Definition of a Roof Diaphragm vs Wall

[sgs114]

Hello,

I have a building with a very steep roof. It makes an angle of approximately 79 degrees to horizontal. We are using braced frames and I am looking for some definition about walls vs roof diaphragms so we can figure out the required extents of our frames. Anybody have any information on this? Severely sloping roof diaphragms?

Thanks,

SGS

 

[KootK]

That is a very steep roof. Your roof and your walls are both diaphragms and both need to be restrained from racking. So I'm not sure that it matters to much what you call them. Is there a particular aspect of the design that concerns you? Effective seismic height etc?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I would also be concerned about the wall/roof interface buckling in or out with out-of-plane wind loads. Geometry is not doing you any favors here. Your ridge beam and connection at the base of the wall are going to take a hit from the resulting substantial up and down loads.

 

[KootK]

Further to XR250's insightful comment, it would be nice to have a flat, ceiling level braced plane if the building use would permit that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

At 79 degrees, it is closer to a shear wall than a roof diaphragm, will not see much if any snow, but will see wind.

Connections aside, it is screaming "WALL" to me.

Mike McCann, PE, SE (WA)

 

[Triangled]

kind of agreeing with Wall vs Roof here...thinking you're beyond the codeaphiles here and coming face to face with Mama Nature whose laws seem to generally prevail over the code's estimations.....

 

[CBSE]

I would agree, this should be treated as a wall.

 

[oldestguy]

What do you name the feature on the top part of a church steeple?

 

[KootK]

What do you name the feature on the top part of a church steeple?

Liability.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sgs114]

Given that we are proceeding treating this as a wall, we are planning to use BRB's for the lateral system. Does anyone know of any restrictions or installing/designing these not perpendicular to the ground? Are there any restrictions on the inclination angle of the frame? Again it will be about 80 degrees from horizontal. Thanks.

SGS

 

[msquared48]

BRB ?

Mike McCann, PE, SE (WA)

 

[sgs114]

Buckling Restrained Braced Frame, guess it's BRBF. Sorry.

 

[KootK]

Are there any restrictions on the inclination angle of the frame?

I've never seen anything in code that addressed this particular situation. An interesting feature of the design will be bracing the point where your two stacked BRB's meet and there is the angular change. Monster out of plane load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Is there a floor diaphragm at the change in inclination to take the out of plane loads to transverse frames or shear walls?

Mike McCann, PE, SE (WA)

 

[sgs114]

msquared48,

There is no diaphragm at the interface to take the OOP loading. So yea, those loads will be quite high and we will have to figure out a way to deal with that.

SGS

 

[wannabeSE]

I don't know of any restrictions on sloping the frame. But, I would call the brace manufacture. Last I heard, there are only three companies that manufacture buckling restrained braces.