Existing Concrete Columns - Confinement Ties

[abusementpark]

When evaluating the capacity of an existing reinforced concrete column, what do you do if the stirrups/ties provided in the member do not meet the minimum requirements per modern code? Is that reason enough to rehab the column if the member's capacity is otherwise adequate?

 

[BAretired]

You bet. If you don't do something to support them, the vertical bars can buckle between ties, spalling off the concrete cover.

BA

 

[hokie66]

Is the capacity adequate without the unconfined vertical bars? If so, the logic of acceptance may be just to treat those bars as aggregate.

 

[shin25]

If the lateral ties are truly needed for strength, then I may recommend to retrofit the columns with FRP wrap, etc to provide concrete confinement.

 

[BruceSPT]

Could you explain further about why the confinement ties don't meet modern codes? Is it spacing? Or is it tie bar size? Or is it both?

Code confinement ties for columns tend to be a rather arbitrary conservative choice. ACI 318 notes that "Limited tests on full-size, axially-loaded, tied columns containing full-length bars (without splices) showed no
appreciable difference between ultimate strengths of columns with full tie requirements and no ties at all". See reference below.

J.E. Pfister, "Influence of Ties on the Behavior of Reinforced Concrete Columns" ACI Journal, V. 61, No. 5, May 1964, pp. 521-537.

I would consider whether the column was part of the gravity load-resisting system or part of a lateral load-resisting system. If it was for earthquakes, then I'd definitely wrap it. Also, consider the capacity ratios for the column to determine how heavily loaded it is.

Then the non-structual part: (1) if the column ties need to be brought up to modern Code what about other beams and slabs, typically newer Codes added detailing requirements for structural integrity to improve the redundacy and ductility in structures that older, existing structures usually won't meet, (2)what is your responsibility to meet the modern Codes, and your duty to the client, and finally (3) what is your liability.

 

[BAretired]

ACI 318 notes that "Limited tests on full-size, axially-loaded, tied columns containing full-length bars (without splices) showed no appreciable difference between ultimate strengths of columns with full tie requirements and no ties at all".

This is contrary to literature I have read on the subject. See conclusions in the following link:

http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDQQFjAA&url=http://

http://www.utoronto.ca/sheikh/Sheik...fhq09vmzN03sMNg&bvm=bv.46340616,d.cGE&cad=rja

BA

 

[VoyageofDiscovery]

BruceSPT,

Code is Code! ACI is not a Code by itself, however if it is referenced in your Building Code then it becomes a legal reference while complying with Building Code. Yes, many structures obviously will not meet current Code, however if you are making mods, extensions to buildings then it is incumbent on you to meet Code. In seismic areas, seismic requirements will override much of the existing designs in any case, and as such this is the perfect opportunity to bring it up to Code as, in my opinion, you will be fully liable if you touch the structure(the original engineer may have passed away already!) So as a building engineer, be aware of the age of structure, type of construction, condition and the likely issues you will encounter prior to engaging with the Owner.

VoD