Asymmetric Building Structures Research Articles

Reducing the torsional behavior in irregular special moment resisting frames with steel dampers

Torsional behavior occurs during the earthquake and it caused displacement on the structure. This study evaluated the used of the metallic damper to the asymmetric three-story building structure in various ground motions scaled to response spectrum. Significant strength and stiffness enhancements were achieved in structure incorporate with damper. Seismic performance evaluation revealed that the used of the damper can reduce lateral displacement approximately 57-65%. The damper could dissipate the energy 28-36%. The use of the damper effectively justified proposed method applicability to seismic structural design.

English

English

Fuzzy neural network control algorithm for asymmetric building structure with active tuned mass damper

Asymmetric structures experience torsional effects when subjected to seismic excitation. The resulting rotation will further aggravate the damage of the structure. A mathematical model is developed to study the translation and rotation response of the structure during seismic excitation. The motion equations of the structures which cover the translation and rotation are obtained by the theoretical derivations and calculations. Through the simulated computation, the translation and rotation response of the structure with the uncontrolled system, the tuned mass damper control system, and active tuned mass damper control system using linear quadratic regulator algorithm are compared to verify the effectiveness of the proposed active control system. In addition, the linear quadratic regulator and fuzzy neural network algorithm are used to the active tuned mass damper control system as a contrast group to study the response of the structure with different active control method. It can be concluded that the structure response has a significant reduction by using active tuned mass damper control system. Furthermore, it can be also found that fuzzy neural network algorithm can replace the linear quadratic regulator algorithm in an active control system. Because fuzzy neural network algorithm can control the process on an uncertain mathematical model, it has more potential in practical applications than the linear quadratic regulator control method.

Damage detection in asymmetric buildings using vibration-based techniques

In recent times, esthetic and functionality requirements have caused many buildings to be asymmetric. An asymmetric building can be defined as one in which there is either geometric, stiffness, or mass eccentricity. Such buildings exhibit complex vibrations as there is coupling between the lateral and torsional components of vibration and are referred to as torsionally coupled buildings. These buildings require 3-dimensional modeling and analysis. Despite recent research and successful applications of damage detection techniques in civil structures, assessing damage in asymmetric buildings remains a challenging task for structural engineers. There has been considerably less investigation on the methodologies for detecting and locating damage specific to torsionally coupled asymmetric buildings. This paper develops a multicriteria approach using vibration-based damage indices for detecting and locating damage in asymmetric building structures. These vibration indices are based on the modified versions of the modal flexibility and the modal strain energy methods. The proposed procedure is first validated through experimental testing of a laboratory scale asymmetric building model. Numerically simulated modal data of a larger scale asymmetric building obtained from finite element analysis of the intact and damaged asymmetric building models are then applied into the modified modal flexibility and modal strain energy algorithms for detecting and locating the damage. Results show that the proposed method is capable of detecting both single and multiple damages in the beams and columns of asymmetric building structures.

Optimal control of a plan asymmetric structure using magnetorheological dampers

Although building structures can be perceived as a combination of primary frames in two orthogonal directions, they are three-dimensional systems that usually present a very complex dynamic behavior due to irregular geometric configurations, in particular due to plant stiffness or mass eccentricities. This asymmetric geometry results in coupled lateral–torsional motion produced by wind and seismic loading with consequences in the design of lateral and corner columns. A considerable amount of research effort has been devoted to develop structural control systems to reduce the effects of plan asymmetries and to improve the dynamic behavior of these buildings. This paper presents a numerical analysis of a semi-active control system with MR dampers designed to reduce lateral–torsional responses of a plan asymmetric building structure excited by El Centro NS earthquake ground motion. A parametric study comprising passive and semi-active control modes is given to demonstrate the effectiveness of the proposed control system with respect to uncontrolled case. The numerical results prove the efficiency of the semi-active control system and its potential use in mitigating coupled lateral–torsional structural responses.

