Big opening in existing wood diaphragm

[nickky]

Hi,

I have an existing warehouse building 62'x60' in floor plan. The structure at this moment is an "empty box" i.e. four walls with an old roof on it. The roof has four 60 feet long big trusses with arched top chord.

The Client wants to construct some offices inside. Along with this internal improvements the architect wants a big (8' radius) semi-circular opening as skylight. (please see the plan attached)

For now, I assumed the opening to be 8'x16' rectangular and specified 4x drags on two sides (parallel to framing) and 4x blkg + strap along the other two sides (perpendicular to framing)to redirect the shear around the opening. I extended the drags/straps 8' beyond the opening to make sure the shear will be distributed safely. The original roof sheathing is 1x flat.

I am trying to do a more detail analysis of this situation to justify my design by numbers but I am not sure about the type of assumtions and approaches that I have to adopt. Any hints or guidance is deeply appreciated.

I did looked at a short discussion about openings in diaphragms in "Wood engineering handbook" by Keith Faherty but I am not sure I can use the same method here.

Thank you very much.

 

[msquared48]

I would recommend two things to start:

1. Extend the drag struts to the exterior walls in both directions around the opening.

2. A 1X flat sheeting (I assume planking) is generally good for only about 100 #/ft in shear for planking that is non-diagonally laid, more if diagonally laid. Depending on your diaphragm shears seen, you may have to top the planking with 7/16" CDX plywood and re-roof.

Mike McCann
MMC Engineering