Horizontal Cold Joints in Torsion controlled Concrete Beam

[iengineerstuff]

I have been doing quite a bit of research on Cold joints in concrete beams, however, I have a more particular problem than i found that can be addressed in some of the forums. Looking for a second opinion.

I have a concrete beam that is torsion controlled. There is a 5' cantilevered balcony, that was supposed to be poured monolithically as noted on my plan. The contractor went rogue and poured the bottom portion of the beam across the full span. Currently leaving the top portion exposed and the cantilever slab unpoured. I was on site by chance last week and noticed the glaring issue.. Now the contractor is hoping he can proceed with as is, rather than tearing out the beam and having to repour the section.

Here is where I stand:
Because the design is torsion controlled, I have concerns that my overall torsional section is going to be reduced by half, unless there is enough shear friction at the cold joint to allow all stresses to develop across the entire beam section. So, there has to be enough shear friction to resist the combined effects of torsional and shear stresses across the depth of the beam, as well as, maximum torsion across the width of the beam.

Considering, this issue was not approved prior to pouring, my gut feeling is to have them tear it out. I am not comfortable with the design, and I have yet to start crunching numbers. Also any reinforcement that needs to be added will be post-installed which leaves the potential for damaging the beam. Which I cannot guarantee won't happen.

Does anyone have any advice on this? Has anyone been in a similar situation with shear friction for a torsion controlled beam?

 

[MotorCity]

If you are not comfortable with condition, it is certainly within your right to have it removed and built correctly. As I see it, you have two issues: (1) Getting the numbers to work such that you get two individual beams separated by a cold joint to work as a single, composite section to resist torsion, and (2) keeping water from finding its way into that cold joint. In my opinion, the water infiltration is the bigger challenge, and, it becomes increasingly magnified since it is part of a cantilevered balcony. Designing and detailing balconies can be a challenge when done correctly, let alone when you build in another path for water to find its way to the reinforcing steel. If it were me, I'd probably crank the numbers to see if I could make it work structurally and put the burden on the contractor to convince me of how he plans to waterproof the cold joint.

 

[Settingsun]

I think it will be OK because torsion design is based on concrete compression struts in a notional space truss. I assume you have close-spaced closed ties as part of this.

If you're not going to enforce demolish-rebuild, you could require that a wet-to-dry epoxy be applied to the joint, and silane coating for durability.

 

[KootK]

I vote "no" on the proposed demolition.

1) Are you quite sure that this is not a compatibility torsion situation where the torsion in your beam can be redistributed as flexure in a slab element? Equilibrium situations do crop up from time to time but they are pretty darn rare when it comes to balcony supports. And compatibility torsion is generally the easier path when it's available.

2) I don't feel that you need to fix anything here. That, fundamentally, because of the closed space truss design methodology that we use for torsion and that steveh49 mentioned. That method does not place any reliance on the unreinforced shear capacity for torsional resistance of concrete as shear design does. There's no torsional equivalent of Vc. So all that you really need to do is to make shear friction work across the horizontal joints across the space truss strut elements on the vertical faces of the beams. And that's little different from when you're dealing with a cold joint for plain old vertical shear which, as you know, is usually manageable.

3) With respect to making things composite for vertical shear, it really doesn't take much to get that job done. ACI offers guidance in this area. From research in the world of topped, precast things, there's reason to suspect that, in many cases, you don't even need rebar crossing a rough, horizontal interface in order to make it work.