C haptEr 9 design Environments and systems
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plastic, and even the total layperson broadly appreciates how these
different characteristics affect their use in the world around us.
Even within the general class of metals, however, we see huge differences between a material such as steel and cast iron. Strengths
may be comparable, but steel is a ductile material, whereas cast
iron is a brittle one, and failure modes are extremely different. An
understanding of these differences has long influenced the use and
design of structures in both architectural and mechanical design
spheres. In the formative years of bridge design in the 19
th century,
it was common to make compression elements out of cast iron and
tension elements out of wrought iron (a ductile material) or steel,
when it became available. The whole form, visual appearance, and
performance of the resulting bridges were in turn critically influenced. This notion of using or exploiting specific material properties in a design context is both a rich approach and a common
one.
The overall goal of a structural design activity is to assure that the
stresses and deformations induced in a structure by forces generated by its environmental or use context are maintained at a desired
level consistent with safety and operational objectives. The structural analysis process is aimed at numerically quantifying—for a
structure already defined in terms of geometry and materials—
these stresses and deformations for identified loadings, forces, and
other considerations associated with the use context. The structural
design process is directed toward altering the size, shape, and material selection for a structure to maintain these same quantities at
desired levels. Designs are typically based on meeting both strength
and stiffness criteria. Once magnitudes and distributions of the
forces and bending moments acting on a member are known, analyzing the member for stresses and deformations and determining
whether it is safe are fairly straightforward processes. Structural
analysis techniques are available to predict stresses, deformations,
and failure modes in complexly shaped structures.
In today’s world, the structural designer uses many specific analytical tools that were developed in relation to particular definitions of
material properties that were discussed in Section 4.3, including:
■ The elastic modulus (E—Young’s modulus) of a material that
relates stress levels to strain levels below proportional limits
in a material for tensile and compressive stresses (E = stress/
strain). Materials with high E values are considerably stiffer
and less prone to elastic deformation than materials with low
values.
294
plastic, and even the total layperson broadly appreciates how these
different characteristics affect their use in the world around us.
Even within the general class of metals, however, we see huge differences between a material such as steel and cast iron. Strengths
may be comparable, but steel is a ductile material, whereas cast
iron is a brittle one, and failure modes are extremely different. An
understanding of these differences has long influenced the use and
design of structures in both architectural and mechanical design
spheres. In the formative years of bridge design in the 19
th century,
it was common to make compression elements out of cast iron and
tension elements out of wrought iron (a ductile material) or steel,
when it became available. The whole form, visual appearance, and
performance of the resulting bridges were in turn critically influenced. This notion of using or exploiting specific material properties in a design context is both a rich approach and a common
one.
The overall goal of a structural design activity is to assure that the
stresses and deformations induced in a structure by forces generated by its environmental or use context are maintained at a desired
level consistent with safety and operational objectives. The structural analysis process is aimed at numerically quantifying—for a
structure already defined in terms of geometry and materials—
these stresses and deformations for identified loadings, forces, and
other considerations associated with the use context. The structural
design process is directed toward altering the size, shape, and material selection for a structure to maintain these same quantities at
desired levels. Designs are typically based on meeting both strength
and stiffness criteria. Once magnitudes and distributions of the
forces and bending moments acting on a member are known, analyzing the member for stresses and deformations and determining
whether it is safe are fairly straightforward processes. Structural
analysis techniques are available to predict stresses, deformations,
and failure modes in complexly shaped structures.
In today’s world, the structural designer uses many specific analytical tools that were developed in relation to particular definitions of
material properties that were discussed in Section 4.3, including:
■ The elastic modulus (E—Young’s modulus) of a material that
relates stress levels to strain levels below proportional limits
in a material for tensile and compressive stresses (E = stress/
strain). Materials with high E values are considerably stiffer
and less prone to elastic deformation than materials with low
values.