Wave dispersion in high‐rise buildings due to soil–structure interaction

Nonparametric techniques for estimation of wave dispersion in buildings by seismic interferometry are applied to a simple model of a soil–structure interaction (SSI) system with coupled horizontal and rocking response. The system consists of a viscously damped shear beam, representing a building, on a rigid foundation embedded in a half‐space. The analysis shows that (i) wave propagation through the system is dispersive. The dispersion is characterized by lower phase velocity (softening) in the band containing the fundamental system mode of vibration, and little change in the higher frequency bands, relative to the building shear wave velocity. This mirrors its well‐known effect on the frequencies of vibration, i.e. reduction for the fundamental mode and no significant change for the higher modes of vibration, in agreement with the duality of the wave and vibrational nature of structural response. Nevertheless, the phase velocity identified from broader band impulse response functions is very close to the superstructure shear wave velocity, as found by an earlier study of the same model. The analysis reveals that (ii) the reason for this apparent paradox is that the latter estimates are biased towards the higher values, representative of the higher frequencies in the band, where the response is less affected by SSI. It is also discussed that (iii) bending flexibility and soil flexibility produce similar effects on the phase velocities and frequencies of vibration of a building. Copyright © 2014 John Wiley & Sons, Ltd.     

1D System identification of a 54‐story steel frame building by seismic interferometry

A 54‐story steel, perimeter‐frame building in downtown Los Angeles, California, is identified by a wave method using records of the Northridge earthquake of 1994 (M_L = 6.4, R = 32 km). The building is represented as a layered shear beam and a torsional shaft, characterized by the corresponding velocities of vertically propagating waves through the structure. The previously introduced waveform inversion algorithm is applied, which fits in the least squares sense pulses in low‐pass filtered impulse response functions computed at different stories. This paper demonstrates that layered shear beam and torsional shaft models are valid for this building, within bands that include the first five modes of vibration for each of the North–South (NS), East–West (EW), and torsional responses (0–1.7 Hz for NS and EW, and 0–3.5 Hz for the torsional response). The observed pulse travel time from ground floor to penthouse level is τ ≈1.5 s for NS and EW and τ ≈ 0.9 s for the torsional responses. The identified equivalent uniform shear beam wave velocities are β_eq ≈ 140 m/s for NS and EW responses, and 260 m/s for torsion, and the apparent Q ≈ 25 for the NS and torsional, and ≈14 for the EW response. Across the layers, the wave velocity varied 90–170 m/s for the NS, 80–180 m/s for the EW, and 170–350 m/s for the torsional responses. The identification method is intended for use in structural health monitoring. Copyright © 2013 John Wiley & Sons, Ltd.     

In situ experiments and seismic analysis of existing buildings. Part II: Seismic integrity threshold

The interest of in situ measurements (presented in Part I paper) for a seismic assessment of existing buildings is analysed in this paper. It is shown that the experimental modal characteristics obtained on regular concrete structures are described successfully by suited Timoshenko beam modelling. For a given structure, taking into account the experimental data, the corresponding beam model, and choosing the maximum tensile strain of concrete as damage criterion for key structural elements, a maximum level of the ground acceleration can be determined. This so‐called seismic integrity threshold is directly related to the onset of structural damages. This new approach is illustrated on one of the studied buildings. The advantages of using ambient vibrations survey for the vulnerability assessment of existing buildings are discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Estimation of element‐by‐element demand‐to‐capacity ratios in instrumented SMRF buildings using measured seismic response

This paper presents a methodology to estimate element‐by‐element demand‐to‐capacity ratios in instrumented steel moment‐resisting frames subject to earthquakes. The methodology combines a finite element model and acceleration measurements at various points throughout the building to estimate time history of displacements and internal force demands in all members. The estimated demands and their uncertainty are compared with code‐based capacity from which probabilistic bounds of demand‐to‐capacity ratios are obtained. The proposed methodology is verified using a simulated six‐story building and validated using acceleration data from California Strong Motion Instrumentation Programstation 24370 during the Northridge and Sierra Madre earthquakes.     

