Existing concrete column with undersized ties

[mmodfr2013]

I'm looking at an existing concrete structure, the owner is interested in adding some additional floors. The original structure dates to the late 60's.

-The columns are reinforced with #11 vertical bars and #3 ties, which was OK under ACI 318-63.
-Current ACI code requires #11 vertical bars to have a #4 tie, minimum.
-We are adding enough additinoal vertical load to the columns that some sort of work will be required to strengthen the columns, just based on strength requirements.
-Project constraints limit the amount of width we can add to the column to 4" total in one direction (i.e., we can add 2" to the left and right side), we have more leeway on the top and bottom.


What are my options to reinforce the column to meet the minimum tie requirement, short of adding concrete to all four sides and a #4 tie? I don't really have room to add vertical bars on teh left and right side with the 2" constraint? I suppose FRP is an option, but I don't have the experience to give a cost comparison on that solution.

 

[jdgengineer]

To me this sounds like a good application of FRP. You can reach out to Simpson Strong-Tie who is moving into the FRP market. They will provide the necessary calculations and assist with details. I'd chat with them and get their take on it. They provide this for free when you spec their product.

 

[KootK]

You could neglect the rebar in the faces of the column where you've only got 2" to play with and make up the loss with additional rebar elsewhere.

I know that FRP can be used to confine concrete in square columns but I'm not sure that it can restrain rebar buckling away from the corners.

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.

 

[MotorCity]

May not be an option from an aesthetics perspective, but how about a steel jacket/sleeve around the column?

 

[KootK]

Do you know the existing tie arrangement? If so, you could make a good argument for only worrying about the bars that fall between points of direct tie restraint.

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.

 

[mmodfr2013]

We have existing drawings that give me a bar/tie layout, but when the current code says the tie has to be a #4 I'm not sure how well I could argue that a #3 is sufficient.

 

[KootK]

but when the current code says the tie has to be a #4 I'm not sure how well I could argue that a #3 is sufficient.

Here's how I'd argue it.

The ties are there to prevent vertical bar buckling. And the tie size is governed by the ties acting as girts spanning horizontally between points of discrete lateral restraint (corners & cross ties). As I see it, you've got four cases:

1) Any new bars will be restrained by #4 ties so those are fine.

2) Any old bars on the faces adjacent to the new bars will be restrained by the new #4 ties and a bunch of additional concrete. If we're worried about these, halve the new tie spacing. These bars are fine.

3) Any old bars on the non-adjacent faces that occur at points of discrete lateral restraint (corners & cross ties) are deemed fine as tie stiffness and strength there will be grossly more than at the #4 condition described next.

4) Any old bars on the non-adjacent faces that occur between points of discrete lateral restraint will be discounted and assumed to have buckled at ultimate.

Talk it through with your AHJ early on to ensure that they're on board. For sport, throw in some rational calculations showing that you expect the #3 ties to provide enough strength and stiffness to produce wave-form buckling of the vertical bars between ties. Maybe they actually will buckle out of the column at limit state.

It's tempting to add enough new rebar that the axial forces in the old rebar would never rise above those appropriate for the smaller ties. While I bet one could convince the AHJ of that, I'd never truly believe it myself. With creep, shrinkage, and a hybrid new/existing condition, it would be tough to assess the stress in any of the bars with any accuracy. Better to assume that they all yield I think.

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.

 

[MrHershey]

We've used FRP to restrain bar buckling away from corners when contractor put the column splice transition at the wrong location but the ties restraining the 1:6 sloped bars at the correct location, leaving our 1:6 bar transition without the code-required restraint. The solution was to drill clear through the column near each vertical we needed restrained, bunch up the FRP, thread it through, then anchor/bond on each face to the FRP wrap. We sized FRP for the magnitude of the outward thrust, but don't see why you couldn't size it to match the tensile strength of a #4 bar instead as the concept is basically the same.

 

[KootK]

The solution was to drill clear through the column near each vertical we needed restrained, bunch up the FRP, thread it through, then anchor/bond on each face to the FRP wrap.

Fascinating. I agree with the logic but that's certainly quite a different animal from a straight FRP wrap. you're basically making FRP cross ties. In this case, one would need to take care to match not just the strength of the #4 bars but, also, the stiffness.

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.

 

[hokie66]

With FRP on the outside of the concrete, how do you achieve the required fire rating? Just assume the unrestrained concrete is good enough for fire?

 

[mmodfr2013]

Yes, it's still early on but it looks like that's the plan. I've gotten in touch with Simpson and I need to check some capacity equations from ACI 562, and see where I sit

 

[MrHershey]

hokie-

The fibers themselves actually do great in fires, their temperature resistance is a lot better than steel. It's the epoxy that's the problem, it just melts. Same thing to keep in mind with adhesive anchors.

I know at lease Fyfe (one major US FRP manufacturer) has a few different fire resistance materials/coatings that could go over the top, link. At least a couple of them even have UL potential up to 4 hours. They're expensive so often preferable to somehow justify going without, but it's at least possible. Think one of their systems (don't know if UL rated or not) is really thin, too. Like 1/32" to 1/16". Doesn't seem like that would offer much resistance but I'm no fire protection engineer.

 

[dik]

Have you checked the design to current code? and with changes to limit states design, can the #11 bars be considered as #10? If so, can you approach the AhJ and see if it is OK to assume the bar change?

Dik

 

[KootK]

it just melts. Same thing to keep in mind with adhesive anchors.

That's an interesting point. It makes me wonder about the fire safety of the new ties which will surely be adhesive set into the existing column.

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.

 

[Ingenuity]

The attached file may be of relevance, RILEM paper entitled: FRP CONFINEMENT OF WALL-LIKE REINFORCED CONCRETE COLUMNS

 

[msquared48]

Lotsa labor, but if you were to chip off the current cover, add your extra vertical and tie bars and replace the cover, you could keep to your increase limits and maintain the fire rating.

The only problem I see with this other than the labor is that you could only add new vertical steel at the corners of the column since adding cross ties is not possible.

Mike McCann, PE, SE (WA)