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Volume 29, Number 6, August20 2008

This work deals with the design optimization of tuned mass damper (TMD) devices used for mitigating vibrations in high-rise towers subjected to seismic accelerations. A stochastic approach is developed and the excitation is represented by a stationary filtered stochastic process. The effectiveness of the vibration control strategy is evaluated by expressing the objective function as the reduction factor of
the structural response in terms of displacement and absolute acceleration. The mechanical characteristics of the tuned mass damper represent the design variables. Analyses of sensitivities are carried out by varying the input and structural parameters in order to assess the efficiency of the TMD strategy. Variations between two different criteria are also evaluated.

Key Words
TMD; structural optimization; random vibrations.

Giuseppe Carlo Marano: Dept. of Environmental Engineering and Sustainable Development, Technical University of Bari, Viale del Turismo 10-Taranto, Italy
Rita Greco: Dept. of Civil Engineering and Architecture, Technical University of Bari, Via Oraboma, 4, Bari, Italy
Giuseppe Palombella: Dept. of Structural Engineering, Geotechnical Engineering, Engineering Geology (DiSGG), University of Basilicata, C.da Macchia Romana, 85100-Potenza, Italy

While spatial plastic mechanism analysis has been widely and successfully applied to thinwalled steel structures to analyse the post-failure behaviour of sections and connections, there remains some contention in the literature as to the basic capacity of an inclined yield line. The simple inclined hinge commonly forms as part of the more complex spatial mechanism, which may involve a number of hinges perpendicular or inclined to the direction of thrust. In this paper some of the existing theories are compared with single inclined yield lines that form in flange outstands, by comparing the theories with
plate tests of plates simply supported on three sides with the remaining (longitudinal) edge free. The existing mechanism theories do not account for different in-plane displacement gradients of the loaded edge, nor the slenderness of the plates, and produce conservative results. A modified theory is presented whereby uniform and non-uniform in-plane displacements of the loaded edge of the flange, and the slenderness of the flange, are accounted for. The modified theory is shown to compare well with the plate test data, and its application to flanges that are components of sections in compression and/or bending is

Key Words
inclined yield lines; spatial plastic mechanisms; post-failure behaviour; plastic hinge capacity; unstiffened plates; flange outstands.

M.R. Bambach: Dept. of Civil Engineering, Monash University, VIC 3800, Australia

In this paper, we present a quadratic element model based on non-conforming displacement modes and modified shape functions. This new and refined 8-node hybrid stress plane element consists of two additional non-conforming modes that are added to the translational degree of freedom to improve the behavior of a membrane component. Further, the modification of the shape functions through quadratic
polynomials in x-y coordinates enables retaining reasonable accuracy even when the element becomes considerably distorted. To establish its accuracy and efficiency, the element is compared with existing
elements and - over a wide range of mesh distortions . it is demonstrated to be exceptionally accurate in predicting displacements and stresses.

Key Words
hybrid element; assumed stress; 8-node plane stress element; modified shape function; non-conforming displacement modes.

Kyoung-Sik Chun: Structural Division, BAU CONSULTANT Co., Ltd., 968-5 Daechi-dong, Gangnam-gu, Seoul 135-736, Korea
Samuel Kinde Kassegne: Dept. of Mechanical Engineering, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182, United States
Won-Tae Park: Division of Construction and Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan-si, Chungnam 330-717, Korea

A procedure to analyse the space frame structure fixed at base as well as resting on sliding bearing using total or absolute displacement in dynamic equation is developed. In the present method, the
effect of ground acceleration is not considered as equivalent force. Instead, the ground acceleration is considered as a known value in the acceleration vector at degree of freedom corresponding to base of the structure when the structure is in non-sliding phase. When the structure is in sliding phase, only a force equal to the maximum frictional resistance is applied at base. Also, in this method, the stiffness matrix, mass matrix and the damping matrix will not change when the structure enters from one phase to another. The results obtained from the present method using absolute displacement approach are compared with
the results obtained from the analysis of structure using relative displacement approach. The applicability of the analysis is also demonstrated to obtain the response of the structure resting on sliding bearing with restoring force device.

