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CONTENTS
Volume 33, Number 2, September30 2009
 

Abstract
The seismic induced interaction between multistory structures with unequal story heights (inter-story pounding) is studied taking into account the local response of the exterior beam-column joints. Although several parameters that influence the structural pounding have been studied sofar, the role of the joints local inelastic behaviour has not been yet investigated in the literature as key parameter for the pounding problem. Moreover, the influence of the infill panels as an additional parameter for the local damage effect of the joints on the inter-story pounding phenomenon is examined. Thirty six interaction cases between a multistory frame structure and an adjacent shorter and stiffer structure are studied for two different seismic excitations. The results are focused: (a) on the local response of the critical external column of the multistory structure that suffers the hit from the slab of the adjacent shorter structure, and (b) on the local response of the exterior beam-column joints of the multistory structure. Results of this investigation demonstrate that the possible local inelastic response of the exterior joints may be in some cases beneficial for the seismic behaviour of the critical column that suffers the impact. However, in all the examined cases the developing demands for deformation of the exterior joints are substantially increased and severe damages can be observed due to the pounding effect. The presence of the masonry infill panels has also been proved as an important parameter for the response of the exterior beam-column joints and thus for the safety of the building. Nevertheless, in all the examined inter-story pounding cases the presence of the infills was not enough for the total amelioration of the excessive demands for shear and ductility of the column that suffers the impact.

Key Words
structural interaction; inter-story pounding; exterior joints local damage effect; infill panels; non-linear seismic analysis.

Address
Maria J. Favvata, Chris G. Karayannis and Asterios A. Liolios: Department of Civil Engineering, Section of Structural Engineering, Democritus University of Thrace, 12 V. Sofias street, Xanthi, GR 67100, Greece

Abstract
This paper investigates the behavior of reinforced concrete (RC) circular columns under combined loading including torsion. The main variables considered in this study are the ratio of torsional moment to bending moment (T/M) and the level of detailing for moderate and high seismicity (low and high transverse reinforcement/spiral ratio). This paper presents the results of tests on seven columns subjected to cyclic bending and shear, cyclic torsion, and various levels of combined cyclic bending, shear, and torsion. Columns under combined loading were tested at T/M ratios of 0.2 and 0.4. These columns were reinforced with two spiral reinforcement ratios of 0.73% and 1.32%. Similarly, the columns subjected to pure torsion were tested with two spiral reinforcement ratios of 0.73% and 1.32%. This study examined the significance of proper detailing, and spiral reinforcement ratio and its effect on the torsional resistance under combined loading. The test results demonstrate that both the flexural and torsional capacities are decreased due to the effect of combined loading. Furthermore, they show a significant change in the failure mode and deformation characteristics depending on the spiral reinforcement ratio. The increase in spiral reinforcement ratio also led to significant improvement in strength and ductility.

Key Words
circular columns; combined loadings; confinement; ductility; flexure; interaction diagrams; shear; spiral reinforcement ratio; torsion.

Address
Abdeldjelil Belarbi, Suriya Prakash and Young-Min You:
Dept. of Civil,Environmental and Archictectural Engineering, Missouri University of Science and Technology, Rolla, Missouri, USA

Abstract
The aim of this paper is to give a rigorous framework for the interpretation of limit analysis results including large displacements. The presentation is oriented towards unidimensional media (beams) but two-dimensional (plates) or three-dimensional media are also concerned. A single-degree-of-freedom system is first considered: it shows the basic phenomena of large displacement limit analysis or secondorder limit analysis. The results are compared to those of a continuous system and the differences between both systems are discussed. Theoretical results are obtained using the kinematical approach of limit analysis. An admissible load-displacement plane is then defined, according to the yield design theory. The methodology used is applied to frame structures. The presented results are nevertheless different from those already published in the literature, as the virtual displacement field can be distinguished from the displacement field at collapse. The simplicity of large displacement limit analysis makes it attractive for practical engineering applications. The load-displacement upper bound can be used for instance in the optimal design of steel frames in seismic areas.

Key Words
limit analysis; stability; kinematics; frames; seismic design; geometric nonlinearity; plasticity; geometrically exact analysis; large displacement.

