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Deteriorated concrete ledge

Deteriorated concrete ledge

Deteriorated concrete ledge

(OP)
Hi all,
Need your input please. The attached is a concrete beam with a ledge supporting adjacent floor. The rebars drawn in the beam are what I expect to be, but I don't have that info, because we don't have existing drawings. The ledge has concrete spalled from the soffit and signs of corroded rebars. I am thinking of cutting and replacing the concrete (of course after shoring), and for the corroded rebars provide epoxy anchored new bars with a hook at the end to look like J placed around the horizontal bar.
Should I worried about the existing rebars in the beam if they are not properly details? The concern I have is because when you analyze the ledge with Strut and Tie model, you end up needing to know what is in the beam to verify the model.
Or do you make the assumption that this ledge is a small cantilever and if tension bars are OK and the interface shear is OK then you're OK?

RE: Deteriorated concrete ledge

Do you know why the ledge failed in the first place? Was it mostly (entirely) due to rebar corrosion?

Seems like I'd want to ensure that the "new" condition met current standards with respect to either ACI corbel design (318 chapter 11) or PCI spandrel ledge requirements.

The beam stirrups do participate as a hanging element so you'd have to know what you have there. ACI's corbel design doesn't directly deal with this as I recall.

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RE: Deteriorated concrete ledge

(OP)
Thanks JAE. The deterioration is mainly due to failed expansion joint and the infiltration of salty water. This is a parking floor.
I am in Ontario, so we use the CSA A23.3. Now, if you don't have as-built drawings, how would you know what rebars are in the beam at the ledge? would you remove concrete to left of the ledge to confirm?

RE: Deteriorated concrete ledge

I'd be seriously tempted to cut the existing ledge clean off and provide a steel bracket that anchors into the top third of the L beam and wraps under the stem. The problem with replacing the concrete will be the shear friction (especially with epoxy coating) at the cold joint. I also suspect that the slip pads (2 korolath plastic shims?) aren't performing well enough to keep the tension load off of the top of the ledge and helped cause the initial cracking.

Is this for each double tee stem or is this for an IT beam (uh-oh)?

RE: Deteriorated concrete ledge

It's tempting to trust that the hanger bars were designed correctly but I'd be looking to verify that somehow. You can probably have the beam scanned to get an estimate of the rebar size and spacing. Some other possibilities for ledge reinforcement:

1) The ledge flexural bars might full hoops.

2) There might be a diagonal hanger in there similar to retaining wall T-joint reinforcing.

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.

RE: Deteriorated concrete ledge

(OP)
Teguci,
I think I misled you with the sketch. The inclined line is not a crack, but the line of cutting the old concrete and replacing it with new. A long section of the ledge needs to be replaced. I am told by the restoration firm I am helping, that this is a typical repair that they do all the time. I wonder if there is a "restoration structural engineer" here to comment.
Would you be satisfied if the removing 2" to the left of the ledge from the beam showed the vertical legs of the stirrups. This way you have new ledge and the interface shear is much greater?

RE: Deteriorated concrete ledge

Quote (Robbiee)

this is a typical repair

I've never seen a precast(?) L-beam need this kind of repair. If the chlorides are high in the soffit then I'd think the top of the ledge is also contaminated, and I'd expect the top rebar to have expanded as well.

Quote (Robbiee)

Would you be satisfied if the removing 2" to the left of the ledge

No, for a concrete repair option, I prefer the inclined cutline as you've shown. If the numbers work then go for it. The advantages for concrete replacement are numerous (fire, corrosion, similar construction, etc...). However, lock down why this ledge failed in the first place. During the years I designed parking structures (and some resto), I did not encounter the bottom surfaces of precast beams spalling (carbonation?).



RE: Deteriorated concrete ledge

Teguci - that sounds like a good idea (using a steel bracket) in terms of the confidence I'd have with it.
However, I'd also worry about future on-going salt-water exposure on the steel.

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RE: Deteriorated concrete ledge

This type of ledge is susceptible to cracking due to failure of the bearings to work as intended. Then when cracks form, water gets in, and corrosion results. I wouldn't look at this as a concrete repair, but rather as providing new support as Teguci suggested.

RE: Deteriorated concrete ledge

...so, as a rough analogy:

With functioning friction pads = pinned connection?

