Rusted design 2

[haynewp]

I have a large grating platform that will be supporting mech equipment that will be outside on a roof. The primer coat is supposed to last only so long. Routine repainting is not real economical for this situation. Mechanical equipment is permanent and large.

Grating mfr has no data or suggestion for strength decline due to rusting. Owner does not want to pay for galvanized.

Any advice on long term strength decline due to rust?

 

[SlideRuleEra]

The key, of course, is to detemine the corrousion rate. Once you have an estimate of this value, calculate the dimensions of the grating members, assuming that they are "shrinking" each year based on the corrosion rate. From these dimensions you can calulate the moment of inertia & section modulus of the bearing bars to compute stresses and deflection at various ages. Also consider the effect of corrosion on any welds, don't want the grating to fall apart.

 

[havesealwilltravel]

Generally, 1/8" is added to metal in a mildly corrosive environment such as a steel mill.
This 1/4" addition to the thickness of bearing bars is prohibitive.

It is rare that exterior grating is not galvanized. With its large surface area, grating is prone to rust and is nearly impossible to repaint.

Another alternative is fiber-glass reinforced or plastic grating. However, these gratings are more expensive.

Your only course of action is to warn your customer and specify the most rust resistant paint availabe.

 

[haynewp]

They have chosen to use galvanized now after all.
thanks

 

[Lutfi]

Insist that the owner pay for galvanized steel. Explain to him the re-occurring cost if he does not get galvanized steel. If he refuses, document his refusal. Place the refusal in the file and copy the owner.

I put a disclaimer note on my design documents that requires owners to perform routine and periodic maintenance on any system that I design. The performance and longevity of these systems is dependent on such maintenance. Painting is part of maintenance of steel structures especially if it is exposed to the weather elements.

I usually up size grating and supporting steel to allow for minor corrosion. This provides a structural system with integrity even if some minor carrion takes place.

This is really a hot topic for me and should be for all engineers. We are faced with this almost on Dailey basis with clients with limited budgets. When doctors perform their surgeries, non-of the patients ask for cheap material! Doctors just do it. Just think about this.

I like to hear more from other engineers and their feedback.

 

[SlideRuleEra]

Lutfi - Agree with you 100%. I have upsized grating and other members (typically don't specify any painted exterior structural member with web or flange less than 1/4" thick - even for light loads). Usually specify galvanized grating (without offering the owner the option of painted grating).

 

[boo1]

If you are in the coastal zone consider fiberglass, which performs much better.