What determines how much a material will bend?

Try to bend a ruler. Hold one end tightly on your desk and push down on the other end. You are exerting a tension force on one surface and a compressive force on the other. The amount the ruler bends depends again on the Young’s modulus (Y) of the material as well as its length (L), width (w), and thickness (t). The bending (x) is proportional to the force applied (F) and the length cubed and inversely proportional to its thickness cubed. Or, in the form of an equation, x = FL3/(t 3wY) That is why the joists supporting a floor are much thicker than they are wide. Typically wood 1-1/2” wide but 10” or 12” thick is used. The larger thicknesses are used if the span between supporting walls is longer.

An “I” beam is often used to support weight. The beam is in the shape of the letter I. The vertical member, tall and narrow, keeps it from bending while the top and bottom members keep the vertical member from twisting. While I beams are most often made of steel, wood beams are now used in houses because they are stronger, lighter, and cheaper than steel.


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