Corrosion of galvanized plates

[drewtheengineer]

Two part question.

First, Is there a relatively easy way of stopping corrosion on a metal truss plate? I have wood roof trusses in a building on the beach and the salt air has begun corroding the metal plates within just a few years. I could attach plywood gussets over the plates but this would be quite extensive. I'd rather paint them with a zinc rich paint or something of that nature. Can anyone direct me to a manufacturer that could help me out with this?

Second, does it make sense to use stainless steel nails in a galvanized uplift connector? I'm looking for an argument to support that.

 

[LPPE]

Whoever designed your structure initially should have known to use stainless truss plates for that salt air environment.
Truss plates are rather thin to begin with, so if they're rusted, your only safe alternative would be plywood gussets.

 

[drewtheengineer]

Good point, though the corrosion is in it's infancy at this point. I'm not concerned with the integrity of the plate yet, I just need to stop the progression.

 

[Lutfi]

I agree with pylko 100%. However, some of my clients, influenced by contractors, are reluctant to use stainless steel to save money.

I put a disclaimer on my plans that the structural systems must be maintained by owner for the system to continue to function properly, and that includes periodic maintenance of various sorts.

If the plates are showing signs or corrosion, consider cleaning in them good and covering them with good cold zinc paint/primer.

 

[drewtheengineer]

Thanks Lutfi, and for the record, I wasn't the engineer for the design, they called me in for the repair.

How about the second part to my post, justification of stainless steel nails in a galvanized hardware?

 

[boo1]

In the long term, painting will not last. You are protecting the outer surface and even that will require up keep. REPLACE or fail

For corrosion resistant finishes:
Hot dip galvanided (ASTM A 153) use hot-dip fastener. No galvanic cell between metals.

Stainless Steel (316) use stainless steel fastener. No galvanic cell between metals.


316 SST connector plates should also be used with new treated lumber, coastal zone, indoor swimming pool or corrosive chemical areas.

 

[Metalguy]

If for some reason you can't use 316 connectors, and the galv. nails are corroding faster than the conns., you CAN use SS nails w/galv. steel, but not vive-versa. The nails will be very small cathodic spots surrounded by a large anode. You should be OK with this if it isn't wet most of the time.

 

[drewtheengineer]

Metalguy,

Thanks. Is there anything (Code, Text) I could reference to show this?

Drew

 

[Trussdoc]

Corrosion resistant coatings for use over truss plates are listed in section 6.5 of ANSI-TPI 1-2003, and include epoxy polyamide, coal tar epoxy, zinc chromate vinyl etc, as well as POST MANUFACTURE hot dipped galvanizing.

These are based on research done by some Transportation departments (one of them being Maryland, I believe) on storage facilities for road salt.

I think these would be more effective than the plywood. Unless of course you need the plywood gussets to make up for corrosion losses.

 

[Lutfi]

Stainless and galvanized is not that great either. There will be galvanic action (zinc and Chromium) will react in presence of water/moisture.