Reaming out holes in galvanized members 1

[damorim]

Hello,

We're looking at an overhead sign structure that has a vertical shaft flange to cantilever arm flange connection. All elements including fasteners are galvanized. Bolt holes were only oversized by 2mm and we're finding that the bolts are not fitting into the holes due to zinc pick-up within the hole (this was a mistake, we know holes should have been oversized 3mm which we've done many many times without issues). I've advised to ream out the holes to allow the bolt to fit. Galvanizing within the hole (on the reamed surface) will likely be completely removed... Is this a concern? Will the adjacent galvanizing and the galvanized bolt protect this exposed area? It's way up in the air and connection is generally "sealed" w/ a double washer. Or is some treatment of the reamed holes recommended?

Thanks!

 

[desertfox]

Hi damorim
Can you coat the edges of the hole with some kind of rust protector before inserting the bolts?
To be honest zinc is sacrificial and even a small nick or scratch in the coating will set the corrosion process off which could happen even during normal assembly, is there some guarantee on corrosion of the sign structure?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[bridgebuster]

Usually, we coat with ZRC

Link

 

[dik]

Not heard of the product, but data sheets indicate it's pretty good; thanks for the heads up...

Remedy likely depends on the degree of corrosion you anticipate...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[STrctPono]

Personally, I get a bit annoyed when fabricators do this and then ask to ream the hole and then just coat with ZRC. I typically require them to repair it using a zinc based solder in accordance with ASTM A780. It is a better touch up system for galvanizing than ZRC.

 

[canwesteng]

How much zinc did you put on that the bolts don't fit in 2mm oversize holes?

 

[dauwerda]

The issue with the solder is that it is very hard to apply (well) to anything but a horizontal surface, so if your structure is already (semi) erected, a zinc rich paint is often the best option if the repair is required on a vertical or overhead surface.

I know for tapped holes, it is common practice for the threads to be tapped after galvanizing with the understanding that the galvanizing on the bolt itself still protects the tapped threads through cathodic protection. I don't see why the same wouldn't be true for a smooth bolt shank passing through a hole that has been reamed. That said, as zinc rich paint is very easy to apply, I would still specify it as well.

 

[STrctPono]

dauwerda is correct. Tapped holes typically are retapped after galvanzing which does remove the zinc. Also correct that the zinc on the bolts threads protects that unprotected tapped threads. My issue is that this analogy does not work for an unprotected standard size hole as the bolt is many times not in intimate contact with the hole annulus to provide this protection. With the way in which iron workers jam their spud wrench in the holes during erection and hammer bolts through I have no faith in the ZRC holding up to that kind of abuse.

The issue dauwerda brings up regarding the difficulty in applying the solder correctly has some merit but don't believe for a second that it is not possible. If there are a lot of holes... the job of zinc soldering becomes more arduous.

This may seem all very nit picky but I don't like to mess around with outdoor steel structures. I even require the galvanizing drain holes to be filled with zinc plugs.

 

[BridgeSmith]

For galvanized steel that is cut, drilled, etc. in the field, we (the DOT Bridge design section) specify 2 coats of zinc-rich paint conforming to ASTM A780.

Rod Smith, P.E., The artist formerly known as HotRod10