Rectangular vs. Triangular / trapezoidal bearing for Offset Footings 2

[efFeb]

Good Morning,
I have always used a trapezoidal / triangular soil bearing distribution when checking the maximum bearing on a gravity footing with eccentric loading.
I have been told by others at my office that I can use a rectangular distribution instead, with the average bearing pressure, instead of the maximum bearing pressure, used for bearing pressure checks.
Is there a code clause that allows me to do this? I have looked through my geotechnical textbooks and am seeing that, at least for SLS, I do need to take this triangular distribution. It is a huge difference in the values between these two approaches too.

Thanks so much in advance!

 

[TLHS]

I use rectangular stress blocks, but only when I'm using load factor based methods. If I'm doing serviceability checks or allowable stress checks I use triangular. If I use a triangular block then I get severely punished for using ULS loading. On moment loads, the factored loads also push the point of eccentricity out further. With a triangular load distribution, that kills things pretty badly and hugely increases footing sizes.

I have to admit, though, that I still pretty often am just sizing footings based on a quick service level check and throwing the kern in the middle third. That gets you a semi reasonable footing like 90% of the time for a moment load.

It logically makes sense to me, because the strength of the rectangular block is there, you just have to deflect through some extra movement on the outer fiber to get to it, just like with steel section design. Comparing against factored ultimate loads is a collapse state check, effectively, so it all functionally seems to make sense.

I know that FEMA does it in their design examples for seismic. I have seen it in Canadian literature, just can't find it right this second. Might be a bridge code thing. I thought it might have been a structural commentaries thing, but I'm not seeing it other than specifically in a couple of figures in the seismic section. It's in my emails somewhere I'm sure when I've had this discussion with checkers. I've also got modified checks for suggested eccentricity location for LRFD design somewhere. I think it's keeping the point of eccentricity in the middle half being preferred as an initial check.


Any of the Canadian people seen the new foundation engineering manual? They finally released it a month ago after about a decade. It's something like $450 though and only in a protected electronic format, and they don't seem to have a good list of what they've added. They say there's a bunch more load factor stuff, but I don't know.

 

[skeletron]

@TLHS: My only source for rectangular distribution and seismic is from an SEABC course on seismic steel. There's an example where they design a cantilevered column system and show the footing design using rectangular distribution. The note indicates:
"...We are using the full plastic limit of the soil, generally acceptable for short-term seismic loads only..."
I'd be interested to know if this is ever explicitly stated elsewhere in a Canadian guide.

I did see the new ad for the CFEM. I'm disappointed that there isn't a print copy especially at that price point. Even so, that's a pretty big fee for a reference book. I don't feel like rolling the dice with a protected eletronic document that I can't annotate and, to be honest, I don't get into geotechnical problems. Supposedly it's kind of the "next page" in transitioning the geotech world to Limit States Design. The last CFEM was an interesting read.

 

[ALK2415]

Think they propose the second solution to adjust for soil-footing separation (i.e. minimum contact surface) !
then do presume the rectangular shape as approximation for easy-hand calculation solution.