Two Point Loads at One Third Points on a Simple Beam When specifying a beam or joist three things are usually considered;
First,is the beam strong enough that it doesn't fail by the fibers on the bottom being torn apart, a fiber failure.

Second, is the beam stiff enough that there will not be excessive deflection. A bouncy floor, cracked tile or plaster would be examples of this.

Third, is the beam strong enough that its fibers don't slip past one another, bend a magazine and watch the pages slide past one another. This is a horizontal shear failure.
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