Perpendicular load at top plate

[tricalcim]

What UBC section should I be looking at for design load connection at the top plate. For instance, I calculated the wind load of 340plf normal to the top chord (top plate). Do I need to provide fasteners between the truss and top plate to transfer this load? Any thought are appreciated.

 

[AlohaBob]

The load you have is very large for a shearwall system. Nonetheless....if your trusses are 24"oc you must provide a shear connection to that chord strut element (the truss) for 680 lbs. Truss clips are not good out of plane (H1's only worth 165 lbs) for this load, so you need more than that. Try an A35 ( at least 645 ).

You'll need to engineer a roof diaphram. Spell out carefully the sheathing and nailing and varying nail patterns as required.

The top plate needs a check for bending out of plane. You may need to use a deeper section for bending and shear. Maybe even a triple plate.

It's likely you'll develop significant chord forces from diaphram action out of plane in the top plates, so they need to carry these in addition to bending and vertical load.

The code doesn't really tell you how to derive the forces that get developed, but a good wood engineering book spells out these load path design points. Just keep track of your unfactored loads so you can apply whatever proper duration factors. There's like 10 possible listed and their applications in the NDS for each component.

 

[tricalcim]

thanks AlohaBob - just a follow-up to your response. The A35's will probable be my choice. I believe I can account for some load transfer from the studs @16'' oc with 2-16d into the top plate.

Now, I'll design (or select) a roof diaphram based on the shear reaction and a calculated unit shear. Do you agree?

I am concerned about the bending stress. Your quick assesment is correct, it's over stressed for a double 2x6 top plate. However, I have not seen a triple plate in a 2-story house. I must not be thinking about this correctly, or being to conservative? What do you
think? Also, the double top plates need to function as one piece, so I'd need to design the nailing to make thes act as a composite, right?

Finally, I believe you are cautioning me to look at the interaction of bending and axial forces in the top chord, sounds reasonable, but I'm not quite sure if your thinking I should combine these with vertical, or just mentioning not to forget to check bending due to vertical?

Any comment is appreciated..............

 

[boo1]

I have designed structures with simular loadings. I added 2x4 on edge (toe nailed to the top plate) between the trusses. Then select connector for the load.

 

[AlohaBob]

Okay.

Vertical load: A roof truss might fall right between two studs. The plate must have enough capacity in weak axis to carry bending and shear.

Chord force:The out of plane load transfers to the roof diaphram like a normal vertical load goes over a beam. Only think of it as a truss and the top and bottom members... the chords are the plates. Pressure makes a compression in the chord. Suction makes a tension. Chord force wl*l/8d. l is the distance between shearwall lines for flexible diaphram. w is the out of plane load. d is the depth of the diaphram. Design your plate splices for continuity for this axial force in addition to the other forces.

Bending out of Plane. If you clip the plate to the truss out of plane with the A35 at 4 feet oc, the plate must act as a bending member over supports 4 ft on center. This bending stress must be added to the vertical weak axis bending stress.

I've had very tall studs that I had to clip with A35's to the plates because the shear transfer there was too great for end nail value for 16d nails. I think 2 end nails is the standard practice without mention.

The out of plane loads collect in your diaphram just like a beam with vertical loads, only it's tipped on the side. The shearwall total shear reaction is delivered from the diaphram V/length strut. In the case of the clipped truss, added load to the diaphram was 640 lbs. Added plywood shear was likely v(640)/d(Say 30' plate to plate at a rake wall) about 21 plf. Once you reach maybe 175 plf diaphram shear, you need to be particular about plywood layup and nailing, and shear transfer back out of the diaphram to your shearwall resolving the load to the ground.