When to use Shearwalls in Wood Framing

[Bedrock1977]

What are some ways to tell if a shearwall (diaphragm) is required in a wood-framed structure? We don't really see them all that much in Michigan.

 

[Delchi]

I'm in the Front Range area of CO where we usually have wind speeds between 110 mph and 140 mph 3-sec gust, so we use shearwalls in residential construction all over. It's just like any other building:

1. Determine winds loads on structure per ASCE7;
2. Get a feel for the resistance by adding up wall lengths in each direction. You can use the vaules of the drywall and exterior sheathing. (Per IBC or NDS.)
3. Compare values to see if you need to beef up the exterior sheathing nailing at exterior walls (preferred) or even add shearwalls inside (common here, but makes the constrution sequence harder, and also can be complicated to hold down the wall ends if not on a concrete foundation.)
4. If you have all windows on one wall, we often have to add steel moment frames. However, note that wood floor/roof diaphragms can cantilever somewhat, but there are limitations in the code.
5. Simpson products are very handy. However, I usually avoid Simpson Strong-walls because of the coordination required during the foundation pour. I have not used their new moment frames yet, but they might be available now in some areas.

My experience has been that all structural noises and the feel of momevent in the wind will be eliminated by simply following the code for wind design. It's when you ignore it that the building feel shaky in a strong wind.

Hope that helps!

 

[archeng59]

How do you resist lateral loads in a wood framed building without shear walls? Do you use steel moment frames instead?

 

[frv]

archeng59-

You beat me to it. That was my exact question.

 

[Ron]

If the wind blows.....

 

[Delchi]

Bedrock1977,

As noted by the 3 comments above, wood structures (almost) always have shearwalls of some sort. Often on squatter structure or structures with a lot of wall, the sheathed exterior walls along with the interior gypsum-board covered walls are sufficent to resist wind without special attention given to shearwalls, in which case the word "shearwall" might not appear on the plans at all.
My comment above assumed that you understood this and that you were using the term "shearwalls" to mean extererior walls with tight nailing or wood-sheathed interior walls. Don't misunderstand and think that a wood structure can be built without any MWFRS just because special shearwalls may not be required.

 

[msquared48]

Sheetrock is a nono here in the Puget Sound area - ply or OSB only - as seismic loads control a lot of the time reducing sheetrock allowables by 50%. The holddowns are an important part of the system too.

The only place shearwalls might not be used in wood structures would be in smaller Pole Structures. The larger pole structures MAY require them to work.

Commercial, retail, multi-family and single family residential all exensively use wood shear walls. We have both high wind loads and high seismic loads to resist, so they are the norm.

Mike McCann
MMC Engineering

 

[bylar]

In the prescriptive part of the IBC Residential code you can see the required panel lengths for the type of wall construction. i.e. as a general rule with apa rated sheating you need 4' of panel at each corner and every 25' but there are several options and conditions for example the size of openings.
If you are 90 mph or less and using the IBC you can use those guidelines. Check it out.