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Unbraced length question[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "SEA International List" <seaint(--nospam--at)seaint.org>
- Subject: Unbraced length question
- From: "Dennis S. Wish PE" <wish(--nospam--at)cwia.com>
- Date: Tue, 10 Nov 1998 20:47:23 -0800
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.
Dennis S Wish PE
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