Shear Wall Collectors - CSA 086 1

[TORCHMAN]

I have not been able to find any information on shearwall collectors on the Canadian wood code - CSA086. I see people on the forum talking about it but I found strange that the Canadian code seems to make no provision for it. Am I missing something? Is it a shear wall requirement? Where can I find good literature on it?

 

[jayrod12]

A collector is only required if you don't have enough direct connection between your diaphragm and your shearwall. In those instances, I've used beams as collectors and connect the beam to the wall to transfer the tension force into the shear wall. It's mostly a first principles concept. Not everything is explicitly codified.

 

[TORCHMAN]

Ah! That makes sense! Much appreciated!

 

[jayrod12]

No problem. I get the vibe that you're a younger engineer. Helping out is the fun stuff for most of us "older" guys. In my experience, the most lacking design aspect when coming out of university was lateral systems. They vaguely mention what diaphragms are and what they do, but they never teach any part of designing them. I would say almost the same for shearwalls and bracing, mentioned, maybe the odd question in an assignment, but no real teaching the nuances of design and analysis of the system as a whole.

So when I've got a greener engineer on a project at my office, I always try to get them to start on the lateral. Because honestly, I can get gravity designs to work out 99% of the time as long as I've already got the lateral worked out. Nothing worse than going back to the client late in the game saying you need more shearwalls, or more bracing, or thicker floor sheathing for the diaphragm loading. But changing a beam size late in the game, as long as it's not drastic, no one cares.

 

[TORCHMAN]

Yes, jayrod12, I am 3 years into my engineering journey so I think of myself as very fresh still!

It took me about a year to realize exactly what you said, lateral systems are going to be my big challenge. Most of my posts on this forum concern lateral systems. And I especially agree with what you said about going back to the client.

Cheers!

 

[Eng456123]

@jayrod12 I’ve had most of my bosses say if your shear wall/braced frame/moment frame is shorter than the length of the wall line/line of framing, you need to design everything along that shear wall line for axial load, but sounds like you’re saying the opposite? What gives

 

[jayrod12]

No, what I had tried to indicate is exactly what your bosses have said. If your direct connection between wall and diaphragm isn't enough to work for the diaphragm shear resistance, then you need collectors. Which then need to be designed accordingly including straps to your shear wall segments.

 

[Eng456123]

Sorry I don’t think I worded my last post properly. They say that even if the direct connection between diaphragm and resisting wall/frame is enough to transfer all of the load, you still need to design collectors along the entire length of that line of framing. Which essentially further reduces the diaphragm shear per foot, but now requires the design and detailing of collectors.

 

[Eng456123]

We had found one spot in some SE study book that cited “diaphragm tearing” or localized diaphragm stresses near the edge of the wall, when there is not specific detailing of collectors and their connections along a line of framing into a shear wall, even when the connection between only the shear wall and diaphragm IS enough to transfer the load. I mostly deal with concrete over metal deck diaphragms (idealized as rigid diaphragm) so not sure if this changes things.

 

[jayrod12]

It's not just the connectors because often the true diaphragm to shear wall connection has a bunch of capacity. It's do you only need the diaphragm length provided by the length of shear walls. Essentially I try to design my diaphragm as only as deep as the available shear wall length and see where that gets me.

To be fair, I don't practice in seismic, so keep that in mind.

 

[Celt83]

Jayrod12:
Your method of just checking the unit shear at the interface of the wall is, from my experience, common industry practice but in my opinion not in accordance with the continuous load path requirements of the building code. Collectors/drag struts are required for all partial depth lateral resisting elements. Terry Malone’s book covers this and this is a past post that also has some good info: Link

 

[phamENG]

The code in the OP is the Canadian Wood code, correct?

In most wood structures, we're fortunate in that the collector and the chord are generally the same thing AND are generally braced in both axes every 16 to 24 inches, making their compressive capacity grossly higher than what is needed in all but the most massive and open of wood structures.

So as long as your diaphragm has a chord (which it has to, or it isn't a diaphragm), you probably have a collector - even if only by mistake.

 

[jayrod12]

Pham makes a good point that I didn't do a great job of pointing out, chord continuity is important to maintain as well, and generally speaking when the wind is reviewed in the other direction, your collectors become your chords and therefore you end up tying them all together one way or the other.