Anchor reinforcement for a stem wall 1

[RabitPete]

I need to design a reinforcement to resist moment loads from the column on top of a stem wall. There is a lot of information out there on anchoring to pedestals, but not so much when it comes to rectangular footings. Concrete breakout is a controlling factor, so I am adding vertical reinforcement with loops encircling entire width of the wall on top and standard hooks at the bottom. My issue is that bottom footing is not deep enough to resist the breakout either.

What would be an effect of adding longitudinal reinforcement to the stem wall, may be make a section of it into a beam? Would it help to distribute the load from the anchors over a longer footing area, so steel which is farther away than 1/2Hef from the anchors would also contribute?

 

[KootK]

What would be an effect of adding longitudinal reinforcement to the stem wall, may be make a section of it into a beam? Would it help to distribute the load from the anchors over a longer footing area, so steel which is farther away than 1/2Hef from the anchors would also contribute?

1) I feel that your logic is sound with that and that it could be made to work with the right attention to detail. Even without the beam detailing, it's pretty common to assume a 1:1 load spread from your base plate down to the footing for bending.

2) It's strictly more correct to turn your dowel hooks the other way but, for the proportions that you're showing, I doubt it would make a difference and your arrangement is more constructible.

 

[KootK]

For me this would be a little too exotic relative to simpler solutions but it's doable. You're basically turning the joint into a reinforced beam/column joint rather than breakout style anchorage. You'd need a pretty good footing depth to make the detailing workable.

 

[r13]

I think your sketch shows the cross section of the stem wall. If I am not mistaken, You need to consider the available/effective concrete area bounded in both direction. The example below uses h_ef = 10". Didn't ACI address this situation? Link

 

[RabitPete]

Didn't ACI address this situation?

They did, and I did account for the effective area on all sides. And without reinforcement, φNcbg is roughly 50% of the required capacity

for the proportions that you're showing, I doubt it would make a difference and your arrangement is more constructible

It is not to scale. Stem wall is 14" thick and footer is 12". I might be able to upgrade it to 14", but that is all. It is going right next to the existing structure and digging a deeper footing would undercut foundation

The wall is 20ft long (10ft on each side of the column), so if we could only distribute that moment over a 20ft... Doubt it can be designed rigid enough

 

[WARose]

My issue is that bottom footing is not deep enough to resist the breakout either.

I guess I am not following what is going on. Are you saying you cannot develop the rebar shown because the footing is too shallow.....so you are checking it by Appendix D (or whatever section it is now in 318) and it isn't working there either?

 

[r13]

In that case, you have no choice but to utilize the vertical reinforcement to carry the tension. You need to ensure there is adequate development length above the intercept of the assumed break plane (for hooked bar), lengthen the anchor bolts as required. I think ACI DG 1 does have example for it. For the base slab, ACI does not address breakout cone, but with control in development length. You need to maintain the development length for bars in tension. If Ld is inadequate, you can add concrete thickness (most effective), or use smaller bars to reduce the development length requirement (note the minimum requirement you can't violate though).

 

[RabitPete]

ACI does not address breakout cone, but with control in development length

Technically speaking, for a single bar it makes sense, breakout does not control. But let's consider 4 hooked bars (#4) closely spaced (on 4in square). Each one is fully developed and hooks oriented in 4 directions (+). Would you really get 4 times the capacity? I would think that break out failure will occur shortly after breakout capacity of a single bar is exceeded.

 

[KootK]

I've investigated that and come to the same conclusion:

1) Generously spaced bars: development is usually enough/.

2) Gaggles of bars: development is often not enough.

 

[r13]

When you design spread footing, is there any suggestion to check the breakout from base pad? I couldn't recall any, but the requirement on development length. Same logic here. Similar concept as develop the rebars of concrete walls in a T joint. Sketch be low is to make sure we are on the same topic - develop rebars in tension. Note the different breakout mechanisms of rebar and headed stud/bend smooth dowel in concrete.

 

[WARose]

Agreed r13. That is what was behind my question above. Wasn't sure why we were talking breakout with rebar development (two different animals).

