Torsional Rigidity of a Metal Deck

[GalileoG]

Does the behavior of a deck (whether it is flexible or rigid) make a difference on whether or not a deck can transfer torsion? (or possess torsional rigidity) or are they independent concepts?

I am working on gas station where the canopy is L shaped and is attached to the store. The store is braced with x-bracing and the canopy has cantilever columns. Because the store is more rigid than the canopy, I see the building taking all the lateral load. However, the center of mass is ways from the center of rigidity and I have significant eccentricity. I am curious if an L-shaped metal deck will be able to transfer full torsion into my store.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

Draw a plan view of store and canopy with roof elevations.

BA

 

[GalileoG]

Hello BAretired,

Please see attached sketch.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

I would not attempt to tie the two together for lateral forces. Make the canopy stand on its own, but because it is so flexible relative to the store, hinge the short wall between the two roofs top and bottom.

BA

 

[Ron]

Clansman...BA is right..I've designed hundreds of canopies...almost alway free standing. Make it independent of the building.

As for torsion in the deck...not usually a consideration. Biggest controlling element is moment in the columns.

 

[rowingengineer]

agree with BA, I doubt the deck can transmit the torsion.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[GalileoG]

Thanks all for the great comments. I should have probably mentioned this in my original post: the building is existing and I am working on an addition to the building and the canopy (making both bigger.) The existing connection between the canopy and the building is not hinged (all around weld at the bottom and a bolted cap plate at the top.) I would rather not touch anything on the existing structure. So my model will have to reflect reality when I'm checking the bracing capacity in my building (which, because of the rigid connection between the building and the canopy, will take lateral load from the canopy.) So from my understanding from the responses posted here so far, a metal deck (flexible diaphragm) does not possess any torsional rigidity and therefore I will have no quantifiable torsional effects on my structure. Did I misunderstand the responses? I will however design the addition to the canopy as if it is acting alone.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[Ron]

Correct

 

[vandede427]

I've done a few of the gas station buildings (not the canopies) and usually the two columns on either side of the main entry door are used as canopy columns. it's not something I did, the architect did it. I personally don't have a problem with it.

I usually design metal deck as a flexible diaphram that doesn't transmit torsion, but I think the definition of a rigid diaphragm is based on diaphragm deflection. If your story deflection is really small, you could consider the deck as rigid.