Using mode superposition method, find the seismic base shear of a two$-$storeyed $(G+1)$ structure located in Kolkata at medium site. The lumped mass of $1^{st}$ floor is $1000$ kN and $2^{nd}$ storey is $500$ kN $.$ Storey height is $3$ m $.($Assume any missing data$).$ Take $E=2{10}^{10}\frac{N}{m^2}.$ Take $K1=$K$2=K,$ size of column $=60cm30cm.$