2). An increase in the height of the beam gives the beam more material "to dissipate the tension"; and when the beam is quite tall, due to the depth of the river or gorge it needs to cross over, there is then the need to add "…latticework, or a truss," to the bridge's beam.
With strong latticework or a beefy truss, the ability of the bridge to dissipate the compression and tension is greatly enhanced, Lamb asserts on page 2. Even with a strong truss, a beam bridge has limited use: a beam bridge can only be expected to go certain distances. Even with stronger trusses, a point is eventually reached where "…the truss can't support the bridge's own weight" (Lamb, p. 2).
In fact, according to the Apliut Association of Professors at technology universities in France, the father apart the supports are "…the weaker a beam bridge gets." Because of this engineering reality, it is rare to see a beam bridge span more than 250 feet, the Apliut website explains. But that doesn't mean that beam bridges cannot span fairly great distances because it is possible for beam bridges to be "daisy-chained together," and this creates what Apliut terms "a continuous span."
As an example of how a beam bridge can become very long indeed is the Lake Ponchartrain Causeway (in southern Louisiana, U.S.) which is the world's longest bridge of any kind. It is nearly 24 miles and actually provides a pair of two-lane sections that are parallel to one another (Apliut). The southbound lane (completed in 1956) of this massive, lengthy beam bridge consists of 2,243 "separate spans"; the northbound lane (finished in 1969) is "pieced together from 1,500 longer spans" (Apliut). Indeed there were cross-over lanes built into this huge beam bridge, seven cross-over lanes in all, and these serve motorists as emergency pull-over places in moments of accident or breakdowns.
The Apliut association suggests that the best material to be used in a beam bridge is "pre-stressed concrete," because it tends to withstand "…the forces of compression well," and with the steel support rods built into the concrete allow the bridge to "resist the forces of tension."
Author Loretta Hall explains that when engineers -- at the outset of the planning process -- are designing a beam bridge, a careful analysis of the forces that will be acting on the bridge is conducted. There are three kinds of "load" that contribute to the forces on a beam bridge, Hall explains (p. 3): a) "dead load' alludes to how much the bridge weighs; b) "live load" refers to the weight of the traffic that will be crossing the bridge, either train traffic or motor vehicles; and c) "environmental load" means the external forces like the wind, rain, bitter cold or steamy hot weather, possible earthquakes, or even traffic accidents which can place a heavy burden on the bridge integrity (Hall, p. 3).
In the Design Technology site, the authors insist that the beam bridge is "the simplest kind of bridge" and while it is simple on paper, it needs to be able to resist "twisting and bending" under the heavy load it is expected to carry. Several kinds of beam bridge construction are presented in the Design Technology site: the beam can be hollow, or solid.
Comparing and Contrasting Arch and Beam Bridges
First of all, every author writing about bridges asserts that beam bridges are easily the simplest bridges to design and construct. They are comprised of a simple, solid horizontal beam, supported at each end by piers (abutments) that can absorb the weight of the beam. Yes, arch bridges also rely on abutments at either end of the bridge, but the arch bridge design is far more complicated that the beam bridge because of the dimensions and arc of the semicircle, along with other issues.
The beam bridge, according to Harlan Bengtson, is designed to be relatively light in weight, strong, and long-lasting. Using pre-stressed concrete beams in the beam bridge creates a strong and durable bridge, Bengtson explains (2011). Using a "meticulous analysis of the stress distribution" in advance of building the beam bridge can assure that the twisting and bending forces, which the beam bridge will come into contact with, will not weaken the bridge. This is one of the only serious engineering issues with beam bridges, while arch bridges have a number of other considerations vis-a-vis stress and force on its superstructure.
While the beam bridge may cost less and have fewer issues regarding engineering and design, Henry Tyrrell writes that arch bridges -- when "properly designed" -- may be created in ways that are "beautiful in outline" (Tyrrell, 1909, p. 181). The advantages of beam bridges, covered thoroughly by Tyrrell, are: a) designers and engineers can locate a beam bridge floor "…much lower and nearer to the high water level or other clearance line than can be done with an arch is used"; b) the foundations for beam bridges can be constructed on soil that is "more or less yielding" and that can't be done with arch bridges (unless "hinges are used at the center and spring" of the arch bridge) (Tyrrell, 181).
