Anyone notice this a lot on foundation drawings now?

[WARose]

Lately, I've seen a lot of foundation drawings that have some kind of note like "Bearing capacity of foundations assumed to be [something low] ksf. Geotechnical investigation should be conducted by licensed professional to assure this assumed bearing capacity is adequate." Or something along those lines.

In other words: we couldn't/didn't get the geotechnical investigation (9 times out of 10 the owner doesn't want to spend the money).....but we need the foundations anyway.

Would this fly if something went wrong? I know the code has presumptive load bearing values, but I've never been sure if running with that or not would be ok.

 

[msquared48]

The use of the word "assumed" should never be used, although I have seen it many times over the years.

The3 drawings should state the "design" bearing pressure, and state whether or not a Geotechnical report was available and used.

Mike McCann, PE, SE (WA)

 

[jayrod12]

But to answer your question about whether this would fly if something went wrong, yes and no.

If your design values are within the generally accepted range for the area (Around here the AHJ has a maximum value that they accept without a geotechnical report) then I believe yes it flies. By that I mean the liability for the failure at that point is shifted to the owner/contractor. Your name will still get dragged through the mud but in the end I can't see it affecting your registration. If you refused to do work on every job without a soils report you would likely go out of business.

Mike, if you don't have a geotechnical report then what do you call the value you choose? It is an assumed value. Call it an educated guess if you would like but I don't think that sounds any better.

 

[jayrod12]

I forgot to add the second part of the yes and no.

No it would not fly if the number you used for design was outside the generally accepted value.

 

[msquared48]

jrod12:

No. It is a design value based on the IBC.

Mike McCann, PE, SE (WA)

 

[KootK]

I've seen it because I do it on smaller projects. We assume a very small value and specify that it is to be confirmed by a geotechnical engineer. My experience with this is identical to Jayrod's. If I remember correctly, Jayrod works north of the 49th, like me. I don't think that we have IBC style presumptive soil bearing values for commercial work.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[aggman]

As a general rule we get soil reports on 20% - 30% of the jobs we do. I wish it was on all of them, but we can lead the horse to water, but can't make them drink. We build industrial (mining) structures. As a rule we try to visit the job-site, look at whats going on, and try to adhere to building code allowables for visible soil types. We discuss with the owner the importance of soil testing, follow those conversations up with written correspondence. We originally put language on the drawings putting in assumed values for design but have since moved away from it, now stating "foundation designed for X,XXX psf, owner to verify capacity with licensed geotechnical engineer". We haven't been hit yet with any issues (some of this is because we use even lower values most of the time than what we say we designed for), but I worry more about settlement that anything. Fortunately most of our stuff can handle some settlement because it's industrial and we use mat foundations on most structures for cleanup purposes, but it doesn't mean it will not catch up with us. I just hope that our written and verbal correspondence will "set us free" when the time comes.

 

[WARose]

...but I worry more about settlement that anything.

Me too (especially if a site was landfill at one point).

 

[manstrom]

We do a lot of houses with an assumed 2000 or 1500 psf. The owner doesn't want to pay for it. In most cases 1500 is all you need.

The downside is that you don't know how bad the soil is. It could be fat clay or crap fill.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[msquared48]

jrod12:

No. It is a design value based on the IBC.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

What Mike said. Without soils data we design to the minimum allowable bearing values in the IBC for the soil type expected. If the design requires a higher bearing capacity then we will note on the drawing that such a bearing capacity must be found and verified in the field. Typically we keep design soil bearing values within reasonably typical values. If it's outside reasonable values we make sure that the contractor understands that a geotechnical report is required and to expect some geotechnical improvements to be required.

Maine EIT, Civil/Structural.

 

[jayrod12]

What Koot said. Up here in the cold northern areas the NBCC doesn't have dictated capacities for commercial projects.

The AHJ does require reports for the majority of new commercial construction however in a retrofit/reno situation they are usually willing to accept assumed values provided they are below a certain range (around here it's 1500 psf for end bearing and 250psf skin friction)

 

[msquared48]

Well, it's up to you, but lawyers have field days over engineers' assumptions...

Mike McCann, PE, SE (WA)

 

[MacGruber22]

A "better" word is presumptive - it is in the IBC and it implies "until there is evidence to contradict otherwise" - it isn't perfect but it is wayyy better than "assumed". Also, the soil should be performance specified like an engineered product with values (bearing pressure, etc.) that you (the structural engineer) require in order for your design (that interfaces with the product) to work. The contractor shall be required to provide you a product (soil) that meets the criteria. Also, low-ball the allowable pressure - 1.5 ksf, 1 ksf, etc. Maybe they will think otherwise when your footings are larger than desired.

You we get the liability, somebody else gets to pay for that liability.

My 2 cents.

 

[charliealphabravo]

I follow M^2.

State the design bearing pressure and perhaps the allowable differential settlement, reference the geotechnical report if available, and qualify that the engineer is to be contacted if the criteria cannot be achieved. I think that the use of "assumed" or "presumed" gives the impression that the GC or the Owner are also allowed to assume and presume. The responsibility of the Owner/GC to meet the design criteria or notify the EOR should be clear.

 

[JStephen]

It seems to me that once you put the drawing out there, it doesn't much matter what disclaimers you put on it, it'll get built that way. Nobody reads the notes.

As a side note...long ago, I worked part time for a company that did trackhoe and dumptruck work. That work included demolition. At some point, they bought some property a couple of miles out of town. This property had a pond on it. So for a couple of years, any concrete or other rubble that got demolished, got recycled as gravel or fill or riprap. But any wood that got demolished went into that pond. Until there wasn't a pond, that is. So somebody sometime is going to be wondering "what's under here" and what's under there will be eight feet of rotten wood.

 

[SteelPE]

My old boss used to provide notes on his drawings in a similar fashion. As he was very cunning, I'm sure he was covered if something happened. Today, if I don't have a soils report I will use values listed in IBC table 1806.2 and state as such in my general notes with another note requiring the GC to obtain this bearing pressure and if he can't..... to notify the EOR.

I find the larger the project the more likely there will be a report. Unfortunately I sometimes design small metal buildings (2,0000 square feet to 10,000 square feet) where the client does not want to pay the extra money for the report. So something needs to be done in this instance. On rare occasions I have refused to provide design services if I know the building is in a low area surrounded by wetlands and the owner refuses to get a report done.

 

[MacGruber22]

SteelPE - I agree with using the IBC to its fullest.

 

[ztengguy]

I had a post some time ago sort along these lines. Just design and state the bearing pressure you need. That puts it back on the owner/contractor to verify, not just also assume. If they dont, they get what they pay for.

Too many owners just want a finished product, but not pay for anything. I design precast bridges, and get the same thing, "what do you mean you cant put a 42' bridge on 1500 psf soil with 6' of cover over it...thats good soil, I built my garage on it last year"

Cover your ass, and if you need to walk from the job, do it.

 

[BUGGAR]

I think there's some semantics here. You don't design to assumptions, you design to a calculated bearing pressure. You're then passing that specific design value on to someone else so they can "deal with it" and make their assumptions that their soil is good enough or hire a geotech.