plywood shear walls 1

[rittz]

Attached is a sketch of a simple wood frame building with a copuple of simple questions (my brain stalled) Also how do I calculate the shear in the roof diaphragm?

 

[rscassar]

The same way you calculate the shear in a beam. The roof diaphragm distributes the lateral force to the shear walls based on tributary area so the shear stress in the diaphragm will be greatest immediatley adjacent to a shear wall.

 

[rittz]

Where my paralysis is manifested is getting the contribution of the roof reaction from wind to the top of the shear wall adding to the wind on the side wall

 

[DaveAtkins]

1. Calculate the wind load (plf) to the roof diaphragm. This is equal to the wind on the upper half of the wall, plus the wind on the vertical projection of the roof.
2. Calculate the reaction at the end of the diaphragm (lbs). This is equal to the load found above times 1/2 of the diaphragm length.
3. Find the shear (plf) in the diaphgragm. This is equal to the reaction found above divided by the depth of the diaphragm.

DaveAtkins

 

[rittz]

Dave

That would give us the max shear in the in the end shear wall and the max shear in the roof diaphragm ...... where the roof is flat. Do we not want the shear in the roof diaphragm parallel with the rafter?

 

[msquared48]

"That would give us the max shear in the in the end shear wall and the max shear in the roof diaphragm"

This is true only if the length of the shear wall is equal to the depth of the diaphragm. If there are any openings in the shearwall or sections where it does not exist, then the load per foot in the shear wall will be proportionately higher than the diaphragm.

"Do we not want the shear in the roof diaphragm parallel with the rafter?"

Shear can go in any direction, but we normally analyze two for simplicity, and they are usually perpendicular to one another. The rafters will see both parallel and normal shear forces, but at different times.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rscassar]

Where my paralysis is manifested is getting the contribution of the roof reaction from wind to the top of the shear wall adding to the wind on the side wall

Sidewall pressures and internal pressures won't cause racking. It is analysed like a balloon with uniform pressures on each face. The walls are subject to pressures but the global force on the building is zero.

Are you concerned with the shear stress in the diaphragm in addition to the tensile stress caused from side wall pressures?

 

[a2mfk]

Not to sound like a jerk, but these are some very fundamental structural questions that I had in undergrad... Do you have a PE mentor at your office you can consult with?