I am designing a 15 storey RCC building in ETABS. i am modelling Lead rubber bearing of stiffness required to match my idea responce. how should i determine weather my bearing is capable of resisting the vertical loads generates on it due to gravity loads as well as earthquake loads.
Dear Dhaval,
generally, lateral load analysis is conducted for evaluating seismic response of the base-isolated structures, wherein the isolators are typically modeled using (equivalent) spring-dashpot elements. These elements have their stiffness and damping corresponding to the lateral degrees of freedom, and designed to provide some specific damped natural frequency (desirable time period lengthening). Thus, during the excitation due to the horizontal (bi-directional) components of earthquake ground motion their effects are evaluated, i.e. dynamic response of the base-isolated structures is evaluated in two orthogonal horizontal (lateral) directions.
In your case, if you are interested in modelling the base isolation systems in ETABS software, only to evaluate its quasi-static response in vertical direction, the above-mentioned spring element provided for "vertical direction stiffness" of the isolator might suffice (obviously, modeling the damping in vertical direction using the dashpot is unnecessary because it is a quasi-static loading). The quasi-static response evaluated in this manner is then required to be checked against code-specified limit-states of failure of the isolator under the vertical loads. In this case, the vertical stiffness of the isolator needs to be separately calculated based on the mechanical and geometric properties (number and thickness) of the rubber layers and steel-shims provided on a specific plan-area of the laminated rubber bearing (LRB) without or with lead-core. However, if you are interested mainly in studying the mechanical behavior of an isolator unit, detailed finite element (FE) model of the LRB is recommended to be developed, which can be acted upon by the reaction imposed from the column being supported by the bearing.
Hope this information helps in your research.
All the best,
Vasant Matsagar.
To the best of my knowledge, the stiffness characteristics of rubber bearings are influenced by the axial load they carried. Previous research has demonstrated that the bearing lateral stiffness decreases with increasing axial load and the axial stiffness decreases with increasing lateral deformation. However, the axial load effect is not usually accounted for in dynamic analysis. In other words, you may use constant axial and lateral stiffnesses of the rubber bearings. Please review the work of Keri et al. “EXPERIMENTAL OBSERVATION OF AXIAL LOAD EFFECTS IN ISOLATION BEARINGS”, 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, 2004.
Depending the locations and values of axial loads of columns transferred to foundation or to LRB, you may divide them in two categories: small deformation and large deformation. For the case of small deformations you may control the force and comparing the forces by the area multiplied to bearing capacity. In the cases of large deformation in addition to control the force, the displacement should be controlled as well as the uplift force on the external tension columns due to overturning moment over the external columns generated by the earthquake overturning moment.
Your best solution is to visit multiple manufacturer's web sites and obtain their data sheets, which show their tested vertical capacity along with displacement capacity. Specifications often require that the vertical capacity be determined through testing.
If you have properties you have assumed and want to check stability under vertical load, Section 5 of MCEER 11-0004, LRFD-Based Analysis and Design Procedures for Bridge Bearings and Seismic Isolators contains excellent discussion, theory, and design equations suited for isolators in bridges, much of which should be applicable to buildings.
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