Dual‐plastic hinge design concept for reducing higher‐mode effects on high‐rise cantilever wall buildings

This paper explores the notion of detailing reinforced concrete structural walls to develop base and mid‐height plastic hinges to better control the seismic response of tall cantilever wall buildings to strong shaking. This concept, termed here dual‐plastic hinge (DPH) concept, is used to reduce the effects of higher modes of response in high‐rise buildings. Higher modes can significantly increase the flexural demands in tall cantilever wall buildings. Lumped‐mass Euler–Bernoulli cantilevers are used to model the case‐study buildings examined in this paper. Buildings with 10, 20 and 40 stories are designed according to three different approaches: ACI‐318, Eurocode 8 and the proposed DPH concept. The buildings are designed and subjected to three‐specific historical strong near‐fault ground motions. The investigation clearly shows the dual‐hinge design concept is effective at reducing the effects of the second mode of response. An advantage of the concept is that, when combined with capacity design, it can result in relaxation of special reinforcing detailing in large portions of the walls. Copyright © 2009 John Wiley & Sons, Ltd.     

A methodology for defining seismic scenario‐structure‐based limit state criteria for rc high‐rise wall buildings using net drift

As a result of population growth and consequent urbanization, the number of high‐rise buildings is rapidly growing worldwide resulting in increased exposure to multiple‐scenario earthquakes and associated risk. The wide range in frequency contents of possible strong ground motions can have an impact on the seismic response, vulnerability and limit states definitions of RC high‐rise wall structures. Motivated by the pressing need to derive more accurate fragility relations to be used in seismic risk assessment and mitigation of such structures, a methodology is proposed to obtain reliable, Seismic Scenario‐Structure‐Based (SSSB) definitions of limit state criteria. A 30‐story wall building, located in a multi‐seismic scenario study region, is utilized to illustrate the methodology. The building is designed following modern codes and then modeled using nonlinear fiber‐based approach. Uncertainty in ground motions is accounted for by the selection of forty real earthquake records representing two seismic scenarios. Seismic scenario‐based building local response at increasing earthquake intensities is mapped using Multi‐Record Incremental Dynamic Analyses (MRIDAs) with a new scalar intensity measure. Net Inter‐Story Drift (NISD) is selected as a global damage measure based on a parametric study involving seven buildings ranging from 20 to 50 stories. This damage measure is used to link local damage events, including shear, to global response under different seismic scenarios. While the study concludes by proposing SSSB limit state criteria for the sample building, the proposed methodology arrives at a reliable definition of limit state criteria for an inventory of RC high‐rise wall buildings under multiple earthquake scenarios. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of fundamental period of low‐rise and mid‐rise reinforced concrete buildings

Fundamental period of vibration appears to be one of the most critical parameter for the seismic design of buildings because this period strongly affects the magnitude of seismic forces. In this paper, an empirical formula for estimating the fundamental period of reinforced concrete structures is recommended, on the basis of the vibration analysis of 20 different real building configurations. These structures have already been constructed in Greece, and they are analyzed by using in detail 3‐D finite element models and modal eigenvalue analysis. These models take into account the presence of external and internal infill walls, which are usually ignored as nonstructural elements. This neglect leads to unreliable evaluation of period because the infill walls' contribution to the lateral stiffness and therefore to the fundamental period of vibration is also ignored. Furthermore, taking into account that the flexibility of soil elongates the fundamental period, the soil–structure interaction effect is also considered. To achieve a unique, simple, and effective empirical expression for the fundamental period of vibration, a comprehensive nonlinear regression analysis is applied for the datasets of buildings under consideration. This empirical expression is also compared with the similar expressions from the pertinent literature. Copyright © 2013 John Wiley & Sons, Ltd.     

