Concrete column strengthening

[Stormin74]

We designed the concrete column for our new 14 stroies apartment building,the first two floor are parking garage. couple of the columns were designed 8ksi but the they were built by 5ksi concrete. We tried to use FRP Composites to strengthen the column but those columns are size 24inches by 12 inches because of the limit space of parking lots. From ACI 440-- the high aspect ratio of the column sections (B/H) of 3.0 or higher exceed the limit allowed by ACI 440 for FRP confinement so the FRP may not be effective in confining the columns.
Except the approach for strengthening columns with lower strength is by enlarging them, which is hard for the parking space, could any other solution work in this situation?

 

[shin25]

That's surely a gross mistake on the part of the contractor! Why don't you ask him to come up with a repair with his own engineer's stamp.

 

[csd72]

Get them to do-in-situ testing, maybe the actual strength of the concrete is more like 8ksi anyhow.

csd

 

[civilperson]

Wrap the columns in 50 ksi steel, with embeds at appropriate intervals to assure composite action. 32 square inches of steel around your column will double its gross capacity and a few braces to keep the unbraced length small will allow use of the majority of available strength.

 

[shin25]

civilperson,

I am not sure, how placing steel jacket will enhance the capacity of the column. All the dead load is already on the column. For the steel jacket to be effective, it needs to be secured with the column in the absence of the dead load. He probably can choose to jack the floors above while wraping the steel jacket.

 

[civilperson]

The embeds will allow the steel jacket to share the load with the concrete and by false work and jacking, the dead load can be remove from the column under repair. If no jacking is done, the jacket will contribute to the additional load after dead load and will help with dead load as the concrete creeps ever shorter. If an over size jacket is used, the annular space can be filled with expansive grout to assure composite action.

 

[hokie66]

I agree with civilperson. You are concerned with the ultimate capacity of the column, and the external reinforcement will achieve that. You need to investigate the fire rating of the new column design.

 

[UcfSE]

If you're in a seismic area, the steel jacket will add stiffness to your column, attracting more load. You could have a problem with that, especially by attracting more shear. Be careful if you go that route.

I would recommend testing the in-place columns, and replace them if they are not correct.

 

[Zambo]

what is 8ksi? are we talking 50N/mm2 (or 50MPa) against designed 80N/mm2 (80MPa)

If so 80N/mm2 concrete is not so easy to achieve, the R&D is certainly not worth it for a couple of columns 600 x 300mm.

If this is only 2 columns each 2 storeys high why not strut and then replace with concrete encased steel columns. You would need shear studs into the foor slab.

 

[csd72]

Zambo,

8ksi = 55N/mm^2
5ksi = 35N/mm^2

Both are fairly readily available these days.

csd