Best steel material for rooftop mounted solar array?

[StructureMan44]

A small project I am helping with requires structural support for a roof mounted solar panel array. For the 25ft span between parapets I have found a W5x19 to work assuming A992 steel. I have very limited outdoor experience; if A992 is mounted outside I assume it needs to be galvanized? Is this an expensive process? Is there a different type of steel that is also inexpensive/readily available and has better weathering properties than A992? I realize the section size will likely have to increase if better weathering steel has a lower yield strength.

 

[KootK]

I would galvanize the member and no, it's not prohibitively expensive to do so. I expect that galvanized steel is probably the right answer here. Some other possibilities:

1) Non-galvanized steel oversized to accommodate section loss. It' will rust and bleed onto adjacent stuff.

2) Weathering steel. Still might bleed onto adjacent stuff.

3) Stainless steel. Too expensive.

4) Aluminum.

5) Galvanized cold formed steel. Reduced longevity compared to hot rolled steel I would think.

6) Properly treated wood?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

A W5 is a skinny bouncy beam for 25 feet.

Use galvanized

or

Specify a high grade coating such as something from TNEMEC. We use something like this:
ONE COAT: 2.5 MILS MIN. OF ZINC RICH PRIMER (SSPC PAINT-12.01 - TNEMEC SERIES 90-97 OR EQUAL).
FINISH PAINT: TWO COATS (4-5 MILS MIN. PER COAT) OF EPOXY-POLYAMIDE PAINT (TNEMEC SERIES N69 OR EQUAL).

There are many other combinations out there - call a local rep to see what they recommend.

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[racookpe1978]

What wind zone are you?
What snow zone?
Hurricane or earthquake zone?

 

[XR250]

Around here galvanizing is about 60 cents per pound with a $300 minimum

 

[StructureMan44]

Thank you for the feedback guys. It looks like galvanized is the answer. Is A588 steel a good option, other than rust bleeding? This needs to last 30+ years.

I am assuming the aluminum frame that supports the panels themselves will provide the lateral bracing between both W5x19s. But bounciness will be an issue? How do I quantify/calculate this?

Also, is there a concern for galvanic reaction between a galvanized A992 touching the aluminum channels that directly support the panels?

 

[StructureMan44]

racookpe1978: It's in Chicago; wind speed = 90mph, wind velocity pressure=12.3psf, snow=25psf, I believe it is SDC=B but I need to confirm.

 

[KootK]

Is A588 steel a good option, other than rust bleeding? This needs to last 30+ years.

Weathering steel is pretty durable.

I am assuming the aluminum frame that supports the panels themselves will provide the lateral bracing between both W5x19s.

I'm not so sure. It would depend on the connection details etc.

Also, is there a concern for galvanic reaction between a galvanized A992 touching the aluminum channels that directly support the panels?

Yes, depending on the service environment. Isolation might be prudent.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

A rule of thumb for purlins - depth to span = 1/30, so say in this case a W10 might be more appropriate for stiffness. But obviously you need to check deflections/stability

 

[racookpe1978]

"Weathering steel" as used in bridges and highway structures is "messy" when used above houses. The "rust" is a dirty, orange-red mixture that keeps getting brushed and washed off, staining everything below the steel (trim, ornaments, plants, planters, concrete), white paint, grey paint, black paint, yellow paint or anything-except-other-orange-red paint.

 

[BUGGAR]

As JAE notes, a W5 may be a little light. Span to depth/width ratio is 60:1

W5's don't detail politely if you use AISC details. I presume you are detailing specially for the W5. (These things are tiny!)

 

[dhengr]

StructureMan44:
You certainly get a crazy variety of design problems, without the engineering experience or judgement to know what to do with them or when you are getting yourself into trouble, what gives here? Why don’t you give us a bit of info. about your educational and engineering work background, and your current job situation. The guy/boss who’s giving you these wild assignments should be working with you and mentoring you, since the two of you can see the actual situation, and we can’t, from here. He should know what you know and what you don’t know so as to help keep you and the company out of trouble.

Any structural steel will work for this support system, and then use a good paint system to protect it. There really is no need for some exotic material for something like this, except to make getting the material difficult and expensive. The beams do have to be sized properly, and some questionable experience shows when you think a 5" beam will work. The more important things might be paying attention to all the possible loading possibilities, and the support detailing at the parapet walls, so as not to do any damage there, and keep them dry.