| Product(s): | SACS |
| Version(s): | All |
| Area: | Post |
Problem
I see my actual minor axis bending is lesser than allowable minor axis bending. But SACS is reporting a higher UC ratio. I am using an I-shaped member and my design code is AISC 9th edition. What is the reason for such results?
Solution
Let us walk through this discussion with an example, from one of our users sample model.
In the sample model, the minor axis bending UC ratio for a member is 2.29 when fbz is 37.09 MPa & Fbz is 187 MPa respectively.
To resolve this, I would request you to please check in the result that the member is experiencing a torsion (Mx) of -3.54 kN-m. As you are using AISC 9th edition for member code check option, and it does not refer any specific torsional stress calculation, SACS converts this torsion into minor axis bending and shear stresses. Please refer SACS Post manual 3.2.1.2 for the calculations of stresses due to torsion in I-shaped beams. You may also refer AISC design guide 9.
What you are seeing as fbz in SACS output, is the pure minor axis bending stress. But when the torsional component gets added with the minor axis bending stress component, it shoots up the value. Thus, the UC ratio reported is a higher value than 37.09/187.
To ensure this pattern, I quickly made some modifications in the sample model to lock this Mx. This time, to some extent the torsion value is reduced even though the critical load case remains the same.
This time, you will see a drastic change in minor axis bending UC even after an increase of Mz (from 9 to 68).
As you see this minor axis bending component contains the part of torsional stress into it hence the UC ratio gets shoot up according to SACS calculation. Since the member section type is an I-section, even a feeble amount of torsion can become critical.