Perforated Roof Diaphragm 2

[LPPE]

I've got a one story commercial structure (metal deck, bar joists, steel girders), and the roof is essentially perforated with sky lights all over the place. How many penetrations (maybe as a percent of the overall roof area?) are allowed before you have to reevaluate the diaphragm strength? Or can you make the frame around the openings rigid enough to transfer the diaphragm forces?

 

[ishvaaag]

I would go the analysis way. You are getting structures akin to shearwalls, and there the links if not the main "members" behave as beams, that is you have compression and tension. In the plied direction compression is not feasible except you reinforce all along the length.

Hence it may turn cozier directly analyze with membrane or so elements and see what is happening.

 

[JAE]

I would use a reduced amount of diaphragm length across specific lines of shear when checking my diaphragm shear capacity. The openings will simply reduce the amount of deck participating in the total shear strength.

On the stiffness/deflection side you can break down the diaphragm into individual pieces and add together their individual portion of stiffness to get a somewhat accurate prediction of diaphragm deflection.

So lots of openings do reduce the shear capacity and stiffness, its just the means to calculate it all that get a little tricky.

 

[dik]

If there are a large number of openings, in particular in the 'middle' of the structure, you may want to consider going to the old fashioned x-bracing in the plane of the roof... The analysis time for wierd and wonderful openings will approach the cost of the skylights <G> and, all said, it may be less expensive to use bracing...