Civil Engineering Reference
In-Depth Information
elements, which are smaller and easier to transport. In short, nearness of
industry, nearness of labor skills, and construction-site features can all affect
bridge selection.
Traffic projections, maintenance methods over time, hazard prevention
(as from coastal surge due to hurricane), and esthetic judgment all count.
Most of all, there is always the master consideration, cost. To give just one
rule of thumb: adding one more span to a multispan bridge adds one more
pier plus the work of attaching the added girder. It is usually more economi-
cal to build with one fewer span and instead to lengthen each girder along
the bridge. But the longer girders then have to be made with deeper webs
and heavier flanges to resist bending forces.
Asked whether to rehabilitate a bridge, replace it, or build a new one,
decision makers must contend with just these kinds of complex choices and
tradeoffs. The two major considerations are cost and safety. Before we get to
analyzing costs, we should recognize one more matter, one that nonengineers
may have difficulty appreciating. It is that engineers do not have perfect
knowledge of how a bridge will perform. In deciding the kind of bridge that
will fulfill travel needs at least cost, the engineers face uncertainties about
future live loads and the proposed bridge's capacity to resist them.
Sources and Further Reading
In the advanced engineering text
Design of Highway Bridges: An LRFD
Approach
, 2nd ed. (Hoboken, NJ: John Wiley, 2007), authors Richard M.
Barker and Jay A. Puckett also classify bridge types and discuss the selection
of bridges to suit site conditions. Brian Hayes provides an interesting guide
to bridge types and other man-made scenery in
Infrastructure: The Topic
of Everything in the Industrial Landscape
(New York: W.W. Norton, 2005).
Another useful, though dated, source is M. S. Troitsky,
Planning and Design
of Bridges
(New York: John Wiley, 1994).
