Rust on Steel Joists

[McQSE]

I am involved in a renovation project where a lot of the existing bar joists have some rust on them. The roof has a lot of leaks. It appears to mainly be surface rust and not to the point where it looks like the joists have been weakened. We are going to be replacing some but would like to try to keep as many as possible. What are my options with the rust? I am thinking about blasting in order to clean the rust off. It seems that would be cheaper than replacing a joist because it has some rust on it.

Thanks for your input.

I tried to attach a couple of pictures. Maybe it will work.

 

[civilperson]

Steel brush on grinder wheel and paint brush is most economical if access is available.

 

[jike]

The deck has "white rust" which should also be delt with.

 

[Ron]

Doesn't look like you have any section significant section loss,so wire brush (labor intensive) or grit blasting would be indicated. Have the existing paint tested for lead content. If these are older joists, good chance of high lead in the primer.

If you find lead, consider using dry ice blasting to keep down the quantity of contaminant. You'll have very little to dispose if you do this. If you use sand or abrasive grit, you'll pay a lot to dispose of all the contaminated sand.

Do some thickness testing on the deck to check its integrity. The "white rust" looks like it results from condensation. You might consider some mitigation for this as it will produce long term deterioration.

 

[JAE]

Ron - how prevalent is "dry ice blasting" around the country? I've never heard of it. I assume that because the dry ice will simply evaporate, the only residue left to dispose of is the lead/paint itself?

 

[hokie66]

Unless there is appreciable section loss, I wouldn't worry about the rust on the bar joists. The paint that was there was just a primer, only intended to protect the joists during construction. There are plenty of buildings with unpainted steel in internal environments. To do a good job of protective coating on the joists, you would have to replace the roof deck. Is that intended? The roof deck would be in much worse shape than the joists.

You should concentrate on ensuring that the roof doesn't leak anymore.

 

[Ron]

JAE...that's correct. It isn't used that often except in areas where sand or grit would cause more damage than good, and where you have contaminated paint. I've specified it on two projects...one at a "major theme park in Central Florida" and one was to remove all the paint on a "golf ball/tee" water tower that had several coats of lead paint.

It was less expensive than I anticipated and the dry ice can be made on site.

It's also a lot less messy than water blasting.

 

[McQSE]

The metal deck is going to be replaced.

hokie66:
Will it continue to rust if the existing rust is not removed?

I attached another picture. I have these joists marked to be replaced. They don't look like they have any section loss but they are rusted pretty bad. Any thoughts?

 

[darthsoilsguy2]

i wouldn't replace any joists. that's an interesting joist-series with the double smooth rod bottom chords. i haven't seen that one. i would think adding top chord reinforcement would do (at least per the photo). what's the deal with the joist layout? is there an expansion joint that i'm not seeing.

If you do feel you must replace the joists, is it not a better option to sister joists with additional ones and keep the old ones. (or that is what you meant)

 

[Ron]

I agree. There's no reason to replace the joists...repair in place.

One thing you might check though...if you're in a higher wind area, many times older joists were not checked for uplift, so lateral bracing and 1st diagonal should be checked. With small "wiggle bar" joists, the buckling in the first diagonal can be a problem under load reversal.

 

[hokie66]

There is a marked difference between the two photos. The first one, just a bit of surface rust. If kept dry, I wouldn't worry about it, as steel doesn't continue to rust just because it has a rusty layer on the surface. But the second photo, under the area where the roof leaked badly, has more extensive rusting. I can't tell from the photo how severe the pitting is, but you may be justified in replacing or sistering those joists. In my opinion, it would depend on your assessment of the appropriateness of the original joist design.

 

[McQSE]

The double joist is supporting a rooftop AC unit.

The joists are strong. The roof currently holds about 4" of water when it rains. It looks like a big lake up there. I found two joists that were damaged due to the extra weight. I will be able to look closer when they get the deck off.

I don't know a great deal about rust. They are building a new roof with a proper slope. The joists will be dry and in a low humidity environment. Will this stop the joists from rusting further? My concern is that I don't want the joists to continue to rust and lose strength. I have read about rust converters that basically sound like they seal the rust to keep out moisture and oxygen. It would go on like paint.

Thanks for all your input.

 

[civilperson]

The rust is nominal and the joists need only cleaning and paint. Replacement is a waste.