Wood Roof Diaphragm to Bond Beam Connection

[dengebre]

Walls constructed of CMU supporting wood-framed roofs is common. The roofs are often sloped, have an overhang, and the roof diaphragms can be large for buildings such as assisted living facilities.

SDPWS states that “diaphragm boundary elements shall be provided [and]… diaphragm chords and collectors shall be placed in, or IN CONTACT WITH, the plane of the diaphragm framing.” The bond beam at the top of the CMU wall is typically used as the chord member because the reinforcing steel can resist the tension chord force easily, and using the subfascia as a chord member is not practical. The gap between the sloped roof sheathing and the bond beam below is the heel height of the roof truss and is often in the 6”-12” range. The question therefore is: how to connect the diaphragm sheathing to be in contact with the bond beam?

There are numerous design examples for flat roofs but not for sloped roofs. I have surveyed the work of colleagues, as well as past posts in this forum, and find proposed solutions include providing continuous blocking along the length of the boundary element, providing intermittent blocking (e.g., every fourth truss), and providing no blocking whatsoever. The last option relies on the transfer of the shear force through the truss heel and connector, and also assumes that the trusses will not roll over. Providing continuous blocking intuitively makes sense but is in direct conflict with the building code requirement to provide an airflow path for vented attics. True, ventilation holes can be cored in the blocking but this is impractical for shallow heel heights. Intermittent blocking solves the air flow problem but if the boundary element is continuous with the diaphragm, is the connection between the two also required to be continuous?

Thank you in advance for your insight.

 

[DBuker]

I've always used continuous blocking but you are correct that providing the required amount of ventilation can be tricky with a large roof and shallow heel. Are the trusses at 2' oc? If so, I think leaving a block out every 4th bay would still meet the continuous attachment intent. Really, as long as the stresses in the diaphragm are low and you distributed the diaphragm stress out uniformly over the length of the wall then there shouldn't be any issue...

An alternative would be to provide roof vents for venting rather than provide all of the venting at the eaves. This would allow you to use standard 2x continuous blocking with holes drilled (like the truss manufacturer usually provides).

 

[KootK]

I've been omitting heel blocking for heels under 6" per a WTCA article that I have somewhere. For larger heel, I've bee ripping off a TJI detail that seems to satisfy my architect friends.

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.

 

[dengebre]

Thank you gents! KootK, I believe you are referring to "SBCA Tech Note: Heel Blocking 2008" and I have included the link for future readers of this thread:

http://support.sbcindustry.com/images/technotes/T-HeelBlocking08.pdf

I suspect the 6” maximum unblocked heel height you are referring to is based on the HUD 2002 testing (referenced by the TechNote) which used a standard heel height and a standard connector clip (i.e., Simpson H2.5). The moment arm for the cross-grain bending calculation used by HUD works out to about a 6” heel height using these typical components. Based on the TechNote, assuming trusses spaced at 2’-0” o.c., it appears the maximum shear transfer (wind) is 114 plf of diaphragm.

The TechNote most certainly addresses the situation but there are a couple of items that require further consideration:

1. Trusses are designed by the truss engineer for in-plane forces only—not for cross-grain bending.
2. The HUD testing is based on two clips per truss, one each side, so that a couple can form to resist the truss rotation.
3. Simpson states that “when cross-grain bending… cannot be avoided… mechanical reinforcement to resist such forces may be considered”.

It makes one think it is cleaner just to use full depth blocking and be done with it. Thanks again for pointing me in the right direction.

 

[KootK]

Definitely cleaner. The other issue that sticks in my craw is that, without blocking, your diaphragm boundary nailing effectively becomes 2' feet on center. It's like the cantilevered steel joist top chord problem but without even the boundary fastened continuous edge angle. To my knowledge, fascia boards are neither continuous nor boundary nailed.

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.

 

[dengebre]

I concur and your angst is shared by Paul McEntee (Simpson Strongtie) in his blog on October 18, 2012. Personally, I believe that full depth blocking along the entire diaphragm boundary is sound engineering practice, but it does not appear to be the consensus among our peers.

 

[KootK]

That's because vendors in a free market always cave in the face of client pressure unless there's legislated code in place to protect them. This won't change until the code mandates full depth continuous blocking. There should even be a little diagram to go along with it to avoid misinterpretation.

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.