Factor of safety

The safety factor, also called security number, indicates the factor by which the failure limit of a structure, component or material is designed to be higher than it needs to be determined by theoretical calculation such as Static calculation.

The safety factor can be defined as:

The reserve factor is the reciprocal of the safety factor.

A safety factor of 1 means that the component has no safety margins against failure. Underlying mechanisms for failure are common: bend, break, kink or fatigue failure ( failure of the fatigue strength ).

By a safety factor is avoided that failed due to tolerances in materials, manufacture, design loads and not proven, lower influences the component. The safety factor is depending on the material used and safety Relevance usually 1.1 to 2.1, for materials with large variations in properties at 3.0 (eg, wood ) and for highly security-critical components at 10 (eg elevator ropes ).

With constantly acting loads ( such as dead load ) will usually require a security of about 2 in the relevant standards. When buoyancy is sufficient in some cases already have a safety of 1.05 (according to DIN 1054 ).

Also in the design against earthquake is expected to security factors. In this load case is sufficient ( as is generally the case of exceptional and rare load cases ) usually a relatively small factor (eg 1.2 ).

In "Disaster load cases " such as accidents or fire, the safety factor is not applied, instead, for example, instead of the elastic limit of the tensile strength is then used as a basis of a material, wherein these stresses permanent deformation of the component is permitted.

In determining the stress often conservative assumptions are made, resulting in the actual existing security even further.

With the new generation of standards that are based on the EU standards, not global safety factors, but partial factors on material and manufacturing side to one side and the load side be given to another, taken together, but give approximately the same safety factor.

  • Structural Analysis
  • Engineering Mechanics
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