Eccentric Footings

[CF144]

Hello all. I'm trying to understand eccentric wall footings a little better. When doing eccentric wall footings (L-shaped, property line footings) where the allowable soil bearing pressure is high, I sometimes end up with a footing that's only slightly wider than the wall itself. This makes sense to me because the eccentricity is minimized using narrow footings. My question is: are there are any practical issues with using a narrow footing? For example, say, a 14" wall using an 16" wide footing?

What if you don't even use a footing? My understanding is that the purpose of a footing is to distribute the load into the soil. What if the bearing area under the wall itself is satisfactory? Would you ever omit a footing entirely? I ask this because I have a case where the bearing area under the wall is satisfactory (walls are bearing on good soil), and if I add a footing, say a 24" wide footing with a 14" wall, I would be introducing an eccentricity of 5". And when I check using P/A +/- MC/I, I end up with a "negative" minimum pressure (negative being uplift), which I think means I'd have to deal with it somehow. Perhaps a strap? In this case, would you omit the footing or provide a footing & strap back?

Editing to add more information:
These are 12 feet tall cast-in-place, reinforced concrete foundation walls in the cellar of a mid-rise building supported on good rock. Footing will be well below grade. Groundwater table is below the footing and the site is not a flood zone so I believe that means moisture changes will be minimal.

 

[1503-44]

You do not mention what type of wall you are building. Are these brick or concrete block walls? Do the walls have reinforcement? Soil types? Height of wall? Is it exposed to frost freezing? Will there be moisture changes? Is it a wall of a building, where the building will most likely change moisture content of the soil, or a free standing fence type of wall? Type of building? One, or two stories? A tool shed, or just a fencing wall, I might do it. A house, probably not.

But true, eccentricity is not good, especially since uniform soil bearing is not possible. It has to be a balanced decision, but a narrow strap grade beam would keep that to a minimum too. There is not much of a reason not to use at least do that option.

I would always use a footing, or strap, or a lintel beam at the base of a block or brick wall, simply because block walls often (always) can easily have issues with cracking at the slightest amount of settlement. Long term differential settlements and soil swelling with changes of water content are difficult to predict at shallow depths as well and can easily upset your assumption of uniform soil bearing. Resulting problems are often difficult to fix.

In some cases a wall can be supported with course of heavier stone capped with concrete running below the wall.

All that said, I would generally use a reinforced strap beam to ensure any spanning over weak spots in the soil can be achieved, but that decision is very much dependent on the answers to all the questions I raised above. Trying to make a general rule would most likely lead to failures. I hardly ever see a wall that survived very long without cracking at some point, probably because potential spanning issues over soil weak spots or soil swelling with water content change was ignored. I'd think, in general, no strap of some kind is just asking for trouble.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[CF144]

Thank you 1503-44 for your comprehensive response. I edited my original post to add additional information if anyone else wants to chime in. Based on your information it seems prudent to have at least a narrow footing and strap beams bracing it.

 

[1503-44]

With a direct rock bearing you may be OK just with concrete topping to provide a level surface.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[driftLimiter]

I am curious whether your wall is retaining any soil. You say its a property line basement wall but that you were able to get a footing to work that is 16" wide. Something seems off to me here.