Metal sheeting 2

[phuduhudu]

I have a situation where I have very large windloads on metal sheeting. For the IBR sheeting I have always used the manufacturers tables of purlin spacing previously. However, they have a limit on the windloads and my loads in this case are far in excess of those limits. Has anyone worked out metal sheet spanning capabilities from scratch. I guess that the limiting factor is local buckling of the thin sheet. Any ideas on how to figure out the limiting stress in a profiled thin sheet?

 

[Ron]

I usually look at such on a unit width basis. Treat as simple span or two span condition.

 

[phuduhudu]

I don't have a problem with the analysis of it but how do you determine the critical buckling stress for the thin profiled sheet.

 

[hokie66]

I don't see why anyone would want to push metal cladding beyond the manufacturer's recommendations. The tables published by the leading manufacturers are based both on calculation (which is very complex) and on testing. The testing involves not only capacity of the sheets but also the fasteners.

 

[apsix]

I can't see that you can calculate meaningful capacities, for reasons stated by Hokie.

Contact the manufacturer; they may be able to advise closer purlin spacings or do a special run with thicker sheet.

 

[Ron]

Why are you worried about buckling stress? That's the least likely stress direction for wall sheets. More of an issue with bending and loss of contact with adjacent sheets allowing water intrusion during wind/rain events.

 

[hokie66]

Ron,

Not column buckling, but local buckling. There are many local buckling failure modes in thin profiled sheets.

 

[BAretired]

Do not push the design of metal deck to the theoretical limit. You will find that theory and practice do not precisely agree.

BA

 

[phuduhudu]

I do not want to push the design to the limit - hence the question. The manufacturer provides tables of purlin spacing based on a wind load of 1.5kN/m2. Now I have a situation where I have windloads more than double that amount so obviously I can't use the published spacing. What I hear you guys saying is that you wouldn't ever try to calculate the capacity from scratch and presumably that means the manufacturers spans are based on test data rather than calcs. I will contact the manufacturer and see what I can get.

 

[hokie66]

The manufacturer should be able to provide you with tables for higher wind loads. In Australia, our tables commonly go up to at least 4 kPa.

Remember the fasteners. They are often the controlling case for suction pressure.

 

[phuduhudu]

Thanks Hokie. Yes, the fasteners of course are critical. I will follow up the manufacturers. It's just that in this part of the world there isn't a great service ethic and I suspect there isn't a great technical team behind these products either. Maybe I should contact some Australian manufacturers.

 

[BAretired]

To design steel deck from scratch, you could use CSA S136 "Cold Formed Steel Structural Members" or you could refer to a text such as "Cold-formed Steel Structures" by Wei-Wen Yu.

BA

 

[BAretired]

If the strength is controlled by flexure, you could modify the tables by calculating the span with increased wind pressure to give the same bending moment as the table value.

So L2 = (L1*w/W)^0.5



BA

 

[hokie66]

phuduhudu,

Can you post a description of the sheeting material? Or a link to a site where it is shown? I could post a link to an Australian manufacturer if that would help.

 

[BAretired]

Oops, I meant to say:

L2 = L1(*w/W)^0.5

where w is load in tables with allowable span of L1
L2 is the allowable span with load increased to W

BA

 

[phuduhudu]

Thanks BA,

I figured out the typo in the equation. That is the method I am using. Hokie, I found useful stuff on the Lysaght website but unfortunately they don't do the same IBR profile that we have here.

 

[hokie66]

What does IBR stand for? Can you describe it?

 

[msquared48]

Any way to install additional drag links/lateral elements to limit the diaphragm stesses and make the table values work for the situation?

Mike McCann
MMC Engineering

 

[phuduhudu]

IBR - 0.6mm thick sheet with 102mm wide flat troughs and 35mm wide flat ribs at 172mm centres. Ribs are 37mm above the troughs.

Msquared - don't understand the question. The table gives purlin spacing to support the sheet for a lower wind load than I have and so I can make it work by reducing the purlin spacing and that is what I am looking for and currently using BAretired's suggestion which gives me my best estimate so far.

 

[dik]

"You will find that theory and practice do not precisely agree."

this could be a whole new thread... but, you might want to look at the theory, if there is a large divergence... <G>

Dik