Column Reinforcing at Lap Splice

[StructuralJoe]

We have a condition where the our columns are reinforced at over 4.5% at the pile cap to base of column location and therefore at our lap splices we exceed 9% area of steel. The columns are already extended from the top of the pile cap to the underside of the 2nd floor slab. We can adjust the floors up the 16 story building and stay below the 8% requirement at all future locations, but at the base of the columns the area is exceeded and the columns is already poured.

Is there any literature or experience dealing with this? Does the column need to be removed and replaced? Is there a retrofit or repair that can be used to correct this issue?

My initial thought is it will be fine as it is a gravity column but I hesitate to say that without evidence or experience with this issue?

Thanks

 

[phamENG]

Take a look at the commentary for the 9% max requirement. I don't have it in front of me, but I believe the primary concern is consolidation of the concrete with so much interference from the bars. Unless you have photos showing you tighter performance, I'd draw the section assuming sloppy workmanship (maximum placement tolerances) and make sure your bar spacing is good with your aggregate size. Then, I'd probably have the column scanned to verify there are no voids. I'm not overly familiar with this, so you may want to speak to your testing lab about their GPR capabilities. If you feel confident that the aggregate can fit in the joint and you have data showing that the concrete mix consolidated, then I'd say you're well on your way to making an informed judgement call.

Hopefully others will chime in with other concerns I may have missed.

 

[r13]

Can you stagger the splices, or use couplers, to minimize the congestion?

 

[phamENG]

retired - he mentioned that this is already cast. He's trying to determine if there's a way to justify acceptance or if he has to have it torn out, redesigned, and reinstalled.

 

[StructuralJoe]

Thanks phamENG, I reviewed ACI-318-11 (the code that the building was engineered to) and it doesn't mention anything beyond 8%? Which code were you referencing? I agree that as long as there isn't any honeycombing or void of the concrete around the exterior of the column then the columns should be fine. Hopefully someone else has experience with this?

 

[phamENG]

Sorry - I read 9% in your original post and latched onto it. You are correct - the max is 0.08A[sub]g[/sub]

 

[StructuralJoe]

Unfortunately the only article referenced in commentary (of ACI 318-11) speaks to the older report of maximum 5% from not 8% or higher (Reinforced Concrete Column Investigation - Tentative Final Report of Committee 105 -from 1931)

 

[StructuralJoe]

Understood, thanks phamENG

 

[r13]

My initial thought is it will be fine as it is a gravity column

If it indeed is a "gravity column", provides there is adequate spacing between splices, and well consolidated, I don't see much to worry about for the 12.5% over reinforcing, since the ductility concern is minimal. But, maybe you should make sure there is no honeycomb present at the splice level. Try talk to a testing lab, see what they suggest.

The quote below said well.

Values of maximum reinforcement ratio is specified differently in different countries. This is to ensure enough ductility in the column. Although higher amount of reinforcement increases the moment capacity, it can result in lower curvatures and deflections at failure. One needs to make sure that we receive enough warning before the structure fails and avoid brittle failure. Also, a practical aspect is that you can only place a limited amount of reinforcement till you run out of space to successfully cast concrete.

 

[KootK]

If it's already poured, I'd be inclined not to worry about this if the concrete looks decent from the outside. Column starter dowels will be inboard of the column vertical, pseudo radially, rather than placed side by side. I feel that this makes for a more favorable condition than would be the case if the bars were side by side.

I do, however, feel that dismissing this because it's a "gravity column" is erroneous. In a multi-story building ground floor column with that much reinforcing, it's a damn safe bet that reinforcing was put there to deal with high compression loads rather than flexure. As such, the primary job of the column reinforcing may well be to satisfy compression demand that cannot be transferred via the concrete alone.

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