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CONTENTS
Volume 38, Number 1, April10 2011
 

Abstract
A new nonlinear model was developed to evaluate the shear resistance of steel fiberreinforced concrete beams (SFRCB) using linear genetic programming (LGP). The proposed model relates the shear strength to the geometrical and mechanical properties of SFRCB. The best model was selected after developing and controlling several models with different combinations of the influencing parameters. The models were developed using a comprehensive database containing 213 test results of SFRC beams without stirrups obtained through an extensive literature review. The database includes experimental results for normal and high-strength concrete beams. To verify the applicability of the proposed model, it was employed to estimate the shear strength of a part of test results that were not included in the modeling process. The external validation of the model was further verified using several statistical criteria recommended by researchers. The contributions of the parameters affecting the shear strength were evaluated through a sensitivity analysis. The results indicate that the LGP model gives precise estimates of the shear strength of SFRCB. The prediction performance of the model is significantly better than several solutions found in the literature. The LGP-based design equation is remarkably straightforward and useful for pre-design applications.

Key Words
fiber-reinforced concrete beams; linear genetic programming; SFRC beam; shear strength; formulation.

Address
A.H. Gandomi: Department of Civil Engineering, The University of Akron, Akron, OH, USA
A.H. Alavi: School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran
G.J. Yun: Department of Civil Engineering, The University of Akron, Akron, OH, USA

Abstract
This paper presents the results of a study on the capability of nonlinear quasi-static finite element modelling in simulating the hysteretic behaviour of CFRP and GFRP-retrofitted RC exterior beam-column joints under cyclic loads. Four specimens including two plain and two CFRP/GFRPstrengthened beam-column joints tested by Mahini and Ronagh (2004) and other researchers are modelled using ANSYS. Concrete in compression is defined by the modified Hognestad model and anisotropic multi-linear model is employed for modelling the stress-strain relations in reinforcing bars while anisotropic plasticity is considered for the FRP composite. Both concrete and FRP are modelled using solid elements whereas space link elements are used for steel bars considering a perfect bond between materials. A step by step load increment procedure to simulate the cyclic loading regime employed in the testing. An automatically reforming stiffness matrix strategy is used in order to simulate the actual seismic performance of the RC concrete after cracking, steel yielding and concrete crushing during the push and pull loading cycles. The results show that the hysteretic simulation for all specimens is satisfactory and therefore suggest that the numerical model can be used as an inexpensive tool to design of FRPstrengthened RC beam-column joints under cyclic loads.

Key Words
reinforced concrete; joints; strengthening; fibre reinforced plastics; finite element method; nonlinear analysis; cyclic loads.

Address
Seyed S. Mahini: Department of Civil and Environmental Engineering, The University of New England, Armidale, NSW 2351, Australia (Formerly Assist. Prof., Yazd University, Iran)
Hamid R. Ronagh: School of Civil Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

Abstract
Referring to the formulation of physical stochastic optimal control of structures and the scheme of optimal polynomial control, a nonlinear stochastic optimal control strategy is developed for a class of structural systems with hysteretic behaviors in the present paper. This control strategy provides an amenable approach to the classical stochastic optimal control strategies, bypasses the dilemma involved in Ito-type stochastic differential equations and is applicable to the dynamical systems driven by practical non-stationary and non-white random excitations, such as earthquake ground motions, strong winds and sea waves. The newly developed generalized optimal control policy is integrated in the nonlinear stochastic optimal control scheme so as to logically distribute the controllers and design their parameters associated with control gains. For illustrative purposes, the stochastic optimal controls of two base-excited multi-degree-of-freedom structural systems with hysteretic behavior in Clough bilinear model and Bouc- Wen differential model, respectively, are investigated. Numerical results reveal that a linear control with the 1st-order controller suffices even for the hysteretic structural systems when a control criterion in exceedance probability performance function for designing the weighting matrices is employed. This is practically meaningful due to the nonlinear controllers which may be associated with dynamical instabilities being saved. It is also noted that using the generalized optimal control policy, the maximum control effectiveness with the few number of control devices can be achieved, allowing for a desirable structural performance. It is remarked, meanwhile, that the response process and energy-dissipation behavior of the hysteretic structures are controlled to a certain extent.

Key Words
physical stochastic optimal control; polynomial controllers; exceedance probability; generalized optimal control policy; hysteretic structures; structural performance.

