Column Retrofit

[ticas]

a building has columns with ties distance 100mm (4") apart with close ties. But in the construction in the ground floor columns, one of the hooks are not bent 135 degrees but just 90 degrees (it is corrected in second to fourth floor which is the maximum storey). What kind of column retrofit can make it perform equal to the hook bent 135 degrees. Can putting flat bars around the finished column distance 4" apart do the trick or even using pure metal plate and welding them at the sides?? Who has encountered this before?

 

[FishTheStructure]

You could consider wrapping the columns with a wet lay-up fiber reinforced polymer (FRP). This type of column reinforcing confines the concrete column in a similar manner as the column ties "confine" the inner concrete core.
In Canada, concrete confined FRP is covered under two codes: CAN/CSA S6-06, Canadian Highway Bridge Design Code and CAN/CSA S806-12, Design and Construction of Building Components with Fibre-Reinforced Polymers.
In the United States, concrete confined FRP is covered under ACI440.2R-08, Design and Construction of Externally Bonded FRP Systems.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[ticas]

how much would this add to the column thickness in your experience? I need the thinnest one available.. this is just to compensate for column tie hooks bent 90 degrees and 135 degrees and not both 135 degrees.

 

[BAretired]

Column ties spaced at 100mm is very unusual. Why does the column require ties so close together?

Maybe you could add 'L' shaped bars at the hooked corner and weld to the ties in place.



BA

 

[FishTheStructure]

The thickness of the FRP jacket required will vary with the specific column requirements. A typical ply of FRP is nominally 1 mm thick and the jacket may require 2-6 plies. This should add less than half an inch to the column diameter.
FRP is commercially available from several companies. In my M.A.Sc. experimental work I used several products from a company called Fyfe and had a good experience. They have engineers on staff and offer free design support.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[ticas]

The column is 3 meters height. The first 1 meter has close ties 100mm apart, while the 2nd 1 meter has ties 150mm apart, and the upper last 1 meter is again 100mm apart. It is for maximum base shear resistance.

But how do you weld the L shape in place when the column already has concrete poured already. Therefore it has to be outside, maybe flat angle bar wrapped around the column at a distance of 12" each would be sufficient to make up for one of the hook ends bent 90 degrees instead of 135 degrees?

 

[hokie66]

Before you go changing something that is already built, perhaps you can advise us more details of the design requirement. What code are you using, and what provision of that code requires the ties to have double 135 degree hooks at that spacing? Is this strictly a column provision or is it because your building is a moment resisting frame? Or is it a specific earthquake engineering detailing requirement?

 

[ticas]

I read in United States provisions on Special Moment Frames that column ties need to be closed at 135 degrees inside. Also the 100mm ties distance is to be safe for maximum seismic forces. I just compute for the column steel reinforcements required for axial loads and moments but didn't really compute for the tie distance. Why, is there provisions that for closely spaced ties, one of the hook ends can be 90 degrees and one can be 135 degrees? Where did you read that provision?

 

[hokie66]

Now that you have revealed that it is a Special Moment Frame, I am not surprised that the ties have to have 135 degree hooks.

 

[ticas]

any idea how strong an earthquake before the 90 degree hook comes off? any paper or study about this? I wonder if putting flat bars wrapped and welded around the already concreted columns can offer the same effect as fully 135 degrees hooks.

 

[UcfSE]

I wouldn't expect flat bars to do much good. You could try wrapping with a steel jacket (round), but that would not only increase your column dimensions but it would also increase your column stiffness and attract more load. I would go with the frp wrap solution instead. It can provide confinement without sigficantly increasing column stiffness.

 

[ticas]

I noticed the 135 and 90 degrees hook ends are put alternatively in opposite sides of the columns so that in each 100mm distance there is alternating pure bar bent and the hooks so I guess this is safe. The hooks are not on the same sides. I guess it is standard practice not to put the hooks on safe sides?