Building Column Footing Combined W/Retaining Wall

[Eng_Struct]

Hi All,

I see there are similar questions posted on the forum before but my situation vary slightly. As shown in the attached sketch, the soil being retaining by the wall is on the outside. I am currently thinking of combining column footing with the retaining wall as shown. I do not want to place the column on top of the wall because given the retained height, the deflection at top of the wall may cause problems for the building structure. Note that there is no support at the top of the wall so the wall will be acting as cantilever.

I wanted to get an opnion if there is a way for me to seperate the coloumn foundation from the retaining wall. My concern with the below concept is that, the contractor may want to precure the steel very early even before the construction of the wall. If column pedestal is poured together with the wall as shown, it might move during backfilling and compaction operation which may cause issues with steel erection above. Note that the building roof is another 12ft above the top of the retaining wall.

Thanks!

 

[XR250]

Can't you just form it out with some pt wood to separate them separate them during the pour? I can't imagine losing that small part of the retaining wall footing would be an issue and can be designed for.

 

[Eng_Struct]

If I understood you correctly, what you are saying is to provide a standard spread footing for the column at same depth by notching the retaining wall footing. The wall however will need to span the width of the spread footing in horizontal direction, something similar to the sketch below. Certainly an option but seems a bit complicated. Something to consider though in case if my origional concept is going to cause major issues.

 

[Tomfh]

Don’t bother trying to separate them. Just make it sufficient sturdy that the wall isn’t going to lean over and causes issues.

 

[Eng_Struct]

I have been doing some research on impacts of compaction during backfilling operation. For externally stability, typically I have desgined the retaining wall using active earth pressure and live load surcharge to satisfy sliding and overturning requirements. I ignore any impact of compaction pressure for sliding and overturning considering that the wall will move during the backfilling operation. Ofcourse when it comes to the design of the wall stem in bending, I consider at-rest pressure with live load and compaction surcharge.

One thing that I do not know how to quantify is how to determine the amount of wall movement during backfilling operation. FOr some reason I think it is important for me to have an idea on the amount of movement during backfilling operation caused by sliding. Becasue if the wall shifts horizontally say by 25-50mm, building steel is no longer going to fit.

Perhapse I am overthinking this issue and the amount of movement during backfilling are minor that it will not be a concern.

 

[dhengr]

StrucEng1992:
Beef the retaining wall up a bit to improve its deflection and lateral movement a bit. The pour your conc. column pier as an integral part of the retaining wall, up to the top of the retaining wall, and set your steel col. at that elev. with epoxied A.B’s.

 

[JoelTXCive]

I'm think it would be easier to keep the footing continuous across the region where the column is.

But do you have enough room to increase the footing size? For example, if your overall footing (toe+heel) is 8ft wide, do you have room to increase it to 12ft wide at the column. Maybe the footing bumps out 5 ft before and after the column?

 

[SlideRuleEra]

....if the wall shifts horizontally say by 25-50mm...

Perhaps I am overthinking this issue and the amount of movement during backfilling are minor that it will not be a concern.

Assuming the wall is continuous along it's length, has "significant" length (say, over 20'), and is correctly designed / constructed, you are overthinking the issue... but that's understandable:

Loads on retaining wall are thought of in two dimensions, but the wall reacts to loads in three dimensions. Backfilling will take place at moving "spots" along wall length, not everywhere simultaneously. Consider how the wall "might" shift 25+ mm:

A "chunk" of wall might shear off from the remainder of the wall and shift 25+ mm...

All at one time, the entire wall might shift uniformly 25+ mm...

The wall might "bend" so that only a portion deflects 25+ mm...

In all three cases... I DON'T think so.
In real life, load is distributed along a length of wall, even thou we often don't take that into consideration.

 

[Eng_Struct]

Space is also a challenge. I can extend the footing towards the inside of the building but restriced on the out side.

 

[Eng_Struct]

[In all three cases... I DON'T think so. In real life, load is distributed along a length of wall, even thou we often don't take that into consideration.]

So you think that I can set steel framing dimensions based on the wall location on paper, design the wall cosidering active pressure for sliding, and the amount of movement will be so small that there will not be any issues with steel erection that is fabricated based on the plan dimensions on the drawings?

If I consider at-rest earth pressure to size the footing for sliding just to get some assurance that movement will be minimal during compaction, I end up with ridiculus footing size even with a shear key.

 

[SlideRuleEra]

1) So you think that I can set steel framing dimensions based on the wall location on paper, design the wall considering active pressure for sliding, and the amount of movement will be so small that there will not be any issues with steel erection that is fabricated based on the plan dimensions on the drawings?

2) If I consider at-rest earth pressure to size the footing for sliding just to get some assurance that movement will be minimal during compaction, I end up with ridiculous footing size even with a shear key.

1) Well, if the wall meets the criteria I gave (length, continuity & quality) and movement less than 25mm is acceptable (your limit)... yes, I do.

See Pressure On Retaining Walls From Compaction Effort. Just reading the paper's abstract is informative, and I believe correct.

2) Keep in mind that size of the footing has negligible impact on the friction force (to resist sliding). Friction force has just two variables (coefficient of friction and normal force). COF is beyond your control. Normal force is primarily weight of the backfill:

Make a larger footing outside the garage (which can't be done) and get additional backfill (weight). Or look into other anchorage.

 

[jerseyshore]

You show a SOG in the first picture, is there a reason you aren't utilizing this to resist against sliding?

 

[milkshakelake]

The majority of sliding or flexural deflection from backfilling would have occurred during the backfilling process. So if a concrete pier is built integrally with the wall, it would have already deflected at the top when you install the column. You can use adjustable, groutable anchor bolt sleeves or use post-installed anchors so the column is back into alignment with the rest of the structure. Any additional deflection would be from incidental surcharge. Unless you have giant 18 wheelers running next to the structure, I wouldn't worry too much about that surcharge causing more deflection.

Also, the pier itself has additional stiffness, so that would mitigate some flexural deflection specifically at the column location.

This is anecdotal, but I've done what you're thinking and haven't had issues. Building is standing after 5 years. Not a long time, but I'd give it another 100 years or so.

By the way, at-rest pressure doesn't apply because technically, the wall is free to deflect. I'd use active pressure.

 

[Eng_Struct]

Thank you everyone for super usefull feedback. I have a few take aways based on all of your comments. I more confident now that I can make my original concept work.

 

[structSU10]

I have analyzed these in the past for stability by doing the following:

I have my column vertical load cases. Generally the pier is integral with the wall so I feel pretty good with spreading that load over my first guess as a footing width. I will generally assume a 45 degree load spread from top of pier / wall as the maximum footing width I can assume.

With that effective width I take my column load and divide it by the effective width so it is treated as a line load. In this way I can check a 1' strip the same as any normal retaining wall.