UNDERSTANDING STRESSES AND STRAINS - Bridges - page 36

time, that our beam is composed of thousands of vertical layers of cubes. As
the load bears down on the beam's upper forward edge, it tips the vertical
layers under it toward us, and tips the adjacent layers slightly less, and the
layers next to them even less. Now, in addition to causing basic bending,
compression, tension, and shear, the load imparts a kind of twisting.
Note that when the middle layers angle forward more than their
neighboring layers do, they slip past their neighbors, stretching the imagi-
nary glue between them. They undergo torsion. With enough rotation, tor-
sional strain can combine with several other kinds of strain to cause failure
in the structural member.
TOWARD ENGINEERING DESIGN
When an engineer designs a particular bridge type, she must be aware that
the future structure will encounter multiple loads: the structure itself, the
deck and pavement, automobile traffic, intermittent heavy truck traffic,
wind, earthquake, loads imposed by thermal expansion, and effects of set-
tling soil, among others. These loads will exert varied kinds of stress at each
location along the bridge. The engineer's task is to ensure that the structure
transfers the various loads to the ground, without failure or collapse.
Toward that end, each structural member must be selected so as to
withstand the compressive, tensile, shear, basic bending, torsional and com-
bined forces that the loads will exert on it. What is more, not just each
component on its own, but the whole interconnected assemblage that makes
up the bridge structure must stand up to the loads it will carry. To turn
these abstract concepts into real structures, engineers have relied on series
of typical structural types. It is to these that we turn in the next chapter.
Sources and Further Reading
Mario Salvadori's Why Buildings Stand Up (New York: W. W. Norton, 1980)
is a well-known and pleasant introduction to structures underlying architec-
tural form, though a few of the topic's parts (as about earthquakes) are no
longer considered correct. A step up in complexity but still highly accessible
is Waclaw Zalewski and Edward Allen's Shaping Structures: Statics (New York:
John Wiley, 1998). A more challenging topic but one still accessible with
elementary mathematics is R. E. Shaeffer, Elementary Structures for Architects
and Builders, 5th ed. (Upper Saddle River, NJ: Pearson/Prentice Hall, 2007).