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Re: cantilever column system

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Tim,

While your approach is more conservative as far as the load getting to the shear walls, I don't totally agree with the introduction of the Omega overstrength factor.  How can this be a discontinuous system if you are starting at the roof line with a cantilevered column with an R= 2.5, then transferring to a shearwall with R= 6.5?  Again, more conservative, butI don't believe it is entirely correct.

On Sun, Sep 20, 2009 at 8:53 AM, Tim Rudolph <timrudolph(--nospam--at)att.net> wrote:
Paul,
see the comment by David Topete -But instead of the factoring the R factor you would use factor the load to design the column by the Omega overstrength value which is 3.0 for wood framed shearwalls - this is typical in steel and in the 97 UBC was in for wood framed construction but had an exception that got rid of it in most applications.

Tim Rudolph PE
Bishop CA




Paul,
I don't see why you wouldn't be able to do as you've done before.  I think
you can argue that the columns are cantilevered above the 8'-0" wall plate.
Therefore, use R = 6.5 for sheathed walls, but amplify the point load at
the top of columns by a factor of 6.5/2.5 = 2.60, and check the bending and
deflection of the column with a height of 3'-0."  The lateral force from the
11'-0" plate height is adequately transferred to the shear wall, and the
column is effectively braced for a height of 11'-0" in one axis.

Also, depends what jurisdiction you'll be jumping through hoops for.  But,
this approach seems very logical and reasonable.  Engineering judgment.
Hope this helps.  Good luck.




--
David Topete, SE