Cee & Zee Flexural Members

[pioneer09]

Are there any published values for Cee & Zee sections used as purlins when attached to standing seam roofs. Per AISI, there is a reduction factor determined in accordance with AISI S908 that is to be used. I would think pre-engineered metal building companies either have their systems tested or utilize their own derived/calculated values. From past experience, I have typically seen values in the range of R=.6-.7

Wondering what others are using when testing/resources that the metal building companies have are not available.

 

[rowingengineer]

Why not design them directly using the cold formed code? Maybe a software to help? Testing does give an advantage but these days the design codes are fairly good.

 

[271828]

There are two options: use discrete bracing and ignore bracing from the panel system or use the bracing from the panel system.

The AISI Specification I6.2.2 says to use Appendix A for USA and Mexico or Appendix B for Canada.

Appendix A I6.2.2 says to either use discrete bracing or use the reduction factor, R, established using AISI S908 Base Test Method. R varies depending on the test results for a panel system.

Appendix B I6.2.2 says to use discrete bracing and sends the reader to C2.2.2.

 

[SandwichEngine]

Some companies test their systems but most don't. Most specify discrete x-braces spaced at whatever intervals and ignore any contribution from the standing seam system. Standing seam systems for roofs of any length over about 50 ft utilize floating or sliding clips which allows the panels to move over the purlins due to thermal expansion of the roof panel. In reality, for the purlin flange to buckle, it would rotate, not just slide horizontally so the panel would offer some resistance. Companies that have tested their systems rely on this somewhat but most just provide bracing to prevent LTB on both flanges. Companies that have tested these systems consider them proprietary so they don't give them out. Even if they did, it would only apply to their specific assembly.

 

[pioneer09]

I have computed values based on AISI S100-2012 and the distortional buckling strength seems to be controlling the design over the LTB capacity with discrete bracing. The member sizes become a little large than what one would expect if went the route of a PEMB supplier, but this was bound to occur as it seems the PEMB suppliers have their own propritary system to justify designs in some cases. Just for reference, the computed reduction factor from yield capacity with discrete bracing at 6' oc was .65 for this project which correlates similar to the R values I have seen from PEMB suppliers in the past.