Stepped Footing 2

[JStephen]

Is there some standard way to handle the transition area in a stepped footing/foundation?
See the attached pdf. Footing shown could be round or square, reinforced concrete.
Summing moments about a centerline as in Section A allows you to solve for steel tension and area, concrete compression, etc., and all is well.
Summing moments at Section C does the same for the reduced section beyond the step.
The problem is at Section B. The cross section is similar to Section A, but obviously, near the edge, you can't support the concrete compression at the upper surface.
A similar issue would arise if the transition were sloped.
Various handbooks and textbooks in the office mention stepped or sloped footings, show diagrams of them, but neglect to mention how the step itself would normally be addressed in the design. Google turns up similar references that also omit details of how to handle the step.
Any input or references would be appreciated.

 

[KootK]

If you're counting on load spread as shown in your sketch, you would need to treat it as you would a beam step. You'll have a compression strut diving down out of the deeper section at the transition. CRSI has a good white paper on beam steps.

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.

 

[manstrom]

I think strut and tie method is your friend here.

Another approach would be to consider the corner of the footing as sloped at a 1:4 slope. That's how RAM concept does it for drop panels.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[SlideRuleEra]

The "step" feature of a stepped footing does not have a specific structural purpose. It real use is to save concrete and the shape is to make the formwork simple. As long as the footing's concrete thickness is adequate to resist shear, the step can sized and located as desired.

Here is a quote and diagram from a 1921 ASTM paper:

The whole idea of having a stepped footing is a carryover from pre-concrete days when masonry footings were the norm. The masonry steps were sized based on conservative rules-of-thumb that in turn came from experience. When reinforced concrete came into use, soil mechanics was advanced enough to allow informed decisions to be made, instead of following tradition.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JStephen]

Thanks for the input, folks, that was exactly the information I needed.

 

[BigH]

You might try Tomlinson's book Foundation Design and Construction, 6th Edition; he has a nice presentation, too, of a stepped footing. Also BS 8103