How can I account for Accidental torsion and it’s amplification as per IBC (ASCE )

For non flexible diaphragms, the IBC/ASCE requires accidental torsion to be accounted for. The accidental torsion is computed by multiplying the shear at a story by a lever arm equal to 5% of the building dimension orthogonal to the direction of the earthquake.

The screenshot below shows how one can account for the 5% accidental torsion.

Let us say you have seismic load acting in X and let “b” be the building dimension orthogonal to X. So your accidental torsion moment would be computed as = F x 0.05 x b where F is the seismic shear at the story. So essentially you are just entering the factor 0.05 and the software internally multiplies it by the dimension b and the story shear F to calculate the accidental torsion moment.

Now let’s say you need to do an amplification and you calculate the amplification factor to be Ax = 1.16. So your total amplified accidental torsion = F x 0.05 x b x 1.16.

In other words the F x b needs to be multiplied by the factor ( 0.05 x 1.16 ) = 0.058. So you enter this value as the Multiplying Factor for Accidental Torsion Moment in the dialog box above.

Note: This wiki assumes that a rigid diaphragm has been defined using a Floor Diaphragm command