Lipped Channel Cold-formed purlins design

[Enhineyero]

Hi everyone, I have a couple of questions regarding cold-formed design, (because the book that i have didn't address this questions)

1.Is there an easier method to design Cold-formed Lipped channels? Because using the reduced section properties of the lip, Flange, and stem of a section is quite tedious. I know DSM but i don't think it can be applied when bending occurs on both axis.

2. if there is a major and minor axis bending, can i use the reduced section properties (using the reduced section properties when bending is at the major axis) for both major and minor axis moments?

I wonder why AISI did not bother to make the analysis of the section simpler, I think using a certain factor to reduce the section capacity is more practical, rather spending so much time computing the effective properties of a section.

 

[hokie66]

Designing light gauge purlins by hand is too cumbersome for me. Best to use load tables from a dependable source. These are a commodity item in Australia, similar to bar joists in the US. Here is one site.

http://www.lysaght.com/go/product/lysaght-zed-and-cee-purlins-and-girts

 

[hokie66]

And another one.

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

 

[SteelPE]

There are some simplifications of cold formed sections in bending located in AISI. Of course there are some requirements of the design which are very restrictive. If you have the 2007 AISI book look at section D6.1 for the design of Girts. I was told these simplifications came from specific tests done on wall systems.

There is also another table located in the AISI manual which gives moment capacity based upon unbraced length of the compression flange. I believe it is Chart II-1b. I sometimes uses these tables.

There is also a computer program you can get which will give you the capacity of various sections. I metal building engineer I know swears by it..... and, oddly enough, you only need the part that is free (go figure)... but I can't remember the name at the moment.

I agree that is it very complicated, but the complications comes from the fact that the sections are so thin. You end up with local buckling before global buckling which really throws a wrench in things. Remember, most of the sections in the AISC manual comply with the buckling requirements.

 

[JedClampett]

I'm just getting started in designing these sections. I agree that designing using the formulas is tedious, but the tables I've seen make assumptions (like fully braced compression flange) that need careful review. My advice is to limit the number of sections you have to check (repeat the same member size as much as possible).
I had my company buy all the AISI books that apply. They work out a lot of examples, which gave me a lot of confidence that my approach was right.

 

[ajh1]

Take a look at the CFS package that RSG software (

http://www.rsgsoftware.com)

offers. They have a "light" version that is a free download so you can see how it feels for your use.

 

[dcarr82775]

The strength equations are pretty reasonable, the pain comes from calculating the gross and effective section properties. Since it seems every manufacturer makes a slightly different shaped product I often will pick a manufacturer and use their gross and effective section properties.

 

[Enhineyero]

Thanks for all the reply. If there is bending in both directions, do i have to calculate the effective section properties for both direction? Very time consuming.( I dont have the patience to do the calculations all day unless it is neccessary)

Sometimes I just reduced the Section modulus by 40% to avoid the lengthy effective section properties calculations.

Has anyone have another rule of thumb in quick calculations?

 

[MikeHalloran]

I have used CUFSM with credible results:

http://www.ce.jhu.edu/bschafer/cufsm/

Mike Halloran
Pembroke Pines, FL, USA

 

[hokie66]

I don't know why you would use these sections to resist bending about the weak axis. There are better ways of doing it.

 

[Enhineyero]

Thanks for the tip mike...I am going to check this out.

Hookie66, since purlins cross-section are paralllel to the roof slope. The purlins are subjected to weak axis bending if the roof slope is large maybe 20 deg or more. if less the moment to be resisted at the weak axis is, in my judgement, insignificant.

 

[hokie66]

You need to provide another means of support for the purlins rather than depending on bending about the weak axis. For a gable roof, you normally provide bridging continuously over the ridge. If it is a single slope, there needs to be a member at the top or bottom to take the horizontal component.

Same as for wall girts...they don't rely on bending to carry the gravity load of the cladding and self weight. You hang them from an eave strut or else support at the bottom.

The links I provided show how it is done in Australia.

 

[Enhineyero]

Thanks hookie, I would like to ask if: Isn't it that the roof loads is transmitted to the purlins then to the bridging? so the points where the bridging is not present, there will deformation at the weak axis on those porti, right? it is like we are reducing the "unsupported length". We can only ignore the effects of weak axis bending when the bridging is spaced closedly? Am I correct?

 

[hokie66]

That is the idea. The closer you place the bridging, the less weak axis bending you have. The bridging also braces the compression flange when the purlin is in uplift. The tables in the links I listed allow for differing amounts of bridging, and are based largely on extensive load testing rather than calculation.

Another factor in designing purlin systems is to load the purlin as closely as possible to the shear centre. So for C purlins or channels, where the shear centre is outside the web, the top flange should be up the slope. Z shape purlins are more efficient, as they can be lapped at the supports. Top flange up the slope for Z's as well, as the shear centre is at the section centroid.

 

[Enhineyero]

can sagrods be called a bridging? they basically have the same function, only that the contact area of support and purlin web is very small because it is just a rod. can you consider this as an effective support/bridging?

 

[hokie66]

In my opinion, sag rods don't provide the same stability to girt and purlin sections as do purpose designed bridging. However, there are differing opinions, and this issue has been discussed here several times before. These discussions are easily found by using the search functions. Here is one, and some of the others are referenced therein.
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