Seismic nonlinear static new method of spatial asymmetric multi‐storey reinforced concrete buildings

In order to obtain the seismic demands of spatial asymmetric multi‐storey reinforced concrete (r/c) buildings, a new seismic nonlinear static (pushover) procedure that uses inelastic response acceleration spectra is presented in this paper. The latter makes use of the optimum equivalent nonlinear single degree of freedom system, which is used to represent the general spatial asymmetric multi‐storey r/c building. For each asymmetric multi‐storey building, a total of 12 suitable nonlinear static analyses are needed according to the new proposed procedure, whereas at least 96 suitable nonlinear dynamic analyses are required in the case of nonlinear response history analysis (NLRHA), respectively. In addition, the present paper provides answers to a series of further questions with reference to the spatial action of the two horizontal seismic components in the static nonlinear (pushover) analyses, as well as to the documented calculation of the available behaviour factor of the asymmetric multi‐storey r/c building. According to the paper, this new proposed seismic nonlinear static procedure is a natural extension of the documented equivalent seismic static linear (simplified spectral) method that is recommended by the established contemporary seismic codes, with reference to torsional provisions. Finally, through a restricted parametric analysis carried out in this paper, a relevant numerical example of a two‐storey r/c building is presented for illustration purposes, where the seismic demand floor inelastic displacements are compared with the respective displacements obtained by the NLRHA. Consequently, the new proposed seismic nonlinear static procedure, which uses inelastic response acceleration spectra, can reliably evaluate the extreme values of floor inelastic displacements (on the flexible and stiff side of the building), as is shown by the above comparisons. Copyright © 2010 John Wiley & Sons, Ltd.     

Identification of the mode shapes of spatial tall multi‐storey buildings due to earthquakes: the new ‘modal time‐histories’ method

In the present article, a new method is presented which attempts to identify the dynamic characteristics (eigen frequencies, eigen periods, mode shapes and modal damping ratios) of spatial asymmetric tall multi‐storey buildings through measured seismic responses (accelerations). This new method is entitled ‘method of modal time‐histories’, because its main target is to identify the modal time‐histories of accelerations that are obtained by accelerograms recorded on the points of buildings where suitable accelerometers have been installed. In the case of an earthquake, the multi‐channel local network of accelerometers records the time‐histories of the accelerations of the building. In addition, in order to have a successful outcome, the instrumentation form of the multi‐channel local network on the building can potentially play the most important role. This paper, first, presents a relevant mathematical analysis that is adapted to the instrumentation form applied to the multi‐storey buildings and, second, outlines the new method, which consists of nine steps. Finally, in order to illustrate the theory, a suitable numerical example of an instrumented asymmetric five‐storey r/c building that has been oscillated by a weak earthquake is also provided. On the one hand, the identification of the dynamic characteristics of spatial asymmetric buildings contributes to the removal of the uncertainties of building models in order to perform advanced non‐linear analyses about inherent building seismic capacity. On the other hand, this method supports the simple monitoring of a building's ‘structural integrity’. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of soft‐storey mechanism caused by infill elimination on displacement demand in nonlinear static procedure using coefficient method

In recent earthquakes, many buildings have been damaged due to the soft‐storey mechanism failure. The seismic design codes for buildings do not contain enough criteria to predict the real displacement of such buildings. This paper focuses on evaluating the nonlinear displacement of buildings that fail in soft‐storey mechanism form. Results show that the nonlinear static procedure with coefficient method, which is described in Chapter 3 of ASCE/SEI 41‐06, does not have sufficient accuracy for estimation of structure displacement demand in such buildings. In this paper, the coefficient methodology is used for evaluating the target displacement for 5‐storey, 8‐storey and 15‐storey special moment resisting steel frames. For this purpose, dynamic nonlinear time‐history analysis has been applied for the mentioned structures having a soft‐storey mechanism failure form. The numerical results of storey displacement and interstorey drift were compared with those values obtained from the coefficient method described in Chapter 3 of ASCE/SEI 41‐06. Copyright © 2012 John Wiley & Sons, Ltd.     

