1b. Factor of Safety  (SSES Ch. 1.6)
Factor of SafetyProof Testing
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Factor of Safety
The Factor of Safety FS is the strength of a system divided by its maximum working load:

  • Pf is the Failure Load (or Ultimate Load), i.e., the strength. Failure may be defined as the system (1) breaking into two parts, (2) permanently deforming (yielding), or (3) deflecting too much. This load is determined from tests or from calculations based on the system's properties and geometry.
  • PD is the Working Load (or Design Load or Allowable Load), i.e., the maximum load expected in service. This load is generally a result of specifications or codes, or tests on prototype systems.
  • The strength and load must be given in the same units - force, stress, etc.).

Proof Testing
We usually test a system before placing it into service to prove that it works. Proof testing is especially necessary when the system or construction methods are new, the structure is difficult to analyze, or when the failure can cause significant damage, especially in human life. The Proof Load Pp is greater than the Working Load PD but less than the Failure Load Pf.

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Updated: 05/16/09 DJD