Cold Formed Z with Unequal Legs 2

[azogr]

When calculating the design strength of a cold formed Z-section with unequal legs is it acceptable to average the leg lengths and create equal legs to simplify calculations? I seem to remember in the past seeing this done and the justification was that the average flange width was still within the allowable fabrication tolerance in AISI? I can't seem to find that reference at the moment and was wondering what you guys thought of this approach.

 

[SteveGregory]

I assume that you mean unequal flanges. I would not average the flange widths. If you think the calculations are too involved, you could use the narrower flange width for both flanges.

 

[paddingtongreen]

Why do that when the actual calculation is simple enough?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[DHKpeWI]

I suggest you use the CFS program to analyze the section. You can download it for free from the following web site.

http://www.rsgsoftware.com/

 

[azogr]

Thank you all,

I am using the CFS program. My problem is that I am analyzing exsiting mini storage buildings. There are so many different span configurations in each building and I don't know which purlins were turned up and which were turned down when installed. Spot checking throughout the buildings there seems to be no rhyme or reason to how they were installed. Multiple breaks in the continuous spans as well as variable lap lengths are complicating the issue even more. I was just looking for reasonable ways to simplify the analysis to control costs for the customer. Does anyone know where AISI references the tolerance for flange width dimensions?

Thanks!

 

[paddingtongreen]

My approach to this type of problem is to look for boundaries. What is the limiting span for the weakest orientation, single span, double span, interior span, etc. Same for strongest orientation.

With a schedule of this information, it becomes simple to look on-site for cases that exceed these limits.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rb1957]

i'd go with steve's answer ... if you don't know which way round they were installed, use the smaller flange for both ... should be conservative ... and if it makes a difference, then things have been cut a bit too fine (IMHO)

 

[azogr]

To follow up...

I did a little research and it looks like assuming equal legs of the smaller dimension is not always going to be conservative. I had several occasions where the section with smaller equal legs was actually less stressed than the actual section with unequal legs. It turns out this is because the equation for calculating the elastic buckling stress (Fe) changes depending on whether or not the section is singly symmetric or point symmetric. I ended up running two scenarios for each number of span conditions. The first one with the larger leg down on the first span and then alternating up then down as you go down the building and another starting with the larger leg up in the first span then alternating the orientation as in the first scenario.

I hope I am explaining it coherently. This is going to be the most accurate and in some cases conservative way to analyze the purlins. It does, however, create a lot of work. With my various span, loading, and lap conditions I am well over forty scenarios. It is worth it though for the peace of mind.

Thanks