Gypsum board for uplift?

[Jake927]

I am designing a multistory apartment building that using pre-fab wood trusses for the roof. The diaphragm is able to span to the exterior walls, but we need to use an interior bearing wall as shown in the attached detail. So, this wall does not need to be a shear wall, but it does need to be used to transfer uplift from the trusses (The building is in a high wind zone along the east coast). My question is if I need to sheath up the walls with OSB or if I can rely on the gypsum board for the uplift. My thinking is that this should be fine since GB is rated for unit shear. I'm just not sure how to show that it is okay for the uplift in a calculation though. Thanks in advance for any insights here.

 

[driftLimiter]

I wouldn't rely on the GB, someone could come along one day take all the gyp off and let it sit.

You have a perfectly good load path through the studs. I would either have the studs anchored to the DBL top plate with screws or simpson connectors, or align the studs with the roof framing and use a connector to get them talking.

 

[Jake927]

That's a good point @driftlimiter. I think that some angles from the studs to top plates would be the way to go since screws from the top plates to studs shouldn't be relied on for end grade tension.

 

[Jake927]

Or something like a Simpson H6 stud to top plate connection

 

[driftLimiter]

There are some screws similar to timberloks that are tested for this application and installed like toenails.

In my experience the guys in the field may be somewhat opposed to changing something that appears as simple as wall framing. So the simpler you can make it the better.

*edit** Example Simpson SDWC truss screw

 

[KootK]

Where does the uplift tension in the wall studs ultimately need to wind up? Is it the floor framing at the level below that you need to grab?

 

[Jake927]

@KootK
The uplift will need to go into the studs at the top story then also through the floor system and into the studs at the level below. There will be a 1.5" concrete topper on each floor, so Once it gets down a couple stories, the dead load of the structure should be able to resist the uplift force. The bottom wall sole plate will still have anchorage to a continuous footing though.

 

[phamENG]

I practice on the East coast just north of what is generally considered "high wind zones", but get the occasional project with 130+mph wind speeds.

My go to for interior walls is the SDWC screw the drift mentioned and then floor to floor screws from sole plate down to top plate. Simpson, with there wonderfully efficient marketing and engineering departments, have all the information you need in handy tech bulletins.

 

[Tomfh]

Use the studs for uplift. The only reason to use gypsum for shear is it comes in big panels.

 

[phamENG]

I think the OP is referring to the use of OSB/Plywood for transferring uplift between components of a wall, particularly from top plate to stud, stud to sole plate, and also stud to stud between floors. (For those not familiar, the SDPWS has a special section in the back devoted to details and shear wall capacity adjustments if you use shear wall nails and sheathing to also transfer this uplift.)

The issue is the edge distance. Having a bunch of screws spread over the whole board, you get plenty of strength for lateral loading as long as a) it's not cyclic and b) it doesn't get wet. With the uplift deal, you're ultimately relying on a row of fasteners 1/2" from the edge of the panel. Ever worked with drywall? Those will do okay to hold the edge of the panel to the studs, but the gypsum between the fastener and the edge is going to be pulverized on 50% if they guy swinging the hammer or using the driver has been doing it for less than 10 years. In other words, those few critical fasteners will have 0 lateral capacity for uplift in most situations.

 

[Tomfh]

Yes he wants to use use the gypsum as a tie down mechanism. But why would you when you have studs which can be fastened together directly. Why take the loads thru a flimsy bit of drywall. As you point out a screw can be pulled off the edge of drywall sheet very easily.

Drywall sheet is a good shear member compared to studs, but a worse tension member.

 

[phamENG]

I feel like we're getting pedantic...

The studs are the tension member regardless. Even when using plywood on the exterior, the tension passes back and forth between studs and sheathing at joints in either. So it's a question of how you hold your studs in tension together, not if the studs will be in tension.

Gypsum can't do it the way plywood can. You need discreet connections at interior bearing walls with uplift.

 

[Tomfh]

So it's a question of how you hold your studs in tension together,

Yes I understand that; it's a question of how it is all strapped together. OP is proposing gypsum strapping, so to speak. This is a bad idea, as we both seem to agree. The studs themselves are fine. Just fix them together directly. Or yes, with plywood sheeting.

 

[RontheRedneck]

The note on the section view bothers me: "Truss ends tight fit abv CL stud wall below."

That detail looks nice on paper. But in reality it doesn't work well.

Walls aren't perfectly plumb. Top plates aren't perfectly straight. Truss members aren't cut to exact lengths. The ends of trusses aren't perfectly square.

I strongly recommend lapping trusses over bearing walls. That gives the framer some play to work with.