A basic criterion by which to select a bridge type for a site is the length to be spanned, but the matter is not as straightforward as it may first appear.
For other than the longest spans, a number of bridge types can do the job.
Any of four types of bridges might suit a 500-foot span. And even if we know the gap to be bridged, we may still have decisions to make about the length of the main span. A bridge could, after all, cross a 2000-foot-wide river with one span, two spans, or three or more all equally spaced, or with two multiple-span viaducts each extending 600 feet into the channel and then connected with a single 800-foot main span.
To be sure, community preference is always a prime consideration, but here we describe only technical factors, which citizens participating in bridge decisions should keep in mind. Span is one geometric criterion;
others are clearances, angles of ascent and descent, and curves. Over water, the bridge may have to give clearance for tall ships; over a highway, for tall trucks. The need for curvature often restricts bridge type to girders, with box-girders preferred over I-girders, since the former are usually better at handling torsion forces imposed by traffic, wind, and earthquake.
For a long crossing that must be curved, one option is to have mul- tiple girder spans near shore, each girder angled more than the previous so as to form a curve, after which the mainspan, which might be a through- arch, remains linear. If the deck must reach high above the surface, room is needed to bring ramps up to the proper elevation, so the geometries of road connections must be studied. Nearby roads are of special concern dur- ing construction, when bridge work can disrupt and endanger traffic, while the traffic itself can put construction workers at risk.
Geological conditions always matter. Rocky canyon walls and palisades might especially suit an arch bridge, providing it with natural abutments.
As bridges are heavy, soils must be investigated for suitability as founda-
tions. Depending on findings, the bridge piers may be made to rest on wide concrete platforms called spread footings; or on piles, which are posts driven into the ground; or in shafts drilled into the soil and filled with concrete (figure 4.11). If the soil is likely to settle, the bridge type selected must be one that can withstand some irregular settlement over time. The potential for earthquakes is always a concern.
Water is a concern and not just because of needed clearance. Rainfall and soil drainage affect the stability of embankments on which a highway bridge rests. Floods increase current pressure against piers or pose the danger the fast waters will scour out soil at the bottoms of piers and destabilize them. In designated flood plains, more piers may not be an option since they may obstruct drainage. And many environmental concerns come into play: preservation of natural shorelines, protection of endangered species, protection of wetlands, and others.
Let us say that planners call for a bridge made of a certain kind of concrete girder, but there are no nearby plants that make such girders. Yet there is a steel fabricating plant nearby. That might be a good reason to change the bridge design. Then again, let us say that the construction is to occur in a narrow gully to which large prefabricated parts are difficult to deliver. Then it might be better to assemble a truss bridge from steel
Figure 4.11. Piers—footings and foundations.
Pile Pedestal Drilled
Pile
Driven
Pile Spread Footing
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 Book 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).
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