Testing facility for experimental evaluation of non‐structural components under full‐scale floor motions

The seismic vulnerability of non‐structural components and equipment with their expensive recovery and/or replacement costs has been demonstrated during past earthquakes. With the exception of the nuclear industry, the limited data collected from past earthquakes are not sufficient to completely characterize the seismic behaviour of non‐structural components and to develop effective mitigation measures. To address these limitations, the University at Buffalo has commissioned a dedicated non‐structural component simulator (University at Buffalo non‐structural component simulator (UB‐NCS)) composed of a two‐level testing frame capable of simultaneously subjecting both displacement and acceleration‐sensitive non‐structural components to realistic full‐scale floor motions expected within multi‐storey buildings. Current codes now require seismic qualification of important non‐structural components, and protocols for experimental qualification have been developed considering existing laboratory limitations. The new testing capabilities provided by the UB‐NCS are described to demonstrate the improved and more realistic qualification procedures that can be achieved. Results are presented from a test on two parallel partition walls aimed to verify the capabilities of the UB‐NCS. Copyright © 2008 John Wiley & Sons, Ltd.     

A new pushover procedure for two‐way asymmetric‐plan tall buildings under bidirectional earthquakes

Conventional pushover analyses despite of extensive applications are unable to estimate the general responses of asymmetric‐plan tall buildings because of ignoring the effects of higher modes and torsion. A consecutive modal pushover procedure is one of the recent nonlinear static pushover procedures that used to analyse the seismic response of one‐way asymmetric‐plan tall buildings under one‐directional seismic ground motions. In this paper, a modified consecutive modal pushover procedure (MCMP) has been proposed to estimate the seismic demands of two‐way asymmetric‐plan tall buildings under two horizontal components of earthquakes simultaneously. The accuracy of the MCMP procedure is evaluated using different buildings and comparing with the results of FEMA (Federal Emergency Management Agency) procedures, the practical modal pushover procedure and nonlinear time history analyses as an exact solution. The results show the proposed MCMP procedure is able to estimate the displacements and storey drifts accurately and introduces a great improvement in predicting the plastic hinge rotations. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling of two‐cell cores for three‐dimensional analysis of multi‐story buildings

The reliability of simplified models for single‐cell cores, and particularly for open and semi‐open U‐cross‐section cores, has been the subject of many research papers in the recent past. In contrast, on an international level, only little mention has been made of the efficiency of such models for multi‐cell cores of multi‐story R/C buildings. This paper evaluates and comments on the reliability of several simplified models for open two‐cell cores that are often used in practice. The models examined are: (a) models composed of equivalent columns in alternative configurations; (b) models composed of panel elements; and (c) finite shell element models with one element for each flange in each story. These models are compared with one another and with the solution considered accurate, which is the one obtained by using a finite element method consisting of an adequately dense mesh of finite shell elements. The conclusions obtained refer to both the simplified modal response analysis and the multi‐modal response spectrum analysis, while the specific assumptions for the numerical investigations are compatible with the provisions of modern seismic design codes. Copyright © 2000 John Wiley & Sons, Ltd.     

Integrated multi‐type sensor placement and response reconstruction method for high‐rise buildings under unknown seismic loading

Structural health monitoring system has been implemented on high‐rise buildings to provide real‐time measurement of structural responses for evaluating their serviceability, safety, and sustainability. However, because of the complex structural configuration of a high‐rise building and the limited number of sensors installed in the building, the complete evaluation of structural performance of the building in terms of the information directly recorded by a structural health monitoring system is almost impossible. This is particularly true when seismic‐induced ground motion is unknown. This paper thus proposes an integrated method that enables the optimal placement of multi‐type sensors on a high‐rise building on one hand and the reconstruction of structural responses and excitations using the information from the optimally located sensors on the other hand. The structural responses measured from multi‐type sensors are fused to estimate the full state of the building in the modal coordinates using Kalman filters, from which the structural responses at unmeasured locations and the seismic‐induced ground motion can be reconstructed. The optimal multi‐type sensor placement is simultaneously achieved by minimizing the overall estimation errors of structural responses at the locations of interest to a desired target level. A numerical study using a simplified finite element model of a high‐rise building is performed to illustrate the effectiveness and accuracy of the proposed method. The numerical results show that by using 3 types of sensors (inclinometers, Global Positioning System, and accelerometers), the proposed method offers an effective way to design a multi‐type sensor system, and the multi‐type sensors at their optimal locations can produce sufficient information on the response and excitation reconstruction.     

