Curved Rafters

[jrbaus]

Dear Colleagues,

I am looking to design some small portal frame buildings (open sides). The client has asked that the roofs be curved (i.e. barrel vault design). Are there any references out there that deal with the design of curved rafters in regards to determining the effective length for lateral torsional buckling? I am not sure what (if any) special considerations are required due to the rafters being curved.

Example of curved rafter (by others):

http://valentineengineering.com/val...ads/2011/12/CurvedSteelPorchBeam2-940x360.jpg

Appreciate your thoughts on the subject.

Regards Jake
Australia

 

[Trenno]

Google is a wonderful thing!

Link

 

[Trenno]

I designed something similar a few months ago, however the purlins were quite heavy and at close spacing. I guess checking it for sectional capacity is a good starting point.

 

[jrbaus]

Trenno, your google foo is much better than mine. Thanks for the article - much better than anything else I managed to turn up on my own. I will have a detailed read.

My purlins will also be fairly closely spaced, although I would prefer to reduce the amount of fly bracing required for aesthetics. Its mainly uplift I am concerned about when there is compression in the bottom flange.

Many thanks,
Jake

 

[Trenno]

Seems as though the worst case is when the outer flange is in compression.

I'm interested to see what you come up with for the effective lengths. Did you see that the bending section said it did not cover portal frames?

If anyone else has a reference with an AS4100 flavour that would also be much appreciated!

 

[canwesteng]

Having done curved steel before, I second the value of SCI P281. Pretty much all there is out there for building structures.

 

[Signious]

Semi-off topic tangent:

I have designed both Glulam and Steel for curved arch support, but have never done cost comparison. Anyone have any info on this? I have been in more of a 'do what is requested' position, but may be asked for cost advice on this eventually.

 

[jrbaus]

I see section 6 states that it does not apply to portal frames however section 7 goes on to give a methodology and examples for portal frame design. I will report back with what I come up with for my effective lengths once I complete the detailed design (I have only done a very rough analysis to determine prelim member sizes and prepare a construction cost estimate for the client at this stage).

Appreciate the help.

Regards Jake

 

[Trenno]

How did you end up, Jake?

 

[jrbaus]

Trenno,

Like 90% of what I have been quoting on lately - I still haven't got the green light from the client to do the detailed design.

I managed to undertake some preliminary member sizing using the literature you put me on to (thanks for that). This allowed the client to get an estimate to fabricate and I suspect the $$$ were more than they were expecting.

Things seem very quiet on the structural front (Queensland, Australia) at the moment so I hope a few of these smaller jobs come off eventually.

Regards Jake

 

[Trenno]

Yep, I'm in Brisbane too... know the pain.

Thanks for the update. I just found curved steel and the associated stresses interesting.

 

[jrbaus]

Indeed it is an interesting subject - one which I don't yet fully grasp to be honest.

I remember deriving the shear forces, axial force and bending moment equations for a catenary shaped beam in university and had a few big 'aha' moments when it all came together. I suspect the same would occur for a curved beam if I were to go back and examined it from first principles.

 

[asixth]

I blame the federal governments budget for the recent slow-down.

That and the flailing AUD.

 

[Trenno]

...blah blah blah...Infrastructure Spending...blah blah blah....