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CONTENTS
Volume 7, Number 6, June 1999
 

Abstract
Optimal design of raft-pile foundations is examined by combining finite element technique and the optimization approach. The piles and soil medium are modeled by three dimensional solid elements while the raft is modelled by shell elements. Drucker-Prager criterion is adopted for the soil medium while the raft and the piles are assumed to be linear elastic. For the optimization process, the approximate semi-analytical method is used for calculating constraint sensitivities and a constraint approximation method which is a combination of the extended Bi-point approximation and Lagrangian polynomial approximation is used for predicting the behaviour of the constraints. The objective function of the problem is the volume of materials of the foundation while the design variables are raft thickness, pile length and pile spacing. The generalized reduced gradient algorithm is chosen for solving the optimization process. It is demonstrated that the method proposed in this study is promising for obtaining optimal design of raft-pile foundations without carrying out a large number of analyses. The results are also compared with those obtained from the previous study in which linear analysis was carried out.

Key Words
optimization, raft-pile foundation, finite element, semi-analytical method

Address
Tandjiria V, Univ New S Wales, Sch Civil Engn, Sydney, NSW 2052, Australia
Univ New S Wales, Sch Civil Engn, Sydney, NSW 2052, Australia

Abstract
This paper presents some simple thinking on an age-old question that given a bridge of a certain span and loading, from the point of view of the structural efficiency, where should the bridge deck be positioned? Generally, this decision is made for other reasons than structural efficiency such as aesthetics and the analyst is often presented with a fait accompli. Using the recently invented Evolutional Structural Optimisation (ESO) method, it is possible to demonstrate that having the deck at different vertical locations can lead to a very different mass and shape for each structural form resembling cable-stayed and cable-truss bridges. By monitoring a performance index which is the function of stresses and volume of discretised finite elements, the best optimised structure can be easily determined and the bridge deck positioning problem can be efficiently solved without resorting to any complex analysis procedures.

Key Words
bridge deck positioning, evolutionary optimal design, performance index, shape and topology optimization

Address
Guan H, Griffith Univ, Sch Engn, Gold Coast Campus, Nathan, Qld 9726, Australia
Griffith Univ, Sch Engn, Nathan, Qld 9726, Australia
Univ Sydney, Dept Aeronaut Engn, Sydney, NSW 2006, Australia

Abstract
A C*-convergence algorithm for finite element analysis has been proposed by Bigdeli and Kelly (1997) and elements for the first three levels applied to planar elasticity have been defined. The fourth level element for the new family is described in this paper and the rate of convergence for the C*-convergence algorithm is investigated numerically. The new family adds derivatives of displacements as nodal variables and the number of nodes and elements can therefore be kept constant during refinement. A problem exists on interfaces where the derivatives are required to be discontinuous. This problem is addressed for curved boundaries and a procedure is suggested to resolve the excessive interelement continuity which occurs.

Key Words
stress analysis, the finite element method, nodal degrees of freedom, C*-convergence

Address
Bigdeli B, Univ New S Wales, Sch Mech & Mfg Engn, Sydney, NSW 2052, Australia
Univ New S Wales, Sch Mech & Mfg Engn, Sydney, NSW 2052, Australia

Abstract
A boundary element method is applied to the analysis of crack trajectory in materials with complex microstructure, such as discontinuously reinforced composite materials, and systems subjected to complex loading, such as indentation. The path followed by the crack(s) has non-trivial geometry. A study of the stress intensity factors and fracture toughness of such systems must therefore be accompanied by an analysis of crack trajectory. The simulation is achieved using a dual boundary integral method in planar problems, and a single boundary integral method coupled with substructuring in axisymmetric problems. The direction of crack propagation is determined using the maximum mechanical energy release rate criterion. The method is demonstrated by application to (i) a composite material composed of components having the elastic properties of aluminium (matrix) and silicon carbide (reinforcement), and (ii) analysis of contact damage induced by the action of an indenter on brittle materials. The chief advantage of the method is the ease with which problems having complex geometry or loading (giving rise to complex crack trajectories) can be treated.

Key Words
crack trajectory, energy release rate, stress intensity factor, boundary element method, toughness

Address
Bush MB, Univ Western Australia, Dept Mech & Mat Engn, Nedlands, WA 6907, Australia
Univ Western Australia, Dept Mech & Mat Engn, Nedlands, WA 6907, Australia

Abstract
This paper presents the development and applications of the software package JIFEX, a new finite element system which can be used for structural analysis and optimum design by the modern computer hardware and software technologies such as MS Windows95/NT and Pentium PC platforms. The complete system of JIFEX is programmed with C/C++ language to make full use of advanced facilities of MS Windows95/NT. In the system, the finite element data pre-processing, based on the most popular CAD package AutoCAD (R13, R14), has been implemented, so that the finite element modeling could be integrated with geometric modeling of CAD. The system not only has interactive graphics facility for data post-processing, but also realizes the real-time computing visualization by means of the Dynamic Data Exchange (DDE) technique. Running on the Pentium computers, JIFEX can solve large-scale finite element analysis problems such as the ones with more than 60000 nodes in the finite element model.

Key Words
finite element software, design optimization, MS Windows95/NT, AutoCAD based modeling, computing visualization

Address
Gu YX, Dalian Univ Technol, Dept Engn Mech, State Key Lab Struct Anal Ind Equipment, Dalian 116024, Peoples R China
Dalian Univ Technol, Dept Engn Mech, State Key Lab Struct Anal Ind Equipment, Dalian 116024, Peoples R China

Abstract
The mechanical behaviours of the structure made from composite materials or the structure with periodic configurations depend not only on the macroscopic conditions of structure, but also on the detailed configurations. The Two-Scale Analysis (TSA) method for these structures, which couples the macroscopic characteristics of structure with its detailed configurations, is configurations, is presented for 2 or 3 dimensional case in this paper. And the finite element algorithms based on TSA are developed, and some results of numerical experiments are given. They show that TSA with its finite element algorithms is more effective.

Key Words
two-scale analysis method, composite material, structure with periodic configuration

Address
Cui JZ, Acad Sinica, Inst Computat Math & Sci Engn Comp, Beijing, Peoples R China
Acad Sinica, Inst Computat Math & Sci Engn Comp, Beijing, Peoples R China
Hong Kong Polytech Univ, Dept Appl Math, Kowloon, Hong Kong
Zhengzhou Univ, Dept Math, Zhengzhou, Henan, Peoples R China


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