Numerical Verification of the Effectiveness of the “Alpha” Method for the Estimation of the Maximum Rotational Elastic Response of Eccentric Systems

In previous research works, the authors have identified a key system parameter which controls the maximum rotational response under free and forced vibrations of one-story linear-elastic systems representative of asymmetric seismic base-isolated building structures. This parameter (called “ALPHA”) has also led to the identification of a simplified procedure (called “ALPHA method”) for the estimation of the maximum rotational response of such systems. The main goal of this article is to verify the properties of the ALPHA parameter and the predictive capabilities of the ALPHA method, when applied to one-story systems representative of generic asymmetric building structures. The verification is carried out through a comprehensive set of 11,600 numerical simulations developed with reference to different representative structures subjected to historically recorded ground motions, with special attention devoted to the identification of the sensitivity of the ALPHA parameter and the ALPHA method upon the fundamental period of vibration of the structure (not yet considered in previous research works). The results obtained: (a) confirm the effectiveness of the ALPHA parameter to capture the intrinsic propensity of an eccentric system to develop a torsional response; (b) confirm the capacity of the ALPHA method to effectively estimate the maximum rotational response of a given eccentric system under seismic excitation; and (c) indicate that the ALPHA method is only weakly sensitive upon the period of vibration of the structure. The article also introduces a simple code-like provision for conservative estimations of the maximum rotation developed under seismic input by asymmetric structures based upon the confidence interval concepts.

Selection and Scaling of Real Accelerograms for Bi-Directional Loading: A Review of Current Practice and Code Provisions

The seismic behavior of asymmetric building structures is often complex and reductions to plane problems which can be analyzed with a single horizontal ground-motion component are often deemed unsatisfactory. As a consequence, dynamic structural analyses with both horizontal ground-motion components become more common, both in research as well as in practice. A review of code provisions regarding selection and scaling of ground motions for bi-directional analysis has, however, revealed that the guidelines provided are frequently inconsistent or are lacking transparency regarding the underlying assumptions. The aim of this study is to shed some light on a number of aspects involved when selecting and scaling records for bi-directional analysis and post-processing results of such analyses.

A simplified approach for seismic calculation of a tall building braced by shear walls and thin-walled open section structures

In this paper an approximate hand-method for seismic analysis of an asymmetric building structure having constant properties along its height is presented. The building is stiffened by a combination of shear walls and thin-walled open section structures. Based on the continuum technique and d’Alembert’s principle, the governing equations of free vibration and the corresponding eigenvalue problem are derived. By applying the Galerkin technique, a generalized method is proposed for the free vibration analysis of coupled vibration of a building braced by shear walls and thin-walled open section structures. Simplified formulae are given to calculate the circular frequencies and internal forces of a building structure subjected to earthquakes. The utility and accuracy of the method is demonstrated by an numerical example, in which the proposed method is compared with finite element calculations.

Simplified seismic analysis of asymmetric building systems

AbstractThe paper reviews the uncoupled modal response history analysis (UMRHA) and modal pushover analysis (MPA) procedure in the analysis of asymmetric structures. From the pushover curves in ADRS format, showing the relationships of base shear versus roof translation and base torque versus roof rotation, a bifurcating characteristic of the pushover curves of an asymmetric structure is observed. A two‐degree‐of‐freedom (2DOF) modal stick is constructed using lump mass eccentrically placed at the end of beam which is connected with the column by a rotational spring. By converting the equation of motion of a whole structure into 2DOF modal equations, all of the elastic properties in the 2DOF modal sticks can be determined accurately. A mathematical proof is carried out to demonstrate that the 2DOF modal stick is consistent with the single‐degree‐of‐freedom (SDOF) modal stick at elastic state. The bifurcating characteristic of modal pushover curves and the interaction of modal translation and rotation can be considered rationally by this 2DOF modal stick. In order to verify the effectiveness of this proposed 2DOF modal stick, a two‐storey asymmetric building structure was analysed by the UMRHA procedure incorporating this novel 2DOF modal sticks (2DMPA) and conventional SDOF modal sticks (SDMPA), respectively. The analytical results are compared with those obtained by nonlinear response history analysis (RHA). It is illustrated that the accuracy of the rotational response histories obtained by 2DMPA is much better than those obtained by SDMPA. Consequently, the estimations of translational response histories on flexible side (FS) and stiff side (SS) of the building structure are also improved. Copyright © 2006 John Wiley & Sons, Ltd.

