Essay on "Definition and Difference Between Arch Bridge"

Essay 10 pages (3068 words) Sources: 1+ Style: Harvard

[EXCERPT] . . . .

Exposing the Dynamics of Arch and Beam Bridges

Page 2 Brief Summary of the History of Bridges

Page 3 Arch Bridges

Page 6 Beam Bridges

Page 8 Comparing Arch and Beam Bridges

Comparing, Contrasting, and Exposing the Dynamics of Arch and Beam Bridges

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 bridges, including their construction and importance vis-a-vis 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 the development of beam bridges -- has led observers and those in charge of construction to go towards beam bridges when a long span is required. The history of bridge-building is fascinating and explains an aspect that prehistoric humans had to deal with and apparently did deal with effectively -- crossing water ways in safe and secure fashion. The specifics and descriptions in the literature are appropriately utilized in this research paper.

Brief History of Bridges

In prehistoric times, man 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 in the website Made How. Eventually early humans learned to pile stones in the 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 describe ed by Herodotus in 484 B.C. Hall explains that this bridge consisted of "timbers supported by stone columns" and was build 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 explains (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).

Arch Bridges

Among the oldest types of bridges in the world, arch bridges have been used for many thousands of years. They are reliable for what purposes they are intended, according to the literature. Originally, they were constructed of stone or brick, but in modern times arch bridges are made of "reinforced concrete or steel" according to Design Technology. Because modern design materials are sturdier and more resilient to weather and other natural dynamics, arch bridges today can be made longer with spans that are 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," Design Technology explains. The considerable weight of a concrete reinforced arch bridge is transferred to the supports at both ends of the bridge. The supports at each end of the bridge, that receive the weight of an arch bridge, are referred to as abutments. Design Technology explains that the abutments keep the bridge from "spreading out."

The tremendous load at the very top of the "key" stone (that stone in the center at the top) places pressure on the stones next to it, and that pressure extends down to the abutments, which, as was mentioned, receive the load in full. What about the ground around the abutments? The Design Technology explanation is that "For every action there is an equal and opposite reaction," and hence, the ground pushes back on the abutments, creating "resistance" which in turn is passed from stone to stone until ultimately that pressure is pushing back on the key stone.

Based on a very old, ancient engineering concept -- "how to span an opening with a curved structural member" -- the arch bridge, which makes it possible for a road to be build on it so people and materials can cross a river, transmits the load from the bridge "deck" to abutments on both sides of the river (mrdbridges.com). The very early versions of arch bridges used stone blocks, as mentioned above; the stone blocks were wedged tightly together to make the arch perfect.

Modern arches are not only made of reinforced concrete, the MRD Bridges Company explains, but they are also constructed of steel. The steel allows an arch bridge to be built over a wider swath -- a river or lake or gorge that must be crossed -- but in the case of a longer arch bridge span, the ground (or foundation) must be very "solid and stable" to withstand the thrust. The arch can be built above the bridge deck, or it can be built below the deck.

When the deck is built above the arch (which is called "a deck arch bridge"), the space between the bottom of the arch, and the deck, can be made to be solid; in that case, the bridge is called a "closed spandrel deck arch" (mrdbridges.com). If the space between the bottom of the arch and the deck is left open -- in which case supporting vertical members must be put into place -- the bridge is called an "open spandrel deck arch" (mrdbridges.com).

The construction of an arch bridge is not as easy for engineers to design as other bridges, including beam bridges. That difficulty arises from the fact that until both sides of the arch are joined in the middle with some kind of a "key" stone or steel link, the bridge's arch is not stable (mrdbridges.com). For this reason, many arch bridges are built with "wooden falsework" to keep the supports in place temporarily until the solid linking piece is firmly in place.

Robert Lamb and Michael Morrissey estimate that arch bridges have been used for more than 2,000 years, which dates back to the origins of the Christian religion and to ancient Rome, where arch bridges were a common sight. The reason arch bridges were built so long ago, and are still prominent in modern times, is explained by Lamb and Morrissey in the website Science / How Stuff Works.

The arch bridge's "…semicircular structure elegantly distributes compression through its entire form and diverts weight onto its two abutments," and moreover, there are few if any "tensional forces," the authors explain (Lamb, et al., 2004, p. 3). Instead of creating tension the natural curve of the arch enhances the bridge's ability to "dissipate the force outward," massively reducing the tensional dynamics that otherwise would put undue pressure on the underside of the bridge, Lamb continues.

That having been said, Lamb and Morrissey warn that the "greater degree of curvature" designed into the bridge -- that is, the larger the semicircle of the bridge's arch -- the greater amount of "…tension on the underside of the bridge" (p. 3). What happens when the degree of curvature of the arch becomes too great is that "…tension will eventually overtake the support structure's natural strength" (Lamb, p. 3).

Interestingly, the arch bridges made by the Romans in ancient times did not even need mortar; when the great stones are placed together, the natural force of that semicircle -- once the stone is in place in the middle, joining the two sides -- keeps the stones and the bridge intact (Lamb, p. 3).

Beam Bridges

Lamb and Morrissey explain that the simplest kind of bridge building is done with beam bridges. This kind of bridge is also known as a "girder bridge" and all that is required is a "rigid horizontal structure" (a beam) and a pair of supports at either end, to rest the beam on. The supports (abutments) at either end support the huge amount of weight of the bridge including the trains or autos / trucks that pass over the bridge (Lamb, et al., 2004, p. 2).

Lamb and Morrissey provide a simple example as a model, to help convey the dynamics of beam bridges. Take a piece of wood, a two-by-four, preferable, and lay it across two empty milk crates. That makes up a "crude beam bridge," Lamb notes on page 2. By placing a very heavy weight in the middle of the two-by-four, it will bend to some degree. The top side of the two-by-four will "…bend in under the force of compression," and the "bottom side would bend out under the force of tension" (Lamb, p. 2). If a sufficient amount of weight is placed on the two-by-four, it will eventual break -- the top side will "buckle" and the bottom side would "snap" (Lamb, p. 2).

Of course beam bridges are not made of wood, steel beams and reinforced concrete are used on beam bridges to handle the huge load expected of the bridge. What controls the distance the beam can be made to cross a river, lake, or gorge? That is controlled by the "size of the beam, and in particular… READ MORE

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