Wood framed building diaphragm chord forces.

[SteelPE]

I am currently designing a wood framed building with wood bearing walls and a plywood diaphragm (single story). I have design almost everything but I am stumped when it comes to designing the diaphragm chord.

I have calculated the chord force for this structure to be 9,000lbs which occurs in the middle of the building. Currently my trusses are 4’ deep bottom bearing members so there is not “plate” to resist these loads. I have thought about using Simpson “Coil Straps” but I would need a few of them to develop the required capacity. How are these loads handled in this instance?

Code IBC 2009. This is a commercial building.

 

[msquared48]

If there is no "plate", what do the trusses bear on? Are the walls CMU?

Mike McCann, PE, SE (WA)

 

[SteelPE]

The trusses are bottom chord bearing and I am using the diaphragm supported by the top chord to resist lateral loads. The diaphragm is located 4' away from the top plate of the wall. I wouldn't think you could transfer this loads down to the top plate of the wall.

 

[BAretired]

Can you transfer the load down to the top plate by providing adequate cross bracing between trusses?

BA

 

[JAE]

We sometimes specify that the top corner of the trusses be fabricated with a 2x or 4x block out for a continuous set of boards.

For lighter loads these might be a continuous 2x6 or 2x8, running horizontally such that the top of the 2x is flush with the top of the truss.
We then use strapping to tie them together at the splices.

It's a bit of a pain for the truss manufacturer to block out a 1 1/2" wide x 7 1/4" deep notch in the top of their trusses, though.

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[SteelPE]

BA,

I'm not sure I would be comfortable using such bracing to transfer the loads through to the plate.

JAE,

I had a similar idea after I started this thread. However, I am thinking about adding a 2x8 to the underside of the top chord and then using straps to attach them together (see attached). It should work and is no better than what you would end up with if the trusses were gable type.

This problem has been in the back of my mind for a while.

 

[XR250]

Good luck. I usually specify top bearing trusses and balloon frame the wall.

 

[mike20793]

SteelPE,

I do exactly what you show in your detail and it works great as a diaphragm chord and nothing else. I nail a 2x6/8 continuous along the bottom of the top chord of the trusses. I splice that member with a coil strap so they can splice it where ever they like. As the diaphragm deflects it engages the chord member. Getting the shear down to the walls below is another story. For that, you will need a shear panel or some kind of bracing to transfer the load out.

 

[msquared48]

Steel PE:

Comfortable or not with the bracing, if you do not use it, or a shear panel, how are you going to transmit any shear forces from the roof diaphragm, wherever the chord is (it really doesn't matter), to the shear wall(s) below? You have to have a physical mechanism through X bracing, blocking or shear panels, or the trusses will just roll over.

Mike McCann, PE, SE (WA)

 

[SteelPE]

BA,

There is bracing there, I'm just not using it to resist the chord forces. I feel much more comfortable with the detail I provided above. If the X bracing helps transfer these chord forces then I guess it is just a belt and suspenders situation.

FYI, I have a colleague who agrees with you.

 

[msquared48]

In that case, the chord can be either above or below the bracing. Your choice...

Mike McCann, PE, SE (WA)

 

[mike20793]

I guess I don't see why the bracing or shear panels are required in the direction of the chord member since it is perpendicular to the applied diaphragm force. The chord is resisting a tension and compression load cause by bending of the diaphragm. It really has nothing to do with transferring shear at the diaphragm supports; for that you will absolutely need bracing or shear panels. The wall below might not be a shear wall, so it may not have any shear panels at all. This is usually the case for multilevel wood buildings with plan offsets at exterior walls.

 

[msquared48]

Well, I see what you mean, but you do have chord members on all sides of the diaphragm. The shear panels, or end wall truss (rim truss), would still be needed to transfer the shear from the other direction, so they would still have to be there.

Mike McCann, PE, SE (WA)

 

[JAE]

Here's something similar to what I described above:

and another one:

This is much easier than fitting in individual pieces between trusses and using miles of strapping.



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[KootK]

This is a pretty tough nut to crack without splitting the trusses at the bearing wall. The sketch below is essentially just another permutation of the shear panel concept. It seems to me that you'll want the lateral load dragged down to the sill plate level for lateral in the other direction anyhow. Might as well use the same system to enable the top plates to serve as your chords as well.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

On second thought, I think that a more conventional shear panel would be more constructable. The trick would be shifting the vertical web member over top of the support to eliminate the need for tie-downs on the panel. That ought to suit the truss fabricator better anyhow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

KootK - those are nice but isn't the original question about diaphragm chord forces? Your details are dealing with lateral force transfer between a diaphragm and a shear wall I think. Or am I seeing your sketches wrong?

The OP was asking about how to establish a diaphragm chord across the width of the diaphragm, not the sides.

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[KootK]

KootK - those are nice but isn't the original question about diaphragm chord forces? Your details are dealing with lateral force transfer between a diaphragm and a shear wall I think. Or am I seeing your sketches wrong?

No, you've got the gist of it. If SteelPE really and truly only needs a chord, and not a drag strut for orthogonal lateral loads, then he's answered his own question: the continuous 2X strap is the way to go. I find that highly unlikely, however, and suspect that shear transfer to the plate level will be required for drag loads for lateral in the orthogonal direction (ms^2's point as well I believe). And, as I mentioned above, if that's the case then it makes sense to use the sill plates for both purposes rather than install dual systems).

Given the length of the cantilever, one could argue that the chord members should be near the fascia in the interest of strain compatibility. That's an issue for another thread however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

I've used the continuous strap to avoid chord discontinuities caused by plan offsets in the diaphragm and having to detail them.