Discontinuous walls and length of cantilever?

[OUe]

Hello all,
I am new to designing with wood construction. I am a PE and consider myself at a reasonable knowledge of designing the theoretical applications of wood structures. However, there are a few things that I'm still concerned about at my place of employment: there are times when other structural engineers decide to neglect/ignore a small wall projection of say 4'-0" or less. The reason I'm concerned is because they seem to never use a drag strut to force the shear in the wall with the inner dimension - not to mention the fact that the rigidities of the walls will be vastly different thereby creating a thrust in a diaphragm that was never designed to take the load.

Has anyone else used a personal criteria for when to determine when to use a dragstrut in a discontinuous wall? This is something in residential design that I've seen ignored quite often, and I'm concerned because it wouldn't pass in theory.

Thoughts?

OUe

 

[DaveAtkins]

The wall's double top plate usually needs to be designed as continuous, anyway, since it will act as a chord for the diaphragm. If the splices are designed for the chord force, they can probably also take the drag strut force.

DaveAtkins

 

[OUe]

Thanks DaveA,

I am using a rule of thumb of 2' or less for discontinuity. However, if it exceeds the 2' without the use of a double top-plate as required in chapter 23 of the code, I'm going to provide a D.S. for the discontinuity. If it's within 2', I'll see if I can use a blocked diaphragm and use the blocking as another form of a drag-strut. Does this sound good to you?



~ps~ Has anyone ever put a diagonal double top plate for small discontinuities. Is this code-permissible?

Thanks for the replies.

OUe

 

[NKT]

It sounds like you're talking about shearwalls with a horizontal offset between parallel lines of rigidity; I'll answer based on that interpretation.

Part of a complete lateral analysis for a wood-frame structure is designing the diaphragm collectors/chords. If the walls are offset, then you have to design the splice to make them behave like one line of shearwalls. Or, design them as two separate lines and design the diaphragm accordingly.

There are many options for offset collector splices. A few that I have used are:

1. A subdiaphragm between the offset lines (often takes the form of a patch of blocked edges and heavy nailing in the floor or roof diaphragm above the shearwalls). If the framing members are perpendicular to the collectors, you'll end up with a line of blocking with a strap on top to avoid dumping a point load into the diaphragm.

2. For an open beam ceiling with plank floor: A diagonal brace between the two lines with let-in pockets for the brace (compression load path) and straps across the top (tension load path).

3. Design a "sub-collector", not necessarily the full length of the diaphragm, to reduce the unit shear from the relatively high levels in the shearwall to allowable levels for the diaphragm.

Unless you know for SURE that you can demonstrate a load path if/when you are called out on it, you can't say it will be OK without checking (I sometimes see "by inspection" used inappropriately when an engineer should have written "lazy" or "don't care").

 

[tdsull]

I believe there are NDS and APA publications that address this issue with precriptive guidelines.

 

[DaveAtkins]

I didn't understand your original post.

I can't add much new information, except to say that I see various options:
*"rack" shear force from the main diaphragm over to the offset shear wall, using a mini three sided diaphragm
*use a drag strut across the offset (Simpson makes a drag strut connector that works great for this)
*connect the diaphragm separately to shear walls each side of the offset (don't use a drag strut unless you must--if the diaphragm is OK above the shear wall on each side of the offset, then you don't need a drag strut).

DaveAtkins