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CONTENTS
Volume 4, Number 5, May 2013
 

Abstract
It generally accepted that most building structures shall exhibit a nonlinear response when subjected to medium-high intensity earthquakes. It is currently known, however, that this phenomenon is not properly modelled in the majority of cases, especially at the design stage, where only simple linear methods have effectively been used. Recently, as a result of the exponential progress of computational tools, nonlinear modelling and analysis have gradually been brought to a more promising level. A wide range of modelling alternatives developed over the years is hence at the designer

Key Words
material nonlinearity; reinforced concrete sections; lumped plasticity models; distributed plasticity models; nonlinear seismic analysis

Address
GONCALO CARVALHO, RITA BENTO and CARLOS BHATT: Department of Civil Engineering, Instituto Superior Tecnico, Technical University of Lisbon, Av. Rovisco Pais 1049-001 Lisboa, Portugal

Abstract
Performance of a prototype base isolated building located at Indian Institute of Technology,Guwahati (IITG) has been studied here. Two numbers of three storeyed single bay RCC framed prototype buildings were constructed for experimental purpose at IITG, one supported on conventional isolated footings and the other on a seismic isolation system, consisting of lead plug bearings. Force balance accelerometers and a 12 channel strong motion recorder have been used for recording building response during seismic events. Floor responses from these buildings show amplification for the conventional building while 60 to 70% reduction has been observed for the isolated building. Numerical models of both the buildings have been created in SAP2000 Nonlinear. Infill walls have been modeled as compression struts and have been incorporated into the 3D models using Gap elements. System identification of the recorded data has been carried out using Parametric State Space Modeling (N4SID) and the numerical models have been updated accordingly. The study demonstrates the effectiveness of base isolation systems in controlling seismic response of isolated buildings thereby leading to increased levels of seismic protection. The numerical models calibrated by relatively low level of earthquake shaking provides the starting point for modeling the non-linear response of the building when subjected to strong shaking.

Key Words
base isolation; lead rubber bearings; numerical models; system identification; N4SID

Address
Rupam Jyoti Nath, Sajal Kanti Deb and Anjan Dutta: Department of Civil Engineering, IIT Guwahati, Pin- 781039, Assam, India

Abstract
Design spectra for damping ratios higher than 5% have several important applications in the design of earthquake-resistant structures. These highly damped spectra are usually derived from a 5%-damped reference pseudo-acceleration spectrum by using a damping modification factor. In cases of high damping, the absolute acceleration and the relative velocity spectra instead of the pseudo-acceleration and the pseudo-velocity spectra should be used. This paper elaborates on the recovery of spectral absolute acceleration and spectral relative velocity from their pseudo- spectral counterparts. This is accomplished with the aid of correction factors obtained through extensive parametric studies, which come out to be functions of period and damping ratio.

Key Words
absolute acceleration; relative velocity; pseudo-spectral values; damping modification factor;correction factors; seismic motions

Address
George A. Papagiannopoulos: Department of Civil Engineering, University of Patras, GR-26500 Patras, Greece; George D. Hatzigeorgiou: Department of Environmental Engineering, Democritus University of Thrace, GR-67100 Xanthi, Greece; Dimitri E. Beskos: Department of Civil Engineering, University of Patras, GR-26500 Patras, Greece;
Office of Theoretical and Applied Mechanics, Academy of Athens, 4 Soranou Efessiou str.,GR-11527 Athens, Greece
GR-11527 Athens, Greece

Abstract
In this paper, learning capabilities of two types of Arterial Neural Networks, namely hierarchical neural networks and Generalized Regression Neural Network were used in a two-stage approach to develop a method for generating spatial varying accelerograms from acceleration response spectra and a distance parameter in which generated accelerogram is desired. Data collected from closely spaced arrays of seismographs in SMART-1 array were used to train neural networks. The generated accelerograms from the proposed method can be used for multiple support excitations analysis of structures that their supports undergo different motions during an earthquake.

Key Words
generation of artificial earthquake; neural networks; spatially varying earthquakes; response spectrum; SMART-1 array

Address
Hossein Ghaffarzadeh, Mohammad Mahdi Izadi and Nima Talebian: Department of Structural Engineering, University of Tabriz, Tabriz, Iran

Abstract
This paper proposes probabilistic models for estimating the seismic demands on reinforced concrete (RC) bridges with base isolation. The models consider the shear and deformation demands on the bridge columns and the deformation demand on the isolation devices. An experimental design is used to generate a population of bridges based on the AASHTO LRFD Bridge Design Specifications (AASHTO 2007) and the Caltrans\' Seismic Design Criteria (Caltrans 1999). Ground motion records are used for time history analysis of each bridge to develop probabilistic models that are practical and are able to account for the uncertainties and biases in the current, common deterministic model. As application of the developed probabilistic models, a simple method is provided to determine the fragility of bridges. This work facilitates the reliability-based design for this type of bridges and contributes to the transition from limit state design to performance-based design.

Key Words
seismic demand; probabilistic model; experimental design; finite element; highway bridge;base isolation; seismic fragility; importance analysis

Address
Paolo Gardoni: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana,IL 61801, USA; David Trejo: School of Civil and Construction Engineering, Oregon State University, Corvallis, OR 97331, USA

Abstract
The recent huge earthquake ground motion records in Japan result in the reconsideration of seismic design forces for nuclear power stations from the view point of seismological research. In addition,the seismic design force should be defined also from the view point of structural engineering. In this paper it is shown that one of the occurrence mechanisms of such large acceleration in recent seismic records(recorded in or near massive structures and not free-field ground motions) is due to the interaction between a massive building and its surrounding soil which induces amplification of local mode in the surface soil.Furthermore on-site investigation after earthquakes in the nuclear power stations reveals some damages of soil around the building (cracks, settlement and sand boiling). The influence of plastic behavior of soil is investigated in the context of interaction between the structure and the surrounding soil. Moreover the amplification property of the surface soil is investigated from the seismic records of the Suruga-gulf earthquake in 2009 and the 2011 off the Pacific coast of Tohoku earthquake in 2011. Two methods are introduced for the analysis of the non-stationary process of ground motions. It is shown that the non-stationary Fourier spectra can detect the temporal change of frequency contents of ground motions and the displacement profile integrated from its acceleration profile is useful to evaluate the seismic behavior of the building and the surrounding soil.

Key Words
large acceleration; pulse wave; non-linear behavior; soil-structure interaction; settlement; non-stationary Fourier spectra; frequency component ratio; surface soil amplification; occurrence mechanism of pulse wave; amplification factor

Address
Shuichi KAMAGATA : Nuclear Power Department, Kajima Corporation, Tokyo 107-8348, Japan; Izuru TAKEWAKI: Dept of Architecture and Architectural Eng., Kyoto University, Kyoto 615-8540, Japan


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