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Re: Cantilever Columns

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I am unfamiliar with the definition that you presented, but I believe the 
restriction is for "Cantilevered Columns" which includes any fixed base 
column that is free to rotate or translate at the top. The issue is one of 
rigidity rather than the possiblity of failure at the base.
The code requires the structure to use the lowest R value in the direction of 
the applied force. However, the latest Blue Book revisions will recommend 
that only the shear in the line of application be increased by use of the 
lower R value (based upon the descretion of the designer). In your case, I 
would still use the lower R value for each side of the diaprhagm in the 
direction of applied force and design my shear resisting elements to be as 
close to equivalent deflection as possible.

The letter of the code also requires the designer to design using torsion 
analysis unless the structure is regular in features and the designer can 
justify the uniformity of the distribution of shear (my wording not the 
codes). I would argue that nothing is to be gained on a 200 square foot 
structure that would justify torsion - Go for the real issue, the stiffness 
of the elements and their connection to the diaprhagm.

You may wish to obtain a copy of the proposed blue book revisions and present 
them to the building official to substatiate your case.

Personally, I think the final judgment is yours. If you believe the structure 
will not beinfit for torsional design and you have taken precautions to 
insure relative balance in the resisting elements then the structure should 
be safe.

Dennis Wish PE


In a message dated 8/10/99 10:44:36 AM Pacific Daylight Time, 
FSRahbar(--nospam--at)aol.com writes:

<< 
 The e-mail that I had received, indicated that for the cantilever steel 
 columns, only the elements in that line may be designed for the lower "R". 
 UBC, requires a "R" of 2.2 for cantilever columns. The whole business of 
 cantilever columns started with 1996 Accumulative Supplement to the UBC 
1994. 
 In that Supplement, it clearly states that the lower "R" is for Isolated 
 columns with Isolated pad footings.
 
 Most of our columns are not isolated. There are at least two columns and the 
 columns are embedded in continuous concrete grade beam. DOES THE LOWER "R", 
 STILL APPLIES TO THIS CONDITION OR NOT?
 
 Does anyone know the answer? I have asked the question from the Technical 
 Representatives of ICBO. I have spoken with two gentlemen. None, knew the 
 answer.
 
 If someone knows the answer, please e-mail me.
 
 Farzin S. Rahbar
 fsrahbar(--nospam--at)aol.com >>