Shear Reinforcing for Replacement of Sluice Gate 4

[AThor]

Just a quick introduction, I have found this forum very helpful and interesting since starting my first engineering job this past year, so I decided to register, and this is my first post here. Anyways...

I am looking at a project involving replacing a sluice gate embedded in a concrete wall. The gate is fixed into the wall by attaching to an "E" type thimble, an iron fabrication with three flanges that embed into the wall around the opening. For the replacement, the concrete holding the existing thimble will be cut out, and new concrete will be poured against the existing mass, with the new thimble in place.

I am trying to figure out the needed reinforcement in the area. This seems like a shear friction problem, since new concrete will be poured against existing concrete exposed in the cut. Do the provisions of ACI 318 11.6 apply to this situation? i.e. can I calculate the required area of shear reinforcement as Vn/(fy*mu)? I assume this required area of reinforcing would then just be distributed around the perimeter of the opening in the wall, across the shear plane formed by the line between new and exposed concrete? I also understand that the roughness of the exposed surface, and the angle of reinforcement will impact the calculation.

If anyone has experience with similar situations, or any recommendations on other things to consider regarding the rebar for this situation, it is greatly appreciated.

 

[JAE]

Hi AThor and welcome.

You might post a sketch or drawing to illustrate the condition and geometry of the cut-out and replacement.

But yes, shear friction could be an approach to look at here. The difficulty would be making the assumption that any resulting force on the new thimble is uniform around the patch/interface.
That may not be true but I don't know the layout so don't know if the stress on it could possibly be non-uniform or not.



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[AThor]

Here are a few sketches. Hopefully they will help clear it up. It took me a while to understand the whole geometry of the project initially.

 

[SlideRuleEra]

AThor - Welcome to, ET. Yes, shear reinforcement is needed between the existing concrete and the new concrete. However, don't try to optimize the details and placement of this shear reinforcement as if it were new work... it is not new work and to treat it as such is a mistake. In large part, steel placement will be dictated to you by existing conditions. For example:

1. Existing rebar in the concrete wall will probably limit the locations where holes for new bars can be drilled.

2. Holes drilled and the anchors (perhaps Hilti type) will have edge distance, hole depth, and spacing requirements that override other consideration.

3. In ACI 318 11.6, I see discussion of angling the rebar for optimum performance. Forget that... drill perpendicular to the concrete face.

4. No matter how thorough the design, unexpected field conditions will require design changes and compromises.

Any structure retaining water, such as a sluice gate, is not something to treat lightly. Calculated loads (e.g. hydrostatic pressure) are real, not statistical probabilities such as typical building wind, seismic, and to a certain extent, live load. Also the consequences of a failure can be sudden, dramatic, and costly.

In all probability, the best approach is to install as much shear reinforcement as can be reasonably be installed... not a theoretical (minimum) quantity.

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[JedClampett]

If you have to take out the thimble, you're on the right track. Listen to SlideRuleEra.
But if you can avoid this, they make all kind of surface mounted gates, where you can have the gate frame bear on the concrete. For example, check out the Whipps 923. Now of course, this changes the location of the operator and a lot of other changes that you have to worry about.
But pouring concrete in a small space like that with a thimble blocking one surface runs the risk of getting porous concrete. It's hard to vibrate the concrete, plus the top of any embedded item is a rock pocket magnet.
Follow the Hippocratic Oath for engineers. "First Do No Harm."

 

[AThor]

SRE - I think you point out an important factor, the difference between new construction and work on existing structures. As a new engineer, I am constantly looking for references to read, such as codes, textbooks, and design manuals. I have gotten the sense that most of these are geared towards new work designs, and don't always capture the nuances and feasibility involved with designs on existing structures.

I agree with your note about hydrostatic pressures. Building code load combinations call for fluid loads to be treated with the same load factors as dead loads, which didn't seem appropriate to me. I have been referencing Army Corps Manual 1110-2-2104 for guidelines on this issue. They provide a load combination discussion more applicable to these types of structures. In general, the Corps manual calls for a 2.2 load factor on Usual case hydrostatic loads, which is what I have been using for preliminary calculations.

The rebar specified will likely end up factoring in the contractor's feasibility constraints, existing conditions, and requirements from the gate and anchor manufacturer. I am just trying to get a complete understanding of the mechanics involved.

Jed - The owner has pretty specific requirements to follow, including replacing the thimble. I agree, it would make things a lot easier to not have to deal with that. There are procedures we are using to eliminate air pockets forming in the concrete pour.

Thank you for the responses!

 

[KootK]

One important feature of shear friction is that it requires the reinforcement to be developed for fy on both sides of the shear plane. Here, with small bar sizes, that probably means that you'll need to cut area about 16" of concrete and drill rebar into the existig concrete 12" or so.

