Diaphragm Boundary Nailing Revisited (Again...)

[StrEng007]

I practice in an area with extremely high wind demands (170MPH and above) and have always required full height blocking to maintain [highlight #FCE94F]diaphragm boundary nailing[/highlight]. Whether this be through nailed blocking w/plates, or using something like a (Simpson) roof boundary clip, it's always there. It's alarming how often I get push back on this from the contractors.

Having combed through this time and time again, I often go through the following sequence of design aspects and responsibilities. In the end, I consistently end up requiring the full height blocking. For those of you who have solidified your design on an alternate solution, do you mind discussing how you get around requiring full height blocking at truss heels? If you're going to suggest a structurally sheathed hybrid soffit/diaphragm, do you have any industry standards or references to back it up (besides your own manual calculations).

Thought sequence:

[li]For both "blocked" and "Un-blocked" diaphragms, the AWC's SDPWS require nailing at diaphragm boundaries.[/li]
[li]However, prescriptive sections of the IRC provide details to ignore continuity for boundary nailing based on an acceptable truss heel height... in some cases omitting any blocking at all.[/li]
[li]However, truss designers assume loading in the plane of the truss. Truss designs do not account for cross-grain bending.[/li]
[li]The SBCA and Wood Truss Council piggyback on the suggestions of the IRC and go a step further by providing an analytical approach to partial height blocking. They assume manufactured connectors can do the trick.[/li]
[li]However, manufacturers' like Simpson do not warranty connectors to replace diaphragm boundary members or prevent cross grain bending.[/li]
[li]GC's claim, "in all their years of building, they never had to do it that way." They also want cost savings.[/li]
[li]Architects are more concerned about ventilation issues... v-notching, blocking holes, blocking every other bay, etc. (rightfully so).[/li]
[li] The building designers must connect the dots and takes responsibility for the truss bracing and load path continuity.[/li]

What am I missing here?

 

[KootK]

What am I missing here?

Very little it seems.

- If I'm genuinely concerned about diaphragm to sill plate connection capacity, I'll insist upon the full height blocking.

- Intermittent blocking is obviously preferable if the numbers support that.

- I've done it per the SBCA recommendations based on heel height etc.

- SBCA used to recommend "bird blocking" which was full height with a hole drilled in it for ventilation. I've specified this too. The things probably just fall apart during drilling or under any real load so I'm not enamored of this option. Might be better in LSL or something like that.

In general, I don't get too excited about the blocking. And this harkens back to my feeling that "normal" light wood frame buildings probably don't warrant serious lateral engineering for the most part. In a high wind location I would, of course, ensure that uplift was dealt with robustly.

A topic of debate here in the past has been whether or not anyone has actually observed a diaphragm failure in a building of this sort. My recollection of the outcome of that is that such failures:

1) Are extremely rare, usually requiring the participation of a significant hurricane or tornado. Or a testing lab.

2) Tend to be local and not catastrophic. Re-entrant corners and such.

3) Are most often found in connector hardware promotional / educational material.

It's an uncomfortable thing. If there's a clean answer to it somewhere, I'll be interested to hear of it.

 

[StrEng007]

I'd also be interested to know. Almost all my work is located in hurricane zones with buildings that have multiple planes of irregular roof layouts. You'd be surprised how many design issues are not on others' radar. That's why I like coming here to hash it out.

It's hard to explain to the GC that the typical prescriptive designs they "see everywhere else" just don't apply. Our Codes are written to a basic 'box type' level of performance to which luxury properties don't subscribe to.

 

[bones206]

The permanent lateral truss bracing (assuming it’s installed) probably provides a lateral load path that we ignore, but could be helping deliver some of the shears in the bottom chord plane and bypassing the sheathing plane. Actual demand at the heels might be less than we estimate using conventional diaphragm theory, since the truss and bracing is actually more of a space frame than anything. I’d be interested to see some testing or phd thesis that looked at that.

 

[StrEng007]

bones,
Very true. That's another area that lacks a lot of coordination between the truss designer, building designer, and roof contractor. Although the IRC (recently, as in several Code iterations ago) covers the permanent lateral bracing requirements, I rarely see it specified on others' construction documents. I just hope the GCs are using the standard document for "Permanent Restraint/Bracing of Chords & Web Members".

I know it's not just me, but I always seem to find these topics that spiral into an accepted method of practice.