# 12.8.4.1 Inherent Torsion

Inherent torsion is included automatically when performing a three-dimensional analysis with the program using either a rigid or semirigid diaphragm.

For diaphragms that are not flexible, the distribution of lateral forces at each level is considered the effect of the inherent torsional moment, M_{t}, resulting from eccentricity between the locations of the center of mass and the center of rigidity. For flexible diaphragms, the distribution of forces to the vertical elements should be accounted for the position and distribution of the masses supported.

If a rigid diaphragm is choosen in the analytical model, the mass tributary to that floor or roof is modeled as a lumped mass located at the resultant location on the floor or roof (the center of mass). This point represents the resultant of the inertial forces on the floor. This diaphragm model simplifies structural analysis by reducing what would be many degrees of freedom in the two principal directions of a structure to three degrees of freedom (two horizontal and one rotational about the vertical axis).

This inherent torsion is included automatically when * performing a three-dimensional analysis* using either a rigid or semirigid diaphragm. If a two-dimensional planar analysis is used, where permitted, the center of rigidity and center of mass for each story must be determined explicitly and the applied seismic forces must be adjusted accordingly.

For structures with flexible diaphragms (as defined in Section 12.3), vertical elements of the seismic force resisting system are assumed to resist inertial forces from the mass that is tributary to the elements with no explicitly computed torsion. No diaphragm is perfectly flexible; therefore some torsional forces develop even when they are neglected.

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