Zee purlins

[Sawbux]

Just round out from photos that the roof Zee purlins in a 30 Y.O. building were installed with the top flanges pointing down hill. Remembering from 50 years ago Structures class that loads should pass through the shear center to prevent torsion buckling. With Zee section on 2:12 pitched roof, it appears pretty important that the top flanges point uphill.

Did a bit of search on internet to find any mention of this, warnings, etc. as support to present to Owner - but could not find much of anything on this obviously important topic! Anyone know of good info source on this topic?

 

[hokie66]

You are correct, but the torsional issue can be solved with adequate bracing. What bracing/bridging exists?

 

[SteelPE]

You are correct, but the torsional issue can be solved with adequate bracing.

30yo metal building and purlin bracing do not go together in the same sustenance.

 

[Sawbux]

Only bracing is the through fastened roof sheeting.
My main question is - Why no readily available internet sources of info on this topic? How do I discuss this as a concern with Owner? It's stood for 30 years argument...........

 

[bones206]

If there are stiff mounting clips, that could be providing some roll-over resistance. You could just analyze the section and see what the capacity is when you include the torsion induced be the gravity loads.

 

[hokie66]

SteelPE,

Realizing that the OP is in the US, you are doubtless correct. But in some countries, e.g. Australia, purlin bridging has been the norm for longer that that. But then, correct orientation has also been the norm.

 

[human909]

My main question is - Why no readily available internet sources of info on this topic?

Check your relevant supplier. For me digging up the information took me 1minute. In capacity calculations of thin sheet cold formed structural members gets complicated pretty quickly especially when it comes to LTB. So getting rigorous engineering or academic resources on it might be a deep dive scenario. If you are wanting to calculate the capacities of the purlins in their current orientation then have fun and good luck!

How do I discuss this as a concern with Owner? It's stood for 30 years argument...........

As you likely know that isn't always a good argument. How to have the discussion well it depends on you relationship and responsibility. Sometime it might be better to mumble that the roofing doesn't seem to 'specification' and back away slowly.

As an ethical structural engineering you shouldn't walk away from a problem, especially if it could be a life threatening issue. But you also need to recognize the realities of what you can achieve.

Realizing that the OP is in the US, you are doubtless correct. But in some countries, e.g. Australia, purlin bridging has been the norm for longer that that. But then, correct orientation has also been the norm.

I always find it fascinating the few niche areas that Australia seems to be ahead of the world in...

Hokie can you shed any light on why Australia is 'ahead' in the structural 'innovation' of bridging?

 

[hokie66]

I'll try. For one, Australia has never had much in the way of 'pre-engineered' buildings. But it has historically used sloped roofs, not flat roofs. The metal roofing systems were developed with both the roofing and purlins in mind, and one reason for the bridging is to provide stability of the purlins for installation of the roofing. Think of the workers. The roofing and purlins have been typically provided by the same suppliers, and the 'design' has been just by using the manufacturers' tables, for both the roofing and purlins. These tables were developed by a combination of calculation and full scale testing, and have survived the test of time. For anyone interested, search for Lysaght and Stramit purlins and girts.