The cause of unproportionately large higher mode contributions in the inelastic seismic responses of high‐rise core‐wall buildings

In the conventional seismic design of high‐rise reinforced concrete core‐wall buildings, the design demands such as design shear and bending moment in the core wall are typically determined by the response spectrum analysis procedure, and a plastic hinge is allowed to form at the wall base to limit the seismic demands. In this study, it is demonstrated by using a 40‐story core‐wall building that this conventional approach could lead to an unsafe design where the true demands—the maximum inelastic seismic demands induced by the maximum considered earthquake—could be several times greater than the design demands and be unproportionately dominated by higher vibration modes. To identify the cause of this problem, the true demands are decomposed into individual modal contributions by using the uncoupled modal response history analysis procedure. The results show that the true demands contributed by the first mode are reasonably close to the first‐mode design demands, while those contributed by other higher modes are much higher than the corresponding modal design demands. The flexural yielding in the plastic hinge at the wall base can effectively suppress the seismic demands of the first mode. For other higher modes, however, a similar yielding mechanism is either not fully mobilized or not mobilized at all, resulting in unexpectedly large contributions from higher modes. This finding suggests several possible approaches to improve the seismic design and to suppress the seismic demands of high‐rise core‐wall buildings. Copyright © 2012 John Wiley & Sons, Ltd.     

Seismic loss estimation of non‐ductile reinforced concrete buildings

Non‐ductile reinforced concrete buildings represent a prevalent construction type found in many parts of the world. Due to the seismic vulnerability of such buildings, in areas of high seismic activity non‐ductile reinforced concrete buildings pose a significant threat to the safety of the occupants and damage to such structures can result in large financial losses. This paper introduces advanced analytical models that can be used to simulate the nonlinear dynamic response of these structural systems, including collapse. The state‐of‐the‐art loss simulation procedure developed for new buildings is extended to estimate the expected losses of existing non‐ductile concrete buildings considering their vulnerability to collapse. Three criteria for collapse, namely first component failure, side‐sway collapse, and gravity‐load collapse, are considered in determining the probability of collapse and the assessment of financial losses. A detailed example is presented using a seven‐story non‐ductile reinforced concrete frame building located in the Los Angeles, California. Copyright © 2012 John Wiley & Sons, Ltd.     

Viscoelastic coupling dampers (VCDs) for enhanced wind and seismic performance of high‐rise buildings

As high‐rise buildings are built taller and more slender, their dynamic behavior becomes an increasingly critical design consideration. Wind‐induced vibrations cause an increase in the lateral wind design loads, but more importantly, they can be perceived by building occupants, creating levels of discomfort ranging from minor annoyance to severe motion sickness. The current techniques to address wind vibration perception include stiffening the lateral load‐resisting system, adding mass to the building, reducing the number of stories, or incorporating a vibration absorber at the top of the building; each solution has significant economic consequences for builders. Significant distributed damage is also expected in tall buildings under severe seismic loading, as a result of the ductile seismic design philosophy that is widely used for such structures. In this paper, the viscoelastic coupling damper (VCD) that was developed at the University of Toronto to increase the level of inherent damping of tall coupled shear wall buildings to control wind‐induced and earthquake‐induced dynamic vibrations is introduced. Damping is provided by incorporating VCDs in lieu of coupling beams in common structural configurations and therefore does not occupy any valuable architectural space, while mitigating building tenant vibration perception problems and reducing both the wind and earthquake responses of the structure. This paper provides an overview of this newly proposed system, its development, and its performance benefits as well as the overall seismic and wind design philosophy that it encompasses. Two tall building case studies incorporating VCDs are presented to demonstrate how the system results in more efficient designs. In the examples that are presented, the focus is on the wind and moderate earthquake responses that often govern the design of such tall slender structures while reference is made to other studies where the response of the system under severe seismic loading conditions is examined in more detail and where results from tests conducted on the viscoelastic material and the VCDs in full‐scale are presented. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of the seismic response of high‐rise buildings supported on tension‐resistant elastomeric isolation bearings