Fundamentally, for a mission critical connection, I feel that it would be preferable to resist the applied load with reinforcement parallel to that load. See below for illustrations of my concern and suggestion. Note that I've limited experience with hydraulic structures so I don't have a good feel for what normal looks like in such situations.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[AThor]

I don't have experience with what "normal" looks like in this situation either, but I do have a few vague reference drawings from similar projects. They do show some sort of longitudinal reinforcing to go with the shear reinforcing, similar to what you drew. My understanding is that the shear reinforcing would be in tension, creating a normal compression force between the two faces, mobilizing the concrete friction. So the longitudinal reinforcing would be providing an additional AsFy of resistance capacity?

Let's say, theoretically, you had enough longitudinal anchors, adequately developed, to resist the loads without any shear reinforcing. Would there be any need for shear reinforcing, or do the two serve individual purposes besides just being added security? Or is it just as SRE said, the more anchoring you can reasonably put in, the better?

 

[msquared48]

Could the new sluice gate be installed on the upstream side since the reservoir will have to be drawn down to install it anyway?

Just wondering as this would make those nasty tension forces in concrete go away, leaving only compression...

Mike McCann, PE, SE (WA)

 

[AThor]

No, the location of the gate is non-negotiable. It is a slide gate with a stem, and the surrounding structure pretty much limits the gate to go back where the original is. But I agree, the unseating head makes the design a little less convenient.

 

[KootK]

My understanding is that the shear reinforcing would be in tension, creating a normal compression force between the two faces, mobilizing the concrete friction. So the longitudinal reinforcing would be providing an additional AsFy of resistance capacity?

That's one way to think of it. My real concerns with your proposal are two fold:

A1) The method that you've described doesn't give account of the moment that will exist on your new blob of concrete in addition to the shear. You could get that done with the bars that you've shown and, perhaps, that's just another aspect of the design that you haven't tabled here, I don't know. Regardless, it's something that needs addressing.

A2) With your method, I feel that the cracking that I've alluded to in my sketch above (green) is a real possibility. For the sake of durability and permeability, if nothing else, I feel that some reinforcing should cross all of the pour joints.

Let's say, theoretically, you had enough longitudinal anchors, adequately developed, to resist the loads without any shear reinforcing. Would there be any need for shear reinforcing, or do the two serve individual purposes besides just being added security?

I still feel that both are required for the reasons that I listed above. With the longitudinal reinforcing, I think that you've got two nice things working in your favor:

B1) The moment on the blobs are less because the eccentricity between the applied load and the shear resistance (longitudinal bars) is less, perhaps even zero.

B2) Moment can be resisted via concrete compression and shear rather than relying on tension which is less reliable and teds to open up cracks.

Or is it just as SRE said, the more anchoring you can reasonably put in, the better?

While I agree with the sentiment in general, I'd be inclined to express it a bit differently.

C1) No matter how many anchors your use, it still needs to be shown to work by calculation.

C2) I feel that conservatism and redundancy are highly desirable in this situation.

C3) Beyond being conservative and redundant, I wouldn't seek to pound as many anchors in as will fit. Anchor installation damages the existing concrete. Micro cracks and all that jazz. Too many anchors and I'd worry that you'd render your substrate porous and/or too damaged to resist load. The structural refurbishment version of the Hippocratic oath is: be damn sure that you do more good than harm.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

KootK - Perhaps you should read my recommendations more carefully. I take exception to your interpretation (C1, C2, and C3) of my words. For example, note that I specifically address "edge distance, hole depth, and spacing requirements that override other consideration." This is a lot different than your spin on my words: "I wouldn't seek to pound as many anchors in as will fit."

AThor - Suggest you keep in mind...

Note that I've limited experience with hydraulic structures so I don't have a good feel for what normal looks like in such situations.

Back to Business:

What is the size and shape of thimble / gate?
What is the total hydrostatic force on the gate?
How thick is "wall"?
Are there adjacent gates or other features (one one or both sides) that limit the width of concrete that can be removed?

Appears you are still trying to design the fix without consideration existing conditions. I do agree with KootK that calculations are essential, but not the way they are being discussed. In a nutshell, projects like this are best designed "backwards". Find out what is present, what is possible, then perform calculations to figure out how make it work (correctly and safely).

Oh, one more thing... yes, I have worked on projects of this nature. One of them involved the sudden failure, successful in-house recovery and repair of an opening in the reinforced concrete dam at one of our hydroelectric stations. The project received ongoing national news coverage since dam failure (if it had happened) would have flooded a mid-sized city.

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[KootK]

KootK - Perhaps you should read my recommendations more carefully.

Perhaps your should write them more carefully. I re-read your recommendations several times and still feel that these two statements are substantially equivalent:

install as much shear reinforcement as can be reasonably be installed

seek to pound as many anchors in as will fit.