Key Words
absolute displacement; relative displacement; sliding bearing; harmonic ground acceleration; El Cenrto earthquake; space frame structure.

A. Krishnamoorthy: Dept. of Civil Engineering, Manipal Institute of Technology, Manipal . 576 104, Karnataka, India

In this paper, a thermo-viscoelastic problem in an infinite isotropic medium in two dimensions in the presence of a point heat source is considered. The fundamental equations of the problems of generalized thermoelasticity including heat sources in a thermo-viscoelastic media have been derived in the form of a vector matrix differential equation in the Laplace-Fourier transform domain for a two dimensional problem. These equations have been solved by the eigenvalue approach. The results have been compared to those available in the existing literature. The graphs have been drawn for different cases.

Key Words
generalized thermoelasticity; viscoelastic media; vector-matrix differential equation; eigenvalue approach; Laplace-Fourier transform.

Arup Baksi: Dept. of Mathematics, Umes Chandra College, 13, Surya Sen Street, Calcutta-700012, India
Bidyut Kumar Roy: Dept. of Mathematics, Vivekananda College, 269, D. H. Road, Calcutta-700063, India
Rasajit Kumar Bera: Dept. of Mathematics, Heritage Institute of Technology, Chowbaga Road, Anandapur, P.O. East Kolkata Township, Kolkata-700107, India

This study presents a method to evaluate the seismic risk of an extradosed bridge with seismic isolators of lead rubber bearings (LRBs), and also to show the effectiveness of the LRB isolators on the extradosed bridge, which is one of the relatively flexible and lightly damped structures in terms of seismic risk. Initially, the seismic vulnerability of a structure is evaluated, and then the seismic hazard of
a specific site is rated using an earthquake data set and seismic hazard maps in Korea. Then, the seismic risk of the structure is assessed. The nonlinear seismic analyses are carried out to consider plastic
deformation of bridge columns and the nonlinear characteristics of soil foundation. To describe the nonlinear behaviour of a column, the ductility demand is adopted, and the moment-curvature relation of a
column is assumed to be bilinear hysteretic. The fragility curves are represented as a log-normal distribution function for column damage, movement of superstructure, and cable yielding. And the seismic
hazard at a specific site is estimated using the available seismic hazard maps. The results show that in seismically-isolated extradosed bridges under earthquakes, the effectiveness of the isolators is much more noticeable in the columns than the cables and girders.

Key Words
seismic fragility; seismic hazard; seismic risk; LRB seismic isolator; extradosed bridge.

Dookie Kim: Dept. of Civil and Environmental Engineering, Kunsan National University, Kunsan, Jeonbuk 573-701, Korea
Jin-Hak Yi: Coastal Engineering Research Department, Korea Ocean Research and Development Institute, Ansan, Gyeonggi 426-744, Korea
Hyeong-Yeol Seo: Dept. of Civil and Environmental Engineering, Kunsan National University, Kunsan, Jeonbuk 573-701, Korea
Chunho Chang: Dept. of Civil Engineering, Keimyung University 1000 Sindang, Dalseo, Daegu 704-701, Korea

This paper describes a three-dimensional dynamic finite element analysis of two/multiple shots impacting on a metallic component. The model is validated against a published numerical study. An
extensive parametric study is conducted to investigate the effect of shot impacting with overlap on the resulting residual stress profile within the component, including time interval between shot impacts, separation distance between the impacting points, and impacting velocity of successive shots. Several meaningful conclusions can be drawn regarding the effect of shot impacting with overlap.

Key Words
shot peening; finite element analysis; residual stress; shot impacting with overlap.

T. Hong: College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, P.R. China
J.Y. Ooi: School of Engineering & Electronics, University of Edinburgh, King?s Buildings, Edinburgh EH9 3JN, UK
B.A. Shaw: Dept. of Mechanical, Materials and Manufacturing Engineering, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK

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