### Example

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

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 8×51 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

I need formula for si unit

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.

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

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