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RE: Factor of Safety against Uplift

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John,

I am sure you realize that allowable stress design had been used for many
years, so your question about how can one possibly design reinforcing in the
toe of a retaining wall footing can be answered by looking in a text book.

Do you view the recommended soil pressure for design as the limit for design
based on factored or unfactored loads?

When factored loads are used to determine soil pressure, it seems that there
should be a limiting value ... a value higher than that for serviceability.
For this to work, that value is going to have to be more than 1.5 times
greater. I have not seen such a value, but I haven't requested a limiting
value from a geotech engineer either.  

I am jumping in here because there is confusion among new grads, and some
not so new on how to merge "factored" loading and "allowable" soil bearing
pressure in the design process.

Does anyone know if two recommended soil pressures are coming in to use?

Then there is the "net allowable soil pressure" discussion ...  

Paul Martin
Wichita, KS


-----Original Message-----
From: John MacLean [mailto:john_maclean(--nospam--at)pomeroy.ca]
Sent: Monday, November 12, 2001 10:11 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Factor of Safety against Uplift


William Sherman wrote:

<Why do you consider limit states design superior to allowable stress design
for overturning? Personally, I have never heard of using limit states load
factors in stability calculations (overturning, uplift, sliding, etc). In
Chapter 18 of IBC 2000, it states to use allowable stress design load
combinations with formulas in that Chapter and in Section 1610.2 it
recommends a factor of safety of 1.5 against overturning, sliding, and water
uplift of retaining walls. Presumably this safety factor is for service
loads as it would be excessive to apply it to factored loads. (This section
should be clarified that it applies to service loads, and it also should not
apply to seismic design conditions.) IMHO limit states should only be
applied to "strength" design (material stresses) and not to "stability"
design.>

William:

Sorry I'm late in replying, I've missed the list for a couple of days.

With respect to your last sentence, it seems fundamental to me that a
structure should be stable under factored loads. But what does it prove if
you're stable under specified loads (allowable stress loads). Another straw
and the camel's back breaks. That's where the FS comes in. But it's just a
patch to work around the fact that the allowable stress method doesn't deal
with things out at failure loads where we're most interested in them.

I'm not familiar with the IBC and only marginally familiar with UBC 97 but I
note that UBC 97 in article 1611.6 has a similar clause, i.e.

"Retaining walls shall be designed to resist .....overturning by at least
1.5 times the overturning moment, using allowable stress design loads."

I read this and scratch my head!

They're basically telling you to use a load factor of 1.5 on the soil
pressure and 1.0 on the dead loads and then check stability. That's pretty
much the same, in concept, as the LRFD method. At least as far as I can
tell.

Especially confusing if you consider that in section 1612.2 the code gives
an LRFD load case of 0.9D ± (1.0E or W) + 1.6H (I added the 1.6 H in as per
article 16.12.2.2 I think I'm reading the code right here but I'm not 100%
sure). If wind and earthquake are zero you have a load factor of 1.6 on the
soil pressure and 0.9 on the dead load which is more severe than the
requirements in 1611.6.

In LRFD I check stability for overturning by applying the factored loads and
then figuring out where the resultant of the soil pressure will be (e = S M
/ S V). If it's under the footing I'm okay. If the soil resultant is outside
the footing I redesign because the structure is going to overturn. You could
also do sum of the overturning moments <= sum of the righting moments. i.e.
FS = 1 under factored loading.

I like limit states better for design for stability against overturning for
two reasons. First it makes more sense. With LRFD you're basically saying
what's the worst thing that can happen here? Lateral soil pressure could be
60% more than predicted, soil / concrete density could be 10% less than
predicted. Okay, let's look at what happens if that's the case. If you don't
like 60% and 10% then change them accordingly. At least you'll have an idea
why you're changing them. What does a factor of safety of 2 or 1.5 against
overturning mean? Are they just numbers that get dreamed up. What's the
rationale behind them?

Second, I can figure out what the soil pressure under the toe of the footing
is so I can design the reinforcing in the toe of the footing. How do you do
that with allowable stress design?

John MacLean



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