Wind loads on Skylight

[Mudar]

Hello,

I need to find out the wind loads an aluminium framed skylight in the shape of a hexagonal pyramid. I would like to know which parts of the ACSE code deals with this shape of structures so that I can figure out the external and internal pressure coefficients for each face at any wind direction.

 

[Mudar]

Can Anyone help please?

 

[WARose]

To me, it's somewhere in between a hip roof and a domed roof. I'd figure the loads for both and run with the worst case.

 

[JedClampett]

To keep code books under 5000 pages and $5000 each, they can't check every possible case. Thank god for that. For cases not explicitly shown, we need to use judgment. WARose is giving some good advice.
Or you could do a wind tunnel analysis. But I bet you don't have the cash for that.

 

[XR250]

I am guessing the 10 PSF (ASD) min. load on the projected area will control.

 

[NorthCivil]

Engineering judgement?

in my area, snow governs gravity, I use 20psf for wind uplift (not a huge wind area), though you wouldn't find that anywhere in any codes.

 

[SAIL3]

I agree entirely with JedClampett......when I encounter unusual cases, I usually use Cp=+/- 2.0 rather than jump thru allot of hoops unless it has a major impact on the cost....I would assume the OP is not designing a 100 of these...if it is a one-off, why finese it....

 

[BridgeSmith]

"I use 20psf for wind uplift (not a huge wind area), though you wouldn't find that anywhere in any codes."

That's the wind uplift pressure specified for bridges in the AASHTO bridge design spec.

 

[MotorCity]

I would cut a slice through the pyramid shape such that you have a gable roof. Calculate the wind based on that cross section and apply it accordingly around the pyramid.

 

[Mudar]

Thanks for your input guys. Much appreciated

JedClampett,
I am doing the skylight analysis on Robot Structural Analysis, which has a wind simulation tool. I have used this tool but the pressures generated were a lot lower than expected.

Anyway, I have done the analysis for the skylight assuming it is a hipped roof, and found the net design pressures. However, I am a bit confused on how to apply these pressures as the code does not explain if the negative and positive pressures are two separate cases.

Below are the net design pressures found, and I have attached an a copy of the page from the code that deals with hipped roofs.

 

[Ron]

I would treat as a domed roof. I've done a few of these for a canopy fabricator and that's the way I approached it. My reasoning is that it has symmetry in the same way as a dome and the slight shape difference is irrevelant.

Wind load should be based on components and cladding at the wind speed for the area. All of mine were in high wind areas.

 

[Mudar]

Ron,

I have done the analysis with treating the skylight as a dome roof. However, my previous questions still stands. Do I consider that faces of the skylight under negative pressures as one loading case, while the second case being positive pressures on all faces of the skylight as illustrated in the attached file?

I have always thought that some faces (windward) would have positive pressures, while other faces (leeward) would have negative pressures under one loading case.

Thanks
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1529318243/tips/Dome_Roof_pyguv0.pdf[/url]

 

[Mudar]

Hi,

Can anyone answer my question regarding load cases for negative and positive pressures?

Thank you

 

[SAIL3]

again, Cp=+/-2.0..
1...All positive press
2...All neg press
3...pos on windward, neg on leeward
4...wind @45deg...pos on windward, neg on leeward
5...any other combination that would give max loads

or
wind tunnel test

 

[Mudar]

Thank you SAIL3.