In many applications of seismic isolation, such as in high‐rise construction, lightweight construction, and structures with large height‐to‐width aspect ratios, significant tension forces can develop in bearings, raising concerns about the possible rupture of elastomeric bearings and the uplift of sliding bearings. In this paper, a novel tension‐resistant lead plug rubber bearing (TLRB) with improved tension‐resisting capabilities is developed and experimentally and numerically assessed. This TLRB consists of a common lead plug rubber bearing (LRB) and several helical springs. After describing the theory underlying the behavior of the TLRB, the mechanical properties of reduced‐scale prototype bearings are investigated through extensive horizontal and vertical loading tests. The test results indicate that TLRBs can improve the shear stiffness and tension resistance capacity even under significant tensile loads. A series of shaking table tests on scaled models of high‐rise buildings with different aspect ratios were conducted to investigate the dynamic performance of the TLRB and the seismic responses of base‐isolated high‐rise buildings. Three different cases were considered in the shaking table tests: a fixed base condition and the use of TLRB and LRB isolation systems. The results of the shaking table test show that (a) base‐isolated systems are effective in reducing the structural responses of high‐rise buildings; (b) an isolated structure's aspect ratio is an important factor influencing its dynamic response; (c) TLRBs can endure large tensile stresses and avoid rupture on rubber bearings under strong earthquakes; and (d) the experimental and numerical results of the responses of the models show good agreement. Copyright © 2016 John Wiley & Sons, Ltd.     

Collapse simulation of reinforced concrete high‐rise building induced by extreme earthquakes

Collapse resistance of high‐rise buildings has become a research focus because of the frequent occurrence of strong earthquakes and terrorist attacks in recent years. Research development has demonstrated that numerical simulation is becoming one of the most powerful tools for collapse analysis in addition to the conventional laboratory model tests and post‐earthquake investigations. In this paper, a finite element method based numerical model encompassing fiber‐beam element model, multilayer shell model, and elemental deactivation technique is proposed to predict the collapse process of high‐rise buildings subjected to extreme earthquake. The potential collapse processes are simulated for a simple 10‐story RC frame and two existing RC high‐rise buildings of 18‐story and 20‐story frame–core tube systems. The influences of different failure criteria used are discussed in some detail. The analysis results indicate that the proposed numerical model is capable of simulating the collapse process of existing high‐rise buildings by identifying potentially weak components of the structure that may induce collapse. The study outcome will be beneficial to aid further development of optimal design philosophy. Copyright © 2012 John Wiley & Sons, Ltd.     

Modeling of environmental influence in structural health assessment for reinforced concrete buildings

One branch of structural health monitoring (SHM) utilizes dynamic response measurements to assess the structural integrity of civil infrastructures. In particular,modal frequency is a widely adopted indicator for structural damage since its square is proportional to structural stiffness. However,it has been demonstrated in various SHM projects that this indicator is substantially affected by fluctuating environmental conditions. In order to provide reliable and consistent information on the health status of the monitored structures,it is necessary to develop a method to filter this interference. This study attempts to model and quantify the environmental influence on the modal frequencies of reinforced concrete buildings. Daily structural response measurements of a twenty-two story reinforced concrete building were collected and analyzed over a one-year period. The Bayesian spectral density approach was utilized to identify the modal frequencies of this building and it was clearly seen that the temperature and humidity fluctuation induced notable variations. A mathematical model was developed to quantify the environmental effects and model complexity was taken into consideration. Based on a Timoshenko beam model,the full model class was constructed and other reduced-order model class candidates were obtained. Then,the Bayesian modal class selection approach was employed to select the one with the most suitable complexity. The proposed model successfully characterizes the environmental influence on the modal frequencies. Furthermore,the estimated uncertainty of the model parameters allows for assessment of the reliability of the prediction. This study not only improves the understanding about the monitored structure,but also establishes a systematic approach for reliable health assessment of reinforced concrete buildings.     

