Adding opening in roof of restaurant

[Euler07]

Hi All,

One of our clients has a restaurant, about 15m x 30m with a 5 degree roof pitch, which is currently fully enclosed. Our client wants to remove about 20 square metres of roof sheeting to allow part of the restaurant roof to become open.

I'm wondering if anyone knows how this will affect the wind pressure on the roof and walls? Generally the internal pressure would be equal to the external pressure at an opening. Therefore, having an opening in the roof with negative suction pressure should reduce the net wind pressure on the rest of the roof? Does anyone have any references for this type of situation? Thanks.

 

[DaveAtkins]

I would be more concerned with the effect the new opening has on the roof diaphragm.

Can I assume the new opening will be covered by a skylight? If so, there won't be any effect on internal and external pressures on the roof.

DaveAtkins

 

[CURVEB]

I agree with DaveAtkins. You'll need to analyze and detail the roof diaphragm with new chord members to get around the opening and make sure it has adequate collectors to handle the increase in shear stresses.

Regarding the roof pressure, ASCE7 has a procedure for determining if a building is partially enclosed based on the area of openings in the envelope. Partially enclosed buildings generally have higher pressures on components and cladding due to the increased internal pressure.

 

[RontheRedneck]

My first concern when reading this is that the framing members would have no lateral bracing.

 

[Euler07]

Thanks all. Just to clarify as I was unclear in my post; the purlins, diagonal bracing and roof frame will remain and only the roof sheeting will be removed. It will not be replaced with a skylight or anything similar to resist wind pressure or seal the building. I will be checking the purlin buckling (now without top flange restraint) and also the other framing and bracing members.

However, I was just wondering what the difference in wind pressure would be between a fully enclosed building and a building with an area of roof sheeting removed? I've looked for references and can't find any. Thanks.

 

[hokie66]

Euler07,
May be best to ask this question in the AS/NZ forum. The comments about diaphragm action are from US contributors. Australians don't use metal roofing for bracing. I don't know the answer to your question but would think the hole would make the remaining roofing more susceptible to being torn off. May have to use edge coefficients for the whole roof.

 

[XR250]

How is rust/rot going to be addressed?

 

[Euler07]

Thanks for all the help everyone. I will take this all into consideration and tell the client what I think. Also, rust is definitely something that needs to be considered as XR250 points out.

 

[hokie66]

Corrosion need not be more a consideration for an internal edge than for an external edge. That requires flashing, guttering, etc.