CSA Wood Design - shear wall max stud spacing 2

[atrizzy]

Hi all,

I'm designing a blocked shear wall with studs (well, posts) at 4' o.c. I see that section 8 of the wood design manual indicates that the maximum stud spacing for unblocked shearwalls is 600mm. Does anyone know where I can find a maximum stud (in my case, post) spacing for blocked shearwalls? The arch really wants to keep an open space between the 4'o.c. posts...

Even if blocked, it seems hard to believe that an unsupported 4'x8' panel has the same shear resistance as one supported by joists at 2' o.c. Though the notes to the shearwall selection table DO state that panel buckling strength is considered for a panel size of 2440x1220... a full panel.

Any thoughts or insight?

Thanks in advance!

 

[dik]

I'll check with my wood expert... I suspect the 24" is to take advantage of 'load sharing' and nominal dimensions of sheathing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[XR250]

Even if blocked, it seems hard to believe that an unsupported 4'x8' panel has the same shear resistance as one supported by joists at 2' o.c.

I believe you answered your own question

 

[atrizzy]

XR250, hardly.

In fact, in the notes to the wood design manual shear wall tables suggest that the "panel buckling strength is considered for a panel size of 2440x1200."

 

[jayrod12]

I don't believe you can use those tables for 48" stud spacing. Nor would I want to. Find a different lateral load path if the architect doesn't want standard walls.

Can you even get the plywood to meet deflection criteria for the wind loads at 48" spans without going to something ridiculous like 3/4" sheathing?

 

[CivilSigma]

I just quickly read through the CSA O86 standard (Clause 11 for LFRS), and I believe the limit of 600 mm spacing (2') is based on the research testing referenced in the standard for unblocked walls.

The limit for unblocked shear walls is in place to prevent to prevent designers from under quoting fastens which are the main shear resistors. You can see 600 mm is the limit since many tables only address design factors for spacing less than 600 mm with unblocked walls. For examples Tale 11.5.4 only address strength factors for gypsum boards at 600 mm spacing or less.

I couldn't find anything relating to maximum blocked walls stud spacing. However, if you are designing blocked walls, you will be providing more fasteners along the blocking and additional wall stiffness.
I believe the spacing would be governed by the gravity load design with blocked walls.

 

[jayrod12]

I disagree that blocking the panel edges will magically get you the blocked shear wall capacities. There's no way that panel buckling would not be a concern at a 48" square panel.

But if you run the numbers of 11.5.1.(c) (CSA O86-14) and the panel buckling works, then I don't see why you couldn't do it.

 

[atrizzy]

jayrod, I'm not sure there's anything magical about it...

Correct me if I'm wrong, but if the panel doesn't buckle, the shear capacity of the panel is the same regardless of interior supports. Or am I missing something?

Of course, I will need to confirm the shearwall table note with 11.5.1c.

 

[JoshPlumSE]

The arch really wants to keep an open space between the 4'o.c. posts...

Not sure I understand the arch's desire. You still have the sheathing there. So, there really isn't any open space. This seems non-sensical to me....

 

[atrizzy]

JoshPlum, the blocking will actually be done on the exterior of the wall, in the cladding space, so nothing but plywood and posts are visible.
It's a long story.

 

[jayrod12]

All of this seems like we're venturing well outside the tested assemblies used to develop the code values.

I'd say you're correct, if buckling of the panel is not the failure mode, then yes the panel capacity should be the same.

However, I have concerns with your blocking on the outside. What is the blocking fastened to? How is it fastened? It's not just there to be a nailing point, it is intended to provide restraint for the panel. If the blocking can roll, then I'd feel that it isn't performing it's task properly.

We would like to know the reason the arch has for keeping the 48" spacing on the posts with nothing in between. If it's for insulation (to minimize cold strikes) I feel this is a terrible way to go about it. especially considering batt insulation comes in 14.5" and 22.5" wide strips intended to be used in conventionally framed walls.

 

[CivilSigma]

I played around with the software Woodworks (which is based on CSA O86) and it only considers 600 mm as the maximum stud wall spacing

 

[jayrod12]

In the wood design manual 2015 edition, page 436, first paragraph after table 8.1. Emphasis added by me.

"...Framing members in shearwalls and diaphragms constructed with plywood or osb must be at least 38mm wide and spaced no greater than 600mm apart."

 

[atrizzy]

There it is, jayrod!
Problem solved. Thank you kindly.