presented by:
PARIK GARG

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STUDY OF UNIAXIALLY ASYMMETRIC SEISMICALLY ISOLATED RC BUILDING
Need for seismic isolation of a Structure
Increased building safety and post earthquake operability are desired.
Reduced lateral design forces are desired.
Alternate forms of construction with limited ductility capacity (such as pre-cast concrete) are desired in an earthquake region.
An existing structure is not currently safe for earthquake loads.
RB(Laminated Rubber Bearing)System
LRB Lead Rubber Bearing System
Distribution of Forces along Height
Base Isolated Structure
Conventional Structure
Minimum design displacements
Four distinct displacements are calculated using simple formulas used for static analysis. These values also serve as the IBC5 permitted lower bound values for dynamic analysis results.
These are:
DD: the design displacement, being the displacement at centre of rigidity of isolation system at design basis earthquake (DBE);
DM: the design displacement, being the displacement at centre of rigidity of isolation system at maximum credible earthquake (MCE);
DTD: the total design displacement, being the displacement of a bearing at a corner of the building and includes the component of the torsional displacement in the direction of DD;
Effective isolated system periods
where W = the weight of the building;
g = acceleration due to gravity; KD,min = minimum effective horizontal stiffness of the isolation system at the design displacement (DBE). KM,min = minimum effective horizontal stiffness of the isolation system at the maximum displacement (MCE). The values of KD,min and KM,min are not known during preliminary design phase and hence the design process will begin with an assumed value which is obtained from previous tests on similar components. After moulding of prototype bearing, the actual values of KD,min, KD,max, KM,min and KM,max will be obtained from the results of shear test on bearings. KD,max and KM,max are maximum effective stiffness at displacement corresponding to DBE and MCE, respectively.
The total design displacements, DTD and DTM are given as:
where b and d are plan dimensions at the isolation plane, e is the actual eccentricity plus 5% accidental eccentricity, and y is the distance to a corner perpendicular to the direction of seismic loading.
Design forces
The superstructure and elements below the isolation interface are designed for forces based on DBE design displacement, DD. The isolation system, the foundation and structural elements below the isolation system must be designated to withstand the following minimum lateral seismic force:
If other displacements rather than DD generate larger forces, then those forces should be used in design rather than the force obtained from above equation.
Problem for study:
A typical C shape building has been selected for study to incorporate uniaxial asymmetry. The study has been carried out with reference to its response against applied forcing function in the form of El-Centro Earthquake. The building has been modeled in the eminent structural analysis tool called SAP-2000(computers and structures Inc. university of Berkley)
Size of column: 30 X 30 cm
Size of beam : 30 X 50 cm
Load applied: dead load +live load= 15 kN/m