Wood Flexible Diaphragms

[TXEng-USA]

thread507-388238

RFreund did you have a chance to discuss these items with Terry Malone.

I have been reviewing numerous structural drawings of multi-family construction in my area (Texas) that other consultants have put out and I have yet to see a single collector or drag strut called for. It is clear to me that WoodWorks (Terry Malone) is saying that they are required but I don't want to get fired from ever doing another multi-family wood frame project.

Has anyone seen a multi-family project in a wind dominant region where these items have been specified?

 

[sponton]

We do a lot of wood structures in my office in Kentucky. I don't know about the rest of the people here but I do call them out and make sections as required.


I think it becomes more difficult since we tend to simplify plans because we know how they work in the field [some contractors are careless, and they don't even go by the plan] so i will say that we erroneously not provide a floor sheathing pattern specifying the location of each plywood sheet and the nailing as well as the locations of collectors with the specified nailing pattern and such and such but what we would provide are the sections [specially when you're using floor trusses] that specify a shear design load and the proper connection to the shearwall [this is to be designed by the truss manufacturer].

 

[KootK]

Has anyone seen a multi-family project in a wind dominant region where these items have been specified?

It'll get you shot in my area unfortunately. We're just starting to warm up to hold downs. Some places where it seems to be getting done:

1) British Columbia sort of.

2) RFreund seems to be doing it (IL/WI).

3) Mike20793 seems to be doing it.

I've no doubt that there are others, I just can't remember them.



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.

 

[mike20793]

Koot is right, I do the detailing and it actually gets installed correctly and without too much complaint...most of the time. I've done both high wind and high seismic for these types of systems. Terry will be the first one to admit that you really only need to the calculations one or two times to get a good feel for the forces caused by offsets, etc. (they are usually relatively small and even negligible in some cases) and then you end up using the same detail at every location. The most common ones I employ are the full depth shear walls with blocking and straps and "lapping" two offset shear walls together across the corridor and using the corridor shear walls to resist the unbalanced force couple. I think the common misconception is that you need the collectors or drag struts every time you have a small offset or jog, but Terry agrees that there are cases that the forces are small enough that it's not practical to provide all the straps and blocking.

 

[sandman21]

Don't feel to bad when a CA engineers design in TX, we get all kinds of crap. We just return the favor when someone from TX designs or builds in CA.

 

[TXEng-USA]

Mike20793, do you specify drag struts across the corridor?

We typically have trusses perpendicular to the corridor walls so they could easily transfer the axial load across but how would they know to increase the diaphragm nailing unless you specify that it is a boundary element? Besides calling out each of the drag struts I assume you could have a typical detail that specified for all trusses in-line with a shearwall to have boundary nailing, but we all know this would never get done.

Another situation, in the longitudinal direction we typically only use the corridor walls as shear walls. If there is not a wall inside the units that closely lines up with the corridor wall then would a drag strut not also be required in this situation to drag the load to the corridor walls? The drag strut would typically be perpendicular to our floor framing so it would require that they add drag trusses or blocking between the trusses.

 

[mike20793]

Yup, almost all shear walls will have a collector across the corridors either to provide a full depth collector element or to lap the shear walls together across the corridor. I use a typical detail and some kind of plan note and symbol to denote. You'll find that the drag force is usually low enough that the typical corridor trusses/joists work no problem and a 22 gauge strap is almost always enough. I almost always utilize just the corridor walls and use pretty much every single one since the architect will usually have to fur out the non-shear walls anyways. Terry Malone is working on getting tall, slender shear walls codified so you can use them as an outrigger in conjunction with the corridor shear walls.

Edit: In the rare event that shear walls line up across the corridor, I still use a collector to tie them together. And by collector I mean joist or blocking and a strap. The coil straps can be cut to length.

 

[KootK]

Terry Malone is working on getting tall, slender shear walls codified so you can use them as an outrigger in conjunction with the corridor shear walls.

You don't mean outrigger in the steel and concrete building sense do you? Just that you can use the little bitty walls around the exterior between openings?

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.