Steel coating and chlorine environment

[ATSE]

I am working on an existing recycled water storage basin, which has a structural roof. About 80ft x 80 ft in plan. Beams will be supported at the ends, and also by some interior concrete columns. Spans about 20ft.
Previous roof was treated wood framed. Completely corroded in 12 years, and must be replaced.
Roof does not have to be perfectly waterproof, and will be vented at the ends.
Chlorine concentration is between 1 and 5 mg/L (ppm), and not well controlled. The recycled water is nearly potable, except high chlorine at the basin.
Because of corrosion potential, my first choice is FRP beams and covers (roofing). Like Strongwell.
However, the strength capacities are low, FRP is not a ductile material (seismic SDS = 1.2g), and connections are a pain (I don't think 316L fasteners do all that well in a chlorine environment).
Should I consider steel with a robust coating? If so, what coating? Connections?

 

[TVP]

If you want a longer lifespan than 12 years, I would not use either 316 or coated steel. Use a more corrosion-resistant stainless steel like 2205 duplex or maybe the Bumax A4 grade from Bulten Stainless, which is really just Type 316L with more Mo for enhanced pitting resistance:

http://www.bumax.se/eng/teknisk_1.htm

 

[cloa]

What do the chloride levels reach? If the chlorine levels are poorly controlled does mean that they spike higher than 5 mg/L?

 

[ATSE]

Chlorine may temporarily spike above 5mg/L, but that would be rare. Most of the time, chlorine will be between 1 - 2 mg/L. However, the water is unregulated and tested only periodically.

 

[EdStainless]

Since this is ambient temp they probably don't even need 2205. A lean duplex like 2101, 2404, 2003, should cut it.
There are people starting to make structural shapes in these grades.
Connections? Weld. This stuff welds well.
Make sure that the joint designs don't allow for trapped moisture.

What failed in the wood? The joints? I have seen wood structures with FRP joint supports and 2205 fasteners.

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Plymouth Tube

 

[ATSE]

Photo attached.
The tank cover roof got corroded from above, below, and the sides.
Site doesn't look like it was completely retrofitted only 12 years ago. Been abandoned a few years. Your taxes at work.