pre-engineered metal building purlins

[jmy61]

I am structurally evaluating the roof framing of a pre-engineered metal building to verify its ability to support the additional weight of a new roof added on top of the existing roof. I have checked the light gauge steel roof purlins for flexure, shear, and deflection. Can anyone tell me where I can find equations, etc. for checking web crippling of the purlins? Thanks.

 

[msquared48]

If I were you, I would be more concerned with the mainframe and foundation.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

Many(most) of those sections are propietary to the mfg. Unless it is a common size - you will have to do it by hand or there exists software you can use to derive - after you draw the outline.

BTW - many (most) of these metals buildings are well engineered - to the point there is not an extra ounce of steel in them. Retro-fitting or adding loads can sometimes (almost always) be a real problem.

Good luck...

 

[JAE]

Go to the AISI website if you are in the US:

AISI Link

 

[Ron]

confusedPE...I'm confused. Usually when a "pre-engineered" metal building needs a new roof you replace the panels. Why would you consider adding a roof over the existing panels? Makes no sense.

 

[audeuce02]

Roof huggers or retro zees over the existing roof panel is actually a fairly common practice now. The major issue with the added load is having to bring the structure up to the current code. With that you lose the 1/3 stress increase for transient loads in the older buildings which typically kills all the primary framing, especially the connections. You will also end up having to address unbalanced snow loading, possibly alternate span snow/live loading of the purlins. If the original manufacturer is still in business, I would go back to them to start with.

 

[hokie66]

Are we talking the same thing? Where I am, a bit of extra mass on the roof always helps, as uplift controls 99% of the time. I understand with snow loading that wouldn't be the case, but surely metal roofing is not enough to make much difference. But I agree with Ron, it is much better to replace the roofing than add some more on the top. I don't like the idea of leaving defective metal roofing in place.

 

[jmy61]

Thanks to all for your responses.
The Client is installing the new roof over the existing using the roof hugger system. They have done so on other pre-engineered buildings with which I was not involved.

The added dead load is approximately 2 psf. The main frames and foundations can handle the additional load. Calculations indicate the purlins are adequate in flexure, shear, and deflection, albeit some require composite action with the roof hugger system to be adequate.
I am concerned that web crippling could be a problem. I will go to the ANSI site and see what I can find.