Aluminum Grating with Steel Beams 4

[JAE]

I've got a situtation where I need to design a "roof" over a series of cooling towers. The roof is to be composed of some type of open grating....preferrably aluminum as the units need a lot of open air space for ventilation. A very wet environment is anticpated. I need to span across this width and would like to do it with structural steel, probably galvanized or painted.

I'm concerned that the interface between the aluminmum grating and the steel beams could be a very good situation for severe oxidation due to dis-similar materials so I don't want them to be in contact. I've devised a plan as follows:

The grating would be supported directly by a series of aluminum wide flanges space about 5' o.c.
The aluminum wide flanges would then sit on top of the steel beams but be seperated by a neoprene pad.
The only problem is - how to bolt together without negating the seperation. Gaylord & Gaylord suggest using hot-dipped or electro - galvanized bolts. Does this make sense?

The structure over the units is intended to provide some protection against severe storm damage and projectiles so I must use grating. However, I really don't want to use steel grating as I can't imagine it ever being galvanized properly to withstand the wet environment. The grating also has to be partially removable in the future if the units ever need to be replaced. Any ideas would be appreciated.

 

[dik]

A scattergun approach...

To prevent galvanic corrosion, it is necessary that the aluminum and steel be isolated. This can be achieved by a good coating system or a dielectric material; because of the high moisture exposure, either system would likely require periodic review to see that the coating/insulation is still effective.

There are many high quality paint systems that require near metal cleaning, and an appropriate primer and topcoat(s). I get involved in the odd sawmill project where they may have corrosion as well as abrasion concerns and there are some high quality coatings that work well. I don't have my files at home, but I can post a couple of quality systems for consideration.

Isolating with neoprene would provide the insulation, but if the grating is loaded or subject to vibration, the grating could 'cut' through the neoprene. There may be other materials that could be less expensive such as EPDM or a butyl roofing material; these may be 'tougher'. If there is something conductive/acidic in the condensate, then a coating system would likely work better than the dielectric approach.

Is the aluminum used for weight reduction only or for corrosion resistance. If the latter, you might want to consider galvanizing only and rely on a mechanical clip connection to secure the galvanized grating to the structure. In any event, the fastener with galvanized grating is less likely to be problematic.

Galvanized grating is likely less expensive than aluminum. If high moisture, then could be long term problems with galvanizing and an effective life expectancy might be considered in the costing cycle.

The pH of the condensate, if basic (ie. non-acidic) then aluminum could be a problem too. Also, aluminum doesn't fare well with some acidic environments and zinc doesn't fare well with most... Nice to know what the condensate will be.

If the affected area is small, then review and replacement or the cost of a high quality coating system is not as significant and the whole exercise may be moot.

 

[FSS]

What about using some type of composite material grating? I have seen them used in many paper mill environments and they may be a viable solution depending on your spans and loads.

 

[Ron]

JAE...agree with FSS that composite worth looking at. Be careful of temperatures, though. Their working range is not great and their creep range even less. Also, composites will "corrode" in presence of certain chemicals.

Your aluminum approach is a good one, with one exception. Don't use galvanized fasteners, hot dipped or electroplated. They will both rust and will contribute to the galvanic action. Use stainless steel fasteners (preferably 316).

Also, though aluminum is anodic to steel, the steel will actually corrode faster. Aluminum passivates quickly and easily, then the process is reversed and the steel corrodes like a #!!&&.

I agree with DIK that whatever you do will have to be checked periodically so build the ability to maintain into your design. The isolation pads should be quickly and easily changed, i.e. without complete removal of fasteners, so that you don't drop the nuts or bolts when changing.

Neoprene is not good in hot, wet environment (take a look at the neoprene flap valve in your toilet). You might consider Teflon or Delrin isolation pads. If designed in "sandwich" between two flat surfaces, not much of a problem with cutting into them by the grating. You can always modify the grating if necessary with a flat plate welded onto it for bearing.

Good luck.

Ron

 

[evelrod]

Composite grating (fiberglass type), fastened with SS hardware, is very effective in acidic environments. It's use in chem plants is fairly universal, but it is somewhat difficult to install in odd shaped areas and it is more costly than coated steel or aluminum. It is light weight and easy to move for access. Some engineers resist using it, I am not sure why. I liked using it myself.
We installed an aluminum dome over an MTBE tank using TFE pads, seemed to work ok.(Again using SS hardware).
Most cooling towers I ran into were wood structures with hot dipped, galvanized steel parts. They obviously were not intended to last forever.
Just some memories, hope they help.

Rod

 

[JAE]

Thanks a lot for all the quick, and informative answers.

 

[Hariharan]

FRP Pultruded gratings are available for a decent load range
and could be quite attractive. I am not sure about the projectile
aspect though. No grating is designed for such conditions?!
IT would solve most of the corrosion related problems. Yes,
SS 316 bolts and nuts should be used.

Hariharan