Keywords: seismic behaviour, base isolation devices, asymmetric 3D metallic frames, parametric studies.

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 base-frame (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 10-20% 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 pre-norm pr EN 1337.

The parametric analysis is based on the variation of the structural elements lengths (beam spans , and inter-story heights between floors ) and in the definition of the space geometry (typology) of the asymmetric three-dimensional structure represented in Figure 2 already used by the authors for characterizing its non-linear 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 top-floor node) for a set of geometric-typological properties (, , typology) and for the three-dimensional 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 three-dimensional 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).

1

Barros, R.C. and César, M.B., "A Parametric Study on the Seismic Behaviour of an Asymmetric Three-Dimensional Steel Frame with Base Isolation Devices", CONVIB Network Final Meeting, CD of Final Presentations and Conclusions, Ed.: L. Faravelli and F. Casciatti, University of Pavia, Italy, 2006.

2

Naeim F. and Kelly J.M., Design of Seismic Isolated Structures: from theory to practice, John Wiley and Sons, New York, USA, 1999.

3

SETRA, "Appareils d'appui en caoutchouc fretté: Utilisation sur les ponts, viaducs et structures similaires - Guide technique", Service d'Études Techniques des Routes et Autoroutes (SETRA), ref. 0032, France, 2000.

4

César M.B. and Barros R.C., "Parametric Study of the Non-Linear Geometric Behaviour and Carrying Capacity of 3D Asymmetric Steel Frames", Civil Engineering Computing, Ed.: B.H.V. Topping, Paper 155, 17 pages, Civil-Comp Press, Scotland, U.K., 2005 (also submitted to Computers & Structures). doi:10.4203/ccp.81.155

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