Steel Column Retrofit 5

[sigma1]

We have an existing steel column W12x106 that has lost about 3/8" to 1/2" of its flange thickness (tf=1"). The column is loaded. Any recommendations on how to reinforce it without shoring will be appreciate it.

 

[MarcbSE]

Why not just weld reinforcing plates to the flanges? Using intermittent stitch welds shouldn't heat up the flange too much if given a chance to cool a bit between welds.

 

[DaveAtkins]

But without shoring wouldn't the new plates only take live load? The dead load would be completely taken by the old, corroded section.

DaveAtkins

 

[msquared48]

Beyond the repair, I would be thinking as to what I will do to prevent the section from deteriorating further once it is repaired. What is causing the corrosion?

Can the section be encased in concrete, i.e., turned into a concrete column with the proper vertical reinforcing and ties. If this new column is designed to take all the vertical load, it is a moot point what load the existing steel column takes.

Mike McCann
McCann Engineering

 

[MarcbSE]

Good point. DaveAtkins is correct that the dead load would still be taken by the remaining original section.

You would need to verify that the reduced section is adequate for the applied load. I would also want to make sure that the corrosion does not continue.

 

[dik]

That's the marvel of limit states design... once the old section yields, the new material takes up the load... If it's a small area affected, then stability issues disappear also...

Dik

 

[sigma1]

Thanks to everyone.

The column was encased in plane concrete and the cracked concrete is what revealed the problem. Creating a concrete column/composite column is not an option due to space restrictions. We do not know much about what is supported by this column. We do know that the column is pin connected at top and bottom with sideway inhibited. Therefore, K=1 on both directions and rx/ry=1.76.

It appears that the slab on grade was placed directly against the face of the column and this is where the problem started. It appears that de-icing salt and water may be the the cause. Stoping it may not be that easy. Any recommendations?

 

[msquared48]

Once metal is exposed to salt, it's just a matter of time before the deterioration is complete. This must have been going on for some time for the flanges to be reduced by 50%.

Is there no way to steel jacket the column since it is already encased in concrete? From your post, I assume that the concrete encasement is unreinforced?

What about pressure epoxy grouting any cracks/voids in the concrete, then steel jacketing it?

Mike McCann
McCann Engineering

 

[MarcbSE]

It sounds like the previous plain concrete wrap was removed to free up space, thus exposing the deterioration.

Perhaps reinforcing the column with new steel plates and then a good coat of a corrosion inhibiting paint. Beyond that it becomes a maintenance issue for the owner to make sure the corrosion does not continue.

 

[shin25]

before you propose a repair, if you are not stopping the root cause of the problem then probably the repair method will not work.

First thing, you need to figure out if the corrosion is going to continue even after the repair. If it does, your repair will eventually fail.

Secondly, you cannot perform a repair on a rusted surface. You have to remove the rust before you do the repair.

If you are not able to use a steel jacket as msquared is sugegsting, then probably you can consider shotcrete. But, before you consider all these you have to find out the root cause of the problem and stop it.

 

[MarcbSE]

Absolutely shin25! No point fixing the result if you haven't stopped the cause.

 

[sigma1]

msquared48 & shin25
Steel jacket is one possible option. The un-reinfroced concrete wrap has been removed (up to about 3 ft above the base plate) and will be replaced.

MarcbSE
Flange reinforcment with steel plates welded on the inside flange is something we are leaning towards. Obviously the rust will be removed and corrosion inhibitor will be applied. However, I am not convinced that this will stop it from happening again in the future.

shin25
Not sure what cuased the problem. Because the deterioration is at the slab level and because it is a garage, I asumed that the de-icing salts and surface water had something to do with it. ??

Once again. Thanks everyone

 

[shin25]

sigma1,

Besides steel jacket, you probably can also consider FRP (fiber reinforced polymer) jackets. These are extremely high strength, flexible and durable in corrosive environment.

 

[Gumpmaster]

Sigma 1,

Is cathodic protection or an active corrosion protection system a possibility. Either of those would probably be something you'd want a good corrosion engineer to look at.

 

[msquared48]

Any way to find out what the connection detail at the top of the concrete portion of the column looks like? If you can find out, you may answer your question as to where the leak is coming from. If the steel is exposed above the concrete, the rust could have started there, and, as we know, when steel rusts, itexpands. Put that with freezing weather, water and more salt, you have quite a recipie...

Mike McCann
McCann Engineering