Students will learn:

The 5 types of loads that can act on a structure are tension, compression, shear, bending, and tortion.

1. Tension: two pulling forces, directly opposing each other, that stretch out an object and try to pull it apart (ex. pulling on a rope, a car towing another car with a chain - the rope and the chain are in tension or are "being subjected to a tensile load").

   Figure 1

2. Compression: two pushing forces, directly opposing each other, which squeeze an object and try to squash it (ex. standing on a soda can, squeezing a piece of wood in a vise - both the can and the wood are in compression or are "being subjected to a compressive load")

   Figure 2

3. Shear: two pushing or pulling forces, acting close together but not directly opposing each other - a shearing load cuts or rips an object by sliding its molecules apart sideways (ex. pruning shears cutting through a branch paper cutter cutting paper (the branch and the paper are "subjected to a shear loading"))

   Figure 3

(ex. pulling on two pieces of wood that have been glued together (the glue joint is "being subjected to a shear loading")

Figure 4

A Moment of A Force

Before you can understand the last two types of loads, you need to understand the idea of a moment of a force. A moment is a "turning force" caused by a force acting on an object at some distance from a fixed point. Consider the diving board shown below. The heavier the person, and the farther he walks out on the board, the greater the "turning force" which acts on the cement foundation.

Figure 5

the force (F) produces a moment or "turning force" (M) that tries to rotate the diving board around a fixed point (A) - in this case the moment bends the diving board

The stronger the force, and the greater the distance at which it acts, the larger the moment or "turning force" it will produce.

A moment or "turning force" (M) is calculated by multiplying a force (F) by its moment arm (d) - the moment arm is the distance at which the force is applied, taken from the fixed point:

Figure 6

(As long as the force acting on the object is perpendicular to the object)

If you have a force measured in Newtons multiplied by a distance in meters, then your units for the moment are N-m, read "Newton-meters". If your force is measured in pounds and you multiply it by a distance given in inches, then your units will be lb-in., read "pound-inches". The units for moments can be any force unit multiplied by any distance unit.

4. Bending: created when a moment or "turning force" is applied to a structural member (or piece of material) making it deflect or sag (bend), moving it sideways away from its original position - a moment which causes bending is called a bending moment - bending actually produces tension and compression inside a beam or a pole, causing it to "smile" - the molecules on the top of the smile get squeezed together, while the molecules on the bottom of the smile get stretched out - a beam or pole in bending will fail in tension (break on the side that is being pulled apart) (ex. a shelf in a book case (& the diving board from previous example)

   Figure 7

5. Tortion (twisting): created when a moment or "turning force" is applied to a structural member (or piece of material) making it deflect at an angle (twist) - a moment which causes twisting is called a twisting or torsional moment - torsion actually produces shear stresses inside the material - a beam in torsion will fail in shear (the twisting action causes the molecules to be slid apart sideways) (ex. a pole with a sign hanging off one side)

   Figure 8

Assess students' understanding, individually or as a group, using the investigating questions located in the associated activity.