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Wood Diaphragm Connection v.s. Architect's Ventilation 1

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bigmig

Structural
Aug 8, 2008
401
I have always been taught that the correct way to transfer lateral loads from a sloping roof diaphgram is to provide full height blocking at the rafter/stud plate interface. The wood diaphragm sheathing is nailed to the full height blocking which as the term implies, extends all the way to the underside of sheathing.

More and more I am finding that carpenters and architects do not like this detail and pretty much ignore it. My research of products on the market that provide ventilation show detail after detail of improperly installed blocking (partial height, held down to allow a "rafter baffle").

Well known wood product manufacturers on the other hand, show the correct details with "v" notches, alternating bays blocked etc.

When I point this out I am greeted with the following arguments:

1. We have never ever done it that way
2. Our propietary underwriter approved "rafter baffle" won't fit with your detail so we ignored it
3. Our roof can't breathe with your detail so we ignored it

It seems pretty elementary to me that you build a roof with full height blocking, but I am amazed that there are products on the market that not only ignore this, but make it impossible to install full height blocking.

Does anyone else callout full height blocking on their designs? Does anyone else notice architects trying to install partial height blocking and call them on it?
 
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Bigmig:
That they have always done it that way and haven’t had any problems, just means their buildings have never seen the code req’d. design loads and req’d. load paths, not that they are right. It won’t make you popular, but they bid the drawing which showed blocking, so build it that way. Alternatively give the owner some money back and a letter and guarantee against any and all roof problems for the life of the building, covered by a bond, not just their good name and good looks. Second alternative, let them sign the drawing for the building permit. Third alternative, notch the solid blocking up at the roof sheathing; blocking every other bay usually meets load needs, but leaves every other bay unvented.
 
Not providing adequate venting is just bad engineering, architecture, and construction. It will only lead to very expensive repairs down the line. It WILL happen.

Tell the Architect and contractor to provide the blocking with associated vent holes as you have prescribed, or you will have to recommend to the inspector, assuming you are not the inspector here, that the framing certification be denied pending poroper installation of the blocking per your requirements.

Sometimes you have to protect the client in spite of himself. You are the engineer of record here and have the license, the responsibility, and the authority. Use it.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
After the original post we found Section R602.10.6.2 of the IRC 2009 actually shows the partial height heel blocking condition in full view, with some explanation in the preceding section R602.10.2. about conditions where you don't even need blocking.

I guess diaphragms don't need attached and pigs can fly.....
 
And I guess cross-grain bending does not exist either, huh?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
For trusses with low heels (6" or so), blocking is probably not required. I did a structual analysis once, assuming the truss heels and roof sheathing act as a portal frame, and I was able to justify this.

But when the heels get tall, you do need blocking. When the heels are very tall (like 1' or more), the wall sheathing should be extended up over the heels.

DaveAtkins
 
Mr. Akins - for very tall heels, do you mean the sheathing should be cut around the trusses? I'm thinking specifically at a cathedral ceiling detail with the rafters extending beyond the wall to form and the soffits following the slope of the rafter.
 
yes.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
Mr. Atkins??? He’s gona get a big head if you keep talking that way. :)

But, I do suspect he meant that you would cut the wall sheathing around the trusses or the rafters. I depends on your exact detail and what you are trying to achieve, and how much shear flow you are trying to transfer from the roof diaphragm to the wall sheathing, shear wall. You pretty much need some sort of 2x blocking, ripped to the roof slope, btwn. each rafter pair, for nailing and shear transfer btwn. the two pieces (planes) of sheathing. You might want 2x blocking up each side of deep trusses also, to nail the wall sheathing to. Now, you probably want venting from the soffit, and right under the roof sheathing too, and that’s the labor part and trouble with the detail. Stand a 2x4 or 2x6 block up and slope rip its top to match the roof, in the middle 6" cut a 1.5" deep notch out of the top and hold the wall sheathing down 1.5"; you have venting and nailing surfaces for both plains of sheathing. Watch horiz. shear in the blocking and edge and end distances with the nailing and number of nails in the blocking. Lay the blocking 2x4 flat to the roof and rip its exterior edge, you got nailing etc., and drill the wall sheathing immediately below the 2x4 blocking with a couple 1.5" holes. There are a bunch of variations on this theme. How much shear flow are you transferring, number of nails needed, spacing, edge and end dist., etc. become the design problem. Bring full sheets of wall sheathing up to the double top pl., like normal; then cut 14.5" or 22.5" x ht. wall sheathing pieces for btwn. rafters or trusses. Now the trick is transferring the shear flow across the horiz. jnt. at the bottom of these smaller sheathing pcs. and at the top of the full sheets, otherwise all else is as above.
 
Sorry this is long, this is a great subject because there is not that much out there written about it that I have seen. There was an article in the last couple of years in Structural Engineering about this subject, and though the author missed a couple of points I thought he did a good job.

Doing most of my engineering work in Florida over the last 13 years, sometimes with buildings that have some pretty big heel heights, this has always been an issue I have struggled with, especially since it seems many engineers/archs do not consider this (or at least it was not that common of a practice, thus the funny looks and comments from contractors).

I have had the argument with some pretty decent SEs about the need for blocking for proper shear diaphragm transfer, I never liked that "it just gets there through the truss"... Especially if you have ever read through truss mfr or uplift connection specs and guidelines, they warn about this phenom and that their trusses do not resist out of plane lateral force and neither do most straps.

