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How to Find and use Torsional Constant modification factor in ETABS?

July 04, 2014
In ETABS during the design and analysis of beams there are certain modification factors used to incorporate the uncertainty and other factors like monolithic behavior etc. One of that modification factor is Torsional constant modification factor. In this post I will be sharing with you the procedure along with an example of how to find it and use it properly.


Suppose beam size is 8" x 51" and f'c = 3000 psi

First of all , put tosrional constant value of 1 in all beam sections , 

How to put torsional constant modification factor

torsional constant value of 1 in beams , for this , go to define then frame
sections then select beam sections one by one and click on modify/show property
then set modifiers then there put torsional constant value of 1 in box
How to Find Torsional Constant Modifier in ETABS

Finding PHI Tcr

now Acp = 8 x 51 => Area

and Pcp = 2(8+51) => Perimeter

Now put these values in phi.Tcr = 0.85 x 4 x (3000)^0.5 x (( 8x 51)^2)/(2(8+51))

= 262700.554 lb-inch = 21.89 kip-ft

Finding Tu

Now for Tu value , run model in etabs , then design it , after design , see all beams of size 8x51 then from these all beams right click on beams and see summary then see Tu value one by one and select maximum tu value from all 8 x 51 size beams

suppose your tu (max) from all 8 x 51 size beams comes out to be 40 kips-ft

or you can analyse in etabs then select 8x 51 size beams and from that beams see torsion values at a distance d from face of support and select maximum tu value

then torsional constant value = 21.89 / 40 = 0.547

now put this 0.547 value in all 8 x 51 size beams

Note: If your beam size is different and also f'c is different then do above all steps for that beam size and f'c

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  1. Replies
    1. Dear BDA,
      The formula is for fps as we used mostly this in our country, but stay tuned i will surely update the article as per your demand.

  2. Hello, what if I get a value >1 for the torsional constant, does it mean it the member is oversize?

  3. What if I put torsional constant equal to 1 lr zero.