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1/3 increase in allowable loads

1/3 increase in allowable loads

1/3 increase in allowable loads

I keep hearing things back and forth about the 1/3 increase in allowable soil loads. I thought I'd post what I believe the history to be and let people comment on what I know or don't know (correctly).

I had understood that there were two increases allowed, one structural (for steel structures) and one for soil. AISC reasoned that the likelihood of a code level wind and a total live load and total roof load and, and, and happening all at once was very low. So, in such load cases, AISC allowed a 1/3 increase in allowable strength.

At the same time (and quite unrelated) general soil mechanics practice allowed for a 1/3 increase in soil capacity (for bearing and pile capacities) for any load combination which included transient loads. The reasoning behind this is that in many cases, the S.F. of 2, 2.5, or 3 was there for long term failures. Consolidation due to long term load conditions was the real danger. Since seismic and wind simply didn't behave the same way, the soil didn't behave the same way and didn't consolidate as much as under persistent or repetitive loads. So, it made sense to allow for a smaller factor of safety.

Then with the advent of LRFD and load combinations (which took into account AISC's concerns) the building code & AISC stated that you cannot use the 1/3 increase in strength when using these load combinations. That's double dipping.

Then... At one office, the EOR told me (I never found it in the code myself) that the appendix/commentary section of the earlier editions of IBC explained why the 1/3 increase was eliminated. AND at the same time, they explained that this did NOT apply to any soil properties since that was for a completely different purpose. Those are to be dictated by the geotechnical engineer for the project.

Since many geotechnical engineers never read that disclaimer, they thought that the 1/3 increase was over for them as well. So, we now have a world full of geotechnical engineers who are at odds with each other because of this confusion.

By saying this, I'm not trying to insult any geotechnical engineers out there. I just need to know if this understanding is correct or not. I'd welcome correction to anything that I've said that warrants such correction.

Thank you.

RE: 1/3 increase in allowable loads

I've seen older geotechnical reports reference the increase in allowable bearing capacities for extreme events but that has phased out sometime in the early 2000's. I personally don't see an issue of increasing a bearing capacity by 1.33 for a low probability and short duration event if using ASD. The difference between 4,000 and 5,320 psf isn't that much when you're talking about bearing capacity failure. You may see some small amount of elastic settlement though.

RE: 1/3 increase in allowable loads

I don't know about the background you presented, although it all sounds reasonable. I can say that 90% of all geotechnical reports I have seen from the past 10 years have included the allowable 1/3 increase for transient loads.

RE: 1/3 increase in allowable loads

This is from the Geostrata magazine from the GeoInstitute of ASCE:


I contacted Dr. Turner just after read that article and he added: "...if sound engineering is being exercised and good communication is taking place between the structural and geotechnical designer, then there is no need for an arbitrary increase in foundation resistance by 1/3—everything should just be based on the actual loads, clearly established factors of safety, and the best estimate of soil resistance."

The 1/3 increase consideration will also depend on if you are using LRFD or ASD .... I guess that the 1/3 consideration has more expectations from structural engineers if ASD is used.

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