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CivilComp Proceedings
ISSN 17593433 CCP: 83
PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Paper 252
Seismic Behaviour of an Asymmetric ThreeDimensional Steel Frame with Base Isolation Devices R.C. Barros^{1} and M.B. César^{2}
^{1}Department of Civil Engineering, Faculty of Engineering (FEUP), University of Porto, Portugal
, "Seismic Behaviour of an Asymmetric ThreeDimensional Steel Frame with Base Isolation Devices", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", CivilComp Press, Stirlingshire, UK, Paper 252, 2006. doi:10.4203/ccp.83.252
Keywords: seismic behaviour, base isolation devices, asymmetric 3D metallic frames, parametric studies.
Summary
A parametric study on the use of base isolation devices in frame building
structures is undertaken. Frames were analysed with regular geometry starting from
a baseframe (BF), from which association of BF in height above a certain
asymmetric plant created more complex frames. Energy dissipation occurs in base
isolation (BI) devices, whose characteristics are used in SAP 2000 in order to
develop a parametric study of the behaviour of frames under seismic actions with
reduced forces supported by the structure. Some of the results of a parametric study
are presented graphically, identifying the importance of positioning the seismic
isolation devices used in each structural configuration (Barros and César [1]).
The first approach in the analysis of structures with energy dissipation devices at the base is based on the use of a model with two degrees of freedom (DOF) (Figure 1): one DOF at the level of the base isolators; the second DOF at the floor level (Naeim and Kelly [2]). A system of base isolation devices made of natural rubber guarantees a degree of damping in the order of 1020% of the critical damping, considerably greater than structural damping factors for steel frames (in the order of 2%). The characteristics of the elastomer used in the BI devices are defined according to the recommendations of SETRA [3] and to the prenorm pr EN 1337. The parametric analysis is based on the variation of the structural elements lengths (beam spans , and interstory heights between floors ) and in the definition of the space geometry (typology) of the asymmetric threedimensional structure represented in Figure 2 already used by the authors for characterizing its nonlinear carrying capacity (César and Barros [4]). Qualitative and quantitative comparisons were made on akin variables (the total acceleration and the relative lateral displacement of a topfloor node) for a set of geometrictypological properties (, , typology) and for the threedimensional asymmetric frame modeled without and with elastomer isolators at the base of the columns (Barros and César [1]). From the values obtained in the parametric study it is verified that the behaviour of the asymmetric threedimensional frame is altered when one to five floors of the rigid slabs L2 and L3 exist; the most favourable effect (i.e. maximum decrease of total acceleration) associated with the use of BI devices happens when three rigid slab floors exist, for which the larger difference between the resultant total acceleration without and with BI occurs . The resultant relative displacement stays practically constant for corresponding cases of the parametric study, without and with the BI devices, with respect to the number of rigid slab floors used (slabs L2 and L3). References
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