New nonlinear anti‐seismic steel device for the increasing the seismic capacity of multi‐storey reinforced concrete frames

In order to increase the seismic capacity of multi‐storey planar reinforced concrete (r/c) frames, a new metal frictional device, which the capacity of a restricted rotation around the horizontal axis perpendicular to the vertical frame plane, is presented. The proposed steel device is joined to the four joints of the vertical floor span of the frame via four diagonal steel dual‐hinge bars. During the above restricted rotation, frictional forces develop, due to a suitable synthetic material that is inserted into the rotational frictional connections. When the horizontal relative floor displacement, between two floors, exceeds a desired specific value, then the proposed device locks and the diagonal steel bar becomes fully activated to tension, adding significant additional strength to the frame. This device is installed in a vertical floor span, but it is worth noting that the devices can be placed at floor spans that are not on the same vertical line. The nonlinear numerical (static/dynamic) analyses carried out in the present article, shows that the proposed devices contribute in increase of the lateral stiffness of the frame, the lateral strength of the frame in the inelastic area and the absorption of the inserted seismic energy. Furthermore, this device protects the diagonal steel bars from the buckling or from premature failure of compression. In addition, the proposed device is designed in a way that allows it to operate effectively under large horizontal relative floor displacements due to the cyclic dynamic loads, and can be used instead of the structural r/c walls. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimum structural modelling for tall buildings

It is a common practice to model multi‐storey tall buildings as frame structures where the loads for structural design are supported by beams and columns. Intrinsically, the structural strength provided by the walls and slabs are neglected. As the building height increases, the effect of lateral loads on multi‐storey structures increases considerably. The consideration of walls and slabs in addition to the frame structure modelling shall theoretically lead to improved lateral stiffness. Thus, a more economic structural design of multi‐storey buildings can be achieved. In this research, modelling and structural analysis of a 61‐storey building have been performed to investigate the effect of considering the walls, slabs and wall openings in addition to frame structure modelling. Sophisticated finite element approach has been adopted to configure the models, and various analyses have been performed. Parameters, such as maximum roof displacement and natural frequencies, are chosen to evaluate the structural performance. It has been observed that the consideration of slabs alone with the frame modelling may have negligible improvement on structural performance. However, when the slabs are combined with walls in addition to frame modelling, significant improvement in structural performance can be achieved. Copyright © 2012 John Wiley & Sons, Ltd.     

Seismic analysis of asymmetric multistorey buildings including foundation interaction and P-Δ effects

This paper presents a method of seismic analysis of three-dimensional asymmetric multistorey buildings founded on flexible foundations. The building-foundation system considered in this study is a linear elastic N-storey asymmetric building with a rigid footing resting on the surface of a linear elastic soil half-space. The method of analysis also includes the P-Δ effects, in which the additional overturning moment and torsional moment at each storey due to P-Δ effects have been replaced by fictitious lateral forces and torques. The whole system has 3N + 5 displacement degrees-of-freedom. The necessary governing equations have been developed considering the three motions of each floor and the five motions of the whole building. Recognizing that the superstructure alone admits classical normal modes, the governing equations of the floors are first uncoupled in terms of footing displacements using the mode superposition method. Substitution of structural deformations, in combination with the dynamic soil-structure interaction force-displacement relationships into the governing equations of the whole system results in five integro-differential equations for footing displacements, which are then solved by numerical step-by-step time history analysis. The floor displacement responses, storey shears, storey torque, etc., are obtained by back substitution of footing accelerations into the relevant governing equations. As a demonstration of the method of analysis and in order to obtain the soil-structure interaction effects, P-Δ effects, and the eccentricity effects, a 10-storey asymmetric building on soft soil was subjected to El Centro 1940 earthquake excitations. The results show that the soft soil conditions increase the lateral deflections, but reduce the twists, storey shears and torques. Increasing eccentricity increases the floor twists and storey torques, however, it does not modify the lateral deflections at the centre of mass, and the total storey shears. The significance of P-Δ effects along with soil-structure interactions on the response of this building has also been discussed.     

Base‐isolated structures with selective controlled semi‐active friction dampers

This paper investigates effectiveness of selective control strategy in hybrid base isolation systems including isolators and semi‐active variable friction (VF) dampers. According the selective control strategy, VF dampers are activated just if the displacement at the isolators exceeds the threshold value. The slip‐force control is based on the values of floors' accelerations and velocities. By controlling the slip‐force magnitude in the VF dampers, effective energy dissipation can be achieved in a seismic event. Activation of dampers according to the selective control strategy allows high‐energy dissipation by minimum energy required for adjusting the slip‐force. Performance of a multi‐storey frame with a base isolation system and VF dampers under various earthquake records was obtained numerically using originally developed MATLAB routines. Seismic response of the analysed structure with the selective controlled system was compared with that when the VF dampers were active during the whole earthquake. It is shown that adjustment of the slip‐force in a selective manner allows additional reductions in peak displacements and accelerations of the structure. The results also demonstrate that this control strategy yields reduction of the base displacement without increasing the peak base shear forces. Copyright © 2009 John Wiley & Sons, Ltd.     

