Mr. Buys
Avon High School
Engineers In Training
What impacts do bridges have on our communities and cities? Bridges enable roadways
to pass through varying terrain, over waterways and through mountains with minimal
deviation, saving time in transport or commute or even connecting areas that would
otherwise be inaccessible. Who designs these bridges? Civil engineers do. Think about
bridges as a way that engineers help us bring worlds together. An example is the CT.
River. It would be difficult to access anything east of the river if it were not for the two
bridges south of Hartford that ties these regions together.
History of BridgesIn an age of rapid transportation, bridges are very important. They allow for
roads and railroads to cross over otherwise impassable obstacles such as rivers or other
roads. By the end of the 19th century there, were three main types of bridges being used
in the United States, the arch bridge, the bascule bridge (drawbridge) and the cantilever
bridge? These three types of bridges reflect the needs and materials available of those
who built them.
Bridges have been widely used since ancient times and when the colonists from Europe
came over to America they built roads and with them bridges. The bridges constructed
by the early colonists were made chiefly of wood. By the late 1800, bridges were being
built mainly with steel, iron and concrete.
As demand for railroad bridges grew, bridges had to be built in difficult places. In 1867,
James Eads was commissioned to build a bridge across the Mississippi river. Because
the bridge supports had to be built in the bedrock bellow the sandy river bottom he had
men digging in airtight capsules at depths of up to a hundred feet. In 1898, an arch
bridge spanning 840 feet was constructed below Niagara Falls. The bridge was
constructed mainly of steel. Towards the end of the century, they also became combined
with concrete to create a concrete reinforced bridge. The first concrete reinforced bridge
was completed in 1898. One of the largest concrete reinforced bridges in the United
States is the Tunkhannock Creek Viaduct in Pennsylvania. This bridge spans 2,375 feet.
With the expansion of the railroad, however, another type of bridge became widely used.
This bridge is called a cantilever bridge. The bridges complex design allowed it to
support an incredible amount of weight, ideal for the trains that weighed up to several
tons. This advancement made the expansion of the railroad that much faster.
Today a new type of bridge has become widely used the suspension bridge. These
bridges are constructed so that they can be built over longer distances. Several bridges
span over a mile. Bridges today are all designed by a computer with architectural
software before the first stone is laid.
1. What were the three main types of bridges being built at the end of the 19th
century and give two examples?
2. Who was commissioned to build a bridge that crossed the Mississippi River?
What difficulty had to be overcome and how did they over come it?
3. Where is the largest concrete reinforced bridge? How large is this bridge?
4. Why was the cantilever bridge designed? What benefits does it offer?
5. How are the bridges of today designed as a result of history?
Three basic types of bridges used in transportation are: beam / truss bridges, arch
bridges and suspension bridges. To understand how bridges work, we must
understand the loads that act on every bridge. The two types of loads are live and
dead loads that create forces that act on the bridge. Live loads are dynamic and
change over time, examples are traffic, wind and ice. Dead loads are static and
never change, the weight of the bridge is considered a dead load. Two major
forces produced by the loads at any given time are compression and tension.
Compression, or compressive force, is a force that acts to compress or shorten the
thing it is acting on. Tension, or tensile force, is a force that acts to expand or
lengthen the thing it is acting on. As a simple example, think of a spring. If we
push both ends of the spring towards each other, we are compressing the spring.
Thus, a force of compression is acting on it to shorten the spring. If we pull both
ends of the spring away from each other, we are stretching the spring. Thus, a
force of tension is acting on it to lengthen the spring. It is the purpose of the
bridge design to handle these forces without a member breaking or failing in some
manner. Some materials may be used together to utilize the strength of one
material without the drawbacks of the other. An example of this would be using
reinforcing bars in concrete to gain the tensile strength of the steel with the
compressive strength of the concrete.
6-8.) Give three examples of Live loads
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9.) Explain the term dead load. ______________________________________________
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10.) ______________ is the force that pulls a structural member apart.
11.) ______________ is the force that squishes a material together.
12.) If a force of 50 Lbs. is exerted over an area of 15 square inches, What is the PSI over
this area? _______________________________________________________________
13.) If a force of 50 Lbs. is exerted over an area of 5 square inches, What is the PSI over
this area? _______________________________________________________________
14. If a force of 85 Lbs is exerted on a member with a strength of 850 Lbs. What is the
efficiency ratio?___________
15. If a force of 85 Lbs is exerted on a member with a strength of 100 Lbs. What is the
efficiency ratio?___________
Beam and Truss Bridges
Beam bridges are the simplest and least expensive type of bridge to build. The most
simple beam bridges consist of a horizontal beam that is supported on each end by
columns or piers. The weight of the beam and any additional load on the bridge is
transferred directly to the piers. However, the beam itself must be able to support its own
weight and loads between the piers. When a load pushes down on the beam, the top
portion of the beam is pushed together by a compressive force while a tensile force
stretches the lower portion. The farther apart the supports or piers, the weaker a beam
bridge becomes. For larger beam bridges designed for heavy car and railroad traffic, the
beams are substituted by simple trusses, or triangular units, which are more economical
than solid beams. Engineers have used many different truss patterns in bridges.
