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CONTENTS
Volume 16, Number 6, December 2003
 

Abstract
Two real-time modeling prediction (RMP) schemes are presented in this paper for analyzing the behavior of deep excavations during construction. The first RMP scheme is developed from the traditional AR(p) model. The second is based on the simplified Elman-style recurrent neural networks. An on-line learning algorithm is introduced to describe the dynamic behavior of deep excavations. As a case study, in-situ measurements of an excavation were recorded and the measured data were used to verify the reliability of the two schemes. They proved to be both effective and convenient for predicting the behavior of deep excavations during construction. It is shown through the case study that the RMP scheme based on the neural network is more accurate than that based on the traditional AR(p) model.

Key Words
neural network; excavation; real-time modeling prediction; construction.

Address
Li-Feng Ni and Ai-Qun Li
College of Civil Engineering, Southeast University, Nanjing, Jiangsu Province, 210096, P.R. China

Fu-Yi Liu
First Institute of Shenzhen General Institute of Design and Research, Shenzhen, P.R. China

Honore Yin
ENPC-LAMI, 8 Avenue Blaise Pascal, Cite Descartes, Champs-sur-Marne-77455, Marne La Vallee Cedex, France

J.R. Wu
Department of Building and Construction, City University of Hong Kong, Kowloon, Hong Kong, P.R. China

Abstract
Investigated in this study are the natural vibration characteristics of the coaxial cylindrical shells coupled with a fluid. Theoretical method is developed to find the natural frequencies of the shell using the finite Fourier series expansion, and their results are compared with those of finite element method to verify the validation of the method developed. The effect of the fluid-filled annulus and the boundary conditions on the modal characteristics of the coaxial shells is investigated using a finite element modeling.

Key Words
cylindrical shell; modal characteristic; fluid-filled annulus; Fourier series; finite element method.

Address
Myung Jo Jhung and Young Hwan Choi
Korea Institute of Nuclear Safety, 19 Guseong-dong, Yuseong-gu, Daejeon 305-338, Korea

Kyeong Hoon Jeong
Korea Atomic Energy Research Institute, 150 Dukjin-dong, Yuseong-gu, Daejeon 305-353, Korea

Abstract
In order to consider the modified seismic response of framed structures in the presence of masonry infills, proper models have to be formulated. Because of the complexity of the problem, a careful definition of a diagonal pin-jointed strut, able to represent the horizontal force-interstorey displacement cyclic law of the actual infill, may be a solution. In this connection the present paper shows a generalized criterion for the determination of the ideal cross-section of the strut mentioned before. The procedure is based on the equivalence between the lateral stiffness of the actual infilled frame scheme during the conventional elastic stage of the response and the lateral stiffness of the same frame stiffened by a strut at the same stage. Unlike the usual empirical approaches available in the literature, the proposed technique involves the axial stiffness of the columns of the frame more than their flexural stiffness. Further, the influence of the bidimensional behaviour of the infill is stressed and, consequently, the dependence of the dimensions of the equivalent pin-jointed strut on the Poisson ratio of the material constituting the infill is also shown. The proposed approach is extended to the case of infills with openings, which is very common in practical applications.

Key Words
infilled frames; masonry infill; stiffening effect; simplified model; equivalent strut; identification technique.

Address
Dipartimento di Ingegneria Strutturale e Geotecnica, Universita di Palermo, Viale delle Scienze, 90128 - Palermo, Italy


Abstract
The non-linear structural analysis of reinforced concrete beams in fire consists of three separate steps: (i) The estimation of the rise of surrounding air temperature due to fire; (ii) the determination of the distribution of the temperature within the beam during fire; (iii) the evaluation of the mechanical response due to simultaneous time-dependent thermal and mechanical loads. Steps (ii) and (iii) are dealt with in the present paper. We present a two-step computational procedure where a 2D transient thermal analysis over the cross-sections of beams are made first, followed by mechanical analysis of the structure. Fundamental to the accuracy of the mechanical analysis is a new planar beam finite element. The effects of plasticity in concrete, and plasticity and viscous creep in steel are taken into consideration. The properties of concrete and steel along with the values of their thermal and mechanical parameters are taken according to the European standard ENV 1992-1-2 (1995). The comparison of our numerical and full-scale experimental results shows that the proposed mechanical and 2D thermal computational procedure is capable to describe the actual response of reinforced concrete beam structures to fire.

