Combination cantilever retaining wall and column footing 6

[Lingwist]

Hi all, first time posting here so please excuse any faux pas, but I wanted to start with a thank you to the community. I have learned a lot perusing this forum over the last few years.

I wanted to get opinions on a situation where a cantilever retaining wall is also being used as a column footing. I have seen this design used on projects by an engineer who has since retired, and now a customer wants to use the same design, so I want to make sure I understand how to justify it. I've added a photo below, and attached a couple details to clarify (Detail 1 and Detail 2), but basically the cantilever retaining wall is precast, and the column pier is cast in place after the precast piece is set in the field. The only reinforcing actually connecting the two are a set of stirrups which project from the wall into the pier.

The wall works fairly well as a retaining wall, and the downward loads on the building columns seem to transfer to the wall heel in an obvious way. What I'm less sure about is the uplift, which can be up to 25 kips or so. There are vertical rebars in the pier, but they don't connect to the wall heel, and even if they did, they don't seem to have much room to develop anything. So that leaves the stirrups to get the uplift into the wall. Please correct my thinking here: the uplift is transferred to the cast in place pier by the anchor rods being developed or lapped with the vertical pier rebar, and then...? Does the force go from pier to wall via shear friction at the stirrups? Does this seem reasonable?

I also noted that ACI 318 has some requirements that could affect this in 16.3, and in particular 16.3.4 and 16.3.5. I'm not sure if these provisions could be satisfied by the stirrups, since I guess technically they cross the interface between the pier and the cantilever wall.

Any comments would be much appreciated! Thanks.

 

[r13]

In general, I don't like to mix building structures with retaining wall, just too many caveats. However, if it can't be avoid, I suggest to place the CIP column in between the precast panels, and make them an integral wall (similar to counterfort wall). Also, at least locally thicken the base concrete for ease of bar development.

 

[BAretired]

C.I.P. is cast in place. What is I.W.P.? It's only 4" thick, not enough to develop the stirrups in tension.

I don't know why the vertical reinforcing is 'by others'. It is a necessary part of the pier. There should be dowels connecting the heel of the footing with the pier.

I don't have the current ACI 318, so cannot respond to your code question.

BA

 

[KootK]

1) Shear friction is just how it'd transfer the load into the wall and I think that it's reasonable here.

2) In my opinion, you can kind of cheat the shear friction development requirement with typical column ties. As long as the ties are small diameter and detailed with a conventional hook wrapped around a longitudinal bar, I feel that they can be said to develop instantaneous as beam stirrups are thought to.

3) The details are a bit confusing in that they seem to show the stirrup ties terminating inside the exterior wythe of an insulated wall panel. Maybe those are just generic details not yet updated for this project. Regardless, I'd be seeking to terminate the ties within the primary structural wythe of the wall and not have them cross through a layer of insulation.

 

[BridgeSmith]

So that there's no question marks with regard to development of the horizontal stirrups from the column into the wall, I suggest just making them closed ties.

The vertical column bars should extend into the footing and terminate with a standard hook.

Be sure that the reinforcing from the stem wall into the footing is adequate for the uplift on the column and the bending load on the stem wall. You should be able to use a distribution length for the column uplift load of about twice the dimension from the footing to the top horizontal tie.

Be sure that the footing is wide enough for the soil load to resist the uplift on the column and the overturning load on the wall. I would have no idea what the load combinations would be for something like that.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BAretired]

Edit: Sorry, I failed to download Detail 1, so did not see your cross section. Please ignore my comments highlighted in black. [highlight #000000]Please provide a cross section as shown in red below.[/highlight]

1. [highlight #000000]Is there a grade slab with finished floor at or near the top of precast?[/highlight]
2. Is the pier or wall laterally supported at the top?
3. Is insulation needed below floor slab? If so could rigid insulation be provided on the inside face of the precast?
4. Can closed stirrups (ties) be used with vertical bars at exterior corners?

If the wall and pier are not laterally supported at the top, they will move outward under active pressure. Depending on the height of retaining wall, this could create a noticeable gap between grade slab and wall.

Shear friction might carry some uplift, but it will be applied at a considerable eccentricity to the wall. It is mandatory that the vertical bars be anchored in the footing. They should be adequate to carry 25k uplift in direct tension without relying on shear friction.

BA

 

[r13]

The details (1 and 2) provided by you are inconsistent, or detail 2 is drawn incorrectly. My guess is you wanted to incorporate/change the more "doable" detail 1 into detail 2. Please provide the full details proposed by you.

 

[r13]

Additional note:

While detail 1 is more doable, does not mean it is a sound detail without concerns. I would like to think the ties should be developed into the retaining wall, the less than 7" available development length is then questionable at the eyes of the code. Also, you should clearly indicate the tasks to be performed "by others". I am confused on your responsibility for this project.

 

[Lingwist]

Thanks everyone for the help! Great to get some feedback. I should clarify some things as you have pointed out.

r13: I can't disagree with your dislike of combining the building structure and the retaining wall. Argument against: retaining walls could move slightly, and building columns shouldn't. (One thing that helps in this case is that all the retained soil is placed and compacted before the columns arrive.) Argument for: the contractor likes the simplified construction. I'm not sure I understand your suggestion to put the CIP column in between the precast panels and make them an integral wall. Would you mind elaborating a little?

