Tapered Plate Girders

[JMC3178]

I am re-evaluating an existing pre-manufactured metal building with tapered web girders. I am using RAM Advance for the analysis and I have run into some problems with how they analyze certain items. The first being, Qa the reduction factor for stiffened slender members. My first question is what defines a stiffened member? Secondly can anyone clarify the formula for Qa? The formula states its Ag-Sum(h-he)t. Is the term h referring to the nominal height of the web or the section? and the term t, does it refer to the web thickness or the flange thickness? Does the sum of (h-he)t mean the sum of the h of the top flange and t of the top and flange plus the h of the web and t of the web...ect?
I am running the original loading to see if I can verify the CSI of .99, however the fa/Fa is considerably higher than the original calcs show. Do the metal building fabricators calc it any different than I would? Use any other assumptions than I normally would?

Thanks for the Help
Justin

 

[271828]

First: What Spec. are you using?

"Stiffened" doesn't refer to member, it refers to element. The web is a stiffened element because, for plate buckling, it's supported on two sides. Your flanges are unstiffened elements.

Qa, as defined in E7.2 of the 2005 Spec., is the effective area of the entire cross section divided by the total area. Effective lengths don't apply to flanges because they're unstiffened. For your web, if it's slender enough, plate buckling will occur and there will be post-buckling strength. AISC handles this by letting you calc an equivalent web depth. For example, say be from E7-17 comes out to be 17" and your web plate depth is 19". Think of it as the middle 2" is ineffective due to plate buckling.

Have you considered hiring somebody familiar with metal bldg design practices? Metal bldg steel design is many times more detailed and difficult than typical steel design. You'll either do lots of things wrong or face a HUGE learning curve if you start from "can design steel bldgs" and get up to speed wrt metal bldgs.

 

[271828]

Oh yeah, you might contact MBMA to try and get a copy of their new Tapered Member Design Guide. I don't know if it's been released, but it should be close. They've come up with a lot of good guidance for how to handle these members that are mostly ignored by the main AISC Spec.

If you're interested in hiring a metal bldg guy, you might contact Star and ask them who they recommend. I heard a while back that they worked on jobs like this from time to time.

 

[JMC3178]

I am using the ASD 9th edition. I just trying to check the frame for new loading. There was 9psf of collateral on the frame, 5 of this was sprinklers which are not installed. With the removed load, I am just checking that they can add the additional points loads of air handlers and conveyers. The moments, shears, and reactions are the same as the first run, so I know I have it modeled correctly. Its just when I go to the Code check that I have the problem. Would it be advisable to show that the fa, fv, and fb are all less than original, thus the new design is ok?