Address
Jie Li: School of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China; State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, P.R. China
Yong-Bo Peng: Shanghai Insititute of Disaster Prevention and Relief, Tongji University, 1239 Siping Road,
Shanghai 200092, P.R. China; State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, P.R. China
Jian-Bing Chen: School of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P.R. China; State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, P.R. China

Abstract
Concrete hydraulic structures such as: Dams, Intake Towers, Piers and dock are usually recognized as\" Vital and Special Structures\" that must have sufficient safety margin at critical conditions like when earthquake occurred as same as normal servicing time. Hence, to evaluate hydrodynamic pressures generated due to seismic forces and Fluid-Structure Interaction (FSI); introduction to fluidstructure domains and interaction between them are inevitable. For this purpose, first step is exact modeling of water-structure and their interaction conditions. In this paper, the basic equation involved the water-structure-foundation interaction and the effective factors are explained briefly for concrete hydraulic structure types. The finite element modeling of two concrete gravity dams with 5 m, 150 m height, reservoir water and foundation bed rock is idealized and then the effects of fluid domain and bed rock have been investigated on modal characteristic of dams. The analytical results obtained from numerical studies and modal analysis show that the accurate modeling of dam-reservoir-foundation and their interaction considerably affects the modal periods, mode shapes and modal hydrodynamic pressure distribution. The results show that the foundation bed rock modeling increases modal periods about 80%, where reservoir modeling changes modal shapes and increases the period of all modes up to 30%. Reservoir-dam-foundation interaction increases modal period from 30% to 100% for different cases.

Key Words
dam-reservoir interaction; modal analysis; concrete hydraulic structure; gravity dam.

Address
H. Shariatmadar: Department of Civil Engineering, Ferdowsi University of Mashhad, P.O. Box 91775-1111, Mashad, Iran
A. Mirhaj: Department of Civil Engineering, Semnan University, Semnan, Iran

Abstract
In this research, the wavelet transform is used to analyze time response of a cracked beam carrying moving mass for damage detection. In this respect, a new damage detection method based on the combined use of continuous and discrete wavelet transforms is proposed. It is shown that this method is more capable in making damage signature evident than the traditional two approaches based on direct investigation of the wavelet coefficients of structural response. By the proposed method, it is concluded that strain data outperforms displacement data at the same point in revealing damage signature. In addition, influence of moving mass-induced terms such as gravitational, Coriolis, centrifuge forces, and pure inertia force along the deflection direction to damage detection is investigated on a sample case. From this analysis it is concluded that centrifuge force has the most influence on making both displacement and strain data damage-sensitive. The Coriolis effect is the second to improve the damagesensitivity of data. However, its impact is considerably less than the former. The rest, on the other hand, are observed to be insufficient alone.

Key Words
damage detection; wavelet transform; moving mass; cracked beam; strain.

Address
Hakan Gokdag: Department of Mechanical Engineering, Uludag University, 16059 Gorukle, Bursa, Turkey

Abstract
Brick veneer over steel stud backup wall is lighter and easier to construct compared to brick veneer over concrete masonry backup wall. However, due to the relatively low stiffness of the steel stud backup, the brick veneer tends to crack under wind load. This paper briefly introduces a new panelized brick veneer with steel frame backup wall system that is developed to potentially address this problem. The experimental study of the performance of this system under simulated wind loading is discussed in detail. The test setup details and the test specimens are introduced, results of major interests are presented, and performance of the new system is evaluated based on the test results.

Key Words
brick veneer, steel stud; panelized wall; laboratory wind loading; wind-driven rain; fullscale experiment.

Address
Jianhai Liang: Thornton-Tomasetti Inc., 51 Madison Ave., Floor 17, New York, NY 10010, USA
Ali M. Memari: Department of Architectural Engineering, The Pennsylvania State University, 104 Engineering Unit A, University Park, PA 16802, USA

Abstract
This study presents an innovative method to estimate the reliability sensitivity based on the low-discrepancy sampling which is a new technique for structural reliability analysis. Two advantages are contributed to the method: one is that, by developing a general importance sampling procedure for reliability sensitivity analysis, the partial derivative of the failure probability with respect to the distribution parameter can be directly obtained with typically insignificant additional computations on the basis of structural reliability analysis; and the other is that, by combining various low-discrepancy sequences with the above importance sampling procedure, the proposed method is far more efficient than that based on the classical Monte Carlo method in estimating reliability sensitivity, especially for problems of small failure probability or problems that require a large number of costly finite element analyses. Examples involving both numerical and structural problems illustrate the application and effectiveness of the method developed, which indicate that the proposed method can provide accurate and computationally efficient estimates of reliability sensitivity.

Key Words
reliability sensitivity; failure probability; importance sampling; Quasi-Monte Carlo; lowdiscrepancy sampling.

Address
Zhenggang Cao, Hongzhe Dai and Wei Wang: School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China


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