BARetired: I.W.P. is insulated wall panel. In this case, it's an insulated metal wall panel that sits above the top of the precast. The detail could be done better, but the hatched 4" thick wall is above top of precast, and does not interact with the CIP pier. The vertical rebar ended up "by others" because the subcontractor who pours the CIP piers is used to supplying the rebar for them as well. For this project, one subcontractor supplies the precast with the protruding stirrups, and another subcontractor forms up and pours the CIP piers on site. When you say the dowels should connect the pier to the heel, is that for downward loads, uplift, just for prudence, or all of the above?

KootK: Sorry for the confusing details, hopefully my explanation to BARetired helps clear it up. Detail 1 shows a little more clearly that the insulated wall is above the precast, and was shown in plan just for reference. The stirrups terminate in the 7" thick solid stem of the retaining wall.

BridgeSmith: Thanks for the advice. The reason the vertical column bars don't extend into the footing (heel) is because that would complicate forming of the precast piece, but I agree that without that, I'm not sure what those vertical bars are even doing. Good point about combining uplift and overturning. I hadn't really thought of that because the precast weight alone is usually greater than the uplift, but I will look closer.

BARetired 2nd post: The wall and pier are not usually supported. Some earlier projects included slab tie-backs near top of precast, but they were later eliminated due to contractor concerns about the slab cracking; I believe restraining the slab shrinkage is what he was worried about. The slab is sometimes insulated (I have seen these used with and without insulated slabs), and when insulated, that insulation does run down the inside of precast wall. What are you thinking with this question?
Yes, closed ties with corner bars could be used.
Regarding your eccentricity comment, could you help shed a little more light on that? I get that the load is eccentric to the joint, but in my mind the CIP pier lifts up as one mass and the shear transfer happens at the joint. I'll admit I only have a basic understanding of this.

r13 2nd post: Agreed, the details could be clearer, hopefully my explanation to BARetired helps clear up both the intent and the scope of responsibility. These drawings were done by a younger version of me, so I am duly noting your suggestions.


Two more questions:
1. I am new to posting here, but I noticed that someone posted my exact question again this morning on this forum, thread507-475374. Any idea what that could be about? It is a copy of my question but without the photos.
2. Is there a general preference here between inserting (uploading) a photo vs linking to it? I did both because inserting 3 photos in the post seemed excessive. Thanks!

 

[BAretired]

BA[/color]]BARetired: I.W.P. is insulated wall panel. Thanks. In this case, it's an insulated metal wall panel that sits above the top of the precast. The detail could be done better, but the hatched 4" thick wall is above top of precast, and does not interact with the CIP pier. Yes, I would have realized that, had I downloaded Detail 1. My bad.
The vertical rebar ended up "by others" because the subcontractor who pours the CIP piers is used to supplying the rebar for them as well. For this project, one subcontractor supplies the precast with the protruding stirrups, and another subcontractor forms up and pours the CIP piers on site. I assume that the footing is poured after the precast is set in place in order to get the reinforcement properly connected between footing and wall. Is that correct?
When you say the dowels should connect the pier to the heel, is that for downward loads, uplift, just for prudence, or all of the above? Primarily for uplift, although it seems simply like good practice even in the absence of uplift.

BARetired 2nd post: The wall and pier are not usually supported. Some earlier projects included slab tie-backs near top of precast, but they were later eliminated due to contractor concerns about the slab cracking; I believe restraining the slab shrinkage is what he was worried about. Yes, I understand his concern, but tie beams below the slab might be considered; admittedly it would be more expensive, but it would hold the bottom of columns in position. It might even include a beam between piers to tie the top of retaining wall, thereby restraining horizontal movement of the wall.
The slab is sometimes insulated (I have seen these used with and without insulated slabs), and when insulated, that insulation does run down the inside of precast wall. What are you thinking with this question? It depends on the expected climate and also whether or not the subsoil is frost susceptible. I was thinking about the potential for frost heave under the edge of slab due to freezing through the retaining wall.
Yes, closed ties with corner bars could be used. Okay, then I recommend them.
Regarding your eccentricity comment, could you help shed a little more light on that? I get that the load is eccentric to the joint, but in my mind the CIP pier lifts up as one mass and the shear transfer happens at the joint. I'll admit I only have a basic understanding of this. If there is net uplift, combined with lateral pressure on the retaining wall and pier, shear friction of the ties will exert a local eccentric moment on the retaining wall. With a tie directly to the footing, the 25k uplift is transferred by the footing to the wall. It still contributes to overturning, but pier vertical steel is properly tied into the foundation.

BA

 

[r13]

I'm not sure I understand your suggestion to put the CIP column in between the precast panels and make them an integral wall. Would you mind elaborating a little?

The PC panels should be casted/erected with dowels at the interface. The dowels provide positive linkage between the panels and the CIP column. The dowels maybe require to be hooked at the end though.

 

[BridgeSmith]

The dowels maybe require to be hooked at the end though.

Or mechanically spliced...

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Yeah, that's good one.