foundation design

[COEngineeer]

This is what the soil report says:

"We have performed Bearing Capacity Analyses for these footings, and based on these analyses, and engineering judgment, we recommend a Factored Bearing Resistance of 1450 PSF."

What does this mean? Sorry, I usually deal with Allowable Bearing Pressure and I expected that the Factored Bearing # should be higher. 1450 psf is very low even for an allowable bearing pressure #.

Never, but never question engineer's judgement

 

[msucog]

sounds like it's most likely the same thing to me. 1450psf seems a bit odd unless someone spec'd 1450 and they're simply restating what the design called for. hopefully someone is not working on 50psf intervals for foundations.

Ryan Coggins, P.E., S.I.

http://www.linkedin.com/in/ryancoggins

 

[COEngineeer]

Well.. it is rocket science It has to be super accurate. I guess I'll call the geotech.

Never, but never question engineer's judgement

 

[hokie66]

Sounds like some very soft stuff. What does he suggest for settlements? Deeper footings may be in order.

 

[TDAA]

What were the materials at the foundation level and the blow counts?

 

[msquared48]

Perhaps he switched his "factors", dividing instead of multiplying...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

That stuff would show a heel print. Someone must have asked for only spread footing information or you should have received other recommendations.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[weab]

If settlements are expected to be high, then that value may be correct. Don't know where your project is, but that value sounds like southern Louisiana. However, if you approach that value, settlements may be high, need piles.

Think it would be worthwhile to get on the horn and talk to the geotechs.

 

[BigH]

Sounds like an LFRD (or is it LRFD???) statement - factored bearing capacity sounds like it is "shear". Sounds like the site is soft clay - with an undrained shear strength in the order of 600 to 700 psf. If he said that at the serviceability level, the capacity is XX, then this would be a settlement/distortion allowable bearing. Why not talk to the geotech and ask him. Sometimes, with computers, statistics, and the professor's need for research and publication, etc. we are getting a little "too smart" for our own good. I agree that working with 50 psf increments is a bit much.
When I first started, I did an analysis (I was green) and came up with an allowable bearing pressure of something like 1750 psf - I told my principal that perhaps we could kick it up to 1900. He questioned as to whether I wanted to sleep at night - and for the type of project and the cost of an extra 1 m3 of conc, he made the decision to give 1600! Basically, it must be remembered that if we go too skinny on the foundations - it'll cost a whole lot more than the few extra m3 of concrete - of course, I am not talking mega-projects.

 

[fattdad]

I'd say, you got the low bidder and there's an entry-level engineer with a clueless mentor running a spreadsheet based on blow counts or pocket penetrometer data. I've written hundreds of geotechnical studies and reviewed a similar number. I've worked (lived) from coast to coast and never seen this language.

net allowable bearing pressure, sure. Ultimate bearing pressure, yes.

Factored bearing resistance, no.

If they've performed analysis (analyses?), ask to review it to better understand factors of safety and anticipated settlements. Then again, they may not want to show you that stuff.

f-d

¡papá gordo ain’t no madre flaca!

 

[darthsoilsguy2]

Ask for a revision which recommends an Allowable Bearing Pressure.

On the off-chance that any other testing firm out there besides this one were to get the testing job... you need to make sure there isn't "confusion" about what to test for on the CMT side.

 

[JAE]

I'd say, you got the low bidder and there's an entry-level engineer with a clueless mentor running a spreadsheet based on blow counts or pocket penetrometer data.

fattdad - that is just scary, huh?

 

[Ltwine]

Newbie mistakes. I guess he meant to say "the bearing resistance/strength is 1450 PSF with a safety factor of...".
But it troubles me how could this report wasn't checked before release.

Will you please check with his office, and let us know the explanation?

 

[Boondog]

"Factored Bearing Resistance" is the LRFD term used to describe the nominal bearing resistance (bearing capacity) multiplied by the AASHTO LRFD resistance factor for bearing resistance (per the AASHTO 2007 table 11.5.6-1). The bearing resistance (qr = Rf*qn) must be greater than the factored bearing stress.It sounds to me like the "newbie" is providing the LRFD "bearing Capacity" as apposed to the ASD. Welcome to the future. The factored bearing resistance divided by the factored bearing stress must provide a CDR of > 1.

 

[Boondog]

The bearing resistance factor for say...an MSE wall would be 0.65 per the 2007 AASHTO table. This would produce an unfactored bearing resistance of 1450/.65 = 2230 psf. Still a somewhat soft material.

 

[BigH]

Yep - so you have your "Factored Bearing Resistance" - now reduce it for serviceablity requirements.

 

[BigH]

Oops - and I forgot: reduce it more for interaction of adjacent footings.