Do you needs a speedy technique?

Then, alternatively to the theoretical methods you can perform ambient vibration measurements in the center of building and at one end, for which you want to find if torsional effects exist. Then, the ratio between maximum torsional displacement (measured at the end) and maximum translation displacement (measured at the center) is computed. If this ratio exceeds 1.20 the building is subject to torsional effects. 

Hi Vishal:

To calculate the torsion by fundamental modes, you only take the matricial form (modal to real space) to obtain the displacements. The torsion displacement is an angular deformation.

Then, you can derive the angular deformation to obtain the angular acceleration. Calculate the max angular acceleration and finally multiply it with the second moment of inertia.

If you consider more than 3 DoF, you can take a aproximation. For this, I suggest you to read Chopra or any Dynamical Structure book. Mechanical Vibrations by Rao is really good for this too.

Greetings.