Arch Bridges vs. Beam Bridges: Design, History, and Dynamics

Executive Summary

There are certainly major differences between a beam bridge and an arch bridge, and this paper delves into the specific and relevant data on both bridge types, including their construction and importance with respect to safety and practical usage. Recent research in the literature shows that while arch bridges have previously been considered more expensive and less practical than beam bridges, new technologies are changing the way engineers approach bridge design. Still, the history of beam bridges — and their development — has led observers and construction planners to favor beam bridges when a long span is required. The history of bridge-building is fascinating and sheds light on a challenge that prehistoric humans had to deal with, and apparently did deal with effectively: crossing waterways in a safe and secure fashion. The specifics and descriptions found in the literature are appropriately utilized throughout this research paper.

In prehistoric times, early humans began to understand the importance of bridges by walking across a stream on a tree that had fallen over it — at least, this is the theory put forward by Loretta Hall. Eventually, early humans learned to pile stones in a stream and place beams of wood, or fallen limbs, between the columns of stones and the stream bank. The very first bridge to be documented in the historical literature was described by Herodotus in 484 B.C. Hall explains that this bridge consisted of "timbers supported by stone columns" and was built across the Euphrates River three hundred years prior to Herodotus' report (Hall, 1999, p. 1). The earliest bridge built by the ancient Romans is believed to have been constructed across the Tiber River in 620 B.C. (Hall, p. 1). The Romans created concrete using lime and pozzalana, a red volcanic powder; this material set up very quickly, was tough, and resisted corrosion by water (Hall, p. 2).

A Brief History of Bridges

Among the oldest types of bridges in the world, arch bridges have been used for many thousands of years. They are reliable for the purposes they are intended to serve, according to the literature. Originally constructed of stone or brick, modern arch bridges are made of reinforced concrete or steel. Because modern design materials are sturdier and more resilient to weather and other natural forces, arch bridges today can be built longer with spans that are proportionally lower.

"Instead of pushing straight down, the load of an arch bridge is carried outward along the curve of the arch to supports at each end," as Design Technology explains. The considerable weight of a reinforced concrete arch bridge is transferred to the supports at both ends of the bridge. These supports, which receive the weight of the arch bridge, are referred to as abutments. The abutments keep the bridge from spreading out under the load.

Arch Bridges: Structure and Engineering

The tremendous load at the very top of the keystone — the stone at the center of the arch — places pressure on the stones next to it, and that pressure extends down to the abutments, which receive the full load. As for the ground around the abutments, Design Technology explains that "for every action there is an equal and opposite reaction," and thus the ground pushes back on the abutments, creating resistance that is passed from stone to stone until it pushes back against the keystone.

Based on the ancient engineering concept of spanning an opening with a curved structural member, the arch bridge allows a road to be built upon it so that people and materials can cross a river. It transmits the load from the bridge deck to abutments on both sides of the river (MRD Bridges, 2005). The earliest versions of arch bridges used stone blocks wedged tightly together to form a perfect arch.

Modern arches are constructed not only of reinforced concrete but also of steel. Steel allows an arch bridge to be built over a wider span — crossing a river, lake, or gorge — but in the case of a longer arch bridge, the ground or foundation must be very solid and stable to withstand the thrust. The arch itself can be built above the bridge deck or below it.

When the deck is built above the arch (a configuration called a deck arch bridge), the space between the bottom of the arch and the deck can be made solid; in that case the bridge is called a closed spandrel deck arch (MRD Bridges, 2005). If the space between the bottom of the arch and the deck is left open — requiring vertical supporting members — the bridge is called an open spandrel deck arch (MRD Bridges, 2005).

The construction of an arch bridge is not as straightforward as that of other bridge types, including beam bridges. This difficulty arises from the fact that until both sides of the arch are joined in the middle with a keystone or steel link, the arch is not stable (MRD Bridges, 2005). For this reason, many arch bridges are built with wooden falsework to keep the supports in place temporarily until the linking piece is firmly secured.

Robert Lamb and Michael Morrissey estimate that arch bridges have been in use for more than 2,000 years, dating back to ancient Rome, where they were a common sight. The reason arch bridges were built so long ago, and remain prominent today, relates to their structural elegance. The arch bridge's "semicircular structure elegantly distributes compression through its entire form and diverts weight onto its two abutments," and there are few if any tensional forces involved (Lamb et al., 2004, p. 3). Instead of creating tension, the natural curve of the arch enhances the bridge's ability to dissipate force outward, massively reducing the tensional dynamics that would otherwise place undue pressure on the underside of the bridge.

That said, Lamb and Morrissey warn that the greater the degree of curvature — that is, the larger the semicircle of the arch — the greater the tension on the underside of the bridge (p. 3). When the degree of curvature becomes too great, "tension will eventually overtake the support structure's natural strength" (Lamb et al., 2004, p. 3).

Interestingly, the arch bridges built by the Romans in ancient times did not even require mortar. When the great stones are placed together and the keystone is set in the middle joining the two sides, the natural force of the semicircle keeps the stones and the bridge intact (Lamb et al., 2004, p. 3).

Conclusion

When a researcher delves 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 the needs of travel over waterways and other obstacles is safety and economy. There are bridges that have collapsed and caused deaths because the correct engineering was not applied, and because certain factors — such as weather, wind, corrosion, load, and tension — were not adequately considered. Researching the literature reveals ample detail, some of it highly technical, relating to the demanding engineering aspects associated with building bridges. Before a city, county, or government builds a bridge, whether arch or beam, science and engineering must be applied intelligently and rigorously to ensure that the structure serves its purpose safely for generations.