MÉTODO SIMPLIFICADO DE EVALUACIÓN SÍSMICA DE EDIFICIOS ASIMÉTRICOS

En este artículo se presenta un método aproximado de evaluación del comportamiento sísmico de edificios asimétricos. El método se basa en un análisis no-lineal de empujón, en la reducción de la curva de capacidad a una de comportamiento de un sistema equivalente de un grado de libertad, enel cálculo de su respuesta máxima ante una demanda sísmica considerada y en la transformación de ésta a la correspondiente de la estructura original. En la aplicación del análisis del empujón, la estructura se empuja en dos direcciones ortogonales con fuerzas laterales de distribuciones y proporciones como lo recomienda el reglamento vigente. El potencial del método propuesto paraestimar el desempeño sísmico de edificios asimétricos se demuestra al comparar los resultados de dos ejemplos ilustrativos, con los correspondientes a los de análisis dinámicos no lineales paso a paso.

Optimum distribution of added viscoelastic dampers for mitigation of torsional responses of plan-wise asymmetric structures

In this study a strategy was developed for an appropriate plan-wise distribution of viscoelastic dampers to minimize the torsional responses of an asymmetric structure, with one axis of symmetry subjected to an earthquake-induced dynamic motion. The modal characteristic equations of a single-storey asymmetric structure with four corner columns and added viscoelastic dampers were derived, and a parametric study was performed to identify the design variables that influence the torsional responses. Based on the results of parametric study, a simple and straightforward methodology to find out the optimum eccentricity of added VED to compensate for the torsional effect of a plan-wise asymmetric structure was developed using modal coefficients. The results indicate that the torsional response of asymmetric structures can be reduced significantly following the proposed method, and that the viscoelastic dampers turn out to be more effective than viscous dampers in controlling torsional response of a plan-wise asymmetric building structure.

Elastic analysis of asymmetric tall building structures

AbstractA simplified elastic hand method for estimating forces in asymmetric multi‐bent structures subjected to horizontal loading is presented. The structures may consist of combinations of coupled walls, rigid frames, braced frames and wall‐frames with shear walls. Results for structures that are uniform in height compare closely with results from stiffness matrix analysis. The method is developed from coupled‐wall deflection theory, which is expressed in nondimensional structural parameters. It accounts for bending deformations in all individual members as well as for axial deformations in the vertical members and is, therefore, more accurate for very tall structures. A closed solution of coupled differential equations for deflection and rotation gives the deflected shape along the height of the building. The proposed method of analysis offers a relatively simple and rapid means of comparing the shear forces and bending moments of different stability systems for a proposed tall building. The derivation of equations for analysis shown in this paper are for unisymmetric stability systems only, but the method is also applicable to general asymmetric structures. Copyright © 2001 John Wiley & Sons, Ltd.

Some observations on performance-based and reliability-based seismic design of asymmetric building structures

In previous work by the second author and others, a framework for a reliability-based, performance-based seismic design procedure was presented. That framework was based on static pushover analyses of two-dimensional models of building structures. In this paper, some preliminary observations and results are presented from a study in which the framework is extended to be used with three-dimensional analysis models to explicitly account for the torsional behavior of asymmetric building structures.

Triply Coupled Vibration of Asymmetric Wall-Frame Structures

This paper presents an analysis of the triply coupled vibration of asymmetric wall-frame structure in tall buildings. The governing equation of the natural vibration and its corresponding eigenvalue problem, which is a set of equations for flexural-shear vibrations in laterally orthogonal directions coupled with a warping St. Venant torsional vibration, are developed. By applying the Galerkin method, a generalized approximate approach is developed for the analysis of coupled vibration and for determining the natural frequencies and associated mode shapes of the structure in triply coupled vibration. The results of the proposed method for the example structure show good agreement with those of the FEM analysis. The proposed method has been shown to provide a simple and efficient, yet accurate, means of coupled vibration analysis of asymmetric tall building structures.

Torsional response of building structures to spatial random ground excitations

This paper applies the random vibration method to analyse the coupled lateral and torsional responses of symmetric and asymmetric building structures subjected to spatially varying horizontal multiple ground excitations. The responses of a single-storey building model with different eccentricities and varying numbers of supports are analysed. Ground excitations at the multiple structural supports are assumed to be zero mean spatial random processes. Their intensities and spatial variations are modelled by a spectral density function and an empirical coherency function. The standard deviations of various response quantities such as base shear, torque and relative column displacements are calculated. Numerical results are presented in terms of dimensionless parameters for their general application. Discussions on the torsional responses induced by both spatial ground excitations and building eccentricities are made. The adequacy of the torsional provisions of current seismic codes is also examined.

