Design Factor and Safety Factor Explained

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Source: Uni-Bell PVC Pipe Association

Recent changes to AWWA standards have introduced confusion to the concepts of design factor (DF) and safety factor (SF).

This document will clarify the relationship between the two terms by:

 • Referencing AWWA standards

 • Providing a table of examples

DESIGN FACTOR DEFINED

The AWWA C900, C905, and C909 standards define “design factor” as follows:

“Design Factor (DF): The inverse of the safety factor. It is used to reduce the hydrostatic design basis (HDB) to arrive at the Hydrostatic Design Stress (HDS) from which the Pressure Class is calculated.”

Putting this definition into simpler terms:

Design Factor (DF): The inverse of the safety factor. DF is used to reduce the material’s breaking stress to provide a safe design stress.

In equation form:

Design stress = Breaking stress x DF

or

HDS = HDB x DF

RELATIONSHIP BETWEEN DESIGN FACTOR AND SAFETY FACTOR

The AWWA definition is very clear: the design factor is the inverse of the safety factor.

Again in equation form:

DF = 1/SF

or

SF = 1/DF

CONSISTENT SAFETY FACTOR OF 2.0

From the last two columns in the table, it is clear that AWWA has consistently maintained a maximum design factor of 0.50 (equivalent to a minimum safety factor of 2.0).

Uni-Bell recommends that engineers maintain the practice of using 2.0 as the minimum safety factor for design.

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