Lecture 32

Lecture 32:

Beam Failure Modes

Structures fail in many ways; two categories of which have been described: stability failures and strength failures. Stability failures usually relate to structural systems, whereas strength failures relate to the members comprising a structure. Of the many ways which a beam can fail, three will be discussed in this course: bending, vertical shear, and horizontal shear.

three failure types

Bending is probably the most common type of failure. It is illustrated by the top figure in which the "fibers" along the bottom face of the beam are torn and those along the top face of the beam are crushed.

Vertical Shear is an idealized mode of failure. There is a tendency for a short beam to fail in this manner. This is the way in which a pair of "shears" or scissors cuts a piece of paper.

Horizontal Shear is the tendancy for a material to separate parallel to the neutral axis as its "internal layers" try to slide past each other. It is a frequent mode of failure which should not be confused with checking in wood beams.

Our discussion during the next few days will be limited to the stresses that are related to these modes of failure. Bending and horizontal shear are the types of failure most often encountered. Vertical shear is useful to us primarily because of what it reveals about horizontal shear. Excessive deflection can also be a cause of failure.