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CONTENTS
Volume 23, Number 4, July10 2006
 

Abstract
A hybrid structural design and optimization methodology that combines the strengths of genetic algorithms, local search techniques, and parallel computing is developed to evolve optimal truss systems in this research effort. The primary objective that is met in evolving near-optimal or optimal structural systems using this approach is the capability of satisfying user-defined design criteria while minimizing the computational time required. The application of genetic algorithms to the design and optimization of truss systems supports conceptual design by facilitating the exploration of new design alternatives. In addition, final shape optimization of the evolved designs is supported through the refinement of member sizes using local search techniques for further improvement. The use of the hybrid approach, therefore, enhances the overall process of structural design. Parallel computing is implemented to reduce the total computation time required to obtain near-optimal designs. The support of human-computer interaction during layout optimization and local optimization is also discussed since it assists in evolving optimal truss systems that better satisfy a user? design requirements and design preferences.

Key Words
Genetic Algorithms; Pareto Ranking; MOGA; multi-objective optimization; conceptual design; parallel computing.

Address
Department of Civil Engineering, Texas A&M University, College Station, TX, USA

Abstract
Observing the unique stress-strain curves of the superelastic shape memory alloy (SMA) in tension and compression, the primary intention of this study is to investigate the behavior of the shafts made of the same material, under torsional loading-unloading cycles for large angle of twist. Experiments have been performed for the superelastic SMA shafts with different unsupported lengths and angles of twist and the results are compared with those of stainless steel (SUS304) shafts under similar test conditions. As expected for the superelastic SMA, the residual strains are small enough after each cycle and consequently, the hysteresis under loading-reverse loading is much narrower than that for the SUS304. For large angle of twists, the torsional strength of the superelastic SMA increases nonlinearly and exceeds that of SUS304. Most interestingly, the slender solid superelastic SMA shafts are found to buckle when acted upon torsion for large angle of twist.

Key Words
SMA; stress induced martensite; shape recovery; nonlinear stress-strain relations; torsional buckling.

Address
Muhammad Ashiqur Rahman; Department of Mechanical Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh
Jinhao Qiu and Junji Tani; Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Sendai 980-8577,
Japan

Abstract
Two full-scale, precast, pretensioned box girders were subjected to shear-dominated loading, one under monotonic loads to failure and the other subjected to one-half million cycles of fatigue loads followed by monotonic ultimate loads. The number of cycles was selected to allow for comparison with previous research. The fatigue loads were applied in combination with occasional overloads. In the present study, fatigue loading reduced the shear capacity by only six percent compared to the capacity under monotonic loading. However, previous research on flexure-dominated girders subjected to the same number of repeated loads showed that fatigue loading changed the mode of failure from flexure to shear/flexure and the girder capacity dropped by 14 percent. The comparison of the measured data with calculated shear capacity from five different theoretical methods showed that the ACI code method, the compression field theory, and the modified compression field theory led to reasonable estimates of the shear strength. The truss model led to an overly conservative estimate of the capacity.

Key Words
analysis; box girders; failure; fatigue; full-scale; precast; prestressed; shear; testing.

Address
M. Saiid Saiidi; Civil Engineering Department, University of Nevada, Reno, NV 89557, USA
Anita Bush; Nevada Department of Transportation, Carson City, Nevada, USA

Abstract
The influence of reinforcement buckling on the flexural response of reinforced concrete members is studied. The stress-strain response of compression reinforcement is determined computationally using a large-strain finite element model for bars of varied diameter, length, and initial eccentricity, and a mathematical expression is fitted to the simulation results. This relationship is used to represent the response of bars in compression in a moment-curvature analysis of a reinforced concrete cross section. The compression bar may carry more or less force than a tension bar at a corresponding strain, depending on the relative influence of Poisson effects and bar slenderness. Several cross-section analyses indicate that, for the distances between stirrups prescribed in modern concrete codes, the influence of inelastic buckling of the longitudinal reinforcement on the monotonic moment capacity is very small and can be neglected in many circumstances.

Key Words
bars; buckling; models; inelastic action; flexural design; concrete structures.

Address
Luisa Maria Gil-Martin and Enrique Hernandez-Montes;
University of Granada, Campus de Fuentenueva, 18072 Granada, Spain
Mark Aschheim; Civil Engineering Department, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053, U.S.A.
Stavroula J. Pantazopoulou; Department of Civil Engineering, Democritus University of Thrace, Vas. Sofias Street, Xanthi 67100, Greece

Abstract
A Genetic Algorithm (hereinafter GA) based optimum design algorithm and program for plane steel frames with partially restrained connections is presented. The algorithm was incorporated with the refined plastic hinge analysis method, in which geometric nonlinearity was considered by using the stability functions of beam-column members and material nonlinearity was considered by using the gradual stiffness degradation model that included the effects of residual stress, moment redistribution by the occurrence of plastic hinges, partially restrained connections, and the geometric imperfection of members. In the genetic algorithm, a tournament selection method and micro-GAs were employed. The fitness function for the genetic algorithm was expressed as an unconstrained function composed of objective and penalty functions. The objective and penalty functions were expressed, respectively, as the weight of steel frames and the constraint functions which account for the requirements of load-carrying capacity, serviceability, ductility, and construction workability. To verify the appropriateness of the present method, the optimum design results of two plane steel frames with fully and partially restrained connections were compared.