A planar model for the static and dynamic analysis of asymmetric building structures

A new planar model for the static and dynamic computer analysis of building structures is presented. The model is general in its application, and can be applied to buildings that are asymmetric on plan, and that may consist of a combination of orthogonally arranged intersecting bents of any type, including shear walls, coupled walls, braced frames and moment resisting frames. The formulation of the model may be described as a folding down of the bents of a building into a single horizontal plane, and connecting them by rigid links so that their relative magnitudes of displacement in the horizontal plane are the same as their slab constrained displacements in the original structure. No transformations of the bent or member properties are required before carrying out the two-dimensional analysis, and the resulting displacements and actions are applicable to the original structure directly, also without transformation. The results for displacements, actions, and periods of vibration compare very closely with those from three-dimensional structural analyses.

Combined seismic base shear and torsional loading provisions in the 1990 edition of the National Building Code of Canada

This paper evaluates the earthquake-resistant design provisions of the 1990 edition of the National Building Code of Canada (NBCC 1990) for asymmetric building structures subjected to combined lateral shear and torsional dynamic loadings arising from earthquake base excitation. A detailed parametric study is presented, evaluating the dynamic edge displacement response in the elastic range, for the side of the building which is adversely affected by lateral–torsional coupling. A series of buildings is studied, with realistic ranges of the fundamental natural period, structural eccentricity, and uncoupled frequency ratio. These buildings are evaluated under base loadings arising from a total of 45 strong motion records taken from earthquakes in North America, Mexico, Europe, the Middle East, and Southern Pacific, categorized according to site soil conditions and the ratio a/v of peak ground acceleration to velocity. The latter parameter together with the uncoupled lateral period are found to influence strongly the combined dynamic edge response, with the greatest forces on edge members arising from earthquakes with high a/v ratio in structures with natural periods below 0.8 s. In this case the NBCC 1990 loading provisions significantly underestimate the elastic dynamic response. For buildings with periods longer than 0.8 s, the conservatism of the base shear provisions leads to overestimation of combined dynamic edge response in asymmetric systems, and this is also true in the short-period range for buildings subjected to ground motions with low a/v ratio. The NBCC 1990 provisions are reasonably conservative for short-period systems subjected to ground motions with intermediate a/v ratio. Key words: earthquakes, seismic, design, response, spectra, base, shear, torsional, provisions.

Evaluation of factors influencing the inelastic seismic performance of torsionally asymmetric buildings

AbstractThis paper addresses some fundamentally contradictory conclusions drawn by Tso and Ying1 and the authors2 regarding the additional seismic ductility demand in asymmetric building structures and the adequacy of certain code torsional provisions. It also clarifies a number of issues arising from the different approaches employed in the two studies. The Mexico 76 and 87 code torsional provisions are taken as examples. Results show that the structural element at the stiff edge is the more critical and that the Mexico 76 code torsional provisions (among others) are inadequate, substantially underestimating the strength demand of this element. On the other hand, the Mexico 87 code torsional provisions are found to be over‐conservative. Recommendations are also given for improving the form and effectiveness of these code torsional provisions.

Simplified P-Delta Analyses for Asymmetric Structures

Computer oriented and manual procedures are presented that enable a direct second order analysis of asymmetric building structures to be performed by means of first order formulation. This is achieved by modeling the geometric-stiffness of the structure as a fictitious column with negative lateral and torsional stiffness properties. Simplified methods are proposed for structures amenable to some form of uncoupling. The approach is also applied to elastic-plastic analysis of asymmetric structures by combining center-of-rigidity and sway-subassemblage procedures. A numerical example is provided.

Generation of lateral and rotational floor response spectra by an alternative approach

AbstractA procedure is described to compute the lateral and rotational floor response spectra of an asymmetric reactor building structure without a time history analysis. The spectral values obtained by filtering the prescribed ground motion first through the structure and the resulting lateral‐rotational motions through simple oscillators are equal to the maximum lateral‐rotational responses of the structure developed when the order of filtration is reversed. Based on the preceding concept a deterministic method is presented to construct the lateral‐rotational floor response spectra utilizing the response spectrum technique.