Timothy A. Philpot - Mechanics of materials _ an integrated learning system-John Wiley (2017)

CHAPTER
4
design Concepts
4.1 Introduction
The design problems faced by engineers involve many considerations, such as function,
safety, initial cost, life-cycle cost, environmental impacts, efficiency, and aesthetics. In
mechanics of materials, however, our interest focuses on three mechanical considerations:
strength, stiffness, and stability. In addressing these concerns, a number of uncertainties
must be considered and accounted for in a successful design.
The loads that act on structures or machines are generally estimated, and there may be
substantial variation in these loads, such as the following:
• The rate of loading may differ from design assumptions.
• There is uncertainty associated with the material used in a structure or machine.
Since testing usually damages the material, the mechanical properties of the
material cannot be evaluated directly, but rather are determined by testing
specimens of a similar material. For a material such as wood, there may be
substantial variation in the strength and stiffness of individual boards and
timbers.
• Material strengths may change over time, through corrosion and other effects.
• Environmental conditions, such as temperature, humidity, and exposure to rain and
snow, may differ from design assumptions.
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