When would one choose hot rolled purlins/girts over cold-formed?

[GalileoG]

When would the choice for hot-rolled sections be better than going for cold-formed sections for purlins/girts?

For example, right now, I have 0.4 m deep cold formed channels for purlins spaced every 1.0 m. These purlins are 23.5 kg/m. I'm thinking perhaps I can get better economy if I go for hot rolled purlins. Would you agree?

 

[youngstructural]

This depends primarily on the material you are supporting, in my opinion.

Your hot rolled purlins will almost certainly be more expensive, as the material of the roof cladding will have a maximum spanning ability; Typically much more in line with CFS purlins.

As an aside, CFS design is actually much more challenging than regular steel design, but the manuals for sizing purlins, etc, make it easy. When you want to do something unique with a material, then you're analytically best to go with hot rolled unless you MUST choose CFS.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

Cold formed purlins will generally be more economical because of the efficiency of use of the material. I am unfamiliar with the heavy cold formed purlin section you are using, so my blanket statement may not apply. If this is an industrial application, corrosion resistance should be a consideration. Another thing to consider is how the cladding is attached--screwing to a thick flange is not common.

 

[civilperson]

Z shapes in folded cold formed purlins as well as channels with turned edges are the most economical with folded metal decks. The one meter spacing of the purlins makes me suspect a concrete or other roof system which negates my previous sentence.

 

[GalileoG]

Nope Civilperson, there is no concrete. It's a metal deck cladding, reason being for the 1.0 m spacing is because of a 4kPa snow load on the roof. The building is located in Northern Canada, where there is lots of snow.

 

[youngstructural]

Hey Clansman;

I'm a Canadian trained engineer, and worked my first three years in Canada. Given the deflection requirements in addition to strenght, I just don't see hot rolled steel giving you any economy.

My original post still stands, even given snow loading.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[civilperson]

Try a 75 mm deep deck type for the 4kPa snow load. If 3.0 N deck is used with three continuous spans, then the deck, (18 gage), span limit for L/240 deflection can be 3.0 meters.

 

[GalileoG]

Another question: Why on earth would a cladding manufacturer catalogue state that "STRENGTH capacity should be checked against SPECIFIED loads" and not factored?

 

[hokie66]

I wouldn't know exactly how to interpret that statement. Sounds like a nothing sentence to try to protect the manufacture against misuse of the product.