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RE: Overturning Load Combination

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This discussion is good - it points out the disconnect between
ICC-mandated structural codes and the murky world of geotechnical design
values.

We have to do our calcs and show numbers, level of precision, track the
loads, etc.  A plan reviewer can ding us on whatever isn't "perfect".

The geotech submits a beautifully flowing narrative about the
Pleistocene era deposits etc., yada yada, and then gives "recommended
design values".  We can look at his boring logs, but don't have a clue
how he translated that into equivalent fluid pressure for retaining
walls.  Neither does the plans examiner.

Recently I did a small modular wood frame building foundation with
individual spread footings all around the perimeter.  Knowing that the
IBC Table 1804.2 is rediculously conservative for lateral sliding on
stiff clays or silts, I used my knowledge of local conditions and NAVFAC
7.02 to do a fully reasonable engineered analysis, and used 12" deep
footings (the minimum frost depth here).  I had a 1.5 factor of safety
against sliding.  Not good enough.  I got called on it and had to deepen
them to 18" to conform to Table 1804.2.  OK, fine, I take my chances and
pay the price.  But if anyone could take a big picture look, they'd
realize I was dealing with a situation where the wind forces would
literally 'plow' the foundation through the soil.  When has that ever
happened outside of a hurricane or tornado zone?  Never.  And in the
hurricane and tornado zones, the superstructure is more likely to
overturn or rip off the foundation before a lateral sliding soil failure
occurs (sans flood waters - another issue).  There is perfect
justification for 0.6D + W on superstructure components, as well as the
whole building overturning.   And soil factors of safety are very
important, BUT let's not multiply them by another factor of safety to
produce a work of paranoia (or ignorance).

Not only would I "flex" on 0.6D + W for unreasonably high bearing
values, I'd also slacken it for footing uplift as it relates to footing
weight (I apply the 0.6 factor to the superstructure, but not the
footings if I know there's no question about the weight, and there's
adhesion to the soil they're embedded in that's not acounted for.)

With all due respect for building officials,

Ed Tornberg
Tornberg Consulting, LLC
503-551-4165

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Monday, February 13, 2006 20:44
To: seaint(--nospam--at)seaint.org
Subject: RE: Overturning Load Combination

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
stresses. 

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)cdm.com

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

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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