Which Type of Arch Bridge Supports The Most Weight?

Munim Deen

Abstract

 

The purpose of this project was to determine which type of arch bridge supports the most weight.  It was hypothesized that if constructed correctly, then an abutment and spandrel column supported single above-arch bridge would support the most weight, therefore a single arch-bridge will support the most weight.   The Procedure was as follows: First, thirteen (13) bridges of each of the following types were built:  single above-arch, single through-arch, single tied arch and multiple arch and pier.  Then, abutments were constructed to support the bridges at the testing site.  At the test site, the bridge was positioned so that it was supported by both abutments.  A set of objects were gathered to use as the weights.  Then, the first weight was placed in the middle of the bridge.   Additional weights were added by placing each one on top of the preceding one until the bridge collapsed.  The total mass of all the weights needed to collapse the bridge were determined.    The final amount of weight was recorded.  This was repeated for all bridges expect single tied arch types.  These followed all above procedures, but did not come in contact with the abutments.  The calculations performed during this experiment were: Tabulating the total amount of weight in grams supported by each bridge by adding the masses of each weight on the bridge; and finding the average amount supported by each type by adding the total amount of weight supported by each bridge of that type, and then dividing that figure by thirteen (the number of bridges of each type) to obtain an average figure.  The results supported the hypothesis.  The   abutment and spandrel column supported single above-arch bridges supported the most total and therefore, average weight.  Based on the results, it can be determined that arch bridges supported by abutments support more weight than bridges not supported by abutments.  The single above-arch bridges supported an average of about twenty-six kilograms.  The single-tied arch bridges supported an average of about seventeen and a half kilos.  About twenty-three kilos were supported by the single through-arch bridges, and finally the double arch and pier bridges supported about nineteen and a half kilos.      

 

Research

 

The arch is one of the oldest structures known to man.  Historically, the Romans perfected the arch bridge.   No matter what the design is, all arch bridges share one common trait.  That is the ability to support an immense amount of weight.  Almost everything that is built to take great pressure is curved, such as the hull of a submarine.  Arches support heavy objects such as the solid stone walls of the Coliseum in Rome.   What allows the arch to support such large amounts of weight is its structural properties.  The forces on an arch bridge extend outward and downward (a condition called compression).  An arch is higher in the center than the ends, acting almost entirely on compression. That is why Arch Bridges must be built using materials that take compressive loads well. In an arch bridge the main structural system supporting the deck is a curve member (or members), higher vertically at its center than its ends, acting almost entirely in compression with its compressive load being maintained by thrust against immovable supports called abutments.  Usually the inclined faces of the curve members of the arch (called skewbacks) come in contact with the abutments.  Abutments are crucial to most arch bridges.  Abutments are supports at both sides of an arch bridge at the base of the arch. When something pushes down on an arch bridge, its weight is carried outward along the curve of the arch to the abutments. As said before, arch bridges act on compression. The compressive load is maintained by the thrust against the abutments.  Without the abutments, the ends of the arch would spread apart under any weight.  Abutments also help support the arch itself.  It is important to have sufficient abutments for an arch bridge.  If they are too small, the bridge will not support much weight.  If they are too big, they can cause unnecessary construction difficulties.  Abutments are solid structures, if they were hollow, they would be too light to effectively support a bridge. Abutments or other types of supports are needed on all types of bridges, but they are especially important for most types of arch bridges.  To sum it up, there can’t be one cubic meter abutments for a five hundred meter span. 

Arch bridges utilizing a single arch must have spandrel columns.  These columns help to transfer weight to the arch as well as to act as a connector between the bridge deck and the arch.  There are many variations in the proportion and appearance of spandrel columns.  Another lesser known device used on arch bridges is the hinge. Without hinges, arch ribs extend continuously from skewback to skewback and are fixed at skewbacks so no rotation can occur and thus the bridge is called a fixed arch.   There can be two or three hinges in an arch. The three-hinged arch has hinges at the center and at the skewbacks, and the two-hinged arch has hinges only at the skewbacks.  (See next page) These hinges permit the transfer of the main compressive loads from one section of arch rib to another or to the abutments without transfer of bending movements.  Hinges at the skewbacks simplify only the abutment design, but hinges at the crown and the skewbacks simplify the design of the arch rib as well as the abutments, eliminate stresses due to temperature changes, and allow the arch to tolerate minor movements of its abutments.  In scaled-down models, hinges are not easily shown.

