Universal Principles of Design, Updated and Expanded Third Edition

061 Factor of Safety

The design of a system beyond expected loads to offset unknowns and prevent failure.

Design requires dealing with unknowns. No matter how knowledgeable a designer and how thoroughly researched a design specification, basic assumptions about unknowns of one kind or another are inevitable in every design. Factors of safety are used to offset the potential negative effects of these unknowns.1

Increasing the factor of safety is achieved by adding capacity, materials, and redundant components to a system to enable it to exceed anticipated operating loads. For example, designing an Internet service that can support 1,000 users appears straightforward. However, to account for unanticipated uses of the service (e.g., downloading ...