Detailing a RC Beam cast in two phases (years apart).

[Lomarandil]

This one is just off-the-wall enough that I'll feel more comfortable with some outside wisdom.

We're designing a building for phased construction. They're nearly complete with the second story now, and will return to add a third and fourth story once more funding is available (owner is on a shoestring budget). Concrete moment frame building, brick infill walls, tin roof, no seismic and really low wind. No prescribed building code, I default to ACI by habit but am open to others.

In the final configuration, it's a monolithic slab and beam floor system. (Two-way mild steel reinforced CIP)

In the meantime, we want to add two beams in the ceiling of the second story to be able to hide some water storage tanks. Not enough budget to build out the entire third floor slab right now. The rest of the second floor will be open to the tin roof above.

My thinking is to cast the bottom half of the beam now, with enough capacity to carry water tanks. In X years, the contractor can come back, roughen up the top surface, shore the beam and cast the rest with the future third floor slab. Thankfully, my final beam is 500mm deep, so I'm able to develop stirrups (smooth 8mm) on both sides of the interface.

A few questions:
[ol 1]
[li]If I cast in the future beam stirrups (with the top half protruding), they won't be engaging the current top bars (future mid bars) like a typical open stirrup does. In the immediate beam configuration, concrete capacity is adequate for shear (and no torsion). Later on, the t-beam will see torsion, so the stirrup will be closed around the future top bars. Is there any reason this isn't a good idea?[/li]
[li]Can I double dip on the required horizontal shear ties? For beams with sufficient concrete capacity to carry shear, I count provided torsion steel toward the minimum shear steel requirement. My reading of Ch. 17 makes me think the same is also valid here. Thoughts?[/li]
[li]Bonus: would the above question be different if using shear friction provisions?[/li]
[li]Does anyone know of a good corrosion protection solution that is easy enough to remove that I can be confident the future contractor will do so?[/li]
[li]Anything else that comes to mind I should be considering?[/li]
[/ol]


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just call me Lo.

 

[Settingsun]

Build the whole beam now and drill in slab bars later?

Is it common for wherever this is for people to do the later stages? IME people who need to save a thimbleful of concrete in Stage 1 never manage to find the money for Stage 2, so your half beam could be that way for many years to come.

Re corrosion, what environment? Zinc paint (cold galvanising) goes ok in fairly benign environments and doesn't need to be removed.

 

[KootK]

My votes:

1) generally rational.

2) double dipping approved.

3) same for shear friction.

4) would a low strength comcrete, second pour NOW be too fancy?

5) how are you loading the fist pour beams without inducing torsion or damaging the stirrups?

 

[Agent666]

A few thoughts:-

What about making your support for the tanks out of some simple temporary steel beams supported from your columns, and just build the entire concrete beam later on. Can't help but think adding a few steel beams will be cheaper than all the carry on of the insitu concrete, false work, form work, etc. Your stirrups are also subject to damage if left poked out of the concrete for years, especially during removal or demolition of the roof in a few years.

Whether beams build now or later, certainly building half and half is probably more expensive in the long run. Building all of it now is going to be cheaper than building all of it in the future with escalation of construction costs and all that.

If you do the whole beam I'd prefer to simply leave reinforcing couplers (if they exist in your market) for carrying on the columns and also slab reinforcing. Make it as simple as possible to build off in the future.

 

[Lomarandil]

Thanks guys -- I think the zinc paint will be a great solution here. Just humid, no other factors driving corrosion.

Steve -- I agree, I think it's likely that the future stage is never built. That's actually the motivation for getting the water tanks out of the way now.

Agent -- unfortunately no couplers here, and rolled steel is extraordinarily expensive (all imported).

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just call me Lo.