STM Spread Footing Design

[jreit]

Good morning,

I have a 8' x 8' x 3' deep spread footing supporting a 4' circular concrete column supporting one end of a sign structure.

The dead load (~70 kips) is small compared to the moment (~550 kip-ft) and overturning is a concern. The geotech engineer provided 4 post-tensioned (~65 kips) ground anchors in each corner of the footing.

Not having done much in the way of footings or strut-and-tie, I had a few questions.

When I run the STM, is the column rebar coming down ok for use to resist the tension in the vertical ties?

Do I need shear reinforcement? The concrete has enough capacity to resist the punching shear forces but do I still need anything in the way of minimum shear reinforcement as a good practice? Given the dimensions, I didn't check one-way shear. Rebar congestion is an issue - I need top and bottom bars each way and bursting reinforcement around the ground anchors. There is also rebar coming in from the column hooking into the footing plus ties for the column rebar.

Do I need additional spalling reinforcement? There is main bar in both directions to pick up the relatively small spalling force but I'm not sure of that failure plane. I would not want the concrete to suffer large cracks before engaging the main bars.

Thank you in advance.

 

[KootK]

When I run the STM, is the column rebar coming down ok for use to resist the tension in the vertical ties?

This may deliver your tension to the underside of the footing but, by itself, is not sufficient to get the loads over the ground anchors.

The concrete has enough capacity to resist the punching shear forces but do I still need anything in the way of minimum shear reinforcement as a good practice?

I don't feel that shear reinforcing is necessary unless deemed so by way of calculation. Some things that you certainly need to check include:

1) punching shear at the column including the effect of moment.

2) Your ability to deliver the ground anchor tension into the corners given that the situation is a bit different from classic punching shear since the load is delivered to the body of the concrete rather than the back face.

3) I'd take a quick look at one way shear on 45 degree planes in front of the ground anchors. It's easy and would reassure me about performance.

Do I need additional spalling reinforcement?

That's a tough one. Many codes do require this for STM models. That said, I'd be inclined not to provide it here. If you're shear stresses are so low that you don't even need reinforcing, I can't see crack formation being excessive.

 

[Ron247]

When you say the ground anchors were post-tensioned, do you mean they go from the ground, straight up through the entire concrete and then have a fastener that pulls the foundation down to the ground? Essentially prestressing post-stressing the ground under the footing.

I am just trying to picture your situation better in my head.

 

[Ingenuity]

Give some consideration to construction aspects of your ground anchors-to-footing connection because it may influence your ciritical sections/s.

For corrosion protection of the ground anchor hardware (150 ksi PT bar or a few 270 ksi strand anchors) usually the bearing plates and nuts/wedge assemblies are placed in a recess (and after stressing they are concrete encased, or other) and that may 'eat' into your 36" footing depth for shear and other calcs. Such recesses can get rather large.

Alternatively, no recess is used and the bearing plate and assemblies are externally corrosion protected above the top of footing - but this is not too common, IME.

 

[Ron247]

geotech engineer provided 4 post-tensioned (~65 kips) ground anchors

I am curious how the post-tensioning was achieved. I have not had to use many STM but doesn't it mean your 4 supports are at the bottom of the footing such as a pile? If your post-tensioned system is not anchored at the base but rather the top, is the STM still applicable?

 

[jreit]

So I can't get out of using vertical rebar. Looks like I definitely need the stirrups.

The ground anchors are in place with a casing prior to the concrete pour and once the concrete is poured and achieves a certain strength, the casing is removed, grout is poured, grout cures and the anchor is tensioned into the rock underneath the footing.

Spot on. We have a recess in the concrete for corrsion protection. ~ 6" below the surface and this is accounted for in the design.


Assuming one of the anchors in each face would be in tension during overturning, I put a truss support at the top of one anchor and at the bottom of the other anchor as the footing tries to overturn. I think that should cover it but am open to a different procedure.

 

[KootK]

So I can't get out of using vertical rebar. Looks like I definitely need the stirrups.

No, my comment should not be interpreted to mean that. I'm simply saying that, in moving your tension from the column to the ground anchors, there's more to that load path that just taking the column verts down to the bottom of the footing. No doubt, you knew this to begin with.

Perhaps you can clarify your original intent with the question below?

When I run the STM, is the column rebar coming down ok for use to resist the tension in the vertical ties?

