Offset wood shear Walls

[bdlc2k]

Hi, everyone
Attached is a floor plan of a 2story wood frame home. I need to transfer the 2nd floor shear wall (dashed line) to the 1st floor wall to the left and below (garage opening line). I plan on using the floor trusses to act as 'drag struts' where shown.
What I assume I can do is use the floor diaphragm (plywood or OSB) to transfer the wind load from the 2nd floor to 1st. The 2nd floor wall shear is about 250plf.
Am I correct in assuming that all I need to do is develop the floor diaphragm to take the 250plf plus the ordinary wind load with correct nail pattern/blocking? (Blocking between trusses is in 'X'). I plan on using 2X6 joists outside of the truss floor area.
Do I need anything special other than the plywood diaphragm? Please tell me if I'm missing something. Your comments and input is greatly appreciated.
Thanks!

 

[msquared48]

Relatively speaking, the plywood diaphragm should work fine with the blocking for the 250 plf value.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[bdlc2k]

Hi, Mike
I have read your replies before in other forums, and I appreciate your input and experience.
1-If I'm correct, in a flat scenario, the truss would act as a drag strut?

Also, can you look at the section attached? This is what the house looks like (kinda). Since the floor trusses will not align with the top plate, can a partial "side roof" be used to transfer from the 2nd floor shear wall to the 1st floor as shown? I have seen this before, but I'm not sure if it is a proper way. Any suggestions? Thanks again.

 

[woodman88]

No the truss is not a drag strut. You are transfering the shear load from the upper wall into the lower diaphragm to the lower wall. The only shear the truss transfers is the amount it takes to deflect its bearing walls or to receive the wind/seismic loads for the bearing walls.
As for your second detail the edge nailing for force transfer of the wall to lower diaphragm needs to be detailed for the force.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[EngineeringAdam]

bdlc2k,

Your second section is something we try to avoid in our office. i.e. having a second floor wood shear wall shear transfer through the roof diaphragm to an adjacent first floor wood shear wall.

Also, where does the second floor diaphragm shear go? If you had the second floor wood shear wall plywood continue down past the rafters with edge nailing to the second floor you could assume that the second floor diaphragm force would travel up the second floor shear wall, across the roof plywood and to the first floor adjacent shear wall.

However, not ideal in my opinion.


What we do is add floor plywood to the ceiling joists to get a direct shear flow transfer - this is a little tricky because you will need blocking and clips near the end of the eave to transfer the shear from the CJ's to the adjacent first floor wall.

 

[bdlc2k]

Thanks for your input guys.
EngineeringAdam, we are going to use the area on the left for attic storage, so I will be sheathing the deck. Thanks again.

 

[EngineeringAdam]

No problem - that's a welcome solution, using attic plywood for the shear transfer.

 

[bdlc2k]

Hey, Guys
One question about the shear transfer. Say the diaphragm shear from half of the 1st and half of the 2nd floor is 200plf to the wall on the left (1st graphic). How would you "add" the 250plf from the 2nd floor shear wall to the 200plf on the first floor? Would you divide 250plf by 2 and add 125plf to the 200? Thanks again.

 

[msquared48]

Think of the top shear wall as a point load to a simple beam that is supported by the two shear walls below. Just distribute the forces to the walls below in the same manner as you would calculate the reactions from a simple beam with a point load.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com