Science Focus Topic 4 Notes: Forces, Loads and Stresses | Print |
External forces on structures are stresses that act on a structure from outside the structure. These forces produce internal forces, or stresses, within the materials from which the structure is made. These internal stresses can change the shape or size of a structure and is called deformation. This deformation can lead to repair of the damage to the structure, or failure of the structure.

External Forces

A dead load is a permanent force, acting on a structure.
This includes the weight of the structure itself.
A live load is a changing, or non-permanent force acting on a structure.
This includes the force of the wind and the weight of things that are in or on a structure.
Impact forces (things that collide with the structure) are another type of live load.

Internal Forces

Tension forces stretch a material by pulling its ends apart
Tensile strength measures the largest tension force the material can withstand before failing.

Compression forces crush a material by squeezing it together.
Compressive strength measures the largest compression force the material can withstand before it loses its shape or fails.

Shear forces bend or tear a material by pressing different parts in opposite directions at the same time.
Shear strength measures the largest shear force the material can withstand before it rips apart.

Torsion forces twist a material by turning the ends in opposite directions.
Torsion strength measures the largest torsion force the material can withstand and still spring back into its original shape.


A bending force is a combination of tension and compression
Shear and torsion forces are also a combination of tension and compression

Resisting Stress - The Inside View
Strength of materials can be traced to the forces between the tiniest particles of the materials.
(See. Figure 4.39 p. 314)

Topic 4 Review Page 314