Question: I have modelled a simple symmetric structure with channel sections and applied symmetric vertical loading to it. I have used IS800-2007 as design code. The columns doesn't show symmetry in UC ratio. Please help.
The difference in UC ratios for column sections is happening due to different allowable minor axis bending capacities though all the parameters are same for that two identical columns.
In STAAD.pro bending capacity is calculated depending upon the bending diagram of the structure. Generally, the section modulus of compression side is reported for the calculation of allowable bending capacity.
For doubly symmetric members, it is not a matter of concern. But for singly symmetric member like channel section, the minor axis bending capacity is very much guided by this parameter. For channel section, depending on the compression side the section modulus changes.
Section modulus, Z = Moment of inertia (I) / Distance between CG and extreme compressed fiber (y).
As the value of y will be different for 'cy' & 'ey' (See attached figure), the value of Z will also be different which in turn provide different capacities for the same section depending upon the compression side while bending.
As compression side is more susceptible to buckling, this methodology has been adopted and STAAD works as per that methodology. But to be in safer side it has been suggested by some of our users to take the minimum value of Z for conservative capacity calculation. It will be implemented in later release.
For a work around, you can use 180 beta angle for the columns to get same bending pattern and UC ratio.
Please find enclosed files to find the difference.
NOTE: This problem will not be encountered for AISC 360-16 as it has already been changed as per the prohibition of code.