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RE: Overturning Load Combination

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Rich,
 
I asked for opinions on this subject some time back.  My particular situation was isolated footings for equipment (not a building column).
 
In the absence of any code language that clarified the requirements, I felt it necessary to include the 0.6DL+W case in checking the bearing pressure.
 
David Finley
M. David Finley, P.E., P.A.
2086 SW Main Boulevard - Suite 111
Lake City, FL  32025
386-752-6400
-----Original Message-----
From: Rich Lewis [mailto:seaint03(--nospam--at)lewisengineering.com]
Sent: Friday, February 03, 2006 5:17 PM
To: seaint(--nospam--at)seaint.org
Subject: Overturning Load Combination

I have not seen this issue addressed thoroughly.  The building codes don’t discuss this much. I don’t have the IBC commentary, but the ASCE commentary hardly mentions it.

 

My issue is the service and stability load combination of 0.6*dead + 1.0*wind.  My understanding is that it is to formalize stability requirements without using a factor of safety.  This load combination in essence gives a factor of safety of 1.67.  If it is compared to the old equations of 0.9*dead + 1.0*wind then it gives a factor of safety of 1.5.  Overturning and sliding stability are accounted for by this load combination.

 

What is not clear is if the entire design has to meet this load combination.  Specifically, I am looking at a footing with overturning moments.  Stability wise I can apply this equation and size a footing.  However, when this load combination dictates the size of the footing based on soil bearing pressure I believe it is an unreasonable combination.  If the overturning moment is large then the bearing stress controls the size of the footing.  If the footing size meets the bearing allowable stress for dead and live combination and the stability alone is accounted for with the 0.6*dead + 1.0*wind combination, I believe the design may be adequate.  I believe it is unreasonable to expect the designer has overestimate the dead load on a footing by 67%, the soil will never feel the stresses of the 0.6*dead + 1.0*wind combination.  As I read the code though I have to consider this extreme load combination in design. 

 

All this comes about because software for footing design uses all the load combinations of the code and fails a design solution based on allowable bearing stresses from the stability equation.

 

Does the IBC commentary address this issue in any depth?

 

Has anyone else addressed this issue within their office and determined it can have some unreasonable affects on designs?

 

I would appreciate any insight you may have.

 

Rich