Collector requirements in rigid wood diaphragms
Collector requirements in rigid wood diaphragms
(OP)
Despite the popular consensus to design wood diagrams as flexible, in most cases with new construction they are almost always rigid. With irregular shaped buildings the most common method of handling diaphragm horizontal offsets is by using collectors with drag strut connectors to the shear walls. While this is appropriate with flexible diaphragms, with rigid diaphragms, collectors can be overly conservative. Stresses can be high at reentrant corners, but I am wondering with a rigid wood diaphragm how to analyze the stress distribution through the plywood, glue, and floor screws when the collector is absent. The SPDWS is a joke when it comes to "real world" diaphragms and the only literature I can find on more complicated irregular shape buildings always assumes a flexible diaphragm analysis, which in my opinion is not accurate, and usually over conservative. Thoughts?






RE: Collector requirements in rigid wood diaphragms
RE: Collector requirements in rigid wood diaphragms
www.idecharlotte.com
RE: Collector requirements in rigid wood diaphragms
RE: Collector requirements in rigid wood diaphragms
But how I approach these things.
Unless the aspect ratio is large, I design for a rigid diaghram too. (I've heard arguments that the gluing of the subfloor might give more stiffness than the nails, and thus diaghrams are more rigid than we design for)
The CWC gives tables for shear resistance, in lbs/ linear foot for different diaghram types, and shear wall types. I ensure that I run a drag strut long enough above every shear wall, that the diaghram strength in lbs/ft at that drag strut has 20% more capacity than the shear wall.
As long as there are no enormous openings near that drag strut, the diaghram load should effectively transfer into the drag strut.
As for evaluating things more meaningfully than that - I couldn't apply first principles to figure out a shear strength of plywood nailed to studs/joists. I assumed the values given in the tables in CWCs wood design manual came from testing...
Hope this helps
RE: Collector requirements in rigid wood diaphragms
Even with glue and screws, resisting tensile stresses in a diaphragm without boundary elements usually means pulling the tops of the supporting framing members apart in tension perpendicular to grain. And that's a no-no in wood design.
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.
RE: Collector requirements in rigid wood diaphragms
RE: Collector requirements in rigid wood diaphragms
Most jurisdictions I work with require we envelope the design for both flexible and rigid diaphragms. This is conservative but solves the issue of how will the diaphragm really behave.
You can't just assume flexible when you have cantilevered diaphragms all over the place (as architectural layouts seem to be nowadays).
RE: Collector requirements in rigid wood diaphragms
NorthCivil,
I'm not sure what you mean by "I ensure that I run a drag strut long enough above every shear wall". My understanding that a drag strut (or collector) basically accumulates the diaphragm shear forces to the shear wall from any open boundary edges. So on reentrant corners, basically 'L' shaped diaphragms, by flexible analysis you would have two collectors (beams) in the X and Y direction running the full length of the diaphragm at the bend in the L to accumulate the shear force and then a drag strut connector would be required to connect the collector to the shear wall in some way by either running it along top of the shear wall and attaching it, or with a connection of some kind to the corner of the shear wall. In a rigid analysis, in theory, so long as the diaphragm forces unit shear is not exceeded at the shear wall connection and the stresses at the reentrant corners don't exceed the capacity of the diaphragm, then no collectors are needed.
KootK,
I appreciate your answer and I generally agree but I guess my point is the sheathing, glue and members have to have some capacity, and probably quite a bit, otherwise houses would fall down in a stiff wind all around us, and yet they don't.
Thanks for everyone's input by the way. I really do appreciate your thoughts.
www.idecharlotte.com
RE: Collector requirements in rigid wood diaphragms
RE: Collector requirements in rigid wood diaphragms
RE: Collector requirements in rigid wood diaphragms
And I generally agree with that. I guess my point is that there's a substantial chasm between what we both strongly suspect to be true and what we're able to prove based on the formal guidance currently available. I don't actually think that small wood residences should even be designed laterally. The BS factor is just too high to justify the effort. I'd much rather just have something prescriptive (min length & spacing of shear walls). That way at least we all be playing by the same rules.
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.
RE: Collector requirements in rigid wood diaphragms
Don't we have that already in the IRC? Does not apply to about 50% of the house am at tasked at designing as they seem to be all glass.
Unless I am cantilevering a floor diaphragm, I simply base my shearwall designs on tributary area. I could care how flexible or rigid the diaphragm is. Is it correct all the time - probably not. Is it ever going to be a problem - probably not.
RE: Collector requirements in rigid wood diaphragms
The IRC and IBC have prescriptive methods but like you only about 25% of houses we design meet that. In the IRC method, very little about collectors mentioned, especially when horizontal offsets are less than 8 feet, so even they acknowledge some account for the strength of the diaphragm without collectors.
www.idecharlotte.com
RE: Collector requirements in rigid wood diaphragms
I do those prescriptively using wood portal frames (if possible) or detail a three side building design or I call the arch. and have them change the dimensions to make it work without going to extreme measures.
RE: Collector requirements in rigid wood diaphragms
Exactly. Even as a rigid diaphragm you would have to resolve the torsion created by the offset of center of mass and center of stiffness which is what you are doing with a three sided building with a flexible diaphragm.