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blocking at eaves

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Martin:

1. Do you require eave blocking?  2. If the forces at the roof diaphragm were greater or less than about 200 plf would your answer to #1 change?

I would not cave into arguments for eliminating eave blocking such as " lots of older houses without blocking that have withstood the test of time and of course  and "in the lower 48 nobody blocks at the eave". To me it is a case by case basis, and it depends  a lot on heel height of the truss or depth of the rafter at the bearing wall, size of the building, and obviously the shear in your diaphragm. Some engineers think that the force "just gets there" from the diaphragm to the wall. But ask a truss manufacturer if they designed their truss with a 2ft heel to take 400lb out of plane, and they will either hang up the phone or say "uhhhmmmm, NO".

Most of my projects are in wind-only country, but we get some very high shears in single story diaphragms, although I don't do residential. I would think even if solid blocking was required by design or code, that you could allow holes drilled at certain spots in the blocking (for ventilation) and you would still get enough shear out of the member. I am not sure how to quantify that.....

Some options I think work well other then solid blocking:

-Diagonal tension straps (Simpson) to take the shear down to the base of the adjacent truss. But that truss connection has to be sized appropriately for that shear and combined with uplift.

-Mini-shear walls at every third or fourth truss, with independent tie downs for OT. Some may argue that the plywood, laid lengthwise across the trusses, will act as the diaphragm shear strut and carry it to that shear wall (which is the way a diaphragm transfers shear, to my understanding). I prefer a little 2x4 blocking and a flat strap at the diaphragm boundry element that directly ties the roof diaphragm (plywood) into the shear walls.

-Solid blocking at every third or fourth truss space (depends on the shear), tied together as mentioned above. The blocking has its own connection to take the shear into the bond beam or top plate. If the shear is not too high, the truss connection itself may handle the shear.

-Not a contractor favorite, or my favorite, good ol' 2x4 diagonal bracing. This is labor intense and a pain to install. Also getting a good mitre cut and a snug fit from top chord to bottom is pretty difficult. You still have to make sure the truss connection takes the shear and uplift. These are compression only, so you need one at each truss space, unless you combineit with a strap for tension.

I'd love to hear others ideas on this issue, or if they think any of the above are crappy ideas.

5. Do you see any problems with a discontinuous roof diaphragm at the roof peak because it was cut for ridge vent placement?

I think I remember a discussion on this list about ridge vent cuts not affecting the diaphragm, but our APA buddies can probably chirp in about that. One argument is at least in shear in the diaphragm parallel to the ridge, the theoretical shear in a rectangular roof will be zero at the middle (ridge). Perpendicular the truss top chords act as drag struts to carry the shear past the ridge. Just some thoughts...

Andrew Kester, PE

Longwood, FL