Analyzing Steel Roof Purlins with Long Overlaps 4

[andrew705]

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.

 

[MiketheEngineer]

I think there are 3 basic ways to look at this:
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.

 

[csd72]

Yes, the standard analysis method where I come from is to treat the laps as a double stiffness member.

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://www.stramit.com.au/downloads/Purlins, Girts & Bridiging Detailing & Installation Guide.pdf

http://www.stramit.com.au/downloads/Purlins, Girts & Bridging Product Technical Manual.pdf

 

[JedClampett]

I recently had my company invest the money and buy the set of AISI specifications and the design manual for cold formed steel members. They work out many good and applicable examples. Some of the formulae have 15 variables, so the book keeping is a challenge, but in your situation, you need them.

 

[BAretired]

csd72,

Nice link. I don't remember seeing anything so comprehensive before.

BA

 

[Den32]

As Jed stated, AISI Specification, Commentary and Design Manual are the resources for USA.

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.

 

[hokie66]

The links given by csd72 are used by Australian engineers as trusted design data, similarly to the way bar joist data is used in the US. These load tables have been developed based on extensive research. We don't really have PEMB, but we use light gauge purlins in most of our commercial buildings.

 

[SrVaro]

Pre-engineered metal buildings will consider the purlins (and girts for that matter) as continuous. I have seen laps that range from 12" to ~6'-0" depending on the moments within the purlins. If they were designed as simple span, the laps would be ~2" to allow for the connection. Depending on the overall length, individual spans, etc sometimes the laps are not consistent across the entire length. Same would be true for the purlin thicknesses within each span.

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).

 

[andrew705]

thanks for the help, everyone. appreciate the insight.

 

[RFreund]

How about the AISI code/manual. We purchased them this year for a project...Talk about melting my brain. Maybe it just me but I thought it made AISC look like a childrens book
Lots of good examples though.

EIT

 

[ajh1]

The AISI Specification is actually the North American Specification. It applies equally to Canada (labeled S136 there). Note that there is an Appendix A for US and Mexico and an Appendix B for Canada with a few modifications between the countries.
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.

 

[andrew705]

is that requirement in the NAS (appendix a)? or is it in another AISI spec? either way, thanks much for the response.

 

[csd72]

RFreund,

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.

 

[andrew705]

i'm not sure how admin will feel about such an obvious plug, but our office purchased a program called CFS that is honestly a god send when it comes to these sorts of calculations. the question i posted in this thread was the result of me trying to follow through a CFS analysis to make sure i can trust it... and i havent found any issues yet.

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/

 

[ajh1]

The reference is in section D6.1.1 condition (6). Technically, the requirement only applies to that section but it is a good rule of thumb to follow. Most of the metal building industry follows that guideline as a minimum when considering a purlin string continuous. In one sense you have the opposite problem with the longish laps. At some point the negative moment over the support will be over twice the moment at the end of the lap and the support location will control. At that point the laps are too long (or not fully-effective). The usual purpose of the lap is to accommodate the large negative support moment with a doubled section to allow the remainder of the span to be designed for a lower moment.
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.

 

[andrew705]

excellent, thanks much!