All products should be safe to use.  In order to be sure that products and structures are safe, even in extraordinary circumstances, they should be built much stronger than they need to be in normal use.

This extra strength is called a factor of safety.

Structures are tested to make sure that they are safe and that:

  • the structure can carry the loads for which it was designed
  • the structure does not collapse in use
  • the structure is stable, i.e. it does not fall over.

The photograph opposite illustrates a swing being used by more children than it was probably intended for. Without a factor of safety being built into the structure, a terrible accident could occur if the swing is strong enough to bear the load of all the children on it and it would collapse or tip over.

For the swing to be really safe, it must be strong enough and stable enough to bear loads far greater than those imposed by the seven little children in the photograph.

A factor of safety must be built into the design.

Children's swing in Tailand

Factor of Safety

The Factor of Safety is a number that is worked out by dividing the load that breaks a product, (e.g. the swing in the photograph) by the load that the product is usually expected to bear.  In the example of the children's swing, we will say that we require a Factor of Safety of 4, i.e. the swing should be built to withstand loads 4 times greater than those normally imposed by children playing on it.

Factor of safety

If we say that children playing on the swing normally impose a load of 1000 Newtons or less, the calculation would look like this:

The swing would have to withstand a load of 4000 Newtons without breaking for it to be considered safe to use in all conditions.

What is a load? Click the link to view an animation and description of a static load and a dynamic load acting on a structure.

What is a force? Click the link to view an animation illustrating compression, tension, torsion, shear and bending forces.

What is damage tolerance? Click the link to view an animation explaining damage tolerance.

Click here to view the PDF version of this resource. MDTE PDF Factor of safety PDF
Copyright Laszlo Lipot.