Effective Section Properties- Weak axis

[ToadJones]

I was running some analysis on cold formed Z purlins and comparing the results to CFS software.

It looks like RISA is not using the effective section modulus when calculating the weak axis moment capacity of the member?

CFS gives the same results for the strong axis moment capacity but the results do not correlate for weak.

Is there something I am missing or does RISA only calculate the effective properties for strong axis bending.

The controlling design equation is combined bending and axial.

AISI C5.2.1-1

Thanks

 

[JoshPlumSE]

Toad -

Did you contact RISA support about this already? I ran a few tests which led me to have a chat with one of our developers. He said he was already looking at the issue based on someone who contacted us while I was on vacation.

The gist of the conversation is that there is a bug with the calculation of effective section modulus for the weak axis of cold formed Z sections. Not sure how long it's been in the program or how often it occurs. But, it's definitely a bug. The developer will have to dig through the source code in detail because at first glance the code logic is correct.

If I had to guess, I would say we're double counting some sections. Or, maybe we're accidentally increasing the section modulus for segments that are in significant tension.