Without functioning friction pads = mimics a fixed connection at the ledge, for which it was not designed?

Teguci, Hokie66, is that what you gentlemen are saying?




RE: Deteriorated concrete ledge

(OP)
Teguci, Sorry, I should have noted that this is a cast-in-place concrete beam not precast.
I'd be more comfortable cutting along line 2 than 1, because we can confirm the presence of the stirrups, and we have longer surface to transfer shear. One concern with this is the development length of the horizontal tension bar beyond the cut line.


RE: Deteriorated concrete ledge

Archie,
With a properly working slide bearing, the support should act as a roller, neither fixed or pinned.

RE: Deteriorated concrete ledge

(OP)
Archie264, I think what Teguci and Hokie66 are saying is that when these pads, that have high compressive strength but small shear strength, fail to function, the friction between the two causes tension. All ledges and corbels, however, need to be design for a minimum tension unless this tension is somehow relieved (sliding bearing maybe?).

RE: Deteriorated concrete ledge

@ Archie264:
Still missing some detail on this.

If this is a ledge supporting double tee stems with an expansion joint between the double-tees and L beam, then you need to allow for rotation of the double tees and allow for sliding at the joint. Ideally, there would be a neoprene pad adhered to a steel/stainless steel plate over a Teflon surface. In the real world, that's not gonna happen for this lightly loaded support. Typically, 2 plastic shims are stuck in there to allow for some sliding and a very little bit of rotation. It is usually good enough for this redundant and lightly loaded connection. In this case, we have specific evidence of a failure that may be due to the usual not being good enough. Further investigation should be pursued or provide a repair that adds rotational flexibility and better sliding.

Of course, I don't know without additional detail. If its just spalling, then replacing the concrete makes the sense.

Update: - OK this is CIP concrete. I'm sure looking at the big picture, this might make more sense. But, I'm having a tough time figuring out why a CIP deck is detailed this way. Oh well, no matter. As noted above, the bearing may be the reason for the failure and should still be looked at or provide a detail that relieves the needed motion.

RE: Deteriorated concrete ledge

>>>With a properly working slide bearing, the support should act as a roller, neither fixed or pinned.<<<

Yeah, I guess I should have written "roller", though it can't be roller on both ends. (Yeah, I know it can in the real world but I'm just referring to idealized conditions.)

I think I've got it. I guess I was just surprised that a seized up roller would make that big of a difference since a ledge is designed for top tension due to negative moment. I guess it's a matter of magnitude and, not having run any numbers, it surprised me. But clearly the bearing pads are there for a reason.

RE: Deteriorated concrete ledge

Robbiee,

The constructability of your "Line 2" is problematic, due to working in such close quarters. I think demolition and recasting of the entire beam would be simpler. That way, you can do it correctly.

RE: Deteriorated concrete ledge

(OP)
hokie66, do you agree that cutting at line 2 is preferred over line 1 if constructability is not problematic?

RE: Deteriorated concrete ledge

I see no advantage to cutting at line two other than the visual confirmation of the stirrups. And you should be able to accomplish that by way of scanning / non-destructive testing. The limiting shear friction condition will be at line one regardless of whether or not you take your demolition to line two.

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.

RE: Deteriorated concrete ledge

It occurred to me that if CRSI research has demonstrated the need for diagonal bars at the interior joint of retaining walls then perhaps it would also be appropriate for ledges such as this. Just a thought.

RE: Deteriorated concrete ledge

Robbiee,
I wouldn't answer that question because I think the premise is false. Constructability is obviously problematic.

RE: Deteriorated concrete ledge

(OP)
Thanks hokie66. In similar situation that we have, the ledge is supporting a cast in place concrete slab, which can be cut at some distance from the ledge so that conductibility is not problematic. Where would you cut the ledge, at line 1 or 2?

RE: Deteriorated concrete ledge

Without actually being there and examining the situation myself, I would stick with my preference to demolish and recast the entire beam.

RE: Deteriorated concrete ledge

Quote (OP)

I am told by the restoration firm I am helping, that this is a typical repair that they do all the time.

This is an accurate statement in my experience.