Code‐based record selection methods for seismic performance assessment of buildings

The recent concerns regarding the seismic safety of the existing building stock have highlighted the need for an improvement of current seismic assessment procedures. Alongside with the development of more advanced commercial software tools and computational capacities, nonlinear dynamic analysis is progressively becoming a common and preferable procedure in the seismic assessment of buildings. Besides the complexity associated with the formulation of the mathematical model, major issues arise related with the definition of the seismic action, which can lead to different levels of uncertainty in terms of local and global building response. Aiming to address this issue, a comparative study of different code‐based record selection methods proposed by Eurocode 8, ASCE41‐13 and NZS1170.5:2004 is presented herein. The various methods are employed in the seismic assessment of four steel buildings, designed according to different criteria, and the obtained results are compared and discussed. Special attention is devoted to the influence of the number of real ground motion records selected on the estimation of the mean seismic response and, importantly, to the efficiency that is achieved when an additional selection criteria, based on the control of the spectral mismatch of each individual record with respect to the reference response spectrum, is adopted. The sufficiency of the methods with respect to the pairs of M–R of the selected group of records and the robustness of the scaling procedure are also examined. The paper closes with a study which demonstrates the suitability of a simplified probability‐based approach recently proposed for estimating mean seismic demands. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of seismic fragility surfaces for reinforced concrete buildings by means of nonlinear time‐history analysis

Fragility curves are generally developed using a single parameter to relate the level of shaking to the expected structural damage. The main goal of this work is to use several parameters to characterize the earthquake ground motion. The fragility curves will, therefore, become surfaces when the ground motion is represented by two parameters. To this end, the roles of various strong‐motion parameters on the induced damage in the structure are compared through nonlinear time‐history numerical calculations. A robust structural model that can be used to perform numerous nonlinear dynamic calculations, with an acceptable cost, is adopted. The developed model is based on the use of structural elements with concentrated nonlinear damage mechanics and plasticity‐type behavior. The relations between numerous ground‐motion parameters, characterizing different aspects of the shaking, and the computed damage are analyzed and discussed. Natural and synthetic accelerograms were chosen/computed based on a consideration of the magnitude‐distance ranges of design earthquakes. A complete methodology for building fragility surfaces based on the damage calculation through nonlinear numerical analysis of multi‐degree‐of‐freedom systems is proposed. The fragility surfaces are built to represent the probability that a given damage level is reached (or exceeded) for any given level of ground motion characterized by the two chosen parameters. The results show that an increase from one to two ground‐motion parameters leads to a significant reduction in the scatter in the fragility analysis and allows the uncertainties related to the effect of the second ground‐motion parameter to be accounted for within risk assessments. Copyright © 2009 John Wiley & Sons, Ltd.     

我国地震海浪初步分析

总结了我国地震海浪的25次记录,在做初步分析后认为我国的地震海浪多由近海海洋地震引起,其多发地区为渤海沿岸与东南沿海一带.随着对沿海地区开发的深入,这类记载逐渐增多,受灾程度加大.另外分析了在社会历史变迁中,国家对海洋的开发政策、沿海开发下城市近海地理位置、沿海港口及海洋贸易的规模与地震海浪受灾程度之间的关系.     

Experimental behavior of transfer story connections for high‐rise SRC structures under seismic loading

Results from an investigation aimed at assessing seismic behavior of transfer story connections for high‐rise building consisting of steel‐reinforced concrete (SRC) frame and reinforced concrete (RC) core tube are presented. Two types of transfer story connections were experimentally evaluated for adequate strength, ductility and energy dissipation. For each type of connection, two large‐scale subassembly tests were carried out under monotonic and cyclic lateral displacement, respectively. Detailed observations and behavior responses were obtained to contrast the differences between monotonic and cyclic performance of the connections. Test results showed that the SRC column failed before connection collapse and that loading types have little effect on the strength but greatly affect the failure modes and the ductility of the connections. All specimens exhibited good properties for earthquake resistance since they all kept a stable inelastic behavior up to the interstory drift demand suggested by the AISC Seismic Provisions. Based on test observations, support stiffeners with appropriate width‐to‐thickness ratio and mechanical connectors connecting bars with the steel plate are recommended for design purposes in order to achieve more ductile and reliable seismic behavior of transfer story connections. Copyright © 2010 John Wiley & Sons, Ltd.     

