Thursday, January 17, 2008

RE: R Values for Kneebraced Posts and Steel Columns

Some graduate student on this list needs to thier thesis on knee braced
timber frames with full scale tests for residential decks so that we can get
a dad-gum R in the code.

Also the graduate student reading this maybe do tests on log shear walls if
it isn't being done so we can get an R for log homes.

How do you brace a wood deck per the code in seismic design category D?
Install a steel moment frame under it [oh the humanity]? I guess you could
assume a knee braced wood post is an inverted cantilevered column? I regret
the torture I've inflicted in the past as a plan reviewer because the code
cook book wouldn't step up and give the poor designer a recipe.

If you model a knee-braced frame the deflected shape is the exact same as a
pinned end portal frame. There isn't a wood moment frame in the building
code. It is an undefined system.

The UBC used to have an R for heavy timber braced frames. The IBC / ASCE 7
committees in their wisdom deleted this. Also what about an R for log homes?

-----Original Message-----
From: []
Sent: Thursday, January 17, 2008 11:29 AM
Subject: Re: R Values for Kneebraced Posts and Steel Columns

In a message dated 1/17/2008 12:16:53 PM Pacific Standard Time, writes:

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

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

Harold Sprague

> Date: Thu, 17 Jan 2008 10:57:19 -0800
> From:
> To:
> 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
> 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?
> never been crazy about them, but they're a very commonly used
> 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
> detailing consisent with inermediat steel moment frames if we're
> to use them in SDC D.
> Am I reading this right or do other have a different
> Thanks,
> Chris Slater, PE


California has not adopted the IRC so as our company will do what we have
always done we will use cantilevered steel columns......let the clients
complain to the city about the new code requirements. Most clients who wine
about the cost because they have used wood for years are the same clients
whom have a good friend who is a lawyer and be glade sue you for their
ignorance for demanding that you use wood knee braced posts.

Joe Venuti
Johnson & Nielsen Associates
Palm Springs, CA


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the new year.

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