That said, "over the wire" communication is always imperfect. If I've misconstrued your meaning then I apologize for that. Statements C1 through C3 were really my opinions rather than interpretations of your opinions. The only reason that I mentioned your work explicitly is that OP seemed to want me to comment on your general conservatism and I wanted to make it clear that agreed with it with a few clarifications.

edge distance, hole depth, and spacing requirements that override other consideration.

These things do not alleviate my concern as I believe that they address macro-cracking phenomena more than micro. In my opinion, you could follow all of the Hilti/App D spacing requirements and still wind up with a compromised mass of concrete if you take the density of installation too far.

AThor - Suggest you keep in mind...

So, clearly, you don't agree with my suggested detailing. What is yours then? I certainly agree that the detailing of the refurbishment needs to account for what's already inside that mass of concrete. That said, I can't be expected to detail around information I don't have. That part of it is OP's problem. And, while I may not be an authority on sluice gates, I'm pretty hand at making stuff robust. I'd have a hard time accepting that my suggested detailing is somehow less robust than OP's. That, particularly, given that I feel there are some holes in OP's shear friction approach.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[AThor]

I think I should mention that, as a junior engineer, my role on this project currently is taking a preliminary look at the project, and the final detailing we settle on will have input from more experienced engineers. Since this is my first project of this type, I am just trying to gather as much information and input as I can from many sources about general considerations in projects like this. So while I won't be stamping this, as an engineer involved on the project, I need to do my best to understand all relevant considerations.

I appreciate the ideas presented so far and the experience you all have. It seems the consensus (which I agree with), is that conservatism is better on water-retaining structures, due to the potentially catastrophic impacts of failure.

Koot - You raise an important point about the negative impacts of overly dense installation. I can see how cracks are potentially more harmful in concrete hydraulic structures due to the water pressure involved. Again, the Army Corps manual EM 1110-2-2104 that I have been looking at has requirements for bar spacing, development, and detailing etc. It seems like those guidelines would be appropriate to follow? But like SRE said, it will be important to remember that the existing conditions may largely govern what is possible. Do any of you have any other reference documents that might have useful information on this topic?

 

[KootK]

I don't have any solid references to offer AThor. It may even be the case that my concerns regarding densely spaced anchors is unfounded. My opinion is really based on my intuition and my experience seeing a few, over aggressively anchored things get crumbly in the field. I'd recommend placing a call to Hilti technical support. They're typically very helpful, and knowledgeable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

AThor - Concur with KootK about contacting Hilti. We started using their chemical anchors in the 1980's. The products they offer have been steadily improved as time goes by. Ask about their "HIT-RE 500 V3" Adhesive Anchor system for use with the rebar. That will probably be the best for your application - proven for structural loading and good for use in water.

Don't have any references for you but will tell you what I would expect from a junior engineer participating in this project:

1. Unless already known, make inquires and, if possible, a site visit to visually verify the reason for this project. Maybe the sluice gate just needs replacement... or perhaps the concrete is deteriorating (say, from ASR). If the surrounding concrete is "bad", no need to design a replacement per your sketch's concept. Note: If the concrete is "good", it may be really good (hard)... many years of continuous wet curing.

2. Determine how the work area will be isolated from the water source. Any well designed hydraulic structure, built in the last century or so, will have a planed way to do this. A way for you to start this conversation is to ask if "stop logs" are used to isolate the gate. Stop logs exactly that, heavy wood panels that are placed, when needed, in permanent slots upstream of a gate to create a kind of mini-cofferdam. There are other solutions, too. Try to find out what will be done to isolate the gate.

3. Try to get the detailed specifications, including dimensions, for the replacement gate and thimble.

4. See if you can get drawings for the existing structure. Up to date as-builts are best, but any will do. Make sure the original rebar plans are included. This will be a big help in determining places where is may be possible (or impossible) to anchor new rebar. General arrangement drawing will help determine if there are obstructions or unexpected features nearby to complicate the work.

Note that the above suggestions all focus on this project. Continue to read and study generalize publications of the subject. However, this project is a wonderful opportunity for you to get involved in a meaningful way. Maybe not what you had in mind... for now, but if you contribute to a successful project you will gain much more technical insight and experience than you ever expected. Also, even if all the above inquiries are unsuccessful reporting on your efforts will be much more impressive (to your boss) than saying you read a lot about large water projects.

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[bones206]

Besides the reinforcement, another aspect to consider would be the watertightness of the cold joints. Research the various waterstop systems and perhaps consider a crystalline waterproofing admixture for the new concrete mix. In general, if preventing water seepage is a critical issue, providing redundant waterproofing systems is a good principle to employ (when conditions allow for it).

 

[AThor]

Bones, thank you for the idea. The waterproofing strategy for the concrete mix is something I need to look into.

Thanks everyone for all the responses.