Column Rebar Configuration

[amain002]

I have a question about rebar configuration for column which I am not familiar too so hope you guys could help me with this.
This is for a building with 1st floor SOG, 2nd floor slab, 3rd floor transfer slab and wood framing above.

Typically for concrete column, our detail shows rebar from the top of footing to the second floor (extended above slab for as required for lap) and from 2nd to 3rd floor slab. Now someone is asking that is it OK to run the column rebar from top of footing directly to 3rd floor slab without breaking it at each floor level.

I personally have only seen seen doing it the conventional way with rebar from floor level to floor level. Has anyone designed their columns in such a way and if so is there any codes regarding using such method.

Please share your ideas and if you recommend or do not recommend doing it so. Other than workability, are there any issues. Thank you for your help in advance.

 

[KootK]

I see only two issues:

1) Contructability, which you mentioned. All slab and beam bars will need to be threaded through the column cage.

2) Safety. Gotta keep that cage from buckling and crushing our builder friends.

From a structural performance perspective, the full height bars are probably an improvement. Can you share why your contractor wants to do this? It's an odd request to be sure.



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.

 

[amain002]

Thank you KootK,

Well he did not give us a reason why but I am guessing probably because they can save a lot of steel from eliminating lap. Also, save labor for bending steel which will probably add a lot. Well that all I can think of.

 

[KootK]

You're quite welcome. I'll be curious to see if we get any dissenting opinions. Where is the project located? Must be someplace where materials are expensive relative to other costs. I usually estimate laps to add abou 15% to rebar costs.

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]

I agree with Koot on all accounts. Structurally it would be superior. But all contractors I know would be asking to add a splice instead of remove one. It's also going to be tough to pour the entire thing in one go and ensure you have proper consolidation at the base. You're talking a best case 20ft tall piece of form work. That's a tall pour but not unheard of, depending on the column dimensions it might be tight in there.

I guess you could only pour up to each floor level but then you're left with a 10ft cage sticking up swaying while you form and pour the second floor slab.

 

[Brad805]

Did the question come from an experienced rod buster? The savings will be offset by the additional work to support the bar during pour #1 and #2. The concrete placers will not like it either if they are using a pump truck. What a nuisance. Structurally, I do not see a problem as it does satisfy your requirements if they do as Koot suggests.

 

[amain002]

Thanks everyone for you opinions.

Well Brad805, it seems they have been in business for a while and I suppose they have used this method earlier as well.

The Project is located in Northern VA but honestly I do not have an idea of the cost around here compared to other places.

I do believe that structurally it is a better option but as noted by you all there are some constructability and safety factors with this method.

So we all agree that in terms of structural stand point we do not have any issues with it correct?

 

[Brad805]

I would have guessed the question came from a bean counter and not someone like Harris Rebar. I would not have a problem with it. OSHA might have a question during slab pour #1 if they do not cable the cage to anything, but that is there problem.

 

[manstrom]

Sounds like someone has a dumb bright idea to save some money

If they are concerned about rebar splices, use mechanical connectors. You can't run a column cage 2 stories and do two pours. The concrete will never consolidate and the cage would need to be braced while the lower slab is poured.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Lion06]

From a structural performance perspective it's better, as Koot notes. Isn't the decision to do it or not Means and Methods for the contractor to decide?

 

[hokie66]

I have seen this done several times, and other than supporting the tall cages before the second floor is placed, there is no issue. Placement of the floor reinforcement is not different. After the second floor is cast, the column cage support is better than with splices. I don't understand manstrom's concern about consolidation, as only one lift is cast at a time. You might have to temporarily move a tie or two to gain access for the pump hose and vibrator, but that is no big deal.

 

[asixth]

It's commonplace to do the column cages two levels at a time in Jakarta. It's all about material costs there and keeping the material quantities to an absolute minimum. Safety legislations for worker's is a bit lax though.

 

[TXStructural]

Continuous rebar cages are very common. They must be guyed properly at all times to prevent them from toppling or bending.

The industry worked with CalTrans and the University of Nevada Reno to complete research and prepare a report on safe erection of rebar cages.

http://www.dot.ca.gov/newtech/resea.../2010-11_task_1098-geotech_and_structures.pdf

 

[MrHershey]

We've seen this fairly often from contractors and haven't had an issue. Better from a structural standpoint.

It cuts out quite a bit of rebar. If for whatever reason you're using tension laps and large bars (say #8 or #9), then your lap splice using the simple development formula (not reducing for cover/spacing, etc.) is going to be 5'-6' for 4 ksi concrete anyways. If you've got a floor to floor of 10', that's like 50% more bar just due to splice if you splice each floor because each bar has to be 15'-16' foot long.

Over two floors, that means you've got 30'-32' foot of bars. If you only splice once in that period (as opposed to twice) it's more like 25'-26' instead. Still fits on a truck and in a container. We usually see the cages stabilized with internal bracing once a floor or so, they're still relatively stout, especially after you pour the first half. But material cost is 10-20% less. Extend that over a whole building and it's a decent amount of steel/cash.