Seismic response of 20‐story base‐isolated and fixed‐base reinforced concrete structural wall buildings at a near‐fault site

This paper investigates numerically the seismic response of six seismically base‐isolated (BI) 20‐story reinforced concrete buildings and compares their response to that of a fixed‐base (FB) building with a similar structural system above ground. Located in Berkeley, California, 2 km from the Hayward fault, the buildings are designed with a core wall that provides most of the lateral force resistance above ground. For the BI buildings, the following are investigated: two isolation systems (both implemented below a three‐story basement), isolation periods equal to 4, 5, and 6 s, and two levels of flexural strength of the wall. The first isolation system combines tension‐resistant friction pendulum bearings and nonlinear fluid viscous dampers (NFVDs); the second combines low‐friction tension‐resistant crosslinear bearings, lead‐rubber bearings, and NFVDs. The designs of all buildings satisfy ASCE 7‐10 requirements, except that one component of horizontal excitation, is used in the 2D nonlinear response history analysis. Analysis is performed for a set of ground motions scaled to the design earthquake and to the maximum considered earthquake (MCE). At both the design earthquake and the MCE, the FB building develops large inelastic deformations and shear forces in the wall and large floor accelerations. At the MCE, four of the BI buildings experience nominally elastic response of the wall, with floor accelerations and shear forces being 0.25 to 0.55 times those experienced by the FB building. The response of the FB and four of the BI buildings to four unscaled historical pulse‐like near‐fault ground motions is also studied. Copyright © 2013 John Wiley & Sons, Ltd.     

Development of a hybrid simulation controller for full‐scale experimental investigation of seismic retrofits for soft‐story woodframe buildings

Hybrid simulations of a full‐scale soft‐story woodframe building specimen with various retrofits were carried out as part of the Network for Earthquake Engineering Simulation Research project – NEES‐Soft: seismic risk reduction for soft‐story woodframe buildings. The test structure in the hybrid simulation was a three‐story woodframe building that was divided into a numerical substructure of the first story with various retrofits and a full‐scale physical substructure of the upper two stories. Four long‐stroke actuators, two at the second floor and two at the roof diaphragm, were attached to the physical substructure to impose the simulated seismic responses including both translation and in‐plane rotation. Challenges associated with this first implementation of a full‐scale hybrid simulation on a woodframe building were identified. This paper presents the development and validation of a scalable and robust hybrid simulation controller for efficient test site deployment. The development consisted of three incremental validation phases ranging from small‐scale, mid‐scale, to full‐scale tests conducted at three laboratories. Experimental setup, procedure, and results of each phase of the controller development are discussed, demonstrating the effectiveness and efficiency of the incremental controller development approach for large‐scale hybrid simulation programs with complex test setup. Copyright © 2016 John Wiley & Sons, Ltd.     

Performance‐based seismic assessment of conventional and base‐isolated steel buildings including environmental impact and resilience

Sustainability and resilience are issues that are recognized worldwide, and increased attention should be placed on strategies to design and maintain infrastructure systems that are hazard resilient, damage tolerant, and sustainable. In this paper, a methodology to evaluate the seismic sustainability and resilience of both conventional and base‐isolated steel buildings is presented. Furthermore, the proposed approach is used to explore the difference between the performance associated with these buildings by considering the three pillars of sustainability: economic, social, and environmental. Sustainability and resilience are both considered to cover a comprehensive performance‐based assessment content. The uncertainties associated with performance and consequence evaluation of structural and non‐structural components are incorporated within the assessment process. The proposed performance‐based assessment approach is illustrated on conventional and base‐isolated steel buildings under given seismic scenarios. Copyright © 2015 John Wiley & Sons, Ltd.