Therefore, most beam bridges rarely span more than 200 feet (61m), however, old truss
bridges crossing major rivers are often as long as 500-600 feet (152-183m), not including
end supports such as piers.
Truss Design
The truss is a simple skeletal structure. Trusses are comprised of many small
beams that together can support a large amount of weight and span great distances. In
most cases the design, fabrication, and erection of trusses is relatively simple. They have
the ability to dissipate a load through the truss work. The design of a truss, which is
usually a variant of a triangle, creates both a very rigid structure and one that transfers
the load from a single point to a considerably wider area.
Before you can go any further into truss design, you need to learn about the parts
that make up a truss.
Upper Chord - defines the top or a roof or bridge truss and helps carry the weight of
roofing materials or snow.
Lower Chord - determines the distance the truss can span (or reach across), and helps
transfer the load carried by the truss to each end of the bridge or
building.
Web Members - add bracing to the upper and lower chords. They help keep the truss
sturdy, help transfer the load to each end of the truss, and keep the
truss from twisting. They give the truss much of its strength.
Gussets - thin pieces of plywood or metal used to join the parts of a truss together.
15.) What is the general definition for a truss?
16.) Why are trusses stronger than single beams?
17-20.) List the 4 parts that make up a truss and give a brief definition of each.
Arch Bridges
Arch bridges are the easiest type of bridge to recognize. They are one of the oldest types
of bridges and have extraordinary natural strength. Instead of pushing straight down as
beam bridges do, the weight of the arch bridge and any additional load on the bridge is
carried outward along the curve of the arch to the supports at each end. These supports
are called abutments. Abutments distribute the load from the bridge and keep the ends of
the bridge from spreading out. Arch bridges are always under compression. The force of
compression is pushed outward along the curve of the arch toward the abutments.
The Romans were masters of the arch bridge. Many of their arch bridges used little or no
mortar, or "glue," to hold the stones together. The goal of an arch bridge is to carry all
loads in compression, without any tensile loads present.
The stones in the structures stay together by the sheer force of their own weight and the
compression transferred between them. The size of the arch, or the amount of curvature,
has a major effect on the effectiveness of this type of bridge. Sometimes, in very large
arch bridges, the arch is often reduced in size or flattened down, which results in
significant tensile forces that must be factored into the design. Most modern arch bridges
span between 100-1,500 feet (30-457m).
The tension in an arch is negligible. The natural curve of the arch and its ability to
dissipate the force outward greatly reduces the effects of tension on the underside of the
arch. The greater the degree of curvature (the larger the semicircle of the arch), however,
the greater the effects of tension on the underside.
As we just mentioned, the shape of the arch itself is all that is needed to effectively
dissipate the weight from the center of the deck to the abutments. As with the beam
bridge, the limits of size will eventually overtake the natural strength of the arch. Arch
types are few - after all, an arch is an arch is an arch. The only real subcategories come in
the form of cosmetic design. There are, for example, Roman, Baroque and Renaissance
arches, all of which are architecturally different but structurally the same. Arches are
fascinating in that they are a truly natural form of bridge. It is the shape of the structure
that gives it its strength. An arch bridge doesn't need additional supports or cables. In
fact, an arch made of stone doesn't even need mortar. Ancient Romans built arch bridges
(and aqueducts) that are still standing, and structurally sound, today. These bridges and
aqueducts are real testaments to the natural effectiveness of an arch as a bridge structure.
21.) Where is the load carried in an arch bridge?
22.) Arch bridges are always in tension. True / False
23-24.) What two factors have a major effect on an arch bridge?
25.) There are several types of arch bridges. True / False
Suspension Bridges
Two categories of suspension bridges are: modern suspension bridges and cable-stayed
bridges. Modern suspension bridges are characterized by an M-shaped cable pattern.
Cables are strung over two towers and then anchored on both ends. The roadway is
suspended from the cables by thinner cables or rods. The roadway's weight and any
additional load are transferred to the cables, creating a tension force in the cables. The
cables then transfer their force to the towers and anchors. Typical modern suspension
bridges span distances from 2,000 to 7,000 feet (610-2,134m). Cable-stayed bridges are
characterized by an A-shaped cable pattern. Cables are anchored directly to the towers
and eliminate the need for an anchorage system. The same tensile and compressive forces
are seen in a cable-stayed bridge as they are in a modern suspension bridge. Typical
cable-stayed bridges span distances from 500 to 3,000 feet (152-914m), fast becoming
the bridge of choice for medium length spans. Cable-stayed bridges also look cool!
Today, we are going to create simple models of each type of bridge that we just discussed
to help us learn more about how the forces of tension and compression act on each one.
We are also going to think about the situations when an engineer might decide to use
each type of bridge when designing roadways.
26.-27.) What are the two categories of suspension bridges? ____________________
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28.) Great compressive forces are generated in the cables of a suspension bridge.
True / False
29.) Suspension bridges can reach 1 mile in length.
True / False
30.) Cable Stayed bridges have what type of cable pattern? ________________________
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