Calculating Accidental Torsion

[imsengr]

Hi everyone,

Section 9.5.5.5.2 of the ASCE 7-02 states that when diaphragms are not flexible, the design shall include the torsional moment ... plus the accidental torsional moments caused by assumed displacement of the mass each way from its actual location by a distance equal to 5% of the dimension of the structure perpendicular to the direction of the applied forces.

My question is this. If my center of rigidity is located at a different location than my center of mass, I would assume that the torsional moment resulting from this is already considered in calculating the torsional moment. Is this assumption correct?

Now, in calculating the accidental torsion, do I just need to consider the distance equal to 5% of the dimension of the structure perpendicular to the direction of the applied force, or do I still need to add on (again) the torsional moment resulting from the fact my center of rigidity is located different from my center of mass? Wouldn't this be double-dipping as far as the torsional moment is concerned.

Hope that someone familiar with accidental torsion for a structure with center of rigidity and center of mass located at different locations would kindly advise. All other advice and suggestions are also welcome.

Thanks, y'all.

 

[Taro]

The total torsional moment consists of:

1. The inherent torsion, Mt, that results from the center of mass being different from the center of rigidity

PLUS

2. The accidental torsion, Mta, which is a minimum additional 5% offset of the center of mass.

You need to offset the mass 5% in each direction and consider the worst-case effect for a particular element. Also, if the structure has a torsional irregularity, you need to amplify the accidental torsion (i.e. use greater than 5% offset).