# Torsional Frequency for Spine Model

**Question:**

Hello,

I need to calculate the torsional frequency for a single span concrete box girder structure. Due to the 2-D model, the eigenvalue analysis does not calculate a frequency for the Rot X degree of freedom. Is there a work around so that a full grillage model is not required?

**Answer:**Dear User,

It depends on the type of mass distribution considered. Since by default lumped mass is considered for line model, rot X is not significant since the masses are lumped at nodal points.

So by either considering, the off diagonal masses or choosing the consistent mass( which accounts for mass distribution) the required result could be obtained.

For more info on this refer the online help menu.

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