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Re: timber knee braces

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When designing timber frame homes, it is my personal belief that by and
large you need something other than the timber frame itself to resist the
lateral loads.  As such, I rarely rely on timber braces to resist lateral
loads and never for seismic loading.  I defintely don't buy that you can
create a viable moment connection in a timber frame (in theory, many
timber frame joints/connections, even traditional mortise and tenon
joints, will have some moment resistance, but give the nature of the
material [wood], that resistance is not "consistant" or reliable over time
[i.e. shrinkage issues and fracture issues]).  As such, we tend to always
rely on a shear wall system (usually SIPs since the company also makes and
sells SIPs and most of the timber frame projects that we do have SIPs).

Now, I will on occasion use braces to resist MFWRS wind loads...if the
building is in a relatively low wind area (i.e. NOT on a hurricane coast,
etc) with relatively reasonable wind loads and if using the walls a shear
walls is not viable option (i.e. the architect and/or owner wants the
walls perferated with window and door openings...damn desire to see the
outside worlds...who would've guessed! <grin>).

In the end, I just don't get any "warm fuzzies" by relying on timber knee
braces to resist seismic loads, especially if they are siginificant at
all.  I don't envision that such a system has much ductility.  I don't
really buy the R=2.8 that the 1997 UBC seems to allow for use with braced
heavy timber frames and in reality, there seems to be agreement in the IBC
and ASCE 7 as neither of those documents include such an R value any

Sorry to not be much help.


Adrian, MI

On Wed, 28 Jun 2006, Pinyon Engineering wrote:

> Hi
>   I looked at a hillside house built in the early 1980's - the house is 2 story with 8x8 and 8x12 post and beam construction with simpson buckets at the joints to level the floor for the house( like a tall pony wall on the down slope side of the structure).  the post and beam system is a classic soft story.  to brace the structure against earthquake loads A contractor wants me to design knee braces for the post and timber frame below.   to design the timber frame the redundancy factor is R=2.8  -- here's the question in steel design for diagonal bracing you are allowed to do any system on 1&2 story bldgs if you design the brace for Omega times the design force  is there a similar provision for timber knee braces?
> Thanks
> Tim Rudolph PE
> Pinyon Engineering

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