Considering dynamic parameters of damaged structure in MPA and YPS nonlinear static procedures for asymmetric buildings

The yield point spectra and modal pushover procedures have already been used in seismic design of new buildings and vulnerability evaluation of existing structures. This paper initially identifies the similarities and differences of the two procedures. Then, the modal characteristics of damaged structure are used to modify their methodologies. The applications of the procedures in estimating displacement, interstory drift index and plastic hinge rotation responses of asymmetric buildings are investigated for three 5‐story reinforced concrete moment‐resisting frame‐building models. The results show considerable improvement in estimating the responses of those asymmetric buildings. Copyright © 2013 John Wiley & Sons, Ltd.     

Hysteretic energy response of steel moment‐resisting frames with vertical mass irregularities

Dynamic analyses were carried out to study the seismic response of high‐rise steel moment‐resisting frames in 16‐storey buildings. The frames are intentionally designed using three different design procedures: strength‐controlled design, strong column–weak beam controlled design, and drift‐controlled design. The seismic performances of the so‐designed frames with vertical mass irregularities were discussed in view of drift ratio, plastic hinge rotation, hysteretic energy input and stress demand. A demand curve of hysteretic energy inputs was also presented with two earthquake levels in peak ground accelerations for a future design application. Copyright © 2004 John Wiley & Sons, Ltd.     

Vulnerability assessment of a high‐rise building subjected to mainshock–aftershock sequences

Strong aftershocks have the potential to further aggravate the damage state of structures, and much less attention has been given to the seismic vulnerability of high‐rise buildings than that of low‐ to medium‐rise buildings. This study assesses the seismic vulnerability of a 32‐storey frame–core tube building by performing the incremental dynamic analysis on the material‐based three‐dimensional numerical model. A storey damage model based on the material damage is developed using the weighted average method. Eighteen recorded mainshock–aftershock sequences, whose mainshock records match the target spectrum, are selected. The results indicate that the developed stroey damage model can effectively reflect the additional damage induced by aftershocks. Strong aftershocks have high potential to change the location of weak storeys. Notably, shifts of weak storeys are observed in more than 30% of aftershocks with relative spectral acceleration of 0.8. As the mainshock‐induced damage state becomes more severe, the mainshock‐damaged building becomes increasingly fragile to the aftershock excitation and more sensitive to aftershock intensities. The probability of exceeding severe damage state increases from 35.3% to 62.1% due to the effects of strong aftershocks. The results in this study can provide supports to the seismic resilience assessment of this high‐rise building.     

Simplified multi‐degree‐of‐freedom model for estimation of seismic response of regular wall‐frame structures

A simplified multi‐degree‐of‐freedom (MDOF) model is developed for estimation of seismic response of tall wall‐frame structures. By using the continuum technique for the structure and adopting the bilinear hysteretic model for material properties, procedure for the development of the simplified MDOF model is derived. The numerical study for a 20‐storey reinforced concrete (RC) wall‐frame structure is conducted to investigate the accuracy of seismic response predicted by the proposed model. Results from the nonlinear response history analyses based on the proposed MDOF model and the detailed structural model with member‐by‐member representation are compared and show very good agreement. The proposed simplified MDOF model is shown to provide a simple, efficient and accurate method for estimation of seismic performance of tall wall‐frame structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Torsional analysis of multi‐storey building structures under horizontal load

New closed‐form formulae are presented for the torsional analysis of asymmetrical multi‐storey buildings braced by moment‐resisting (and/or braced) frames, (coupled) shear walls and cores. The analysis is based on an analogy between the bending and torsion of structural systems. A closed‐form solution is presented for the rotation of the building. The torsional behaviour is defined by three distinctive phenomena: warping torsion, Saint‐Venant torsion and the interaction between the two basic modes. Accordingly, the formula for the maximum rotation of the building consists of three parts: the warping rotation is characterized by the warping stiffness of the bracing system, St Venant rotation is associated with the St Venant stiffness of the building and the third part is responsible for the interaction. It is demonstrated that the interaction between the warping and St Venant modes is always beneficial, as it reduces the rotation of the structure. It is shown how the proposed formula for torsion can be used for the determination of the maximum deflection of multi‐storey asymmetrical building structures. The results of a comprehensive accuracy analysis demonstrate the validity of the method. A worked example is given to show the ease of use of the procedure. Copyright © 2010 John Wiley & Sons, Ltd.     