Key Words
optimum design; genetic algorithm; refined plastic hinge analysis; plane steel frames with partially restrained connections.

Address
Young Mook Yun; Department of Civil Engineering, Kyungpook National University, Daegu, Korea
Moon Myung Kang and Mal Suk Lee; Sch. of Architecture, Kyungpook National University, Daegu, Korea

Abstract
The aim of this study is to investigate the reliability of strong motion records processed by causal and acausal Butterworth filters in comparison to the results obtained from a synthetic accelerogram. For this purpose, the fault parallel component of the Bolu record of the Duzce earthquake is modeled with a sum of exponentially damped sinusoidal components. Noise-free velocities and displacements are then obtained by analytically integrating the synthetic acceleration model. The analytical velocity and displacement signals are used as a standard with which to judge the validity of the signals obtained by filtering with causal and acausal filters and numerically integrating the acceleration model. The results show that the acausal filters are clearly preferable to the causal filters due to the fact that the response spectra obtained from the acausal filters match the spectra obtained from the simulated accelerogram better than that obtained by causal filters. The response spectra are independent from the order of the filters and from the method of integration (whether analytical integration after a spline fit to the synthetic accelerogram or the trapezoidal rule). The response spectra are sensitive to the chosen corner frequency of both the causal and the acausal filters and also to the inclusion of the pads. Accurate prediction of the static residual displacement (SRD) is very important for structures traversing faults in the near-fault regions. The greatest adverse effect of the high pass filters is their removal of the SRD. However, the noise-free displacements obtained by double integrating the synthetic accelerogram analytically preserve the SRD. It is thus apparent that conventional high pass filters should not be used for processing near-fault strong-motion records although they can be reliably used for far-fault records if applied acausally. The ground motion parameters such as ARIAS intensity, HUSID plots, Housner spectral intensity and the duration of strong-motion are found to be insensitive to the causality of filters.

Key Words
response spectra; signal processing; butterworth filters; strong-motion accelerograms; static residual displacement; signal to noise ratios; long period ground motion; synthetic accelerograms; causal/acausal filters; ARIAS intensity; HUSID plots; Housner spectral intensity; strong-motion duration.

Address
Pelin Gundes Bakir; Department of Civil Engineering, Istanbul Technical University, Pembegul sok. Pelin apt. No:20 D:5 Suadiye Istanbul, Turkey
Richard J. Vaccaro; Department of Electrical and Computer Engineering, University of Rhode Island, 4 East Alumni Ave. Kingston, RI 02881, USA

Abstract
A new incremental finite element model is developed to simulate the frictional contact of elastic bodies. The incremental convex programming method is exploited, in the framework of finite element approach, to recast the variational inequality principle of contact problem in a discretized form. The non-classical friction model of Oden and Pires is adopted, however, the friction effect is represented by an equivalent non-linear stiffness rather than additional constraints. Different parametric studies are worked out to address the versatility of the proposed model.

Key Words
contact mechanics; friction; variational inequality; finite elements; convex programming; incremental approach.

Address
Faculty of Engineering, Zagazig University, Zagazig, Egypt

Abstract
Steel cable network structures are characterized by strong geometric nonlinearities and high flexibility, which are the main causes of their unexpected behavior, especially under dynamic loading conditions (Levy and Spillers 1995). Excluding cable-stayed and cable-suspended structural systems as well as low-tension cable nets, the majority of static and dynamic analysis techniques used are approximate and complex in nature, while their approaches differ significantly. There are five widely accepted methodologies dealing with the foregoing structural systems, with their salient features summarized in the work of Kwan (1998), while combinations, variations and extensions of these, taking into account numerous parameters, such as slackening, friction, pre-stress etc., have also been reported (Goslin and Korban 2001, Kanno et al. 2002, Kanno and Ohsaki 2005, Talvik 2001, Volokh et al. 2003, and others not cited herein). To this end, in the present study a numerical technique for the undamped dynamics of cable networks is proposed, in which pretension of cable elements, uniform time dependent temperature change as well as flexibility and temporal dependence of boundary conditions are accounted for. Applications of the method produce results in very good agreement with existing ones and important conclusions for structural design purposes are drawn.

Key Words
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Address
Dimitris S. Sophianopoulos; Department of Civil Engineering, University of Thessaly, Pedion Areos, 38 334 Volos, Greece
Panagiotis G. Asteris; Department of Structural Design and Construction, Hellenic Ministry of Rural Development and Food, 60 Serafi & 210 Liosion Str., 104 45 Athens, Greece


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