Wood building, roof sheathing cutting 2

[bonbon83]

Hi all!

I had a question regarding the wood building in high seismic region,
a client has asked me to put an equipment (weight around 14kN) on the roof of a 3 story wood frame building . by analysing i found that the roof framing below requires reinforcement and the only way to reach joists is by cutting the sheathing from the roof. however I am afraid of cutting the roof sheathing (diaphragm) to avoid disturbance to seismic response of the roof. I am thinking even if they cut and redo the sheathing (fill the cut portion by ply-wood) there would be stress concentration at the cut edges/corners. I am not sure if I am correct a. that would be great if you guys with similar experience kindly share your thoughts and solutions.

Cheers!

 

[driftLimiter]

You could have them remove & replace full sheets to match the original layup. But I agree that you would want to look into this.

 

[jayrod12]

As long as there's blocking and appropriate nailing at the edges of the repair area for both the existing sheathing to remain, and the new sheathing, I don't see the issue?
Might need some straps to tie the chord forces if that is the case, but I still don't think this would be a big deal. That being said, I don't practice in high seismic areas.

Do you regularly include plywood layouts as part of the drawing package in high seismic? If not, how do you ensure something similar isn't done on a new build?

 

[driftLimiter]

I typically indicate layout requirements for diaphragms that show the long dimension of the panel perpendicular to the framing, and full size sheets, along with exceptions to full size sheets at the edges per NDS. If it is a really low utilization diaphragm then maybe not, but generally I rely on a specific layout pattern for diaphragm strength per the SDPWS.

 

[bonbon83]

Driflimiter, I like your solutions, as long as client is ok with the cost of labor and roofing.
jayrod12, I like your solution as well. is it doable to locate the blocking at the edge of the opening? I am thinking of one side of blocking is accessible to be connected to the joist, what about the other side? this is the first project in high seismic zone I am dealing with.

thanks guys!

 

[jayrod12]

I think you need to remove the sheathing such that you can adequately fasten the blocking at each end. That likely means enlarging the sheathing removal area to the next joist.

I do understand the intent behind an optimal sheathing layout, however I feel depending on the size of the opening to be re-sheathed, that provided you design the transition into and out of the repair area properly in terms of shear transfer at the joint and chord continuity as required, then it shouldn't really be an issue.

 

[MotorCity]

It depends. How big is the opening? How big is the roof? Are you cutting a 5' wide opening in a 20' wide roof?
The seismic load is probably smallest at the roof.
Its also most likely performing as a flexible diaphragm which will distribute the load based on the tributary area of the LFRS.
As such, the load will find an alternate load path around the opening.
That alternate load path could consist of subdiaphragms, blocking, etc

 

[bonbon83]

thanks motorcity, the guy is saying joist are 2X10, 14.5 feet long, 24" apart. I am thinking of putting (2) 4X4 HSS beam b/w joists and to be supported on shear walls and then (4) stubs on hss going out of the roof ,the equipment will be installed on those. therefor the size of the opening would be 14.5 feet x 5 feet ? not sure if there is blocking b/w joists. old building.

 

[jerseyshore]

I don't work in high seismic areas, but most times when sheathing is removed (either cut out and/or by prying out the nails), to re-install, they add a "ledger" to the side of the existing joist for the new sheathing attachment. Then blocking in the perpendicular direction to support the other edges of the plywood. Makes it much easier to re-nail the new sheets without having to worry about how good the original workmanship was when lapping at a joist. So that's what I would shoot for here.