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RE: R Values for Kneebraced Posts and Steel Columns

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Chris,
There are a lot of "commonly used" structural systems that are not allowed by the building code. 
 
If I am understanding this correctly, the knee brace puts bending into the wood column.  This has been precluded by the various codes for quite some time for high seismic demand because of the failure mechanism.  The first mode of failure is a buckled column which is a very undesirable mode of failure in a seismic event. 
 
That said, with this being a residential structure, I doubt you want to be in the ASCE 7 at all.  You may get a more desirable answer if you used the IRC which is indicated in ASCE 7 Section 11.1.2.2. 

If you want to continue to use the ASCE 7, I would suggest the cantilevered column detailed as a special steel or special concrete moment frame. 

Regards,
Harold Sprague

> Date: Thu, 17 Jan 2008 10:57:19 -0800
> From: chris(--nospam--at)jdwylieengineering.com
> To: seaint(--nospam--at)seaint.org
> Subject: R Values for Kneebraced Posts and Steel Columns
>
> I'm looking through the R values in ASCE 7-05 (table 12.2-1) and I'm
> having a hard time figuring out what R values to use for two of the
> systems that a lot of our residential clients request.
>
> We get a small but steady flow of people wanting to support small
> patios using kneebraced posts. The only item I'm seeing that would
> seem to match that is in section G, item 7 - Timber frames. They
> assign an R value of 1.5 and say it's not allowed in Seismic Design
> Category D, which is where a lot of our work falls. Is that to say
> then that kneebraced poss are just not allowed in the new code? I've
> never been crazy about them, but they're a very commonly used system
> for small patio additions.
>
> Even more commonly used are cantilever steel columns. We've used
> these in the past to support porticos and larger patios that won't
> work with knee-bracing. It looks like they want an R value of 1.5 and
> detailing consisent with inermediat steel moment frames if we're going
> to use them in SDC D.
>
> Am I reading this right or do other have a different interpretation?
>
> Thanks,
>
> Chris Slater, PE
>
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