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Seismically Non-Compact Beam

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Dear Matt:
I think adding web stiffeners (vertical plates perpendicular to the web) will not help in preventing local buckling of the flange. As far as I know, web stiffeners can help in preventing web shear buckling.  even with very closely spaced web stiffeners, the flange can buckle between the web stiffeners.

If the web in your beam is compact and only the flange(s) need to be made compact, one solution might be to weld an "edge stiffener" such as a plate or an angle to the edge of the flange(s) underneath the top flange and above the bottom flange. This solution is successfully and routinely done in seismic retrofit of existing steel bridges in CA and elsewhere where the members , especially those with high Fy, do not satisfy current seismic b/t ratios.

By adding the edge-stiffeners to the tip of flanges, the "unstiffened" flange with limit of b/t<  or = 0.30(E/Fy)^0.5 is converted to "stiffened" element with  b/t<  or = 1.49(E/Fy)^0.5.  These are the limits in the AISC Seismic Provisions (2005) Table I-8-1.

I have to add that in bridges, the edge stiffener is usually an angle where the horizontal leg is bolted to the existing steel sections since welding is not allowed in existing bridges.

For buildings, obviously if applicable standards and procedures for welding existing members are followed, edge-stiffeners can be welded to the existing steel members.

The edge-stiffening angles or plates do not need to be  large sections. Usually a relatively small section will do. There is a rational method and quite simple equations suggested in "Buckling strength of metal structures" by Friedrich Bleich (1952)for sizing these stiffeners.  I have a section on this issue in my graduate course Reader and will be happy to share information from it with you if it can be of any help.

Best wishes.

Abolhassan Astaneh, Ph.D., P.E., Professor
University of California, Berkeley


Subject: Seismically Non-Compact Beam
From: "Steiner, Matthew"<MSteiner(--nospam--at)>

We are evaluating an existing SMF building that has a number of beams
that are seismically non-compact because of the flange width to
thickness (Actual is 7.67, Maximum is 7.23).

I was thinking of adding web stiffener plates to stiffen the flanges and
prevent buckling but I'm not sure how to determine the buckling length.
My guess is there is some variation of plate theory that can be used to
find the local flange buckling length and use that to determine the
spacing of the stiffener plates.

Any references are appreciated.

Thank you,


Matt Steiner, P.E., S.E.=20
Thornton Tomasetti=20
2211 Michelson Drive, Suite 460=20
Irvine, CA  92612=20
T 949.271.3300  F 949.271.3301  =20
D 949.271.3314  M 949.910.8130  =20

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