Safety Factor or "Factor of Safety"

Factor of safety (FoS) can mean either the fraction of structural capability over that required, or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected.

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Appropriate design factors are based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually can use a safety factor of four or higher (often ten).

If we assume that the WTC had a factor of safety of four (and this is a typical number used by various sources), this means structural columns could lose 75% of their strength (from heat or other damage) and be just fine. In fact, the columns would be better than fine, as the factor of safety is calculated for the maximum load of a building, which generally is not reached by the weight of the inner contents.

But simply put, the safety factor means that ALL of the columns on one floor of the WTC would have to lose more than 75% of their strength at the same time for the floor to collapse. Given the fact that fires were contained to a fraction of the area on each floor, the fire-induced collapse explanation cannot account for the destruction of the towers.