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CONTENTS
Volume 5, Number 1, July 2013
 

Abstract
Soft storey failure mechanism is a common collapse mode for masonry-infilled (MI) reinforced concrete (RC) buildings subjected to severe earthquakes. Simple analytical equations correlating global with local ductility demands are derived from pushover (PO) analyses for seismic assessments of regular MI RC frames, considering the critical interstorey drift ratio, number of storeys and lateral loading configurations. The reliability of the equations is investigated using incremental dynamic analyses for MI RC frames of up to 7 storeys. Using the analytical ductility relationship and a coefficient-based method (CBM), the response spectral accelerations and period shift factors of low-rise MI RC frames are computed. The results are verified through published shake table test results. In general applications, the analytical ductility relationships thus derived can be used to bypass the onerous PO analysis while accurately predicting thelocal ductility demands for seismic assessment of regular MI RC frames.

Key Words
local ductility; global ductility; coefficient-based ethod; confined masonry; soft storey;nonlinear analysis; period lengthening

Address
R.K.L. Su, T.O. Tang and C.L. Lee:Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, People\'s Republic of China

Abstract
In this study, the effect of flexibility of superstructures and nonlinear characteristics of LRB (Lead Rubber Bearing) isolator on inelastic response of base isolated structures is investigated. To demonstrate the intensity of damage in superstructures, demand response modification factor without the consideration of damping reduction factor, demand RI, is used and the N2 method is applied to compute this factor. To evaluate the influence of superstructure flexibility on inelastic response of base isolated structures, different steel intermediate moment resisting frames with different heights have been investigated. In lead rubber bearing, the rubber provides flexibility and the lead is the source of damping; variations of aforementioned characteristics are also investigated on inelastic response of superstructures. It is observed that an increase in height of superstructure leads to higher value of demand RI till 4-story frame but afterward this factor remains constant; in other words, an increase in height until 4-story frame causes more damage in the superstructure but after that superstructure\'s damage is equal to the 4-story frame\'s. The results demonstrate that the low value of second stiffness (rubber stiffness in LRBs) tends to show a significant decrease in demand RI. Increase in value of characteristic strength (yield strength of the lead in LRBs) leads to decrease in the demand RI.

Key Words
superstructure flexibility; LRB isolator characteristics; inelastic response of base isolated structures; demand RI, N2 method

Address
Rashid Eddin Cheraghi and Ramezan Ali Izadifard:Department of civil engineering, Imam Khomeini International University, Qazvin, Iran

Abstract
This article presents the statistical characteristics of elastic floor acceleration spectra that represent the peak response demand of non-structural components attached to a nonlinear supporting frame.For this purpose, a set of stiff and flexible general moment resisting frames with periods of 0.3- 3.6 sec. are analyzed using forty-nine near-field strong ground motion records. Peak accelerations are derived for each single degree of freedom non-structural component, supported by the above mentioned frames, through a direct-integration time-history analysis. These accelerations are obtained by Floor Acceleration Response Spectrum (FARS) method. They are statistically analyzed in the next step to achieve a better understanding of their height-wise distributions. The factors that affect FARS values are found in the relevant state of the art. Here, they are summarized to evaluate the amplification and/or reduction of FARS values especially when the supporting structures undergo inelastic behavior. The properties of FARS values are studied in three regions: long-period, fundamental-period and short-period. Maximum elastic acceleration response of non-structural component, mounted on inelastic frames, depends on the following factors: inelasticity intensity and modal periods of supporting structure; natural period, damping ratio and location of non-structural component. The FARS values, corresponded to the modal periods of supporting structure, are strongly reduced beyond elastic domain. However, they could be amplified in the transferring period domain between the mentioned modal periods. In the next step, the amplification and/or reduction of FARS values,caused by inelastic behavior of supporting structure, are calculated. A parameter called the response acceleration reduction factor(R acc), has been previously used for far-field earthquakes. The feasibility of extending this parameter for near-field motions is focused here, suggested repeatedly in the relevant sources.The nonlinearity of supporting structure is included in (R acc) for better estimation of maximum non-structural component absolute acceleration demand, which is ordinarily neglected in the seismic design provisions.

