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Re: Steel Jacketing of RC columns

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Serroels, Chris/SAC wrote:
> I am also not a corrosion expert so I can't unfortunately
> offer a suggestion as to the appropriateness of the unpainted
> jacket.  However, in general what I have seen and read
> with respect to bridge columns (can't imagine why it wouldn't
> also apply to buildings) is that rectangular jackets
> are very hard to make effective without through-bolting
> or external stiffeners.  Otherwise the jacket will not provide any
> significant confining pressure until the concrete has dilated.
> At this point, lap splices will have surely degraded - probably
> failed.
> If I interpret the description of your jacketing concept
> correctly, it seems that you will have gaps in the jacket
> along the sides of the column.  I would be leery of this
> as it will provide space for spalled concrete to escape,
> subsequently reducing the ability of the jacket to provide
> a confining pressure.
> In general, Caltrans has avoided rectangular jackets.  They
> typically install elliptical jackets on rectangular columns.
> What is typically done on building retrofits?  Are rectangular
> jackets common?  If so, what is the anticipated performance
> of a rectangular jacket - how is it justified.
> Chris Serroels, PE

Well the main problems are:

1. As I mentionned earlier, I'm in Lebanon, Middle East, where FRP is
not yet available unless something shows up in the Construction Fair due
in a couple of days !
2. The renovation of the project has already started based on
Architectural drawings that changed the layout of the internal
partitions only. Since there were no plans to remove or modify
structural elements, I was not consulted. But after tearing down the old
infill masonry walls, the columns turned out to be smaller than shown on
the drawings with very little reinforcement, especially hoop wise. So
when I saw these columns I suggested they wait a while so I could come
up with some improvement plan. The structural elements were built
without any consideration for seismic requirements since it's not a
legal requirement down here.
3. Since renovation work is under way, I can't stop the work completely,
I can just slow it down for a while, and I can't modify the
architectural plans.

So since FRPs are not available, concrete jacketing is not possible for
the column section would have to be enlarged by at least 4" each face,
replacing the whole column would take too long and require shoring, I am
left with 4 choices:
1. Do nothing.
2. Jacket the column with a full length solid steel jacket then grout
the gap.
3. Jacket the column with steel plates at d/2 or less intervals welded
on the vertical corner angles.
4. Think of something I haven't thought of yet.

Option 1 is not desirable for I might regret it some day.

Option 2 is expensive, grouting can be hard to do for the top part of
the column, below the existing slab, and protecting the steel would be
even harder than for option 3.

Option 3 seems the most reasonable because it's cheaper, faster, and the
plaster coats would bond better to the columns since most of the
existing concrete would still be exposed providing a better bond than
steel. Now maybe I can use wedges between the concrete and the
horizontal steel plates to create an initial stress and preserve that by
dry packing an expansive cement mortar between the concrete and the
steel jacket. I expect (or hope) the jacketing system to provide a
confinement somehow equivalent to that of internal hoops.
Now I understand this is not the best strengthening scheme for all the
reasons all of you guys mentioned, but if it increases the ductility of
the column by say 15 to 20% it might be worth trying.

Option 4 is ongoing but it probably won't catch up with the pace of the
renovation work.

Thanks again for all your input,

Moni Serhal