Are you asking if that is sufficient to anchor the column tension to the concrete struts that will ultimately move the load over to the ground anchors?

 

[jreit]

I'm not sure I follow this. If my vertical bars form the column go down into the footing and are hooked at 90 degrees and extend to the end of the footing, is that not sufficient to transfer the tension over to the ground anchors using the top and bottom rebar in the footing + concrete struts?

Sketch
(Column rebar and ground anchor in pink - concrete struts in yellow - main rebar both ways not shown).


Yes, that's my question. Essentially, can the tension in the vertical ties 9 & 10 be considered to be resisted by the column rebar and then the horizontal tension ties and concrete struts carry it over to the anchors? I have checked the struts for the compressive forces in the absence of any confining reinforcement.

Sketch2
(Loads in Red - Ground Anchors in Blue)

 

[KootK]

If my vertical bars form the column go down into the footing and are hooked at 90 degrees and extend to the end of the footing, is that not sufficient to transfer the tension over to the ground anchors using the top and bottom rebar in the footing + concrete struts?

You tell me. When you crunch the numbers on the STM model, does the affected node calc / detail out? All I'm saying is that the embedment alone, in the absence of other checks, doesn't guarantee success.

Essentially, can the tension in the vertical ties 9 & 10 be considered to be resisted by the column rebar and then the horizontal tension ties and concrete struts carry it over to the anchors? I have checked the struts for the compressive forces in the absence of any confining reinforcement

These two statements seem contradictory. No, the column bars cannot transfer your shear between the column and the ground anchors. But, then, there's no need to do this if you've already proven that the concrete struts could do this job.

 

[Ron247]

one of the anchors in each face would be in tension during overturning

I figured the term "post-tension" meant it was installed and then tightened. It sounds like you meant it is installed, has no load (tension or compression) until it attempts to move or overturn. At that point it is tensioned. Based on your description, the anchors work near upper surface and not the lower surface. You are achieving the "grip" closer to the upper surface.

 

[jreit]

I think I understand now what you mean. I did not phrase my question properly. Sorry about that. I can use the vertical rebar coming down from the column to resist the vertical tension but obviously must run the calculation/detailing for it. I was asking whether the vertical column rebar could be used at all or whether I specifically needed vertical stirrups even if the vertical column rebar was sufficient based on the calculations


Sorry for the confusing statements. This I'd like to better understand. The applied loading on the column is transferred down to the footing and acts as the applied loads on the STM truss. This truss has vertical, horizontal and diagonal members in compression or tension. Together the truss carries the external loads to the anchors. Anything in compression is a concrete strut. Anything in tension is a steel tie. The horizontal steel ties line up with my footing rebar and is resisted as such. What steel resists the vertical steel tie? I assumed it was the vertical column rebar.
Checking all 3 - footing concrete, footing horizontal rebar and column vertical rebar would ensure the STM truss works. That's what I meant by my statement.

Am I picturing this correctly?

 

[KootK]

Ok, I think that I get this now. I'll put it like this: if the numbers and the detailing checks out, you absolutely can use your column bars to deliver the tension to the first concrete strut in compression (nearest the column) in your truss model. You don't need to pound of bunch of stirrups underneath the column or anything like that. At least not for this reason.

How'd I do this time?

 

[jreit]

KootK sounds good to me. Glad we're on the same page.
I did have one more question. What do I do with a diagonal tension tie? No diagonal rebar so can I resolve it into a vertical and horizontal component and use those forces to design my rebar in those two directions?
My concern is that I only have 6 vertical column rebar coming down so it's not as well distributed as I wanted it to be (we need to fit conduits through). I might have to increase that to 8 bars coming down to ensure distribution to prevent excessive cracking.

 

[KootK]

What do I do with a diagonal tension tie? No diagonal rebar so can I resolve it into a vertical and horizontal component and use those forces to design my rebar in those two directions?

I believe that you can resolve it into components and I've done this before. That said, you really need to keep an eye on how your satisfying the various detailing requirements when you do it this way. For your situation, I'd favor the orthogonal STM model shown below. It is true that less direct load paths make for less excellent STM's but, then, I think that the components approach is really the same idea anyhow so I doubt that anything meaningful is lost from switching from one approach to the other.

Will you have control over the bar locations in this or will their orientation about the column perimeter be effectively random?