Key Words
Floor Acceleration Response Spectrum (FARS); absolute acceleration modification factor;seismic design; stiff and flexible moment resisting frames; near-field strong ground motions

Address
Ali Reza Taghavee Kanee, Iradj Mahmood Zadeh Kani and Assadollah Noorzad:University of Tehran, Enghelab st.,Tehran, Iran

Abstract
Laterite blocks are used for construction of masonry walls since ages in the South-western coastal areas of India. The south-west coastal areas of India lie in zone III of seismic zonation map of Indian code IS 1893-2002. In spite of the fact that laterite is the most favored masonry material in these regions of India, the structural erformance of laterite masonry has not been systematically investigated. Again there are no previous studies addressing, in detail,the seismic performance of laterite masonry buildings. Now that these areas are becoming more and more important from point of view of trade and commerce, there is a need for a detailed research on the seismic response of laterite masonry structures located in these areas. The present paper reports the results of such a study of the seismic response of box-type laterite masonry structures. Time history analysis of these structures under El-Centro acceleration has been performed using commercial finite element software ANSYS. Effect of

Key Words
laterite masonry; box-type structure; finite element analysis; natural frequencies; seismic response; time-history analysis

Address
Sujatha Unnikrishnan, Mattur C. Narasimhan and Katta Venkataramana:Department of Civil Engineering, National Institute of Technology Karnataka,Surathkal, Mangalore 575025, India

Abstract
A simple damper optimization method is proposed to find optimal damper allocation for shear buildings under both target added damping ratio and interstorey drift ratio (IDR). The damping coefficients of added dampers are considered as design variables. The cost, which is defined as the sum of damping coefficient of added dampers, is minimized under a target added damping ratio and the upper and the lower constraint of the design variables. In the first stage of proposed algorithm, Simulated Annealing, Nelder Mead and Differential Evolution numerical algorithms are used to solve the proposed optimization problem. The candidate optimal design obtained in the first stage is tested in terms of the IDRs using linear time history analyses for a design earthquake in the second stage. If all IDRs are below the allowable level,iteration of the algorithm is stopped; otherwise, the iteration continues increasing the target damping ratio.By this way, a structural response IDR is also taken into consideration using a snap-back test. In this study,the effects of the selection of upper limit for added dampers, the storey mass distribution and the storey stiffness distribution are all investigated in terms of damper distributions, cost function, added damping ratio and IDRs for 6-storey shear building models. The results of the proposed method are compared with two existing methods in the literature. Optimal designs are also compared with uniform designs according to both IDRs and added damping ratios. The numerical results show that the proposed damper optimization method is easy to apply and is efficient to find optimal damper distribution for a target damping ratio and allowable IDR value.

Key Words
optimal dampers; target damping ratio; added dampers; optimal passive control; interstorey drift ratio

Address
Ersin Aydin:Faculty of Engineering, Department of Civil Engineering, Nigde University, Nigde, Turkey

Abstract
This paper describes effects of infill walls on behavior of RC frame with low strength, including numerical modeling, modal testing and finite-element model updating. For this purpose full scaled, one bay and one story RC frame is produced and tested for plane and brick in-filled conditions. Ambient-vibration testis applied to identify dynamic characteristics under natural excitations. Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods are used to obtain experimental dynamic characteristics. A numerical modal analysis is performed on the developed two-dimensional finite element model of the frames using SAP2000 software to provide numerical frequencies and mode shapes. Dynamic haracteristics obtained by numerical and experimental are compared with each other and finite element model of the frames are updated by changing some uncertain modeling parameters such as material properties and boundary conditions to reduce the differences between the results. At the end of the study, maximum differences in the natural frequencies are reduced on average from 34% to 9% and a good agreement is found between numerical and experimental dynamic characteristics after finite-element model updating. In addition, it is seen material properties are more effective parameters in the finite element model updating of plane frame.However, for brick in-filled frame changes in boundary conditions determine the model updating process.

Key Words
ambient vibration; dynamic characteristics; finite-element model updating; low strength concrete; RC frames

Address
Mehmet Emin Arslan and Ahmet Durmuş:Karadeniz Technical University, Department of Civil Engineering, 61080,Trabzon, Turkey


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