The reason for the fact that beam bridges have fewer issues at the abutment / ground level of design, is that arch bridges have a "lateral thrust," and on softer foundations an arch bridge is "liable to cause serious injury to the structure" (Tyrrell, 181). But as to beam bridges, the amount of "settlement under the abutments" does not -- in most cases -- result in injury to the structure of the bridge (Tyrrell, 181).
One common objection, mentioned hitherto, is that beam bridges are not "susceptible to artistic treatment" in the same sense as arch bridges. Those creative engineers and designers who have been able to think outside the box have been able to design beam bridges that, according to Tyrrell, "…are equally pleasing in appearance to arch bridges" (181). And again, the advantage of a beam bridge over an arch bridge comes down in many cases to cost; beam bridges are generally more economical than arch bridges unless they are designed to be very long, spanning a wide river for example. Then the design becomes more complicated and the stressors are more serious with reference to bridge stability.
When a beam bridge is "non-continuous" and fairly short, the cost of the bridge is less than a typical arch bridge. However, the longer the span in a beam bridge, the greater the cost, which can approach or exceed the cost of an arch bridge.
In the book from the Fourth International Conference on Current and Future Trends in Bridge Design…" Jure Radic, et al., explain that the "competitiveness of arch bridges in comparison with beam bridges…is now increasing" (Radic, 2006). In other words, whereas beam bridges have generally taken precedent over arch bridges because of the relative simplicity of design, arch bridges are now being considered perhaps the bridge of the future. Why? "Very large span arch bridges may be constructed if the dead weight is reduced by adopting the much lighter composite superstructure" rather than the normal application concrete. This can be done, making arch bridges more attractive, when innovative "cross-sections" of concrete are filled with "steel tubes" (Radic).
In conclusion, when a researcher delving into the differences and similarities of bridges -- notably beam bridges and arch bridges -- there is a great deal of contrast to be found. On the other hand, the bottom line when it comes to bridge building and needs of travel over waterways and other obstacles is safety and economy. There are bridges that have collapsed and caused deaths -- albeit not reported in this paper -- because the correct engineering was not included, and/or certain factors like weather, wind, corrosion, load and tension were not taken into consideration. Researching the literature one finds ample details -- some of it esoteric -- relating to the difficult engineering aspects associated with building bridges. Before a city or county or the government builds a bridge, be it arch or beam bridge, science and engineering must be applied intelligently.
Apliut. 2012. Beam Bridge. Retrieved May 28, 2012, from www.apliut.com/pages/.../swapshop.../desparbes%20_bridges.pdf.
Bengtson, Harlan. 2011. Beam Bridges. Bright Hub. Retrieved May 29, 2012, from http://www.brighthub.com/engineering/civil/articles/46079.aspx.
Design Technology. 2004. Arch Bridges -- Design Technology. Retrieved May 29, 2012, from http://www.design-technology.org/archbridges.htm.
Hall, Loretta. 1999. How Concrete Beam Bridge is Made / Concrete Beam Bridge. Made How. Retrieved May 28, 2012, from http://www.madehow.com/volume-4/concrete-beam-bridge.html.
Lamb, Robert, and Morrissey, Michael. 2004. How Bridges Work / Arch Bridges / Beam Bridges. Science / How Stuff Works. Retrieved May 28, 2012, from http://science.howstuffworks.com.
MRD Bridges. 2005. Bridge Engineering / Common Bridge Types / Arch Bridge. Retrieved May 29, 2012, from http://www.mrdbridges.com/engineering.php.
Radic, Jure, Savor, Zlatko, and Kindij, Alex. 2006. Innovations in Concrete Arch Bridge Design. Fourth International Conference on Current and Future Trends in Bridge Design, Construction and Maintenance: Proceedings of the…