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R Values for Kneebraced Posts and Steel Columns

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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?


Chris Slater, PE

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