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RE: Overturning Load Combination

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Rich, Sections 1801.2 and 1804.1 of the 2003 IBC clearly state that the
given allowable soil bearing pressures in the code are to be used with
the ASD load combinations in Section 1605.3.  Thus, it does appear to be
their intent for the given load combinations to control bearing

Nevertheless, I would agree with you that the factors in some of these
load combinations do not seem appropriate for soil bearing stresses when
such stresses already include a significant safety factor.  A reduction
factor intended to account for enhanced stability for overturning or
uplift does not seem appropriate for soil bearing stresses.  In the past
I applied safety factors to overturning and sliding rather than
reduction factors applied to the dead load.  I feel that such factors
actually make the analyses more confusing than the application of
standard safety factors. 

I am also surprised that some engineers use LRFD factored load
combinations for evaluation of stability and ultimate soil stresses.
Where are phi-factors defined for soils?  With the traditionally higher
factors of safety used to derive allowable stresses for soils vs those
used for structural material allowable stresses, it seems that load
factor design should not apply to soils. 

There are two items in the 2003 IBC that further confuse me: 

1. Section 1801.2.1 allows the seismic overturning moment to be reduced
by a 0.75 factor when using strength design.  Why doesn't the same 0.75
factor apply for allowable stress design?  How does the moment know
which design method is being used? 

2. Section 1806.1 specifies a safety factor of 1.5 against sliding and
overturning for retaining walls.  Does this factor of safety apply to
seismic load combinations?  Does the reduction factor of 0.6 on dead
load apply with these safety factors?  (I would assume "no", but the
code is not clear.) 

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

-----Original Message-----
From: Rich Lewis [mailto:seaint03(--nospam--at)] 
Sent: Saturday, February 04, 2006 11:02 AM
To: seaint(--nospam--at)
Subject: RE: Overturning Load Combination


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

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.


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


I assume that you are interested in the commentary for the IBC section
on the load combinations (section 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."


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
> 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
> Specifically, I am looking at a footing with overturning moments.
> wise I can apply this equation and size a footing.  However, when this
> combination dictates the size of the footing based on soil bearing
> 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
> 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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