Kootk has made some interesting points on this over the years (if you do a thread search).

 

[r13]

WARose,

I am puzzled as well.

 

[KootK]

"Interesting points" = it shall be my life's work to shed light on the fact that development <> anchorage.

A couple of the more salient threads, the first of which would surely have seen violence visited upon me if it were a real world pub discussion:

Development by precast78

Anchor Bolt / Pedestal / Foundation Connection by azcats

 

[r13]

1) Failure mechanism of reinforced concrete and round smooth stud/dowel in tension. 2) ACI do address influence of rebar spacing and clear cover in development length calculation.

 

[RabitPete]

ACI table 25.4.2.2 is for straight bars, not hooks. I believe spacing, clear cover and confinement addressed in 25.24.2.3 are more about getting enough concrete to encase the bar on all sides and address 1st failure mechanism illustrated by R13 above.

You wont see a 3" OC vertical rebar in a typical spread footing, but with anchor reinforcement, I got 4 bars closely spaced around each anchor rod. Not sure about moment load and effect of compression side, but if it were a pure uplift, breakout would very likely to occur if each hook barely meets l_dh.

 

[r13]

The calculated l_DH accounts for rebar spacing and cover limitations, after that, it is there to ensure the rebars will develop code specified tensile strength. If you need the 4 bars to resist the load, you shall make sure the l_DH is calculated based on the bars in group, not the individual bar. Also, when utilize rebars to carry load from anchor, you need to pay attention to the distance limits between rebars and the anchor.

 

[RabitPete]

you shall make sure the l_DH is calculated based on the bars in group, not the individual bar

I am not familiar with any provision of ACI which addresses groups of standard hooks. In fact, quite opposite, per R25.6.1.5A: "The development of bundled bars by a standard hook of the bundle is not covered by the provisions of R25.4.3"

And per R25.6.1.7: "The increased length of lap required for bars in bundles is based on the reduction in the exposed perimeter of the bars"

Lets consider the following example:
single #4 90 degree hook in 4ksi concrete, no confinement and Cb of >=1.25in requires Ldh=6.64" and provides 11.78 kips of tensile strength
anchor embedded 6.64" into 4ksi concrete give us 19.48kips of breakout strength.

Now, lets consider 2 of the same 90 degree hooks in 8" OC: Cb is still >=1.25, hence the same Ldh and 23.56kips capacity
2 anchor 8" OC have breakout capacity of 27.3 kips, so still not a controlling factor.

And now, lets consider 4 of the same 90 degree hooks in 8"x8 pattern: Cb is still >=1.25, hence the same Ldh and 47.12kips capacity
While 4 anchor in 8"x8" pattern limit breakout capacity to 38.27 kips. Breakout!

So KootK appears to be correct:

1) Generously spaced bars: development is usually enough/.

2) Gaggles of bars: development is often not enough

 

[r13]

Bar group ≠ bundle bars. The minimum base slab thickness is governed by the longer l_DH calculated for the #8 (in bar group), and the #6 (individual). Hope this ends the discussion.

 

[RabitPete]

I am not seeing any ACI provisions talking about increasing Ldh for a "group" of bars. Only anchorage section considers closely spaced groups. However what we are not considering is a compression force from the moment arm, and its effects on the breakout. Due to that force breakout might not occur, but I am still not comfortable specifying 4 closely spaced bars with minimal Ldh and near their maximum capacity.

I am even more concerned with load distribution and soil bearing. While overall moment per ft of length is relatively low, I got a single concentrated moment applied in a middle.

 

[WARose]

From a practical standpoint....wouldn't other things become more critical before development? For starters, how does the wall footing handle this much shear? Say you've got 4 #5's closely spaced.....that sends nearly 75 kips of force into the wall footing for shear. That's going to get you close to a 3' wall footing. (Which is outrageous in my neck of the woods unless we are talking industrial work or frost depth.)

I've generally observed the 3 bar diameter spacing in the code for hooks (without using the allowed reduction). I don't know if that meets Kootk's "Generously spaced" criteria.....but usually problems show up elsewhere before that becomes one.