Key Words
fire design; heat conduction; Reissner beam; finite element method; reinforced concrete; creep.

Address
University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2,
SI-1115 Ljubljana, Slovenia


Abstract
This paper contains the results of the study on the development of fracture and crack propagation in quasi-brittle materials, such as concrete or rocks, using the Discrete Element Method (DEM). A new discrete element numerical model is proposed as the basis for analyzing the inelastic evolution and growth of cracks up to the point of gross material failure. The model is expected to predict the fracture behavior for the quasi-brittle material structure using the elementary aggregate level, the interaction between aggregate materials, and bond cementation. The algorithms generate normal and shear forces between two interfacing blocks and contains two kinds of contact logic, one for connected blocks and the other one for blocks that are not directly connected. The Mohr-Coulomb theory has been used for the fracture limit. In this algorithm the particles are moving based on the connected block logic until the forces increase up to the fracture limit. After passing the limit, the particles are governed by the discrete block logic. In setting up a discrete polygon element model, two dimensional polygons are used to investigate the response of an assembly of different shapes, sizes, and orientations with blocks subjected to simple applied loads. Several examples involving assemblies of particles are presented to show the behavior of the fracture and the failure process.

Key Words
quasi-brittle materials; discrete element method; polygon block elements; Voronoi; failure.

Address
Jong Seok Lee
Department of Civil & Environmental Engineering, University of Ulsan, Ulsan,Korea

Yoon Bock Rhie
Rhie & Associates, Inc., 1900 N. Vine St. #407, Los Angeles, CA 90068, USA

Ick Hyun Kim
Department of Civil & Environmental Engineering, University of Ulsan, Ulsan,Korea

Abstract
In this paper a thick cylindrical shell with varying thickness which is subjected to static non-uniform internal pressure is analyzed. At first, equilibrium equations of the shell have been derived by the energy principle and by considering the first order theory of Mirsky-Herrmann which includes transverse shear deformation. Then the governing equations which are, a system of differential equations with varying coefficients have been solved analytically with the boundary layer technique of the perturbation theory. In spite of complexity of modeling the conditions near the boundaries, the method of this paper is very capable of providing a closed form solution even near the boundaries. Displacement predictions are in a good agreement with the calculated finite elements and other analytical results. The convergence of solution is very fast and the amount of calculations is less than the Frobenius method.

Key Words
thick cylindrical shell; varying thickness; perturbation theory; finite elements method.

Address
Department of Mechanical Engineering, Tarbiat Modarres University, P. O. Box 14115-143 Tehran, I.R. Iran

Abstract
The structural deterioration of concrete structures due to reinforcement corrosion is a major worldwide problem. Service life of the age-degraded concrete structures is governed by the protective action provided by the cover concrete against the susceptibility of the reinforcement to the corrosive environment. The corrosion of steel would result in the various corrosion products, which depending on the level of the oxidation may have much greater volume than the original iron that gets consumed by the process of corrosion. This volume expansion would be responsible for exerting the expansive radial pressure at the steel-concrete interface resulting in the development of hoop tensile stresses in the surrounding cover concrete. Once the maximum hoop tensile stress exceeds the tensile strength of the concrete, cracking of cover concrete would take place. The cracking begins at the steel-concrete interface and propagates outwards and eventually resulting in the through cracking of the cover concrete. The cover cracking would indicate the loss of the service life for the corrosion-affected structures. In the present paper, analytical models have been developed considering the residual strength of the cracked concrete and the stiffness provided by the combination of the reinforcement and expansive corrosion products. The problem is modeled as a boundary value problem and the governing equations are expressed in terms of the radial displacement. The analytical solutions are presented considering a simple 2-zone model for the cover concrete viz. cracked or uncracked. A sensitivity analysis has also been carried out to show the influence of the various parameters of the proposed models. The time to cover cracking is found to be function of initial material properties of the cover concrete and reinforcement plus corrosion products combine, type of rust products, rate of corrosion and the residual strength of the cover concrete. The calculated cracking times are correlated against the published experimental and analytical reference data.

Key Words
expansive corrosion products; time to cover cracking; steel-concrete interface; porous zone; internal radial pressure; residual strength; tension-softening.

Address
Kapilesh Bhargava and A. K. Ghosh
Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India

Yasuhiro Mori
Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8603, Japan

S. Ramanujam
Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India


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