To borrow a premise from comedian John Mulaney, I think I was supposed to be a nerd. I think in heaven they made 3/4 of a nerd but forgot to flip the final switch before they sent me out the door. So I got all of the obsession with Lord of the Rings and X-men, but unfortunately very little of the useful math and science skills stereotypically reserved for nerds.
This brings me to my topic today: shear. This is a term I’ve heard a multitude of times over my brief construction career. Shear walls, shear failure, etc. So while I always kind of got the gist of it, this particular term in mechanical physics never quite made as much sense as it’s cousins compression and tension.
I think this is about right.
So, on a VERY basic level, shear is when parallel internal surfaces are laterally shifted in relation to one another.
Fun side note: this is actually what my lower back looks like too.
Shear walls are used when designing buildings to resist the lateral loads that are put on the structure (namely wind/earthquakes). The wind or earthquake wants to shift the building one way and the building’s foundation wants to keep it in place. These parallel forces put shear stress on the wall. A shear wall will typically use a structural sheathing (DensElement / OSB) to stiffen it up, or can be constructed of masonry or reinforced concrete to keep the building from falling down at the first breeze.
You might also see shear when talking about concrete bridge construction. Bridges are subject to dynamic loading and needs something to resist the forces placed upon it. On the picture below, the orange arrows can be both the dead load of the bridge itself and any dynamic live load that crosses it (ex. Semi-Trucks filled with dumbbells, buses full of offensive linemen, etc). The bridge wants to push down in one direction and the piers want to resist that force in the other direction. These forces cause shear stress at the circled locations.
Obviously, I’m not an engineer. I’m just a guy with a liberal arts degree trying to make sense of all this stuff.