Deteriorated concrete ledge

[Robbiee]

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?

 

[Robbiee]

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?

 

[hokie66]

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

 

[KootK]

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.

 

[jayrod12]

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.

 

[Teguci]

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.

 

[KootK]

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.

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.

 

[jayrod12]

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.

 

[KootK]

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.

 

[Teguci]

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?