Why do we scale the Base Shear ??
Here is the Complete Answer u are looking for !!
Firstly, The TIME PERIOD (Ta) is inversely proportional to the design lateral force (BASE SHEAR) of the building. i.e
Less the TIME PERIOD (Ta) of the building more the BASE SHEAR of the building
Similarly, STIFFNESS of the building is inversely proportional to the TIME PERIOD (Ta) of the building. i.e
More the stiffness of building less the TIME PERIOD (Ta) of the building.
AGREE ??
Now, Based on the rigorous research from the best engineers across the globe on the behavior of the existing buildings under earthquake, the empirical formula for the calculation of TIME PERIOD (Ta) which is 0.09h/sqrt(d) was proposed (Which we use in the regular day engineering practice). The point to be noted here is that the TIME PERIOD (Ta) formulae was proposed based on the existing buildings which consist of non-structural elements like Partition walls, these partition walls will provide some additional LATERAL STIFFNESS to the building.
AND
We are not modeling any non-structural partition walls in our (ETABS, STAAD, SAP OR ETC.) Analysis model by which we are ignoring the stiffness of the Non-structural partition walls in our analysis calculation.
Now we have two TIME PERIOD’S here
1. Static TIME PERIOD (Ta) of the building based on the research &
2. Dynamic TIME PERIOD (Ta) which is obtained from the analysis model
Static TIME PERIOD is calculated including the stiffness of the non-structural partition wall
And, the Dynamic TIME PERIOD is calculated from the analysis model in which we did not consider the non-structural stiffness of the partition walls
If you observe the above two statements, you can clearly understand that in the second statement i.e. in Dynamic Analysis Method we are underestimating the stiffness when compared to the Static Analysis method.
Less the STIFFNESS more the TIME PERIOD, more the TIME PERIOD less the BASE SHEAR So, this is why our code specifies us to Scale up the BASE SHEAR of the building when performing Dynamic Analysis Method to ensure that we are not underestimating the design lateral load when compared to the Static Analysis Method.
