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CONTENTS
Volume 10, Number 1, July 2000
 

Abstract
It is inevitable that every space truss structure would be under some form of direct member loading. At least the structure self weight certainly affects the members directly, and in structures involving top concrete slabs or cladding, their weight is also likely to apply some lateral pressure on the members. In spite of that, direct member loading is usually ignored in space truss designs and assumed to lead only to a negligible effect on truss performance. This study is intended to explore this point and identify the actual effects that can arise fi-om direct member loading, and eventually provide an answer to the question of whether the current design practice is satisfactory or certain modifications would be needed. After presenting two analytical techniques to allow the study of space trusses with laterally loaded members, the paper describes a wide parametric study involving practical-size space trusses with different configurations, aspect ratios, boundary conditions and number of chord panels.

Key Words
space trusses, direct loading, behaviour

Address
El-Sheikh A, Univ Dundee, Dept Civil Engn, Dundee DD1 4HN, Scotland

Abstract
This paper presents a case study with a multi-degree-of-freedom (MDOF) system where the Floor Response Spectra (FRS) have been derived from a large ensemble of ground motion accelerograms. The FRS are evaluated by the frequency response function which is calculated numerically. The advantage of this scheme over a repetitive time-history analysis of the entire structure for each accelerogram of the set has been highlighted. The present procedure permits generation of FRS with a specified probability of exceedence.

Key Words
ground motion, frequency response function, floor response spectra, exceedence probability

Address
Ghosh AK, Bhabha Atom Res Ctr, Reactor Safety Div, Bombay 400085, Maharashtra, India

Abstract
The Oktalok nodal connection system is an aesthetic and efficient system. It has been widely used throughout Australia. The paper will briefly introduce the concept and application of the Oktalok nodal system. The existing design method is based on the assumption that the joints are pin-ended, i.e., the rotational stiffness of the joints is zero. However the ultimate capacity of the frame may increase significantly depending on the rotational stiffness of the joints. Stiffness tests and finite element simulations were carried out to determine the rotational stiffness of the Oktalok joints. Column buckling tests and nonlinear finite element analyses were performed to determine the member capacity of columns with semirigid end conditions. A simple formulae for the effective length factor of column buckling is derived based on the above experimental and theoretical investigations.

Key Words
buckling, column, semi-rigid joint, space frame, tubes

Address
Zhao XL, Monash Univ, Dept Civil Engn, Clayton, Vic 3168, Australia
Monash Univ, Dept Civil Engn, Clayton, Vic 3168, Australia
Spacetech Pty Ltd, Melbourne, Vic, Australia

Abstract
Identification of damage location based on modal measurement is an important problem in structural health monitoring. The damage index method that attempts to evaluate the changes in modal strain energy distribution has been found to be effective under certain circumstances. Ln this paper two damage index methods using bending strain energy and shear strain energy have been evaluated for numerous cases at different locations and degrees of damage. The objective is to evaluate the feasibility of the damage index method to localize the damage on large span concrete bridge. Finite element models were used as the test structures. Finally this method was used to predict the damage location in an actual structure, using the results of a modal survey from a large concrete bridge.

Key Words
strain energy, damage index method, concrete bridge

Address
Wang ML, Univ Illinois, Dept Civil & Mat Engn, 842 W Taylor St 2095 ERF, Chicago, IL 60607 USA
Univ Illinois, Dept Civil & Mat Engn, Chicago, IL 60607 USA

Abstract
This paper proposes a simple numerical model for use in a finite analysis (FEA) of scaffold-shoring systems. The structural model consists of a single set of multiple-story scaffolds with constraints in the out-of-plane direction at every connection joint between stories. Although this model has only two dimensions (termed the 2-D model), it is derived from the analysis of a complete scaffold-shoring system and represents the structural behavior of a complete three-dimensional system. Experimental testing of scaffolds up to three stories in height conducted in the laboratory, along with an outdoor test of a five-story scaffold system, were used to validate the 2-D model. Both failure modes and critical loads were compared. In the comparison of failure modes, the computational results agree very well with the test results. However, in the comparison of critical loads, computational results were consistently somewhat greater than test results. The decreasing trends of critical loads with number of stories in both the test and simulation results were similar. After investigations to explain the differences between the computationally and experimentally determined critical loads, it was recommended that the 2-D model be used as the numerical model in subsequent analysis. In addition, the computational critical loads were calibrated and revised in accordance with the experimental critical loads, and the revised critical loads were then used as load-carrying capacities for scaffold-shoring systems for any number of stories. Finally, a simple procedure is suggested for determining load-carrying capacities of scaffold-shoring systems of heights other than those considered in this study.

Key Words
scaffolds, shores, scaffold-shoring systems, load-carrying capacities, finite element analysis, failure modes, structural analysis

Address
Huang YL, Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan
Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan
Clemson Univ, Dept Civil Engn, Clemson, SC 29634 USA

Abstract
Critical loads and load-carrying capacities for steel scaffolds used as shoring systems were compared using computational and experimental methods in Part I of this paper. In that paper, a simple 2-D model was established for use in evaluating the structural behavior of scaffold-shoring systems. This 2-D model was derived using an incremental finite element analysis (FEA) of a typical complete scaffold-shoring system. Although the simplified model is only two-dimensional, it predicts the critical loads and failure modes of the complete system. The objective of this paper is to present a closed-form solution to the 2-D model. To simplify the analysis, a simpler model was first established to replace the 2-D model. Then, a closed-form solution for the critical loads and failure modes based on this simplified model were derived using a bifurcation (eigenvalue) approach to the elastic-buckling problem. In this closed-form equation, the critical loads are shown to be function of the number of stories, material properties, and section properties of the scaffolds. The critical loads and failure modes obtained from the analytical (closed-form) solution were compared with the results from the 2-D model. The comparisons show that the critical loads from the analytical solution (simplified model) closely match the results from the more complex model, and that the predicted failure modes are nearly identical.

Key Words
scaffolds, shores, critical loads, elastic buckling, failure modes

Address
Huang YL, Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan
Natl Chung Hsing Univ, Dept Civil Engn, Taichung 40227, Taiwan
Clemson Univ, Dept Civil Engn, Clemson, SC 29634 USA

Abstract
This study is an investigation of the effect of cracks on the dynamical characteristics of a cantilever beam, having multiple open-edge transverse cracks. The flexibilities due to crack have been identified for several crack depths and locations. In the study the finite element method and component mode synthesis methods are used. Coupling the components is performed by a flexibility matrix taking into account the interaction forces. Each component is modelled by cantilever beam finite elements with two nodes and three degrees of freedom at each node. The results obtained lead to conclusion that, by using the drop in the natural frequencies and the change in the mode shapes, the presence and nature of cracks in a structure can be detected. There is some counter-evidence, however, that the effects due to multiple cracks may interact to make detection more difficult than for isolated cracks.

Key Words
structural integrity, damage assessment, defective structures, cracks

Address
Kisa M, Univ Wales, Sch Engn, Cardiff CF1 3NS, S Glam, Wales
Univ Wales, Sch Engn, Cardiff CF1 3NS, S Glam, Wales


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