Geotech report Limit States vs ASD 1

[Domestos_WC]

Hey guys,

A quick one...
Typically we receive geotech reports stating the loads in ASD (allowable), then we know it's unfactored loads vs allowable bearing pressure and it's all cool. We use unfactored loads for foundations sizing.

Anyway, since a while we've been getting geotech reports using limit states (Canada). Limit states are not very different from LRFD so I thought you may be able to help me.

Anyway, the report for the piles says:
"FACTORED ULS End Bearing of 175kPa (that's approx 3655 PSF)" - in my world, ULS means factored already, so it's like being double factored. I understand the whole LSD principles, the ULS is to ensure the max structural loads do not exceed the nominal capacity of the foundation units. The ULS foundation bearing capacity is obtained by multiplying the nominal (Ultimate) bearing capacity by a resistance factor (or call it reduction factor), which is then compared to the factored structural loads. The resistance factor (or reduction factor) can be 0.4 for static analysis (compression), or 0.5 for dynamic testing (compression), or 0.3 for uplift.

Does that mean that the FACTORED ULS End Bearing of 175kPa is already factored by the reduction factor of (0.4 or 0.5 or 0.3) and I can simply take my max column FACTORED load and size and footing for the 175kPa?

The very same report on the strip and spread footings says: The footings may be designed with Limit States Design parameters of 225kPa and 150kPa for the ULS and SLS respectively. Why isn't that consistent and why they don't say FACTORED ULS anymore? Do I have to multiply that 225kPa by the reduction factors (0.4, 0.5 or 0.3) when comparing to my factored loads and if so, how do I know what testing method they took?

It's all pissing me off as every report uses different wording and different design approaches, and all I need to know is whether we're comparing apples to apples, and that their apples are the same apples as mine.

I tried to call them but they're not picking up due to the COVID bs (another excuse to stop picking up your phone), left a message, never heard back.

Help?

Thanks!

 

[r13]

Did you pay the service yet? If not, stop payment may prompt them to call!

 

[Settingsun]

If you multiplied 225kPa ULS by 0.4 = 90 kPa, you're way below the SLS number.

I agree about not paying until you're satisfied. Try email rather than phone message if they're not tending to the phones.

Is it reasonable that pile end bearing would be less than shallow footing?

 

[GeoEnvGuy]

The ULS should be factored the unfactored capacity should be identified as the ultimate resistance.

 

[skeletron]

I can empathize with the frustration (re: geotechnical reports using LSD and ASD). If we are taking bets on what the "right answer" is, here is what I would go with:

Footing design:
1. Design the footing size based on total load vs. 150 kPa.
2. Design the concrete for 225 kPa as a factored pressure.
The factor of safety from the geotech (225/150 = 1.5) vaguely runs equal to the average load factor (1.25DL + 1.5LL ≃ 1.4 Total Load).

For the pile design:
1. Use your factored Pf column force vs. 175 kPa factored bearing resistance to size the diameter of the pile.
2. Use your factored Pf to design the concrete.


Practically speaking, your end bearing at pile depth should be stronger than for a shallow foundation. But the CFEM applies different resistance factors to deep and shallow foundation. Add to that the geotech's focusing more on the settlement issues. Deep and shallow foundations will compare differently because of that (in my experience).

As an aside, and stated in the CFEM:
"...there has been a trend towards the use of reliability-based design and probabilistic methods in geotechnical engineering design. However, complete probabilistic design is difficult to apply reliably and appropriately, in particular in most practical geotechnical design situations, generally because of lack of statistically viable information."

I used to think it was because geotechnical engineers are old lazy unsophisticated but it turns out their data set is based on widely varying but dialed in empirical experience.

 

[GeoEnvGuy]

Your report for the shallow foundation should also state a maximum footing width to limit total and differential settlements.

I did want to add discussion on skeletons comment of geotechnical engineers old/lazy unsophisticated image.

There was a decline in the geotech field during the 90's and there is a significant gap in the 20-30 year engineer range currently, so the last 10 years appeared to have young graduates that didn't last for any length of time working under the direction of 20 plus year engineers.

The lazy appearance may be just do to the standard geotech report feels like picking items off a shelf to fill in the report sections. After a while you feel like you have drilled a hole in almost every commercial property development in your entire region. You get to a point where you have the entire report written before you even visit the site.

The unsophisticated image is likely due to the lowered dress code typically seen in geotech firms. It can also be due to the greater reliance on engineering judgement than code and the fact that geotech engineers are constantly playing with dirt. Even the old guys love getting there boots and hands dirty.

 

[Settingsun]

ULS should be factored the unfactored capacity should be identified as the ultimate resistance

So ultimate limit and ultimate resistance mean very different things. Yuck. We use design and ultimate capacities.

I've always thought the nonlinearity and uncertainty in geotech means limit states make sense. You find out that the margin against overload or insensitivity to parameter variation wasn't actually what you thought it was sometimes.

I like the Australian piling code's variable reduction factor approach. Doesn't solve everything but does address a common shortcoming that even a structural engineer could see.

 

[skeletron]

For the record: I have respect for what geotechnical engineers do. My old/lazy (both should have been crossed out) and unsophisticated comments were mainly included to show the progression of my perception until I really dug into the historical background of geotechnical engineering.

 

[Domestos_WC]

Thanks guys for your input, much appreciated. We have sized the pile end bearing for the 175kPa on factored loads. I did my exercise and checked the same pile for the allowable safe bearing pressure of 100kPa (2000psf) that is commonly used in our region and it came out as 2100psf applicable bearing pressure, so we're pretty much spot on.

We don't have the leverage of not paying them, the report was provided to us by the client.

There's indeed a problem with geotechs right now, we had one company that would regularly provide geotech reports for our client (custom home builder) and it would be literally copy&paste every time. One time we had an allowable bearing pressure of 100kPa stated in the report and we found a bedrock about 6ft below the grade level on-site, the bedrock wasn't even mentioned in the report. The client would have saved a lot of $$$ if the geotech did their due diligence and would have actually tested the site, we would be then able to size the retaining walls for the much higher bearing pressure...

About the "unsophisticated image and lower dress code": I have a lot more respect for the guys who don't mind getting dirty on site and when I see an engineer wearing a flashy suit on a site it typically tells me it's all the meat but no potatoes in their engineering work

 

[Settingsun]

Maybe it was the case that the client would have saved $$$ if the client had paid for a site investigation instead of desktop study.

 

[calculor1]

Divide your factored load by 225kpa to get the area of the footing, design for shear and flexure or you could assume a footing size and then calculate factored bearing stress and compare to the uls capacity. The nbcc does not specify any service load combinations for foundation design. Similarly for pile design.

 

[BigH]

See what happened when the Structural purist types pushed to have the geotechs go to the LRFD (where the factors, anyway have been correlated to the "old" way). Bet you had few problems when the geo reports were written by the old timers. And the big bugger is that seldom does bearing capacity prevail in the bearing pressures - settlements do and you need the real loads for that. Glad I got away to overseas work when I did.