Diaphragm Analysis with overhangs 2

[reverbz]

Hey guys,

I'm working on a building that has roof overhangs on all 4 sides over a porch. How do you typically approach this from a diaphragm design standpoint? Do you treat it as a 3-sided diaphragm? I've attached an image of the general layout.

 

[phamENG]

Whether or not it's 3 sided comes down to the arrangement of vertical LFRS elements.

 

[reverbz]

I have walls as shown below on all 4 sides @phameng I'm just trying to decide how to account for the diaphragm sections hanging over the walls

 

[STpipe]

Conceptually, a diaphragm is just a deep beam. So assuming you have a lateral load in the N-S direction, then the diaphragm is just a beam with two simple supports with a slight overhand beyond each support. You'll end up with load reversal for your chord axial forces since you'll have both positive and negative bending along the length of the diaphragm. Then you just need to add collectors to get the forces from your overhangs into your vertical LRFS elements.

 

[phamENG]

Also depends on the shape of that roof. If it's flat with no parapets, then the in-plane load in those overhangs will be negligible. If it's a hip roof, still pretty small. Gable, one direction might matter, but in the grand scheme of things is still unlikely to make much of a difference. Parapets, then yes - you'd look at the overhangs as cantilevered sub-diaphragms.

The load is going into the diaphragms from the walls below, so if there are no walls, there's no load being added but any that is directly applied to the roof surface.

 

[reverbz]

@STpipe @phameng thank you, that makes sense I'll give it a shot just need to remember the fundamentals like you mentioned.