Axial load capacity of C-purlins

[lager]

I know C-purlins can buckle and so can't officially take vertical loads.. but I'm wondering how to compute for it's buckling and the axial load the top of it (2 meters vertical height) can theoretically take.. assume the purlin is 2 inches by 4 inches in size and 0.07" thick (1.8mm).. would it be able to take say 40 lbs of weight or 20 lbs for its 2 meters vertical length? How to compute?

 

[Ron]

Use Euler's buckling theory.

 

[lager]

C-purlins are not wide flange or tube, they are not strictly HSS (Hollow Structural Sections).. how do you compute for the buckling of C-purlins.. what is the common formulas used?

 

[Retrograde]

Some manufacturers provide this information in their design literature.

For example:

 

[Trenno]

Direct Strength Method & the software CUFSM.

 

[hokie66]

You seem to be using cold formed C sections as struts or vertical beams, not purlins. Getting the nomenclature correct is important. Purlins span horizontally between rafters, girts span horizontally between columns. The chart which retrograde furnished is for resistance of lateral loading as struts due to wind on purlins/girts, typically at gable ends of metal buildings.

Computations of capacity of light cold formed sections is very complex, and won't be explained easily here. In addition to computation, these systems are usually load rated by full scale testing by manufacturers like Stramit.

 

[Trenno]

Going on from this discussion, I've found this really good, simple paper on using a sheeting/purlin setup as a roof & wall diaphragm.

Link

How many designers out there take advantage of this? What are your analysis and design methods? Interested to hear the responses.

 

[hokie66]

You need to distinguish between roof deck and roofing. The North American practice of using roof deck as a diaphragm works fine, but the roof deck is not the waterproofing. Australian practice of using metal roofing, either crown fixed or supported in sliding clips, is not strong enough for diaphragm action, at least that is the philosophy. So you have to weigh up strength vs watertightness.

 

[Trenno]

Surely Lysaght or Strammit can achieve both a valley fixed and watertight fastening. I'll investigate!

I understand Kliplok would be rubbish for in plane stiffness.

EDIT: Seems as though Lysaght calls up use of sealing washers, this should do the trick - right?

 

[hokie66]

The neoprene washers are used on the crest, not in the valley. That in itself is not a perfect solution, as the rubber deteriorates with time, especially in our harsh sun conditions.

Valley fixings are sometimes used on wall sheeting.

If using roofing for diaphragm action were acceptable practice, I think the US preengineered metal building companies would do it, but they don't.

 

[XR250]

Get the free download of AISIWIN software. I use it for simple light gage design.
@Hokie, I have a seen a couple PEMB's in the past that used diaphragms instead of rod bracing.

 

[hokie66]

But how big were they? For small, simple garage sheds that is understandable. What type sheeting and how was it fastened?

 

[Trenno]

They can be used for valley fixed sheeting, but you are cautioned when doing so.

"Fasteners with sealing washers should be tightened only until the washer is gripped firmly enough to provide a weathertight seal. The fasteners should not be over-tightened because this may split the sealing washer or deform the sheet, either of which could lead to water penetration. Take particular care when valley fixing because there is no flexibility with the sheet hard against its support. Take particular care to ensure the fastener is driven perpendicular to the sheeting to avoid deformation of the washer."

 

[XR250]

@Hokie;

Don't recall, but it was probably 50x100. I have a sheet from Butler (a major PEMB manufacturer) somewhere that shows the diaphragm capacities of their roof. If I find it, I will scan it.

 

[hokie66]

I assume you mean 50x100 ft, which is small. If in metres, I doubt that it has been done. My name for diaphragm design is "trickle theory". Perhaps our member who works for Butler will comment.

Without discrete bracing, there are a lot of practical (and safety) issues to consider during erection. Spare a thought for the installers.

 

[XR250]

Yup, 50x100 ft.

 

[lager]

What would happen if a beam is supported by many vertical supports below it.

 

[ajh1]

To respond to Hokie's comment: Diaphragm strengths on screw down panels are quite well documented. The typical means of obtaining a strength is to run a diaphragm test following ASTM and AISI protocols. The AISI test protocol is S907-13 and the ASTM protocol is E455. AISI recently released a new design standard which updates the SDI standard allowing a calculation-based method of establishing diaphragm strengths for specific panel types. That document is AISI S310. All of these AISI documents are available for free download off of CFSEI's web site,

http://www.cfsei.org.

Standing seam panels, which may or may not have much in the way of diaphragm strength, can be tested through the same AISI protocols, however there is no calculation based method for those panels. The strength of the standing seam panel is in direct relationship to the tightness of the mechanically formed seam. Snap together or hand-seamed panels are not logical for diaphragm strength.