Is 100% bearing contact always required?

[LearnerN]

In checking P/A +/- M/S for eccentric loading on a rectangular footing: is it left up to engineering judgment and a client decision to determine if slightly less than 100% bearing contact pressure is acceptable, OR is this never acceptable? This is in regards to a 30' tall vertical vessel on a 9'x9' footing that has 80% bearing contact when it has worst-case horizontal wind loads. But the footing would have to be increased up to like a 14x14' footing to get 100% bearing contact with these wind loads. (As these calculations assume, overburden and concrete weight are disregarded. But when those are factored in, the overturning stability is more than sufficient. So my scenario is that the vessel is safe against falling over, but my question is in regards to the soil bearing contact pressures.) I'll be having the standard design review on this by another engineer, but I wanted to get some input from engineers on this board. Thank you.

 

[KootK]

More often than not, I do utilize less than 100% bearing contact pressure. In many cases, it will lead to a substantially more efficient design as you are finding. If you're going to use a partial contact model, all of the the affected parts and pieces of your structure will also need to be designed for that set of forces but that usually isn't a big deal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

I agree with KootK. Full bearing is not required. Just far more likely to overstress the soil due to the increased leverage.

Mike McCann, PE, SE (WA)

 

[Ron]

Full bearing is not required and often not achieved. Further, unless the client is a qualified geotechnical and/or structural engineer, it should not be his/her decision. It is what it is...from an engineering perspective.

 

[bridgebuster]

Ditto what the others say. In bridge design, foundation "rocking" is acceptable in some circumstances for seismic loading. As Mike pointed out keep an eye on the soil pressure.