low power that humans can provide in steady-state imposes a
strong requirement on the mechanical design of the vehicle. Aircraft, bicycles,
and to some extent boats need to be both light and strong. To achieve this
goal requires a careful selection of the materials from which the structure
is to be made as well as the shape in which the material is fabricated. The
same material with a different shape will have a different mechanical response.
diagram below illustrates this concept. The material (green
free-form) is converted into a structural component (in this case a girder)
by selecting an I shape that distributes the material mainly at the top and bottom
of the structural component. Experiment shows that this shape is good
in sustaining vertical bending loads, and I-beams of this type are employed in
building construction. Selection of different shapes, or the same shape but
with different height-to-width ratios will change the material performance
as a structural element.