I've spent a good deal of my career working in close proximity to some of western Canada's more prominent restoration engineering groups. And, on many occasions, they've consulted me and my purely structural colleagues regarding "serious" structural stuff that crops up I their work.

I've found that there are significant philosophical differences in how your average structural engineer thinks and how your average restoration engineer thinks. In general, without pulling punches, I find restoration guys to generally be lousy structural engineer and, in return, they find me to be impractical and pedantic.

I've seen the ledge repair that you've proposed on several occasions and, to the credit of the restoration folks, so far, so good. Here's how they seem to approach it:

1) Beam torsion was working before the repair so it's reasonable to assume that it will continue to work after the repair. No needs to verify stirrups.

2) Hanger steel was working before the repair so it's reasonable to assume that it will comtinue to work after the repair. No need to verify stirrups.

3) Cut ledge back to your line one with a little back slope.

4) Post install ledge tension steel that works via anchorage principles rather than reinforced concrete / STM principles.

5) Ensure that shear friction works on your plane one.

6) Throw some extra dowels in near the bottom of the ledge for good measure.

7) Do something about the water leakage issue.

So yeah, just like your original plan.

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.

RE: Deteriorated concrete ledge

Koot's experience with restoration engineer's is exactly mine as well. We have an in-house restoration department and even though we say we're only joking about "light side versus dark side" there definitely seems to be some fundamental differences in the way each department assesses problems and determines solutions.

I too have seen this detail approved and performed in the field. The key is ensuring the problem doesn't happen again (i.e. fix the source of the water into the connection) the rest seems to take care of itself.

RE: Deteriorated concrete ledge

The big difference in my mind:
The Restoration engineer has to tell the owner how much damage has been done and how much money he has to pay to continue to operate a slowly failing structure
The New Design Engineer gets to provide a new revenue source to an Owner for only a small, mostly invisible fee.

In both cases, the engineer needs to do his due diligence. For Resto, there is an appropriate assumption that everything was previous designed and built correctly. This assumption is broken when something fails and needs to be addressed.

RE: Deteriorated concrete ledge

Quote (OP)

In both cases, the engineer needs to do his due diligence. For Resto, there is an appropriate assumption that everything was previous designed and built correctly. This assumption is broken when something fails and needs to be addressed.

This is the tricky bit for me with this being a waterproofing failure rather than a failure associated with the design or construction of the structure. Can we assume that the main body of the beam was designed and constructed correctly without drawings or site verification? I find that your typical restoration engineer will say yes while, as evidenced here, your average structural engineer will often say no.

Quote (KootK)

In general, without pulling punches, I find restoration guys to generally be lousy structural engineer and, in return, they find me to be impractical and pedantic.

I should probably clarify this a bit. Back in "the day" most restoration guys seemed to be former structural guys who crossed over. They were, and are, pretty solid structural engineers. The modern trend, however, seems to be engineers coming out of university and going straight into restoration engineering. I've found their structural competencies to be sorely lacking in many instances. I get engineers who've never designed a two way slab trying to refurbish one etc. If anything, I think that structural restoration work requires greater technical prowess than new build work does.

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.

RE: Deteriorated concrete ledge

Quote (KootK)

The modern trend, however, seems to be engineers coming out of university and going straight into restoration engineering. I've found their structural competencies to be sorely lacking in many instances.

Very much so. Luckily at our firm, we work close enough together that most times there's someone from the new design side, working in conjunction with a restoration person to create "the best of both worlds fix". I also don't know how many times I've gone to the restoration guy with a new design building envelope question. I should probably go buy them a box of donuts now that I think about how often it is.

RE: Deteriorated concrete ledge

That sounds like a healthy setup jayrod. At one place that I worked, I'm fairly certain that they intentionally isolated restoration from structural for efficiency reasons. They would refer to our department as having "analysis paralysis". I guess they preferred to suffer from "incompetence-itis". More profitable that way.

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.

RE: Deteriorated concrete ledge

Maybe I'm going loonie here but I still think salt exposure on the bottom of an L-beam (the soffit), is a sign that you have a more serious problem than just a failed expansion joint. If the bottom of the joint is spalling, what is happening on the top of the ledge? Did the ledge crack first and allow chlorides through the ledge? Maybe you meant that just the top of the ledge is spalled?

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