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Re: Unbraced length question

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In a message dated 11/10/98 11:49:47 PM EST, wish(--nospam--at)cwia.com writes:

<< I am designing a combined grade beam that will take the moment from two
 columns 20' tall with a 4.8 Kip load applied to each at the top.
 My problem is that one column is unbraced laterally for the full 20 feet
 (minus about 6" of cover over the gradebeam and assuming that the point of
 fixity occurs at the top of the grade beam). The opposite side of the grade
 beam (and second column) lies 4.5 feet higher at a step in the foundation (a
 split level portion of the home). This causes a couple of concerns for me:
 
 1. The columns (TS14x14x1/2) will have a different stiffnesses considering
 one connects to the grade beam with 4' of soil and a 4" slab above. This
 reduces it's effective length to approximately 16'
 2. The story drift will be different due to the unbraced length  - a result
 of the different stiffness.
 
 The exposed column is an architectural feature. The longer column (20'
 unbraced) is actually set withing 8 to 12 inches from the face of the
 retaining wall which is constructed over (perpendicular to) the grade beam
 (and independent of the grade beam).
 
 Would I be better off trying to maintain adjust rigidity by:
 1. Varying the column sizes and thicknessesKee.  OR
 2. Keep the columns the same and brace the backside of the taller column to
 the retaining wall?
 
 The change is due to a half story level change in the home resulting from
 the natural grading of the site. The architect does not want to move the
 retaining wall to satisfy the structural needs of these columns. The owners
 are adamant about keeping the columns appearing to step down.
 
 The columns are critical for the shear distribution from the high roof and
 are in an area where there are no sufficient wood shear walls.
 
 Any suggestions?
 Dennis S Wish PE
  >>

Dennis,

Are the top of these cantilever columns tied together using the same link
beam.  If they are, I would use the same size columns for both the 20ft and
16ft clear heights, but adjust the load based upon stiffness ( the 16ft column
taking more load).  I think this would be easier than using two different
column sizes, unless you are trying to make both columns carry the same load.
I don't think I would want to anchor the column to the retaining wall.

Mike Cochran