Address
Noel Challamel: Laboratoire de Genie Civil et Genie Mecanique (LGCGM), INSA de Rennes, Universite Europeenne de Bretagne 20, avenue des Buttes de Coesmes, 35043 Rennes cedex, France

Abstract
This paper is aimed at combining wavelet multiresolution analysis and nonstationary Kanai-Tajimi model for the simulation of earthquake accelerograms. The proposed approach decomposes earthquake accelerograms using wavelet multiresolution analysis for the simulation of earthquake accelerograms. This study is on the basis of some Iranian earthquake records, namely Naghan 1977, Tabas 1978, Manjil 1990 and Bam 2003. The obtained results indicate that the simulated records preserve the significant properties of the actual accelerograms. In order to investigate the efficiency of the model, the spectral response curves obtained from the simulated accelerograms have been compared with those from the actual records. The results revealed that there is a good agreement between the response spectra of simulated and actual records.

Key Words
earthquake ground motion; simulation; non-stationary model; wavelet analysis; Kanai-Tajimi model.

Address
G. Ghodrati Amiri and A. Bagheri: Center of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering, Iran University of Science & Technology, PO Box 16765-163, Narmak, Tehran 16846, Iran

Abstract
In this paper structural analysis of nonhomogeneous nanotubes has been carried out using nonlocal elasticity theory. Governing differential equations of nonhomogeneous nanotubes are derived. Nanotubes include both single wall nanotube (SWNT) and double wall nanotube (DWNT). Nonlocal theory of elasticity has been employed to include the scale effect of the nanotubes. Nonlocal parameter, elastic modulus, density and diameter of the cross section are assumed to be functions of spatial coordinates. General Differential Quadrature (GDQ) method has been employed to solve the governing differential equations of the nanotubes. Various boundary conditions have been applied to the nanotubes. Present results considering nonlocal theory are in good agreement with the results available in the literature. Effect of variation of various geometrical and material parameters on the structural response of the nonhomogeneous nanotubes has been investigated. Present results of the nonhomogeneous nanotubes are useful in the design of the nanotubes.

Key Words
nanotubes; differential quadrature method; nonhomogeneous; bending; vibration; buckling.

Address
S.C. Pradhan and J.K. Phadikar: Dept. of Aerospace Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721 302, India

Abstract
This paper introduces an improved modal pushover analysis (IMPA) which can effectively evaluate the seismic response of multi-span continuous bridge structures on the basis of modal pushover analysis (MPA). Differently from previous modal pushover analyses which cause the numerical unstability because of the occurrence of reversed relation between the pushover load and displacement, the proposed method eliminates this numerical instability and, in advance the coupling effects induced from the direct application of modal decomposition by introducing an identical stiffness ratio for each dynamic mode at the post-yielding stage together with an approximate elastic deformation. In addition to these two introductions, the use of an effective seismic load, calculated from the modal spatial force and applied as the distributed load, makes it possible to predict the dynamic responses of all bridge structures through a simpler analysis procedure than those in conventional modal pushover analyses. Finally, in order to establish validity and applicability of the proposed method, correlation studies between a rigorous nonlinear time history analysis and the proposed method were conducted for multi-span continuous bridges.

Key Words
bridges; improved modal pushover; identical stiffness ratio; equivalent modal load; elastic deformed shape.

Address
Kwak, Hyo Gyoung and Shin, Dong Kyu: Dept. of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea

Abstract
In the companion paper, a simple but effective analysis procedure termed an Improved Modal Pushover Analysis (IMPA) is proposed to estimate the seismic capacities of multi-span continuous bridge structures on the basis of the modal pushover analysis, which considers all the dynamic modes of a structure. In contrast to previous studies, the IMPA maintains the simplicity of the capacity-demand curve method and gives a better estimation of the maximum dynamic response in a bridge structure. Nevertheless, to verify its applicability, additional parametric studies for multi-span continuous bridges with large differences in the length of adjacent piers are required. This paper, accordingly, concentrates on a parametric study to review the efficiency and limitation in the application of IMPA to bridge structures through a correlation study between various analytical models including the equivalent single-degree-offreedom method (ESDOF) and modal pushover analysis (MPA) that are usually used in the seismic design of bridge structures. Based on the obtained numerical results, this paper offers practical guidance and/or limitations when using IMPA to predict the seismic response of a bridge effectively.

Key Words
bridges; improved modal pushover; seismic performance; parametric study; A-D relation.

Address
Kwak, Hyo-Gyoung and Shin, Dong Kyu: Dept. of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea

Abstract
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Key Words
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Address
Bin Ji: State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology,
Dalian, 116024, P. R. China
Wanji Chen: State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology,
Dalian, 116024, P. R. China
Dept. of Aeronautics and space navigation, Shenyang Institute of Aeronautical Engineering, Daoyi South Street 37, Shenyang, LN, 110136, P. R. China


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