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RE: diaphragm chord wood roof

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Thanks for your response.  And thanks to all the others responses that came in.


You bring up a good question on the diaphragm chord.  Like I stated in previous posts, I usually consider the double top plate of the stud wall as the chord.  In CMU construction I use the top bond beam.  Since I haven’t designed the tall parallel chord truss projects I never considered much were the chord was.  When I studied this detail require I started questioning how the shear was going to be transferred from the diaphragm to the wall and how it would all behave.  That’s when I started soliciting advice from others.


My understanding is all diaphragms need chords.  If there is not a chord then the typical diaphragm design analogy is not valid and the sheathing must be designed for bending as well as shear.  Chords are the tension and compression members, the flanges so to speak, of the diaphragm “beam”.  I remember a design seminar on  steel roof diaphragms.  The presenter said if you don’t provide a chord member and weld the deck to the chord for the required shear force design then you wasted all the effort you put into the diaphragm with sidelap screws, etc. because it won’t behave as a regular diaphragm, the strength is reduce significantly.  I’m thinking the same is true of a wood diaphragm.  If there is not a chord and shear transfer path then the effort to get the right edge nailing is somewhat wasted because the diaphragm behavior is different.


I’m still hoping someone will respond about the coil strapping mentioned in previous posts.  Is the coil strapping ment to be the tension chord element, or is it doing something else?


Thanks again for all the responses and help.





From: Andrew Kester, P.E. [mailto:akester(--nospam--at)]
Sent: Sunday, July 13, 2008 12:48 PM
To: seaint
Subject: re: diaphragm chord wood roof


We deal with wood trusses with deep heel heights at bearing all of the time, it is necessary with many overhangs especially larger ones, and the steeper the truss slope, the more pronounced...


If we get lucky and it is up to about 16" deep, we use solid blocking with conventional 2x or an LVL. We space it according to the amount of shear, trying to alternate truss spaces if possible. With deeper truss heel heights we use 2x4 X-bracing with horizontal blocking, and straps that wrap over the truss top chords, if we use them in tension too (trying to keep them in compression only is the simplest detail and easiest for the framer to get right).


Similar to this is using diagonal straps (tension only of course) made by Simpson or sim. We may add a Simpson HGAM or angle at the horizontal blocking at the top of the bearing wall to help carry the shear into the wall, if the truss uplift is high and the combined stress on the truss connector is too high.


With a few jobs I have had very high shear in the diaphgram and a pretty tall truss height where X bracing would be tough due to the sharp angle, or congestion of truss webs if you wanted to use a 4ft horiz. run. So we did what I think Gerald M. was explaining, every other truss space or every truss space, if nec., we used a little mini shear wall made of a 2x4 "picture frame", where the top plate was flush with the U/S of roof deck, and nailing thru the sheathing into the mini shear wall made the transfer nice and clean. Then additional connectors were added for the shear wall uplift due to overturning and shear, OR the truss connections can be upsized for this additional reaction plus the uplift. I guess it makes a big difference if you are analyzing a shear due to seismic vs shear due to wind, because uplift then may not be a problem.


Does a diaphgram chord need to be at the plywood sheathing to be effective as part of the diaphgram "deep beam" analogy? I was  never under this impression, as long as the shear and out of plane forces were all properly transferred into the bearing wall properly, this would act as your diaph. chord, such as a bond beam in CMU walls and top plate in 2x wood construction, or a cont angle in steel deck construction (that is then bolted into a bond beam possibly...)



Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Drive, Suite 301
Orlando, FL 32803