Residential Slab on Grade Footing

[karimom]

We've been having discussions at the office and we're looking to solve this once and for all. We have a residential footing here in Canada for a house that will see roughly a line load of 30KN/m2 the soil is around 100KPa. Based on this design, one engineers believes that we still need to consider eccentricity. I think that eccentricity can be ignored based on the fact that the top of the footing is tied into the slab to eliminate overturning.

Can you some one please point us in the right direction to solve this debate? The footing is fine as designed with my assumptions but fails miserably (under tiny loads) if eccentricity is considered.

 

[Dowels93]

Footings in particular are prone to layout inaccuracy, so a footing failing with even a small eccentricity is of concern since you have a good probability of seeing those eccentricities just from construction. I would personally consider the eccentricity unless the footing is tied into the slab to prevent any rotation of the footing (not just overturning as a whole), although at that point it is essentially just a thickened slab at that location. Do you have a sketch of the layout?

 

[karimom]

See attached

 

[karimom]

Any thoughts on this one anyone?

 

[jayrod12]

If eccentric loading needed to be accounted for in a situation like that, many garages are here (almost all) would be in danger of foundation failure... haven't seen one to date.

 

[KootK]

- I do acknowledge that this, and things like it, are done all the time and, apparently, without incident.

- The game afoot is about stiffness as much as it is strength. In that regard, I struggle to see a 3" SOG doing all that much for you, especially sans downturn.

- What is the nature of the "failure" that you are considering? If it's just exceeding allowable soil stress at the peak demand, we might be able to rationally improve upon that.

 

[karimom]

This is a 5"SOG, the failure is based on e<l/6 and exceeding allowable stress at peak demand. I've also seen L-shaped footings where eccentricity was ignore pending that the stirrup legs into the slab could resist the moment.

 

[karimom]

See attached from another post I seen on neg-tips. However, even on that thread they didn't solve this definitely.

 

[Dowels93]

As KootK & jayrod12 said, it's common and hasn't failed yet... but that's probably more to small loads, conservative bearing values, and failures causing settlement rather than catastrophic failures. To quantify though:

These footings are tied back to resist some rotation, so you could probably use the tension developed in the bar that goes into the SOG to reduce the eccentric moment by T*h (T = tension force, h = dist from bar to bottom of footing), but you additionally create a soil friction F = T at the base of the footing by doing so (similar to T and F in your second attachment). However, you would need to actually check how much moment could be counteracted by that tension to ensure that it would remove your eccentricity (or if not, how much your eccentricity was reduced) and that your soil can resist the friction created.

Alternatively if you cannot develop enough soil friction, you could try to use the moment capacity of the SOG to take some eccentricity out of the footing by taking the moment in the SOG, although this will give lower values than the tension/friction couple.

 

[KootK]

Meh, a 5" slab isn't nothing. It probably does come down to loads that don't materialize and soil capacities being grossly underestimated. As far as technically digestible story weaving goes, this is the best that I can come up with.