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CONTENTS
Volume 2, Number 4, December 2011
 

Abstract
The problem of reconstruction of complete building response from a limited number of response measurements is considered. The response at the intermediate degrees of freedom is reconstructed by using piecewise cubic Hermite polynomial interpolation in time domain. The piecewise cubic Hermite polynomial interpolation is preferred over the spline interpolation due to its trend preserving character. It has been shown that factorization of response data in variable separable form via singular value decomposition can be used to derive the complete set of normal modes of the structural system. The time domain principal components can be used to derive empirical transfer functions from which the natural frequencies of the structural system can be identified by peak-picking technique. A reduced-rank approximation for the system flexibility matrix can be readily constructed from the identified massorthonormal mode shapes and natural frequencies.

Key Words
inverse problem; modal identification; orthogonal decomposition; system identification; structural dynamics.

Address
Manish Shrikhande: Department of Earthquake Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, India

Abstract
In this paper, cable-stayed bridges with single pylon and two equal side spans, with variations in geometry and span ranging from 120 m to 240 m have been studied. 3D models of the bridges considered in this study have been analysed using ANSYS. As the first step towards a detailed seismic analysis, free vibration response of different geometries is studied for their mode shapes and frequencies. Typical pattern of free vibration responses in different frequencies with change in geometry is observed. Further, three different seismic loading histories are chosen with various characteristics to find the structural response of different geometries under seismic loading. Effect of variation in pylon shape, cable arrangement with variation in span is found to have typical characteristics with different structural response under seismic loading. From the study, it is observed that the structural response is very much dependent on the geometry of the cable-stayed bridge and the characteristics of the seismic loading as well. Further, structural responses obtained from the study would help the design engineers to take decisions on geometric shapes of the bridges to be constructed in seismic prone zones.

Key Words
cable-stayed bridge; free vibration; parametric study; seismic response; mode shapes; frequencies; bridge response; time history.

Address
Madhav Bhagwat: Indian Institute of Technology Roorkee, India
Saptarshi Sasmal: CSIR-Structural Engineering Research Centre, CSIR Complex, Chennai, India, University of Stuttgart, Germany
B. Novak: University of Stuttgart, Germany
A. Upadhyay: Indian Institute of Technology Roorkee, India

Abstract
Ground motion models predict the mean and standard deviation of the logarithm of spectral acceleration, as a function of predictor variables such as earthquake magnitude, distance and site condition. Such models have been developed for a variety of seismic environments throughout the world. Some calculations, such as the Conditional Mean Spectrum calculation, use this information but additionally require knowledge of correlation coefficients between logarithmic spectral acceleration values at multiple periods. Such correlation predictions have, to date, been developed primarily from data recorded in the Western United States from active shallow crustal earthquakes. This paper describes results from a study of spectral acceleration correlations from Japanese earthquake ground motion data that includes both crustal and subduction zone earthquakes. Comparisons are made between estimated correlations for Japanese response spectral ordinates and correlation estimates developed from Western United States ground motion data. The effect of ground motion model, earthquake source mechanism, seismic zone, site conditions, and source to site distance on estimated correlations is evaluated and discussed. Confidence intervals on these correlation estimates are introduced, to aid in identifying statistically significant differences in correlations among the factors considered. Observed general trends in correlation are similar to previous studies, with the exception of correlation of spectral accelerations between orthogonal components, which is seen to be higher here than previously observed. Some differences in correlations between earthquake source zones and earthquake mechanisms are observed, and so tables of correlations coefficients for each specific case are provided.

Key Words
ground motion; intensity measure; correlation; risk analysis.

Address
Nirmal Jayaram and Jack W. Baker: Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Room 283, Stanford, 94305, California, USA
Hajime Okano and Hiroshi Ishida: Kajima Corporation, 6-5-30 Akasaka, Minato-ku, Tokyo, 107-8502, Japan
Martin W. McCann, Jr.: Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Room 283, Stanford, 94305, California, USA
Yoshinori Mihara: Kajima Corporation, 6-5-30 Akasaka, Minato-ku, Tokyo, 107-8502, Japan


Abstract
In seismic design and structural assessment using the displacement-based approach, real structures are simplified into equivalent single-degree-of-freedom systems with equivalent properties, namely period and damping. In this work, equations for the optimal pair of equivalent properties are derived using statistical procedures on equivalent linearization and defined in terms of the ductility ratio and initial period of vibration. The modified Clough hysteretic model and 30 artificial accelerograms, compatible with the acceleration spectra for firm and soft soils, defined by the Japanese Design Specifications for Highway Bridges are used in the analysis. The results obtained with the proposed equations are verified and their limitations are discussed.

Key Words
equivalent linearization, equivalent damping, equivalent period.

Address
Fernando Sanchez-Flores: Department of Urban Management, Kyoto University, Japan
Akira Igarashi: Department of Civil and Earth Resources Engineering, Kyoto University, Japan

Abstract
In this paper, the seismic response of elevated broad and slender liquid storage tanks isolated by elastomeric or sliding bearings was investigated. The accuracy of predictions of SAP2000 vs. 3DBASIS-ME programs was examined. A comparative study of the performance of base isolated tanks when isolation bearings are placed at the top or at the bottom of the supporting tower structure was conducted. It was found that base isolation is quite effective in reducing the earthquake response of elevated liquid storage tanks in which high reductions of base shear and shaft displacement were achieved. Modeling the isolated tanks in SAP2000 was very successful in producing results that are nearly identical to those of program 3D-BASIS-ME. Placing the isolators at the top of the shaft in elevated tanks proved to be much better than placing them at the bottom.

Key Words
elevated tanks; seismic isolation; liquid storage; modeling; 3D-BASIS-ME; SAP2000.

Address
Ayman A. Seleemah and Mohamed El-Sharkawy: Structural Engineering Department, Faculty of Engineering, Tanta University, Egypt


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