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CONTENTS
Volume 40, Number 6, December25 2011
 

Abstract
Damage detection methodologies based on the direct examination of the nonlinear-nonstationary characteristics of the structure dynamic response may play an important role in online structural health monitoring applications. Different signal processing based damage detection methodologies have been proposed based on the uncovering of spikes in the high frequency component of the structural response obtained via Discrete Wavelet transforms, Hilbert-Huang transforms or high pass filtering. The performance of these approaches in systems subjected to different types of excitation is evaluated in this paper. It is found that in the case of random excitations, like earthquake accelerations, the effectiveness of such methodologies is limited. An alternative damage detection approach using the Continuous Wavelet Transform (CWT) is also evaluated to overcome this limitation. Using the CWT has the advantage that the central frequencies at which it operates can be defined by the user while the frequency bands of the detail functions obtained via DWT are predetermined by the sampling period of the signal.

Key Words
damage detection; wavelet transform; empirical mode decomposition; butterworth filters; hilbert transform

Address
Luis A. Montejo: Department of Engineering Science and Materials, University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico

Abstract
Results from nonlinear time-history analyses of wall-frame structural models indicate that the condition of vulnerable foundations -for which uplifting and reaching the bearing capacity of the supporting soil can occur before yielding at the base of the shear walls- may not be necessarily detrimental to the drift response of buildings under strong ground motions. Analyses also show that a soilfoundation system can inherently have deformation capacity well in excess of the demand and thus act as a source of energy dissipation that protects the structural integrity of the shear walls.

Key Words
soil-structure interaction; vulnerable foundations; performance-base framework; nonlinear analysis; seismic strengthening

Address
J. Paul Smith-Pardo: Department of Civil and Environmental Engineering, Seattle University, Seattle, WA, USA

Abstract
This paper addresses the finite strip formulations for the stability analysis of viscoelastic composite plates with variable thickness in the transverse direction, which are subjected to in-plane forces. While the finite strip method is fairly well-known in the buckling analysis, hitherto its direct application to the buckling of viscoelastic composite plates with variable thickness has not been investigated. The equations governing the stiffness and the geometry matrices of the composite plate are solved in the time domain using both the higher-order shear deformation theory and the method of effective moduli. These matrices are then assembled so that the global stiffness and geometry matrices of a moderately thick rectangular plate are formed which lead to an eigenvalue problem that is solved to determine the magnitude of critical buckling load for the viscoelastic plate. The accuracy of the proposed model is verified against the results which have been reported elsewhere whilst a comprehensive parametric study is presented to show the effects of viscoelasticity parameters, boundary conditions as well as combined bending and compression loads on the critical buckling load of thin and moderately thick viscoelastic composite plates.

Key Words
buckling; effective moduli; shear deformation; viscoelasticity; variable thickness; finite strip method

Address
Nasrin Jafari, Mojtaba Azhari: Department of Civil Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
Amin Heidarpour: Department of Civil Engineering, Monash University, Melbourne, VIC3800, Australia

Abstract
A new experimental technique was developed on the plate-inserted welded tubular joints of spatial grid structures. In the experimental technique, a microcomputer controlling instrument of moire intervention (MCIMI) was adopted. A test was designed on the plate-inserted welded tubular joints of spatial grid structures to show the effectiveness of the MCIMI technique. Both traditional electrical measuring technique and MCIMI technique were employed in the test. The test results showed that the MCIMI technique was feasible in the case of the complicated tests on steel structures. The MCIMI technique not only implemented the limitation of traditional electrical measuring technique, but also improved the accuracy of the test. According to the test results, we further examined the plate-inserted welded tubular joints in the cable-stayed spatial grids of the Binhai International Convention & Exhibition in Tianjin, China. The analysis showed the joints are safely designed with adequate conservatism. The research provided a new application of MCIMI in the field of large-scale structure engineering.

