Allowable soil bearing pressure

[m1208]

Under UBC the maximum allowable soil bearing pressure was 1000 psf (without the need for soils report). Under CBC 2013, TABLE 1806A.2, the same soil bearing pressure is 1500 psf. The same Dead load + Live load governs the design, using the working stress design. Any body know what was the reason for the increase in the allowable soil pressure?

 

[perfectaccess]

Strictly speaking this so general,and you have to follow the soil report.

 

[JStructsteel]

I think it was found, that unless just total trash, 1500 was pretty achievable.

 

[jayrod12]

I'm pretty sure a sandbox can give you 1500 psf

 

[DaveAtkins]

At the beginning of my career, about 30 years ago, we would assume 2,000 PSF for any soil, no matter how crappy. Back then, geotechnical reports would routinely allow 3,000 psf or 4,000 psf.

Now, it seems a lot of geotechnical reports allow just 2,000 psf, even when they have soil borings. And as you said, codes are also more conservative.

Have there been problems over the years with settlement?

DaveAtkins

 

[hokie66]

Or maybe just the soils engineers doing a better job of selling their services?

 

[GeoPaveTraffic]

At the risk of being shouted down by the structural engineers...

It could also be that the owners, architects, or other engineers that are selecting the geotechnical engineers on the basis of cost. So the number of borings goes down, the amount of laboratory testing goes down, the amount of time spent doing any type of analysis goes down and they send basically the same report that they have sent for the last 50 jobs, because no one is willing to pay them to actually do any engineering.

I've been in this business for over 25 years, and the cost of a geotechnical report for typical commercial projects is within 20 percent of what it was when I started.



Mike Lambert

 

[Lomarandil]

I'm with you 100% Mike. Geotechnical reports have become commoditized to a frightening degree.

 

[BigH]

Mike - I did a job in Ontario back in 1993 where the cost of the job as 50% of the cost of a similar investigation next door (same structure and that) that it was back in the mid-60s. - I've gone overseas!

 

[Ron]

One plausible reason for the prescriptive differences in the two codes is the significant differences in the geology of Canada and the US and the resulting soil morphology. I believe the IBC has to accommodate a more diverse range of bearing soils than the CBC would. But then again....it could be as arbitrary as other code provisions!

 

[emmgjld]

I actually agree with all the above.
-- In my opinion, the Geotech services are terribly commoditized (I believe the Architects & Structurals have about the same degree of impact as the cheap developers & the atrocious Public entities).
-- Geotechs do not appear to be doing any significant field or lab work compared to 30 years + ago (In my area of Western Colorado, I am amazed at the vast range of native & fill conditions which ALL result in an allowable of 1300 to 1600 psf).
-- As development has occurred, most towns/cities have been moving into areas of poorer subsurface conditions, probably justifying the pessimism of the Codes.

 

[JStephen]

That kind of provision gets voted into existence by some committee, and a different committee gives you different answers, so it's not surprising that there is not consistency there.

 

[TLHS]

Ron, the CBC is presumably the California Building Code. The Canadian model code is the National Building Code of Canada (typically abbreviated to NBCC).

 

[TLHS]

Also, the surface conditions of Canada vary significantly, as you'd kind of expect for something as big as it is.

 

[Ron]

TLHS....thanks. Didn't think of that. Oddly enough, the statement still applies...perhaps even more so.