Dear Builder’s Engineer,
I’m a framer and I hear all the time, “shear off a wall.” What’s that really mean?
Jerome O., Branchville, South Carolina
If you were in the giant scissor business, I’d have a different answer than the one that follows.
In the world of construction, shear can refer to several things. To engineers it’s a certain type of stress inside a structural member due to some applied load. Shear can also mean a lateral load from earthquake or wind. And shear can refer to a construction method of resisting wind and earthquake loads. To shear off a wall is in reference to this third definition.
Shear is one of several stresses, with bending, tension, and compression being the other main ones. To understand what shear really is, let’s look at how things fail.
- If a member fails in tension, it is pulled apart. For example, a cable being pulled beyond its tensile capacity snaps in two.
- If a member fails in compression, it crushes. For example, a short post supporting too heavy a load mashes.
- If a member fails in bending it breaks due to too large a bending moment. Breaking a pencil in half with your hands is a good example. (Bending moment is another topic altogether.)
If a member fails in shear it rips. A rip is caused by one side of a member going one way and another side going the other way. The most common examples of shear failures are walls that have been through an earthquake. In these you’ll see lots of diagonal cracking especially at doors and windows. That’s from in-plane (in the plane of the wall) lateral (sideways) forces racking the wall. Racking is the top of the wall being forced in one direction while the bottom is held stationary or is forced in the other. Door and window corners are particularly vulnerable to these racking shear forces.
Here’s another seat-of-the-pants example of shear. Say you have a tall stack of long 1x4s. You and a buddy lift the stack, one guy on each end. The boards sag and bounce as you walk. They do this because the boards can slide on top of each other. Now envision that same stack of 1x4s with a layer of stout glue between each board. After the glue has cured, you and your buddy carry them and are astonished that there is zero sag or bounce. The glue is resisting shear forces between the boards. You have just turned a bunch of puny 1x4s into a mighty glu-lam beam.
Wood glue, in fact, is mainly intended to resist shear, which in this sense means pieces of wood sliding on each other. The glue I spec in my structural designs, Liquid Nails LN-940, is good for 450 pounds per square inch, wood-on-wood shear.
In most stick-framed construction, walls provide resistance to the racking, lateral forces brought on by wind, storms, and earthquakes. The amount of racking resistance a wall provides has everything to do with how it is constructed. A wall of just 2x4s, no plywood or drywall, can support a lot of gravity (downward) load but will provide almost no racking resistance. As a framer, you know this because the only way you can true up a wall is to rack it plumb before any sheathing has been nailed off.
A “shear wall” generally means a wall intended to resist the racking loads applied by wind or seismic events. The word “intended” is key because while any wall can resist some lateral load, not all of them are designed to do so. If a non-shear walls does anyway, that’s great and adds to the redundancy of the lateral resisting system. When engineers design buildings, they determine how lateral loads will be distributed and ensure that shear walls are located strategically to resist those loads.
A framed wall with drywall on one or both sides can resist a fair amount of racking force. The building code recognizes this and allows drywall-sheathed walls as shear walls. More typical are plywood or OSB-sheathed shear walls. As you might suspect, wood sheathing has greater shear capacity than drywall.
The capacity of a shear wall also depends a lot on the nailing pattern of the sheathing to studs; whether or not the edges of sheathing have blocking behind them; and other factors.
So when someone says that a wall is to be sheared off, they’re really saying, “Make sure that sheathing is applied to the studs with a certain nail pattern so that the wall can be depended upon to take wind and earthquake loads.”
We all know that a chain is only as good as its weakest link. A shear wall is but a link in the chain of a building’s lateral resisting system. In a future column we’ll tear into that topic, also known as load path.
Tim Garrison is an author, public speaker, and professional engineer. He welcomes correspondence via his blog at ConstructionCalc.com.