Analyzing Steel Roof Purlins with Long Overlaps
Analyzing Steel Roof Purlins with Long Overlaps
(OP)
Hello All;
I am analyzing cold formed C and Z shaped steel roof purlins in an existing pre-engineered portal frame building. Each purlin is only single span (30' spans) but have a 5' lap over every support.
I have been using a program called CFS that analyzes cold formed steel sections, and when I input the system with 5' laps it assumes that the purlins are continuous, and that their stength in negative moment (ie over the supports) is doubled. Thinking about it logically, these assumptions make a lot of sense to me (the lap creates an effectively continuous member, and there is double the area of steel over the support) but I've never heard of this as an appropriate design procedure. Is there anything in the code (I'm in Ontario, Canada, but any reference to an American code would be interesting to me too)that allows these sorts of assumptions? Is there anyone who can explain to me how I can justify this sort of analysis?
Any help is great appreciated.
I am analyzing cold formed C and Z shaped steel roof purlins in an existing pre-engineered portal frame building. Each purlin is only single span (30' spans) but have a 5' lap over every support.
I have been using a program called CFS that analyzes cold formed steel sections, and when I input the system with 5' laps it assumes that the purlins are continuous, and that their stength in negative moment (ie over the supports) is doubled. Thinking about it logically, these assumptions make a lot of sense to me (the lap creates an effectively continuous member, and there is double the area of steel over the support) but I've never heard of this as an appropriate design procedure. Is there anything in the code (I'm in Ontario, Canada, but any reference to an American code would be interesting to me too)that allows these sorts of assumptions? Is there anyone who can explain to me how I can justify this sort of analysis?
Any help is great appreciated.






RE: Analyzing Steel Roof Purlins with Long Overlaps
1. 30' simple span
2. 30' span with two 5' cantilevers
3. 30' continuous
If the overlaps are tied together - then you would probably have a negative moment generating connection over the frame. Which probably leads to smaller purlin sizes. These designs are ALL about saving ounces of steel!!
At least two caveats: unbalanced loads and what happens at the "ends" of the building where the purlin terminates??
You are the engineer - your call.
RE: Analyzing Steel Roof Purlins with Long Overlaps
Though it needs to be double bolted at the ends to ensure they are adequately tied together.
Here are a few Australian links that are more useful than any other ones that I have read:
http://ww
http:/
RE: Analyzing Steel Roof Purlins with Long Overlaps
RE: Analyzing Steel Roof Purlins with Long Overlaps
Nice link. I don't remember seeing anything so comprehensive before.
BA
RE: Analyzing Steel Roof Purlins with Long Overlaps
Yes, they would be considered continuous members since they are lapped 5'-0" (assuming they are bolted/connected properly).
I am not sure how the 'doubling the capacity' thing works. Wouldn't one purlin actually be transferring load to the other purlin to make it act like a continuous member? I would have to sit and think a bit more how this works.......or see this explained.
RE: Analyzing Steel Roof Purlins with Long Overlaps
RE: Analyzing Steel Roof Purlins with Long Overlaps
I do not recall if they will use the double thickness at the laps for calculating the capacity. However, I do know that if the moment is larger in the center of a span they will use a doubler purlin, effecting increasing the thickness. Therefore, it might be reasonable to assume the double thickness (if connected properly as stated above).
RE: Analyzing Steel Roof Purlins with Long Overlaps
RE: Analyzing Steel Roof Purlins with Long Overlaps
Lots of good examples though.
EIT
RE: Analyzing Steel Roof Purlins with Long Overlaps
Yes, these laps would be considered to make the purlins continuous. The requirement within AISI is that the lap must be at least 1-1/2 times the purlin depth on each side of the support to be considered continuous and yes as noted earlier the laps mustbe bolted together, generally both at the ends of the lap as well as over the support. End frame locations would be treated as simple supports.
RE: Analyzing Steel Roof Purlins with Long Overlaps
RE: Analyzing Steel Roof Purlins with Long Overlaps
Yes, much more complicated than for regular structural steel as there are so many more buckling modes.
Makes you appreciate the importance of the dimensional limitations to the regular steel codes.
RE: Analyzing Steel Roof Purlins with Long Overlaps
if you work with cold form steel alot, and actually do those calculations by hand, you should definately check this program out!!
http://www.rsgsoftware.com/
RE: Analyzing Steel Roof Purlins with Long Overlaps
In some other countries for short spans they will simply span a single section over two spans giving you the benefit of the continuous moment curve, but not giving any doubled section benefit.
RE: Analyzing Steel Roof Purlins with Long Overlaps