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RE: Overturning Load Combination

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Scott,

Thanks for the response and research.  I understand the requirement is for
strength of materials and it does in fact create a uniform factor of safety
applied to all elements so that the strength of the elements are all
sufficient.  My gut tells me though that when the 0.6D + 1.0W has to be
applied to bearing values and the size of the footing it is unrealistic.  I
can see it applying to the dead weight of the footing for uplift.  I can see
it applying to overturning moment.  But for bearing value I think it is
should not apply.  JMHO.  The factor of safety in bearing values is already
in excess of 3 (as specified in the geotech reports)  To now add an
additional factor of safety already onto the this seems excessive to me.  If
my 1.0D + 1.0W meets the geotech's FS, or even a 0.9D + 1.0W to account for
overestimating the dead load, then the FS of 3 should still be adequate.
Again, this really only comes into play with large wind moments on a
footing.

Maybe I should propose a change to the code committees.  Let's see, hours of
time writing papers.  Going to meetings to convince people of my case.  Nah,
I'm too busy for that.

Rich


-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Friday, February 03, 2006 5:19 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Overturning Load Combination

Rich,

I assume that you are interested in the commentary for the IBC section on
the load combinations (section 1605.3.1...at least in the 2003 IBC).  If
so, then the commentary (I only have the 2003) only points to the
definitions in section 1602 and the equivalent section (and thus
commentary) in ASCE 7 (section 2.4.1 to be specific).

If you want me to look at the commentary for a different IBC (2003)
section, let me know and I will be happy to look at it and let you know.

Upon further looking (occupational hazard), I did find this statement in
the commentary to section 1609.1.3: "In the code this limitation on dead
load is accomplished through the load combination of Section 1605.3.1.
The applicable combination is 0.6D+W.  This load combination limits the
dead load resisting wind loads to 60 percent (2/3 = 0.67; round down) and
applies to all elements.  In this form, it is clear that the safety factor
on dead load applies to all actions where the dead load is assisting in
resisting wind loads."

HTH,

Scott
Adrian, MI


On Fri, 3 Feb 2006, Rich Lewis wrote:

> 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
>
>
>
>

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