What is Shear?

Shear is a force that acts in the plane of a member and, ultimately, causes adjacent fibers to slip by each other. It is also one of the hardest forces to demonstrate.

Let's take a piece of paper for an example. I can rip the piece of paper or cut it using scissors (or "shears") to cause the fibers of the paper to separate…but this is all occurring outside the plane of the paper. A shearing force would cause separation of the paper fibers without bending it.

Think of the strong man act at the circus. One of the tricks is or used to be ripping a phone book in half. This apparently impossible task, like the ripping of a single sheet of paper, was accomplished through the out-of-plane distortion of the material. The demonstration of a true shear force would require that I glue all the pages together first, place the book in a steel jig, hook elephants to opposite positions, then offer peanuts as a pulling incentive.

Shear In The Plane
The telephone book is held in the jig so that the top of the book is not removed like a bottle cap.  It is only permitted to slide horizontally with all fibers moving at the same time.

As you have seen, a number of materials can be used to resist shear forces. Their rated capacities to resist shear forces without failure are related to their general strength characteristics…in particular their ability to resist loads and return to their previous shape once those loads are removed. To understand this and to help explain shear wall function later on we'll need to visit a testing lab.