Drag truss/diaphragm design?

[ADAMK2310]

Given:
2-story building.
Wind controlled design.
Shearwall located under drag truss.

Is it necessary to design the drag truss to transfer the roof wind loads (wind loading on roof only) OR does it need to transfer the diaphragm loads (wind loading on roof + half of 2nd floor wall height)?

I know that the shearwall load at the top of the 2nd floor is the diaphragm load, but is it necessary to design my drag truss for this total load?

 

[JAE]

Is the truss under the roof or under the floor?

 

[JKStruct]

Do you have any diphragm at the truss bearing elevation? Or is the wind load carried up the diaphragm attached to the top chord of the truss? If it's the latter, then yes, design for the roof wind load and half the wall height. Think of the section of wall exposed to wind between two shear walls. Wind blows on the wall, half travels to the 2nd floor diaphragm, and half to the top of wall/truss bearing. Since the diaphragm is on the top chord of the trusses, your collector truss has to transfer this down to the shear wall.

 

[msquared48]

As I understand the question, the drag truss would only have to drag in the forces from the roof diaphragm to the shear wall underneath, let's call it shear wall "A", shear wall "A" being between the second floor and roof diaphragms.

If there is a shear wall "B" below shear wall "A", then that shear wall would see roof plus second floor lateral forces, but not shear wall "A".

Mike McCann
MMC Engineering