Repair of steel W shape column with corrosion near base

[conradlovejoy]

I got thrown into the fray of an RFI fire drill where the contractor working on a renovation project found an existing steel column displaying corrosion near the base. The bottom of the column is still encased in concrete/topping and not exposed. It appears that the corrosion occurs up to around a foot from the top of concrete and it is unclear how far it occurs below the concrete (I am not even sure how far down the base is below this concrete) and if there is also corrosion at the column anchorage. The Contractor sent some photos but its difficult to tell the degree of damage and impossible to tell if there is significant section loss from review of the photos. I asked a few questions along those lines to try to get him to investigate and get more information but before he could answer one of my company owners (who is an architect) chimed in with a detail showing two angles welded to the web of the column and epoxy bolted to the concrete topping. I can't put my finger on it, but this doesn't seem adequate to me. Assuming the angles (or likely a bent plate since standard angles aren't made that large) could pass the damaged area and be welded to the web, would they then act to transfer all (most) of the column's axial load in bearing to the concrete topping slab? Even if that were the case I still have too many variables in my head. Will that crush the topping (I don't even know what it is)? Will the angles behave properly with that sort of loading condition (I really don't know how to analyze that condition)? What if the web is so damaged that the flanges are now struggling to support all of the load and the repair to the web only doesn't alleviate the new load path created by the damage? I want to say that I don't feel this is adequate but I want to tread carefully around tossing out my company Owner's recommendation as poor, and especially if it is an adequate solution to the problem and I am over-reacting.

I have seen some folks on here talking about steel corrosion repair and there seems to be a good amount of knowledge and experience on the subject so I'd like to get some of that here as this is my first brush with the issue.

 

[straub46]

The repair is going to be dictated by what is damaged. Is it just the web? Just the flanges? Both? You're post only indicates damage, but doesn't specify what is damaged. Is the column part of a moment frame, or strictly a gravity column? What kind of load are we talking about?

The angle idea could be feasible depending on what kind of column this is.

Go Bucks!

 

[canwesteng]

A picture is worth a thousand words

 

[hokie66]

It sounds like you need to expose the column base for assessment. A site visit is required. The structural engineer should be designing any repair, not an architect.

 

[CrabbyT]

I worked on a project that sounds very similar to your's. In my case, there was a small room inside of a building on the ground floor. The contractor temporarily removed one of the walls so that they could replace a piece of equipment, and they discovered two columns with heavy corrosion at their bases.

The webs were entirely rusted through on each column - from the inside-of-flange to inside-of-flange, there was no web. You could literally see from one side to the other. The flanges were rusty, but intact.

In the end, I issued a field instruction that went like this:

1. Temporarily support all the steel above with shoring posts.

I gave them a sketch showing the placement of the posts, and I designed the posts including the base plates. In my case, I spec'd HSS 6x6x5/16 tubes.​

2. Chip out the foundation pedestals down to the base plates.

3. Remove the bottom portions of each column, including the base plates and anchors, to a height of 4' above the floor.

4. Fabricate and install identical columns/base plates to the ones that will have been removed.

I gave them a sketch showing a splice detail. Check out the attached doc.​

5. Replace concrete.

I gave them some concrete specs.​

6. Remove shoring posts.

Looking back on it, I kind of wish I would have used a stub column w/ a bearing plate. Potato, potato I guess.

 

[Agent666]

Make sure you do an assessment of the loads against the potential loss of section. Even though its corroded, if the capacity is adequate still, then all that might be required is removal of the corrosion and reapplying some appropriate corrosion protection. Something like NOXYDE is pretty good at encapsulating and halting the corrosion with minimal effort to clean the section. You should remove the concrete to deal with any deterioration that extends into the concrete.

Usually this interface between embedded steel and concrete requires special attention and additional corrosion protection coatings both into and above the interface are required for good long term performance. This usually comprises a band of non-conductive paint (Inorganic Zinc Silicate is used in these parts). This is even true of galvanised steel, it also requires similar protection in this region. It's surprising how often you might see a contractor consider just embedding the steel with the normal primer and painting the top coats up to the interface (path of least work)!

 

[SKJ25POL]

conradlovejoy (Structural)(OP)
Could you please attach couple of pictures of the problem?

Thank you
Skj

 

[hokie66]

Often, the worst corrosion is of the nuts on the anchor bolts. So you have to get to them as part of your inspection.

Agent666, it is surprising to see that you use inorganic zinc in repairs of this type. Inorganic zinc requires impeccable surface preparation, and is normally used only in new work. Organic primers, including thick build mastic primers, are the typical choices in repair work.

 

[Agent666]

Yes I was referencing the requirements for careful consideration of this area in new work to avoid the potential of similar issues.

For existing, like I noted, I'd consider something like noxyde as the steel just usually needs a good waterblast and loose bits removed to encapsulate what's remaining.

 

[hokie66]

Fair enough. I'm not familiar with that product.