Key Words
moire intervention; MCMI; spatial grids structure; plate-inserted tubular joints; welded

Address
Zhi-Hua Chen, Guo-Jun Sun and Zhi-Shan Luo: School of Civil Engineering, Tianjin University, Tianjin 300072, China

Abstract
This paper provides a numerical solution for a finite internally cracked plate using hybrid crack element method (HCE). In the formulation, an inclined crack is placed in any place of a rectangular element and the complex variable method is used. The complex potentials are expressed in a series form, and several undetermined coefficients are involved. The complex potentials for the cracked rectangle are first suggested in this paper. Based on a variational principle, the element stiffness matrix can be evaluated. The next steps are same as in the usual finite element method. Several numerical examples with computed stress intensity factor and T-stress are presented.

Key Words
hybrid crack element; variational principle; complex variable method; stress intensity factor; T-stress

Address
Y.Z. Chen: Division of Engineering Mechanics, Jiangsu University, Zhenjiang, Jiangsu, 212013 People

Abstract
This paper evaluates the effects of topographical and geotechnical irregularities on the dynamic response of the 2-D soil-structure systems under ground motion by coupling finite and infinite elements. A numerical procedure is employed, and a parametric study is carried out for single-faced slope topographies. It is concluded that topographic conditions may have important effects on the ground motion along the slope. The geotechnical properties of the soil will also have significantly amplified effects on the whole system motion, which cannot be neglected for design purposes. So, dynamic response of a soil-structure systems are primarily affected by surface shapes and geotechnical properties of the soil. Location of the structure is another parameter affecting the whole system response.

Key Words
soil-structure interaction; finite elements; infinite elements; elastodynamics; topography; site effects; ground motion

Address
Oguz Akin Duzgun and Ahmet Budak: Department of Civil Engineering, Engineering Faculty, Ataturk University, 25240 Erzurum, Turkey

Abstract
The objective of this study was to experimentally investigate the bond-related tension stiffening behavior of flexural reinforced concrete (RC) beams made with lightweight aggregate concrete (LWAC) under various high-cycle fatigue loading conditions. Based on strain measurements of tensile steel in the RC beams, fatigue-induced degradation of tension stiffening effects was evaluated and was, compared to reinforced normal weight concrete (NWC) beams with equal concrete compressive strengths (40 MPa). According to applied load-mean steel strain relationships, the mean steel strain that developed under loading cycles was divided into elastic and plastic strain components. The experimental results showed that, in the high-cycle fatigue regime, the tension stiffening behavior of LWAC beams was different from that of NWC beams; LWAC beams had a lesser reduction in tension stiffening due to a better bond between steel and concrete. This was reflected in the stability of the elastic mean steel strains and in the higher degree of local plasticity that developed at the primary flexural cracks.

Key Words
lightweight aggregate concrete; reinforced concrete beam; high-cycle fatigue; tension stiffening; bond-slip

Address
How-Ji Chen, Te-Hung Liu and Wen-Po Tsai: Department of Civil Engineering, National Chung-Hsing University, Taichung 402, Taiwan
Chao-Wei Tang: Department of Civil Engineering & Engineering Informatics, Cheng-Shiu University,
Kaohsiung 833, Taiwan

Abstract
Six reinforced concrete beam-column joint specimens were constructed and tested under reverse cyclic loading to failure. The six specimens were divided into three groups, each group representing a different joint design. The main objectives of this study are to investigate the response of joints with three different design, reinforcement detailing and beam strengths, and to evaluate and compare the responses of beam-column joints reinforced with traditional steel rebar and a recently proposed steel reinforcement called prefabricated cage system (PCS). Each of the three test specimen designs included equivalent amount of steel reinforcement and had virtually identical details. The results of the research show that the PCS reinforced joints had a slightly higher strength and significantly larger deformation capacity than the equivalent rebar reinforced joints.

Key Words
beam-column joints; steel reinforcement; reinforced concrete; shear failure

Address
Matthew J. Fishe: URS Corporation, Cleveland, Ohio, USA
Halil Sezen: Department of Civil and Environmental Engineering and Geodetic Science, The Ohio State University, Columbus, Ohio, USA


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