Which Type of Arch Bridge
Supports The Most Weight?
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
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3.
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The Handy Science
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The New
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The Illustrated
Encyclopedia of Invention. Vol. 3.
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