New corbel on existing concrete structure 2

[tmalik3156]

Good day.

We know how to design a corbel in a monolithic construction.

However, my problem is different. We need to design a new corbel that attaches to an existing structure.
As shown below, the new corbel is a light pole base which gets attached to the existing bridge deck. So the attachment relies on dowels inserted into the deck.
Has anybody ever done such a design? I am looking for comments on design procedure and sketches of reinforcement arrangements.
Thank you

 

[CDLD]

I've never done this design before but you can Hilti those in by the development length and rely on shear friction for the shear at the joint.
You might want to consider the cold joint at the sidewalk.

Hilti has a guide with examples for slab extensions.

 

[hokie66]

Why not use a steel bracket rather than trying to replicate a concrete corbel? Still have the issues of drilling and installing new anchors, but not the mess of formwork and casting concrete.

 

[tmalik3156]

@ CDLD
Thank you. Yes, the dowels will be attached by Hilti adhesive anchor or similar. But I am having difficulties in figuring out the load path, and reinforcement arrangement. If this had been a monolithic construction, I would have used a strut-and-tie model. The light pole (due to wind effect) introduces torsion and moments as well - which makes the load path complicated.

@hokie66
Good suggestion. However, since this a concrete bridge esthetics is important to consider. Probably, a steel bracket won't look good.

 

[lexpatrie]

Design it as integral to the first cast and then navigate through making it work as a retrofit?

 

[fel3]

tmalik3156…

This is not my area of expertise, but I think hokie's suggestion is excellent. For esthetics, you could make the bracket the same size and shape as the proposed corbel, and you could paint it to either blend in to the existing concrete or stand out as an architectural feature, whichever looks better.

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Tomfh]

Do it in steel like Hokie said. This is a bridge and reliability and ease of future inspection should be paramount.

Install anchors, and proof load them. Provide lots of fat (steel lighting poles regularly fall over)

 

[TLHS]

You can't just rely on shear friction. You have some shear from vertical load and wind load to deal with, but the governing issue is more likely to be moments from wind and eccentricities.

The issue with corbels tend to be detailing related to developing reasonable load paths for compression and tension to resolve those couples. Putting a bar in with development length doesn't get the load into the existing reinforcement. You'd need a lap length of some sort with the existing steel, even if it's reduced based on only needing partial capacity.

You need to figure out the couple forces and then develop those forces back into the base structure. Do you happen to know the detailing of the existing? If not, you can likely make a reasonable assumption and just make sure you embed enough to do a non-contact lap or a strut and tie lap analogy with the existing bars. There's probably hooks or bar bends in the existing that give you a reasonably quick development of the existing bars to lap with.


I would also watch out with using the curb as load carrying unless you understand how it was built. It may have a pretty minimal load path depending on what they were doing.

The above is with wind to the left and right. When wind is into our out of the page you have torsion on your system.

 

[tmalik3156]

@ lexpatrie, @ fel3, @ Tomfh
Thank you for your comments. We will see if we can sell the idea of a steel bracket to the owner.

@ TLHS
Very good points about moments and torsion. The load path gets complicated, and developing resisting couples using the dowels may be tricky.
You are right about the curb. it's a sketching mistake by me. We won't rely on the curb to attach the dowels.

 

[cliff234]

I have a couple comments / concerns.

1. Presumably the sidewalk slab was a non-structural second pour. If so all of the tension load at the top of the corbel will have to be resisted by the “curb”. Do you know what the reinforcing is in the curb? How thick is the curb? Can you lower the top of the corbel so that the topmost dowels go into the “existing deck”?
2. What is the construction of the “existing deck”?
3. Drilling those bottom diagonal dowels in the “existing deck will be problematic.
4. Provide a horizontal bar on the inside of the hook on the horizontal top bar in order to more quickly develop that bar.
5. Use #5 top hooked bars to better, and more quickly develop those bars
6. I agree with the suggestion to use a steel bracket (versus a concrete corbel).
7. Lower the uppermost bolts so they are not in the curb, and are at least 4” below the top of the “existing deck”.
8. This assembly will presumably be exposed to weather. I suggest galvanizing the bracket and using galvanized post-installed anchors.

 

[rapt]

You have simply created a new tension zone at the back of the anchors.

Is there other horizontal reinforcement to transfer that tension force further into the kerb/deck?