Existing Column - Upgrade to current code required??

[strguy11]

I have a project where i am adding load to an existing concrete column built in 1950's. The gravity load is being increased by more than 5%. My question is based on an analysis, the column can withstand the imposed loading on the columns, however, the column ties do not meet the minimum size. (the existing plans list them as 1/4" dia, which would be a #2.) Current code requires a #3 minimum tie.

Question is do i have to bring the column up to current code and strengthen, or a minimum bar size not being met even though the design loads can be supported??

 

[JoshPlumSE]

My belief is that this would not require ay strengthening, but I'm not 100% sure.

I had a project where I had to replace some damaged elements. I gave a requirement for how to replace them on the plans. Then I identified all the other locations where similar (but undamaged) elements existed and suggested an optional strengthening (but not complete replacement) of those elements.

I just wanted the owner to know that these issues would likely occur elsewhere in the future. Ultimately, the owner strengthened those elements as well. If you're really concerned about those columns (especially if they're part of a seismic resisting frame) then you might provide a similar optional upgrade (steel or composite jackets?).

Note: in my case those upgraded elements still didn't quite meet current code. But, they were pretty close and way, way better than they were before.

 

[strguy11]

That is kind of my thought... I was able to locate a copy of the ACI 1947 code. Going with the theory that the ties in a column are to confine the reinforcing steel in compression, I went to the existing code and calculated the maximum gravity capacity of my column, and i am under it, so I think i could justify that when the column was built, it would allow for a higher load than i am putting on it, so Im confident im ok structurally, just not sure if i technically meet the code language.

 

[ProgrammingPE]

I think you need to justify that the smaller tie size does not decrease the compressive strength of the column. The compressive strength calculations assume that the longitudinal reinforcing is adequately restrained to prevent buckling of the bars, but that may not be the case here, so your capacity may actually be less than what you calculate.

What size longitudinal bars do you have? What's the tie configuration and spacing? How do these compare to the code minimums? Since a #3 tie works for a #10 longitudinal bar, you may be able to say that a #2 will also work for your size longitudinal bars if the spacing is comparable to the current code minimums.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/channel/UCOj2mXXf3ZbZtgxTc6BtkGg

 

[strguy11]

Programming PE

In my mind, I am justifying the use of the smaller tie by saying I'm under the compressive load that was allowed at the time. My rationale is that there could be other columns in the building that have the exact same tie configuration that was allowed in the code at that time. I have also looked at it using today's code and the allowable loads appear to be a little larger, so from a safety standpoint, I don't think there is a problem.

The longitudinal bars are #6. Ratioing the size of a #10 to a #6 you get .44/1.27 = 0.346 or 34.6%. Using this, you can say that the allowable load in the #6 bars is 34.6% of the #10. Since a #3 can confine a #10 longitudinal bar, it seems to reason that for a #6, I could have a tie that is 34.6% of a #3. (.11 * .34 = .037). A #2 can easily take that....