The author of this article mentioned a method of essentially using the uplift strap that wraps over the top of the truss to resist the overturning moment caused by the shear force at the top of the truss. Maybe this is what Dave means by portal frame but I am not 100% sure. But you would also have to add in the uplift reaction to that tensile force of the strap, and the in-wall-plane shear via an interaction equation. Depending on where you are in the country and the size of your roof span, and your diaphragm forces this could be a a big problem getting it to work. Not something I can see working very often with structures I have designed, but that is mostly in high wind territory.

I have had to use connections that were maxed out in uplift, then put in blocking in every other truss space with a Simpson HGAM or similar just for shear.
Other options with low heels are specifying truss connections with an in-plane (of the wall) resistance so the shear goes through the mechanical connection, but these are limited in their capacities and applications also. Then there is always diagonal 2x4 bracing and tension straps.

I had a project with 2ft plus heel heights in V=130mph plus I=1.15 that I ended up doing little mini shear walls in between every other truss, the diagonal forces and amount of strapping got too ugly for diag. bracing and the uplifts were large enough all by themselves.

Lastly, you have the condition of out-of-plane lateral loading where you must check the strap for uplift and the reaction at the top of the wall from the lateral force into the truss (1/2 height of wall x 2ft x lat wind pressure)... Sometimes one connection just cannot be strong enough in 3-D!!!

Like to hear what others have done in these situations... Talking mostly CMU walls but wood too.

Andrew Kester, PE
Structural Engineer
Florida
 
I had a project with 2ft plus heel heights in V=130mph plus I=1.15 that I ended up doing little mini shear walls in between every other truss, the diagonal forces and amount of strapping got too ugly for diag. bracing and the uplifts were large enough all by themselves.

For really large truss heel heights (>2ft), I like to call for some blocking trusses in between the truss heels to be designed and provided by the pre-engineered truss supplier.
 
I did a structual analysis once, assuming the truss heels and roof sheathing act as a portal frame, and I was able to justify this.

How did you prove that you have fixity in the sheathing connection to the top of the truss heel? Also, did you consider a continuous portal frame with multiple bays? Or did you just look at a single frame?
 
This is a good discussion.

I have used the Simpson TBE (truss bearing enhancer) for short heal heights in the past. Cross grain bending is still present, but to a much lesser degree.

For greater depths, I have been using the Simpson RBC. You can provide a 3/4" gap the full length of the truss space. If more area is need than what the 3/4" provided, I have been doubling the RBC's in a space. The next space or spaces will have partial height blocking. If more than 1 partial height exists in sequence I add a coil strap over the top to act as a drag between the spaces with the RBC's.

I would like to hear some opinions if you have them.
 
Regarding the tall heel situation, I have done what others mentioned above--build mini shear walls between the trusses. Then the wall sheathing just continues up past the double top plate to the top of these mini walls.

Regarding the portal frame analysis, I was looking at some low heels (6" to 8" as I recall) on an existing one story buidling, and I did not want to make the contractor install blocking. I used all of the trusses to make a very long, multiple bay portal frame, and used the Simpson hurricane ties for the uplift and out of plane lateral force (it has been awhile--doesn't Simpson give out of plane capacities for some of their ties?). I assumed the nailing between the roof sheathing and the top chord of the truss could resist some moment.

Ever since then, I haven't asked for blocking between low heel trusses.

DaveAtkins
 
Dave,
Simpson gives uplift, out-of-plane and in-plane allowable reactions, using F1 and F2 type designations with diagrams to clearly indicate the direction. Some connections have low to zero F1 and F2 resistance. My problem has always been when I begin combing F1 or F2 with uplift in an interactive equation. So blocking, strapping, etc. will at least eliminate in-plane shear from the connection making it more workable in high shear and uplift situations.

I can see why you would want to make no blocking work on an existing roof!

Thanks for this discussion guys.

Abusement park- thanks for the link, I was sniffing around the wrong magazine website for 20 minutes... I like how a lot of these magazine articles allow you to save them as PDFs, sure makes making a library a lot simpler.
 
DaveAtkins,

I had the same question in my mind as abusementpark asked...I don't see how you can transfer much moment through a single 2x member with a single row of nails in it. Your fastener arm to resist bending is only 3/4".

Regarding the limits of when to use blocking, when to not use blocking, and when to building "mini shearwalls" between truss heels, IRC 2009 Section R602.10 includes paint by number pictures as well as heel height parameters. But like
Mike pointed out earlier, they all assume the members can perform in crossgrain bending.

Whoever wrote that portion of the code had to balance between ventilation failures and roof shear failures. I personally have seen multiple ventilation failures, but not a single roof shear failure, despite the fact that about 95% of the industry does not building blocking to the sheathing. In all reality the load paths from roof sheathing to top plate and shearwall are going through things like cross grain bending, fascias, soffits and gable end lookouts in a complex indeterminant way (assuming partial height blocking). Combine that with temporary load duration from wind, the chance that the structure has never seen a design wind load etc. and that would explain why it is still standing.

The 2009 IRC pretty much agrees with that which is why they allow a 2" gap between blocking and roof in every case.
 
Attached is a pdf from the WTCA (for wood trusses) on heel blocking which includes a standard heel truss without blocking per HUD, 2002 information. It also states that the EOR is the responsible party for heel blocks. Given this I am not sure if I would allow this as the truss design is per someone else. But, if the truss engineer stated on the truss designs or in a letter that their truss designs will accommodate a certain shear load to the wall, I would accept this.

Also note that Simpson states that "Hurricane Ties do not replace solid blocking."

Garth Dreger PE
AZ Phoenix area
 
 http://files.engineering.com/getfile.aspx?folder=d6d49918-5d45-402d-8cc3-994862ad81ef&file=T-HeelBlocking08.pdf
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