components of a structure be experimentally tested. This paper presents a newly proposed integration algorithm for seismic hybrid simulation which is aimed to extend its capabilities to a wide range of systems where existing methods encounter some limitations. In the proposed method, which is termed the variable time step (VTS) integration method, an implicit scheme is employed for hybrid simulation by eliminating the iterative phase on experimental element, the phase which is necessary in regular implicit applications. In order to study the effectiveness of the VTS method, a series of numerical investigations are conducted which show the successfulness of the VTS method in obtaining accurate, stable and converged responses. Then, in a comparative approach, the improved accuracy of the VTS method over commonly used integration methods is demonstrated. The stability of the VTS method is also studied and the results show that it provides conditional stability; however, its stability limit is well beyond the accuracy limit. The effect of time delay on the VTS method results is also investigated and it is shown that the VTS method is quite successful in handling this experimental error.

Fragility Analysis – A Tool to Assess Seismic Performance of Structural Systems

In the last few decades, there has been a growing trend to utilise reliability based methods of seismic evaluation for assessing the vulnerability of structures subject to ground motion vibrations. The traditional deterministic approach for seismic assessment of structures was found to be inadequate owing to the uncertainties and randomness commonly encountered under the effect of an earthquake [3]. With the advent of digital technology, accuracy of seismic analysis increased which paved way for probabilistic approaches which offer a more realistic and rational approach. Fragility analysis is one such approach which correlates demand and capacity to establish the probabilistic characterization of the demands with respect to certain limit states. Hence the present work is primarily focussed on presenting the potential of fragility curves in performance assessment of structural systems. Building structures of various configurations and irregularities introduced to achieve certain functional benefits are also considered in this study. Nonlinear static analysis is carried out using Capacity Spectrum method on the structural models using SAP2000 and the results discussed in section 5 clearly demonstrate the beneficial aspects of the fragility tool.

モルタル外壁の変形拘束効果を考慮した解析モデルによる木造住宅の実大振動台実験の再現

By the recent researches of the shaking table tests of wooden post-and-beam houses, the effect to the seismic performance by exterior walls was not negligible. However, it is difficult to estimate their effect to the seismic performance of structural system precisely, when the exterior panels straddle multiple stories or shear walls. Moreover, if the exterior walls are made by wet construction (e.g. mortar plastering), the analysis of seismic performance of whole structure is more complex, because the composite effect of plastered wall according to plastering area cannot be ignored. By the analysis of the load-displacement relationships of the shaking table test conducted at E-defense (NIED, Miki-city, Japan) on 2005, it was found that the story shear of the experimental result of earthquake input was much higher than the estimation of the story shear by the summation of the results of the static shear tests of the wall element used in shaking table specimen. This margin was estimated to be due to some differences between static tests and dynamic shaking table tests (i.e. dynamic loading effect, twist behavior). Some analytical approaches for this shaking table tests indicated that it was because of the multi-story behavior of mortar wall or the underestimation of shear performance of structural elements. It was important to understand the effect to the seismic performance by exterior walls for the precise estimation of the seismic performance of real wooden houses. In this paper, the new numerical analysis model for exterior walls like mortar walls in Japanese post-and-beam wooden houses was suggested and the reproductive numerical analyses of the shaking table tests conducted at E-defense were carried to estimate the effect to the seismic performance by exterior walls. The two shaking table test specimens that had the mortar plastered exterior walls and mud plastered interior walls were used as target specimens. In the analytical model of them, shear performance of the shear walls and tensile performance of the joints between frames were decided by the result of element tests and the test of literatures. The two modeling methods for the exterior mortar walls were used. One is the brace substitution (BR-model) and the other is consists of the panel elements and linked springs (PN-model). The panel elements were rigid and connected to the wooden frames by two-direction shear spring. The neighboring panel elements were linked each other by the link springs. By this modeling method, the composite effect of plastered wall according to plastering area was realized. By the numerical analysis of the lateral loading for shear walls and the shaking table tests conducted at E-defense, followings was found. 1) The numerical analysis results of PN-model for the lateral loading of shear walls were agreed with the theoretical results by the calculating method that estimate the rigidity and the yield shear force of shear walls. 2) By comparison of BR-model with PN-model, the multi-story effects and size effect of the mortar walls increase the story shear of the first-story by 20 to 60% in push-over analysis and by 15 to 55% in time-history response analysis. 3) By the numerical analysis of shaking table tests, it was found that the multi-story effects and size effect of the mortar walls were realized by our modelling method. 4) The upper deformation of column end joints of the numerical models was smaller than the experimental results. The improvement of skeleton curves of springs for analytical models is needed.

Model updating method for substructure pseudo‐dynamic hybrid simulation

SUMMARYSubstructure hybrid simulation has been actively investigated and applied to evaluate the seismic performance of structural systems in recent years. The method allows simulation of structures by representing critical components with physically tested specimens and the rest of the structure with numerical models. However, the number of physical specimens is limited by available experimental equipment. Hence, the benefit of the hybrid simulation diminishes when only a few components in a large system can be realistically represented. The objective of the paper is to overcome the limitation through a novel model updating method. The model updating is carried out by applying calibrated weighting factors at each time step to the alternative numerical models, which encompasses the possible variation in the experimental specimen properties. The concept is proposed and implemented in the hybrid simulation framework, UI‐SimCor. Numerical verification is carried out using two‐DOF systems. The method is also applied to an experimental testing, which proves the concept of the proposed model updating method. Copyright © 2013 John Wiley & Sons, Ltd.

To Scale or Not to Scale Seismic Ground-Acceleration Records

Current estimates of seismic structural fragilities are commonly made on the basis of finite collections of actual or virtual ground-acceleration records that are scaled to have the same scalar intensity measure, for example, peak ground acceleration or pseudospectral acceleration. This paper models seismic ground-acceleration records by samples of Gaussian processes X(t) and constructs scaled versions X˜(t) of X(t) by following current procedures. This analysis shows that X˜(t) and X(t) have different probability laws, so that fragilities on the basis of X˜(t) provide limited if any information on the seismic performance of structural systems, that is, fragilities on the basis of X(t). The usefulness of current fragility estimates on the basis of scaled seismic ground-acceleration records is questionable, and scaling ground motions is not recommended.