On the applicability of pushover analysis for seismic evaluation of medium‐ and high‐rise buildings

The assumption that the dynamic performance of structures is mainly determined from the corresponding single‐degree‐of‐freedom system in pushover analysis is generally valid for low‐rise structures, where the structural behaviour is dominated by the first vibration mode. However, higher modes of medium‐ and high‐rise structures will have significant effect on the dynamic characteristics. In this paper, the applicability of pushover analysis for seismic evaluation of medium‐to‐high‐rise shear‐wall structures is investigated. The displacements and internal forces of shear wall structures with different heights are determined by nonlinear response history analysis, where the shear walls are considered as multi‐degree‐of‐freedom systems and modelled by fibre elements. The results of the analysis are compared with those from the pushover procedure. It is shown that pushover analysis generally underestimates inter‐storey drifts and rotations, in particular those at upper storeys of buildings, and overestimates the peak roof displacement at inelastic deformation stage. It is shown that neglecting higher mode effects in the analysis will significantly underestimate the shear force and overturning moment. It is suggested that pushover analysis may not be suitable for analysing high‐rise shear‐wall or wall‐frame structures. New procedures of seismic evaluation for shear‐wall and wall‐frame structures based on nonlinear response history analysis should be developed. Copyright © 2009 John Wiley & Sons, Ltd.     

Shaking‐table test of a friction‐damped frame structure

This paper presents results from an international research project devoted to evaluating the seismic performance of a three‐storey steel frame structure equipped with a friction‐damping device (FDD) recently developed at the Technical University of Denmark. Experimental results indicate that the FDD performed very well in reducing the lateral storey drifts of the test frame. Numerical simulation of the seismic response of the primary and friction‐damped frame was also conducted. This paper also compares the predictions of the displacement demand from the test results with those obtained by the capacity spectrum method. Copyright © 2003 John Wiley & Sons, Ltd.     

Integrated wind‐induced response analysis and design optimization of tall steel buildings using Micro‐GA

This paper presents an integrated procedure for wind‐induced response analysis and design optimization for rectangular steel tall buildings based on the random vibration theory and automatic least cost design optimization technique using Micro‐Genetic Algorithm (GA). The developed approach can predict wind‐induced drift and acceleration responses for serviceability design of a tall building; the technique can also provide an optimal resizing design of the building under wind loads to achieve cost‐efficient design. The empirical formulas of wind force spectra obtained from simultaneous measurements of surface pressures on various rectangular tall building models in wind tunnel tests are verified testified using a published example. Upon the known wind force spectra, the equivalent static wind loads for every storey, such as along‐wind, across‐wind and torsional loads, are then determined and applied for structural analysis including estimation of wind‐induced responses. An improved form of GAs, a Micro‐GA, is adopted to minimize the structural cost/weight of steel buildings subject to top acceleration and lateral drifts constraints with respect to the discrete design variables of steel section sizes. The application and effectiveness of the developed integrated wind‐induced response analysis and design optimization procedure is illustrated through a 30‐storey rectangular steel building example. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimal design and seismic performance of Multi‐Tuned Mass Damper Inerter (MTMDI) applied to adjacent high‐rise buildings

The tuned mass damper inerter (TMDI) is an enhanced variant of the tuned mass damper (TMD) that benefits from the mass‐amplification effect of the inerter. Here, a multi‐TMDI (MTMDI) system (comprising more than one TMDI) linking two adjacent high‐rise buildings is presented as an unconventional seismic protection strategy. The relative acceleration response of the adjacent structures triggers large reaction forces of the inerter devices in the MTMDI, which in turn efficiently improve the seismic performance of the two buildings. By addressing a real project of two adjacent high‐rise buildings connected by two corridors equipped with the proposed MTMDI system, the displacement‐, interstory drift‐, and acceleration‐based parametric optimizations are separately performed by employing Nondominated Sorting Genetic Algorithm II (NSGA‐II) under 44 ground motions from the FEMA P695 far‐field record set. It is found that the frequency content of the seismic input has strong impact on the MTMDI mitigation performance. Adopting realistic mass ratio constraints, the optimally designed MTMDI outperforms both conventional MTMD and single TMDI in acceleration control, while it is not much effective in mitigating the displacement response due to the highly flexible nature of the high‐rise buildings, in contrast to other literature studies generally focused on low‐to‐medium rise buildings.