Continuous Z purlin

[Italo01]

Hello, i work for a steel fabricator as designer and we usually use Channel purlins for roof structures, but i am studying the advantages of the continuous z purlin. Currently, my boss has no interest in using the z purlin because of the cost, but if the weight reduction is substantial, he may advance the purchase of a press brake machine that he is planning for some time. I have two questions:

1 - Talking with my former steel structures professor, he said that i should be cautious about thermal expansion/contraction, particularly beacuse of the difference of temperature between the purlin and the roof, which can put the bolts in shear and tear the roof, causing leakings. Is this a valid concern?
I've run some preliminary calcs and don't consider that this will be a problem. Does anyone had problems with this?
Also, i don't see how the z purlin will behave differently from the Channel with regard to this problem.
I think that the continuous z purlin will develop a good level of compression because of temperature increase. Does anyone check for that? Is it a big problem for continuous Z purlins?

2 - I'm studying the influence of the deck(Through fastened) on the bending strength of the purlins. The R factor method described in the AISI specifications does not take the braced length in consideration and this surprised me. If i use bridging, the bending strength, according to the method, does not increase. How can this be explained?

Thanks.

 

[SandwichEngine]

1. Continuous zee purlins offer substantial savings over simple span cees. That's why PEMB companies use them. To really evaluate them, I recommend you purchase the software CFS. It's only around $150 and allows you to run continuous zees to compare with simple span cees.

https://www.rsgsoftware.com/

Thermal expansion and contraction of a through fastened roof is very real. The surface of the roof can get super hot just like a sidewalk. However the purlins that aren't in the sun's direct radiation will remain close to the temperature on the inside of the building. Over time, this will cause the holes to wallow out and your building WILL leak. I'd never put a through fastened roof on my home or a building I own. You're correct though that it's no different from how a through fastened roof works with Cees.

If using a metal roof, you would want to use a standing seam roof with floating or sliding clips. They allow the roof panel to move relative to the purlins.

2. It's been a while since I hand calc'd cold forms stuff but I know our analysis consider the strength of the deck to brace the outside flange and there are even equations to use that deck strength to brace the inside flange as a torsional brace.

 

[Italo01]

Thanks SandwichEngine. I'll Check the software.

 

[hokie66]

I like concealed fastening as well, but it you fasten a roof through the crowns with the appropriate fastener, it won't leak. That is, provided the profile is selected for the length of run and slope of roof. After all, each valley is a gutter, and slope matters.