Factor of safety
Also called Safety factor
The ratio between a material’s or structure’s actual capacity and the maximum load it is designed to carry, built in to allow for uncertainties in loading, material variability and workmanship. A stated safe working load already has an appropriate factor of safety applied, so it should never be further derated on assumption, nor exceeded on the assumption that spare capacity remains.
In practice
The factor exists to absorb what is not known: how the load really arrives, how the material really varies, how the thing was really made and used. It is not slack to be spent. Once a factor has been applied by the designer and published as a rating, the rating is the number to work to — applying another one on site is not extra caution, it is a second uncontrolled adjustment to a figure somebody already controlled.
Where this comes from: Standard engineering practice. The factor appropriate to a given item comes from the design code or specification it was designed to, and is not a figure to be chosen on site.
Where it matters on site
In ground bearing the factor is on both sides of the comparison and it is easy to double-count. A mat’s safe working load already carries one, and an allowable ground bearing pressure from a geotechnical assessment already carries another. Multiplying in a further margin because the ground “looks soft” is a decision that belongs to the temporary works designer, who can see what has already been allowed for.