The first arch bridges were built of stone blocks to only one specific design.  This design called for the roadway to be above the supporting arch with supporting columns (now called spandrel columns) between the straight road surface and the curved surface of the arch. During the late nineteenth century, construction of masonry arch bridges had reached their peak, although the one basic design was still predominant.  In the mid-twentieth century, a new design of arch bridge appeared.  In this design, the roadway passes through the arch, which results in the roadway being lower in height.  This design uses a system of steel beams to connect the roadway with the weight-bearing arch.  This design necessitates the use of more advanced construction techniques, but produces a striking visual effect; the Sidney Harbor Bridge is an example.  In addition to these two types of arch bridges, there is a third type. This type does not make use of abutments or spandrel columns.  Instead, the curve members of the arch (called the ribs) do the work of supporting the arch.  One end of each rib is tied to the other end, and the thrust of the ribs is supported by structural tension.  The tie is about deck-level and the deck hangs below the curve of the arch.  This type is rare because of its construction difficulties and correspondingly high cost.  The most revolutionary design of arch bridge to date came about in the early 1990’s.  This type has the roadway going above the arch, but unlike the type previously mentioned, it uses no spandrel columns.  In fact, the only supporting structures are two or more arches and sometimes, a pier between the arches.  To support the weight of the roadway better, the tops of the arches are flattened.  This design results in a very aesthetically pleasing bridge. These are the four major types of arch bridges in use today.           

All arches can support a large amount of weight.  Each type of arch bridge discussed above is its own unique design, with its own unique structural elements.  How those elements affect each bridge’s weight–carrying capability is being explored in this investigation.

 

Goals

1.      To determine which type of arch bridge supports the most weight.

 

2.  To determine which, if any, aspects of an arch bridge’s design have any effect on its structural strength.

 

Hypothesis

It is hypothesized that if constructed correctly, then an abutment and spandrel column supported single above-arch bridge will support the most weight, therefore a single arch-bridge will support the most weight.  

 

Procedure

1.      Build thirteen (13) bridges of each of the following types:  single above-arch, single through-arch, single tied arch and multiple arch and pier.  Use identical wooden pieces for all parts of all bridges.  Each bridge should be 25 cm long and 5 cm tall.  Each joint should use exactly 4 sq. cm of glue.

2.      Construct abutments to support the bridges at the testing site. 

3.      At the test site, position the bridge so that it is supported by both abutments.

4.      Gather a set of objects to use as the weights.

5.      Make sure the testing area is not subject to movements of any kind during testing, place the first weight in the middle of the bridge.  Continue adding weights by placing each one on top of the preceding one until the bridge collapses. 

6.      Determine the mass of all the weights needed to collapse the bridge.  Record the final amount of weight.

7.      Repeat for all bridges expect single tied arch types.  These will follow all above procedures, but will not come in contact with the abutments. 

Results

 

The calculations performed during this experiment were:

 

1.                  Tabulating the total amount of weight in grams supported by each bridge by adding the masses of each weight on the bridge.

 

2.                  Finding the average amount supported by each type by adding the total amount of weight supported by each bridge of that type, and then dividing that figure by thirteen (the number of bridges of each type) to obtain an average figure.

 

 

Discussion of Results

 

The results supported the stated hypothesis.  The single above-arch bridges supported by spandrel columns supported the most average weight of 26.136 kg.  The arch bridges supported by abutments supported more weight than the bridges not supported by abutments.  This suggests that abutments are necessary to the design of an arch bridge if it is required to support heavy loads.  The aforementioned results will be very helpful to construction of large-scale projects such as urban bridges and also for small projects such as building a shelf.  The results would help in finding a compromise between structural strength, aesthetics, function, and cost.

 

  Conclusions

 

1.      The results supported the stated hypothesis. 

2.      The   abutment and spandrel column supported single above-arch bridges supported the most total and therefore, average weight. 

3.      Based on the results, it can be determined that arch bridges supported by abutments support more weight than bridges not supported by abutments. 

4.      The other components of arch bridges do not seem to have a significant effect on structural strength.

 

Further Developments

Expansions of this project could include:

1.      Which building material produces the strongest arch?

2.      Do arch bridges with subterranean abutments support more weight than bridges with above-ground abutments?

3.      If this same experiment was repeated in precipitous conditions, would the results differ?

4.      Does humidity affect the structural strength of wooden arch bridges?

Acknowledgements

My gratitude to Mrs. Hunter and Dr. Testa for their guidance and assistance

And also to my parents for their help and support

Works Cited

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2.      McGraw-Hill Encyclopedia of Science and Technology. Vol. 3. McGraw-Hill Incorporated, USA. 1992.

3.      Llewellyn, Claire.  How Things Work.  London: Scholastic Incorporated. 1995.

4.      Nova Online@www.pbs.org. 

5.      The Handy Science Answer Book. Pittsburgh: The Carnegie Library of Pittsburgh. 1994.

6.      The New Encyclopedia Britannica. Vol. 2. Chicago: Encyclopedia Britannica Incorporated. 1995

7.      The Illustrated Encyclopedia of Invention.  Vol. 3. Tarrytown: Webster Unified. 1990.

8.      Salvadori, Mario. Building: The Fight Against Gravity. Saddle Brook: American Books. 1979.