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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Study of the Dynamic and Equivalent Static Analysis Methods for Seismic Design of Bridges: Ranges of Applicability, Effect of Modelling Assumptions, and Support Conditions
M.M. Bakhoum+ and S. Athanasious*
+Faculty of Engineering, Cairo University, Egypt
M.M. Bakhoum, S. Athanasious, "Study of the Dynamic and Equivalent Static Analysis Methods for Seismic Design of Bridges: Ranges of Applicability, Effect of Modelling Assumptions, and Support Conditions", in B.H.V. Topping, (Editor), "Proceedings of the Eighth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 37, 2001. doi:10.4203/ccp.73.37
Keywords: bridges, seismic analysis, dynamic, equivalent static, codes.
The paper focuses on the structural analysis methods used currently in practice for the seismic design of bridges [1,2]. Several Dynamic Analysis Methods (DAM) and Equivalent static analysis methods (ESM) are currently available. The former are more accurate, but require knowledge and training about Dynamic Analysis of structures. Also, they require more advanced analysis packages which are not readily available in many practices. The later (Equivalent static) are better known to the engineers, but their applicability is limited to regular bridge geometry and configurations. It is the first objective of this paper to discuss these methods and the cases where ESM could be applied.
The Modeling assumptions made in either the dynamic or ESM could significantly affect the output results of analysis, particularly: soil structure interaction (SSI), and effective inertia of the cross-section. The common assumptions of fixed supports at the connection of column to the foundation (footing or pile cap) gives higher values of stresses (expressed bending moments in the frame analysis) than what could actually happen in a bridge. Also, the assumption of gross cross-section inertia is not realistic for the design earthquake level. For ductile response of structures to earthquake events, it is assumed that steel reinforcement has yielded, which is necessarily accompanied by cracking of reinforcement. Neglecting both these assumptions could result in a much stiffer response, and larger effect of the earthquakes on the bridges. The paper discusses methods to includes the effect of SSI and effective cross section properties in the different methods of analysis for seismic design.
Moreover, the paper presents a discussion of the response of several bridge structural systems to seismic actions. Several boundary or support conditions are studied including: monolithic connection between deck and column, hinged supports, and elastomeric bearing supports. For each case, the deck horizontal deflection, and bending moments in the columns are evaluated and compared. The analyses are carried with and without considering effect of SSI and effective cross- section. Although it would originally appear that a bridge with monolithic deck / column connection has a stiffer response and higher stresses than one with hinged supports or elastromeric bearings. However, considering more realistic modeling assumptions, and dividing the elastic forces by values relevant to each case, to obtain design forces, the stresses in both systems would be almost equal. The bridge with monolithic connections have the advantages of smaller lateral displacements of the deck, hence smaller expansion joints, and there is no need to worry about replacing the bearing in the future .
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