Column strength if some of vertical bars not present and column ties cut in one face

[ajk1]

Is there any way to determine the strength of a reinforced concrete column if some of the vertical bars in one face have been removed and some of the column ties have been cut? This is the condition we found when we chipped out the hollow sounding delaminated faces. We suspect that it was built this way 40 years ago, possibly because they got the reinforcing cage too close to the surface, so they fixed the exposed bar by cutting them out, we suspect!

Column analysis is based on the concrete being confined by the vertical bars and the ties.

 

[CELinOttawa]

Well this is a situation I handle fairly often - You're always surprised by what you find in old structures, or deteriorated structures, once you start chipping.

Without sarcasm: First principles and older codes. Once the requirements of a current code are breached, you cannot apply that code.

In the case of older columns missing bars (through neglect, odd detailing, or severe rust) I typically prop if nervous and analyze for the post-repair detail only.

You should not leave this condition as found; You should be looking to add the required confinement in order to be able to have some ductility and prevent brittle fracture as the principle column failure mode. As for the corner bars, other than the increased compressive capacity (if utilised), they are there for some bending/flexural strength.

Unless you are going to model to the nines you don't know the real behaviour of the structure. Personally I don't believe the models give you the true behaviour either. Corner bars and ties at d/2 max spacing produce a time tested and robust column. Restore the column to a standard detail and set aside the determination of what strength you might otherwise have.

 

[ajk1]

to CEL - Although nothing is impossible, I feel that it is almost impossible to repair to original intended design given the conditions. But I would be interested in what idea you have for repair. Please see the attached sketch.

Note that there are integral walls which help the capacity somewhat as some of the load can be transferred to them, and there is virtually no moment at the 1st floor.

The column has been like this for 40 years we believe, with no signs of distress. The column, based on Code formula for reinforced tied concrete columns has lots of capacity, even with 3 of the 8 vertical bars missing, but of course the Code formula (as any software ) assumes that the column concrete is confined.

Perhaps the column capacity could be checked based on plain unreinforced concrete....

 

[jayrod12]

That would be the first step I took after noticing this. See how it calcs out assuming plain concrete. If it works then you've got nothing to worry about.

If it doesn't work and you have the space you could pour a 3" concrete collar with new ties either full height or at least where you know you need them.

 

[ajk1]

Jayrod - Based on 3000 psi plain concrete (specified on the 1974 drawings) it is 17% overstressed, but after 40 years the concrete strength would have increased by 15%, so is ok. Bickley told me a long time ago that could generally rely on at least 15% increase in strength after many years.

If I try to add ties, they do not confine the concrete unless I also add verticals. And how do I lap the verticals with the existing unless remove the concrete cover to the verticals in the column above, and also need to drill in dowels to the footing below. And not that easy to anchor the ties either. May end up doing more harm than good I think. One thing going for us is that there were 2 floors of library in the original design, but the 2nd floor has always been just offices, and the one floor of library is just low shelves for children and not really loaded to 150 psf, although anything on main floor open to the public does require a minimum of 100 psf design live load.

 

[jayrod12]

At 17% overstressed you are close to what you need. Depending on the situation and how tightly you were finessing your loads down that may be acceptable. It's a judgement call at that point.

 

[CELinOttawa]

I agree that this is a judgement call, but having always worked in moderate to high seismic zones I've never let this sort of thing slide.

In terms of lapping on the new verticals, this is far easier than most people think. New bars directly on the roughened face of the column and double U-staple ties work thanks to gap-lapped formulae. Effectively your LD increases by the gap between the bars being lapped. Spray-crete I your friend and a good nozzleman can produce a very fine product without formwork. Job done.