FRP versus Steel Beams

[MFJewell]

I am looking at replacing some galvanized steel beams (W8x18 and W8x24) with something more corrosion resistant. The specific application is for a cooling tower walkway/laydown area. The beams support FRP grating around the perimeter of the tower. The galvanized coating is failing rapidly on the metal that gets tower spray (already plan to replace the galvanized handrails with FRP). I believe the design firm didn't account for as much water/evap with chemicals getting on the walkway, thus after a few years in service it is heavily corroded.

I have never designed a structure with FRP and in general my experience with FRP in field use hasn't been great (failed joints, beams shearing under impact, etc.). I have read some of the available internet info on FRP versus steel and it appears that FRP tensile strength is used as the design limit versus yield for steel. That said, it looks like the FRP design strength is nearly equal to steel (which will vary some by actual manufacturer of the FRP beams). Does anyone have any guidance in replacing steel beams with FRP to ensure loading is appropriate?

 

[canwesteng]

Maybe off topic, but if the steel is getting sprayed by corrosive chemicals, galvanizing is not appropriate. Instead the steel should be painted with a high durability epoxy or urethane with a zinc primer.

 

[MFJewell]

I agree, they could have used a different coating system for the railings and/or structural steel. As stated I believe the AE thought these areas would see minimal overspray, but depending on wind conditions, they can see a lot. The tower itself is all FRP. That is why I am replacing the railing for certain and am considering replacing the beams closest to the CT basins. Here is a picture showing the difference between a cell that has been in service for the last 5 years and one that is installed, but has never been put in service.

 

[SlideRuleEra]

We use galvanized steel at the cooling towers for our electric generating stations. It has a limited useful life, but with attention to design / specification details that life can be quite lengthy.

W8x18 and W8x24 are not proportioned correctly for extended life in a corrosive environment. Like all wide flange sections their dimensions are optimized for use as beams - "thick" flanges and "thin" webs (1/4" webs in both cases). The webs are often the first site of structural failure.

Consider replacing the W8s with HP8x36, used as beams. Both the webs and flanges are 7/16" thick.

The original design may not have required a "thick" zinc coating in the hot-dip galvanizing process. Make sure the specification requires an average zinc coating thickness of, say, 4.5 mil (2 1/2 ounces / square foot).

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[Sawbux]

Deflection will control on F/G beams. E is about 1/10 of steel. If you can live with that, then F/G will be OK.