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RE: Steel SCBF Yield Lines

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Title: RE: Steel SCBF Yield Lines

One additional note on this topic....

I spoke with the engineer on this project, who clarified that the schematic bracing details shown in the article unintentionally visually distort the severity of the tilt of the yield line relative to the brace axis. In the actual connections, the engineer indicates they are much closer to perpendicular.


-----Original Message-----
From: Charlie Carter
Sent: Thursday, May 18, 2000 1:24 PM
To: 'seaint(--nospam--at)'
Subject: RE: Steel SCBF Yield Lines

>from David Merrick, SE
>Modern Steel Construction magazine for May 2000
>.... chevron bracing....sketches show yield lines not
>square with the brace. [Shouldn't] the yield line be
>[perpendicular to the axis of the brace]?...

You are correct, David. The yield line for an SCBF gusset is intended to be perpendicular to the axis of the brace to satisfy the "2t free" dimensional requirement in the AISC Seismic Provisions Part I Section 13. There has been a lot of discussion on this topic, particularly because the requirement drives up the gusset size whenever the brace can't be placed at a 45-degree angle.

I've seen details that don't satisfy this geometric requirement on several occasions before. Although the writers of the 1997 AISC Seismic Provisions had the picture clear in their minds, readers of the resulting document did not always pick up on the nuance that the yield line should be perpendicular to the axis of the brace. To explicitly express this aspect of the intent, a clarification was made in AISC Seismic Provisions Supplement No. 1, which was published in 1999.

You can download AISC Seismic Provisions Supplement No. 1 here: