RE: Width/Thickness Ratios[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Width/Thickness Ratios
- From: Charlie Carter <carter(--nospam--at)aiscmail.com>
- Date: Fri, 26 Jan 2001 08:19:08 -0600
>UBC 2211.8.4.b requires that beams and columns in special moment
>frames have a b/t ratio less than 52/sqrt(fy). I have a beam that was
>designed with A36 steel which meets this requirement. The supplier
>substituted A572 steel which does not meet the requirement.
As recognized in the AISC Seismic Provisions and the FEMA 350-series documents, ASTM A36 steel should not be treated as though it has an actual yield stress of 36 ksi. That is the minimum specified value, but the actual yield strength has been much higher for quite some time now. Perhaps it was never really as low as the minimum value, except in the very early days of the
AISC and the SAC Joint Venture (writers of the FEMA documents) have emphasized that designers should be specifying ASTM A992 for wide-flange shapes, particularly in seismic applications, because it is a product with much better definition of its mechanical and chemical properties. See more about it here:
As far as your specific question goes, however, it's a bit of a gray area. Rick Drake posted the technical background for the distinction between the normal ductility limit of 65/sqrt(Fy) and higher ductility limit of 52/sqrt(Fy). Those limits are based upon Fy, which is the minimum specified value, not the actual value. That said, if you plan to be able to fully yield the cross-section, as we do in a special moment frame, something close to the actual yield strength will be activated, not the minimum specified value. The issue is a little more clear in wind and low-seismic (R of 3 or less) applications, where we do not normally expect the frame to be deforming to dissipate earthquake energy. In those cases, I'd say no problem.
For high-seismic systems (R greater than 3), the testing done by the SAC Joint Venture for FEMA may give you sufficient justification for what exists. Their test specimens mostly used two beam cross-sections: W36x150 and W30x99 -- both in ASTM A992 or ASTM A572 grade 50, which both have Fy (minimum specified value) = 50 ksi. The first one meets the higher ductility limitation at Fy = 50 ksi. The second one meets the normal ductility limitation only, not the higher ductility limitation, at Fy = 50 ksi. nonetheless, both beam sizes performed acceptably in their testing.
I personally believe that the SAC testing provides strong justification that the higher ductility limitation may need to be re-examined. The Code requirements still read as they have based upon pre-SAC thinking, however.
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