Corrosion protection of exposed steel

[StructTaco]

Is corrosion protection required per code for exposed steel. I have some cable anchors for a large zipline that are not galvanized, but should be. Is this a code requirement, or can they be painted, or can we assume that rust will take some of the steel section over the life of the structure and account for that in the thickness specified?

Thanks!

 

[BAretired]

I don't know the answer to your question, but I would not feel comfortable in assuming a reduction in cross section to account for rust. I believe you should protect the cable by galvanizing or other protective coating. Check with your steel wire supplier for recommendations.

BA

 

[asixth]

StructTaco

What code are you designing to? If the structural steel is external than it should have some form of corrosion protection. If you have specified that the cables are to be galvanized and they need to be galvanized for the integrity of the design, than you should not allow anything less to be installed.

Today I had to pull up a contractor for not galvanizing dowel bars that pass through a cold joint.

Attached is a photo of a W-beam which was within a building for 30+ years and protected with zinc primer but still experienced a minor degree of rusting.

 

[StructTaco]

sorry, I didn't make myself clear...the cable is galvanized, the anchorage to the foundation is not. The cable attaches to a link plate/take up device that connects to a baseplate which has anchor bolts on top of a caisson/CIDH pile. Everything in between the cable and the anchor bolts are not galvanized.

 

[jhoulette]

Be careful using dissimilar metals, you may find out later that your un-galvanized metal corroded faster than expected. It's always recommended to use similar metals. A bunch of science involved that I'm not going to into and I'm not an expert in that field.

However, you can design your metal material to have a sacrificial amount of material that is basically an outer layer that is expected to corrode over X number of years. Just depends on a lot of factors. Typically, this is done with metals underground where limited corrosion may occur since oxygen is not present (helical piers for example).

I recommend galvanized steel at a minimum if exposed to the elements. A painted protection just doesn't do it, unless you plan on re-painting every couple of years.

Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[BAretired]

StructTaco,

Sorry, I misread your first post. You were quite clear.

Your photo indicates rust on some of the small connectors. I would think these should be replaced with galvanized fasteners. The painted part is easily inspected and I would be inclined to leave it alone.

BA

 

[StructTaco]

thanks everyone

 

[frv]

jhoulette is right; dissimilar metals may drive corrosion in unanticipated ways.

Conversely, you could use this to your advantage if you attach a "sacrificial anode" to the assembly. Generally, the more reactive element will corrode completely before you have to worry about the other elements corroding.

Not sure about cost or whether the fact that three different metals would need to be involved may have unintended consequences, but it may be a solution.

 

[apsix]

Unless you are on the coast painting will be adequate, but will require maintenance. Galvanising would have been better.
Mixing galvanised and ungalvanised steel will cause the gavanising to corrode away quicker, but won't be a problem if it's painted properly.

BTW that split pin doesn't look very tamper-proof, is it in a public area?

 

[StructTaco]

It's inland, and the split pin is not in a public area, good eye. To detension the cables and replace with galvanized parts will be extremely expensive at this point.

Thanks for the input.

 

[dik]

The damage appears to be from entrapped moisture, not galvanic action. It appears to be too extensive.

Coating technology has progressed a lot and for future work, you might consider looking into some of the polyurea coatings.

The work can be protected using an active cathodic protection system; to use passive sacrificial anodes, the work has to be continuously wet.

As it stands, it's a matter of periodically inspecting the area and to provide a new coating as required. A zinc rich primer to minimise future corrosion... once it has corroded, the material is gone...

Dik

 

[hokie66]

I would not allow it to stay in that condition. The nuts and pin are critical, and need to be protected at least as well as the cables (they actually look like stressbars). If you replace those, the brackets can be galvanized at the same time. That will give them a chance to do a decent job of the grouting next time. By the way, what takes the horizontal component of the force into the concrete?

 

[csd72]

Has anyone tried zinc rich paint in this situation? I would expect that it would show satisfactory results.

 

[StructTaco]

The anchor bolts are in shear and tension, 1" diameter A354 Gr BD. These attach the baseplate to the link plate (1" Plate, A572, gr 50). The anchorage is designed for the ultimate strength of the cable, about 80 kips, but the cable is only at 25 to 30 kips max working load. The anchor bolts are embedded 3' feet into the caisson and they have a tight spiral for the upper 3' of the caisson which is 30" diameter, 4000 psi conc.

Hokie, You're right, the grout was not done well...and they had sprinklers wetting everything down until i made them divert the irrigation elsewhere.

 

[hokie66]

Are the anchor bolts galvanized? If so, how did the exposed parts and the nuts get to be red? The nuts on the anchor bolts would be the first thing to rust. The main reason for galvanizing every part of this connection is that once corrosion starts, there is no way to get at all of the surfaces for preparation and coating without disassembly.

 

[StructTaco]

the anchor bolts were cold-galvanized, the rest of the steel was just shop primed only(the red paint you see)...now is the time when the owner is asking me to final everything per the approved/permitted drawings which say galvanized for the rest of the parts. I'm looking for options which won't require them to disassemble everything, but not looking like that's going to happen.

 

[hokie66]

Cold galvanizing is not in any way equivalent to hot dipped galvanizing. The red paint on the ends of the anchor rods and on the nuts looks like it was field applied. The shop primer is intended to protect steel only until it is erected, not during external service. If the approved drawings required hot dipped galvanizing for everything, the contractor has not performed, so should be required to rectify.

 

[Zambo]

Th foundation and the fabrication work look good. A pity the coating and the grouting turned out like that, just a bit more time and very little additional cost and it would be a job well done.

I'm not sure what's being referred to as "cold galvanising" but I don't think this term can be applied to any brush applied coating (i.e a paint) if anything cold galvanising is electroplating.

The problem seem to be how to destress the tension bars and get everything hot dip galvanised at this stage.

 

[hokie66]

To me, cold galvanizing is just an organic (epoxy) zinc rich primer.

http://www.ingal.com.au/FAQ/faq16.html

 

[FFENGINEER]

Does the grout come up around the baseplate? Its hard to realize that its a 1" plate