“Design” Bearing Pressure

[steeletc]

Working on a Canadian retrofit project now with foundation drawings from the late 80s. Foundation drawings call out “DESIGN BEARING PRESSURE 6000PSF”

Like many retrofits, we’ll be adding substantial load.

Maybe it’s my age but “design” means nothing to me. Is this intended to be ultimate or allowable? I want to think allowable but by happen stance that’s telling me the bearing pressure is more than 3 times what it needs to be based on the existing loading conditions. My judgement is therefore telling me ultimate.

Just seems somewhat incomplete to me to call a bearing pressure “design” and would like to be sure before making any recommendations.

 

[dik]

If in the 80s, it's likely allowable unless noted otherwise. Most current projects list SLS and ULS for allowable and ultimate, respectively. You might want to talk to a geotekkie... 6 Ksf if reasonably high, you need quality, likely granular, soil; what is the soil? Is there an available geotekkie report? What is the size of the foundations? Are they small and size could be nominal? like 3'x3' or something of that ilk or maybe for settlement? dunno... need more information.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[steeletc]

Yes I’m familiar with the transition from allowable stress to limit states, and surely this wouldn’t be ULS or SLS. It’s either ultimate or allowable and I expected such language to be used on the drawing instead of design.

No they’re definitely not nominal footings, up to 13’ by 13’ so clearly intentionally sized as such. It’s possible that it’s settlement related but one could guess that would get specified as well.

Site soil conditions vary from solid granular to piss pour dust. There is at least one local geotech who’s knowledgeable of the site though contacting them is less than optimal as they work for a competitor (we don’t have in-house geotech)

Anyway, would you posit that design meant allowable or ultimate at first glance? Maybe someone else has had a similar experience.

 

[BigH]

I truly doubt that this would be ultimate bearing capacity (shear). You say you are in Canada? Ontario? The glacial tills such as the Peel Till Plain would regularly give, for a foundation's allowable bearing PRESSURE, something like a minimum 300 kPa (300x20=6000 psf). This would not be ultimate bearing capacity (shear) for a variety of reasons. Of course one would need to know the soil types - if not the Peel Till Plain or other glacial till areas, the allowable bearing pressure value (based on serviceability) would likely be less - but I don't think that they would put on ultimate bearing capacity (shear) -as that would force the contractor (or inspection geotech) to come up with and apply a factor of safety for an allowable bearing capacity. And, for some projects, likely most projects, it is the settlements that control (serviceability, not shear) and not every project uses 25 mm (or 40 mm depending on country) for the allowable settlement (differential would be different). Anyway this is my take on your problem.

 

[dik]

It's not likely using ULS and SLS so the bearing is very likely working stress design. I cannot explain the anomality of the loads unless there are other concerns. Is there any reference to geotekkie info... with 170 sq.ft. at 6ksf, I cannot imagine a geotekkie not being involved.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Smoulder]

Could be a design assumption to be confirmed on site.

 

[BAretired]

If you are adding substantial loads and the soil is as variable as you describe, you need the services of a geotech to review the soil bearing capacity. In the end, you will be responsible for the design, so making assumptions about the original recommendations is not prudent.

BA

 

[steeletc]

BigH appreciate your perspective. I’m on the east coast, where soils can vary from unfractured rock to peat very quickly. I don’t believe this is one of those sites.

I also held the thought that giving the ultimate didn’t make sense for the reasons you gave. I suppose the general consensus however in this thread is to consult someone familiar with the local conditions.

SlowByrne this is an existing structure, on a site where one bore hole elsewhere is unlikely to provide adequate information.

Was hoping for an “oh yeah that’s definitely just the allowable” but it appears that was wishful thinking. Will investigate further. Thanks for the replies

 

[SlideRuleEra]

Foundation drawings call out “DESIGN BEARING PRESSURE 6000PSF”

Perhaps the note applies only to the structural design of the foundation. That is, the foundation is suitable for allowable bearing pressures up to 6000 PSF and can be used on soils at, or below that value.

We had to do something like this in the early 1980's. For a project on fast-track schedule, the foundations for one of our generating stations had to be designed before we knew anything about site soil properties or exact loads the foundations would support. I don't recall notes like this on the drawings, but would have been a good idea to have used them.

http://www.SlideRuleEra.net

 

[dold]

Did the geotech report mention anything about ground improvements? I have on multiple projects given the bearing pressure i have designed for/'wished for', and an aggregate pier designer must size their piers accordingly. The geotech report would have some paragraph to the effect of: "bearing pressures of up to 4000psf may be achieved on this site with ground improvements such as rammed agg piers or stone columns. capacity to be confirmed by supplier...".

Otherwise, I can only imagine that your 6000psf is an allowable (ASD) pressure.