Bridges & Structural
Failure
Suitable for Grade 8 CAPS
Based on information
by G. C. Accom
sourced from
Shuter’s
TOP CLASS
Grade 8
Technology Text book
+
Adaptation of information
found at
Wikipedia,
the free encyclopaedia
DEFINITION
A bridge is a structure built to span physical
obstacles such as a body of water, valley or
road for the purpose of providing passage over
the obstacle.
Designs of bridges vary depending on the function of
the bridge, the nature of the terrain where the bridge
is constructed, the material used to make it and the
funds available to build it.
WHERE DID THIS IDEA COME FROM?
The first bridges were made by nature itself — as simple
as a log fallen across a stream or stones in the river.
The first bridges
made by
humans were
probably spans
of cut wooden
logs or planks
and eventually
stones, using a
simple support
and crossbeam
arrangement.
Some early Americans used trees or bamboo poles to cross
small caverns or wells to get from one place to another.
A common form of
lashing sticks, logs,
and deciduous
branches together
involved the use of
long reeds or other
harvested fibers
woven together to
form a connective
rope which was
capable of binding
and holding in place
materials used in
early bridges.
Primitive suspension bridge over the River Astore
Types of bridges
• BEAM & COLUMN BRIDGES
•SUSPENSION BRIDGES
•CABLE – STAYED BRIDGES
•ARCH BRIDGES
•CANTILEVER BRIDGES
BEAM and COLUMN BRIDGES
1. The beam and column bridge is the simplest
type of bridge
2. It is usually straight.
3. The beam is supported at each end by
columns.
4. Columns are designed to hold the weight of
a structure and in this case , a beam bridge.
See pages 16 & 17 of Shuter’s TOP CLASS Technology textbook Grade 8
ABUTMENT
L
O
A
D
A beam bridge consists of a rigid horizontal member called a beam
that is supported at both ends, either by a natural land structure,
such as the banks of a river, or by vertical posts called columns.
COMP
There are many forces that act on a beam and column bridge.
R
The bridge's own weight can put a huge strain on the structure.
E
For the beam and column bridge to be strong, it must develop
S
compression along the top and tension along the bottom.
S
I
O
N
TENSION
Sky_gate_bridge
Multispan plate girder bridge deck on concrete piers.
Advantages of beam and column bridges
The beam bridge rests on simple columns.
A beam may be built away from the final position,
and lifted swiftly into place with no disruption to the
traffic below.
Disadvantages of beam and column bridges
The beam bridge experiences large forces and
therefore has to be very big in size.
Suspension bridges
In a suspension bridge the deck or road is suspended or hangs
from the main cable in the bridge.
The columns hold the main cable up.
The anchorages pull the cable outwards and downwards.
The hangers connect the deck to the main cable.
The main cables are fixed to the towers at saddles to avoid sliding
The deck carries the traffic.
See pages 17 & 18 of Shuter’s TOP CLASS Technology textbook Grade 8
ADVANTAGES OF SUSPENSION BRIDGES
1. Suspension bridges can span very long gaps. The design
allows the bridge to be made up of very long sections.
2. If one of the hangers fails or breaks, it can be replaced
without disruption to the flow of traffic and will not have an
effect on the stability of the bridge.
3. Suspension bridges do not have to be built along straight
channels but can be designed along curves and arched spaces
4. Suspension bridges can be built with double decks for cars
and trains. The strength of the cabling system is sufficient to
hold the double decks
DISADVANTAGES OF SUSPENSION BRIDGES
1.
Suspension bridges are very expensive to build
2.
They take a very long time to build. It can take up to
nine months to connect the main cables to the
anchorages.
3.
The concrete towers has to be built in shallow bedrock.
4.
The incomplete decks can be affected by strong winds
Suspension.bridge.bristol.
Golden Gate Bridge
The cables are in tension, and the deck is in compression.
The spans can be constructed as cantilevers until they are joined at the
centre.
A cantilever is a structure that is supported at one end only, while the
other end projects out into space.
The cables are made of high tensile steel.
The columns are often made out of concrete.
See pages 18 & 19 of Shuter’s TOP CLASS Technology textbook Grade 8
Advantages of cable-stayed bridges
The two halves may be cantilevered out from each side.
There is no need for anchorages.
They can be cheaper than suspension bridges.
Disadvantages of cable-stayed bridges
In the longer sizes, the cantilevered halves can be affected by strong
winds during construction.
The cables require careful treatment to protect them from corrosion.
Cable-Bridge_over_Krishnarajapuram_Railway_station.
Rama_VIII_Bridge_at_night.
Arches
An arch is a curved structure across an
opening.
An arch not only spans a gap or space
but it also supports a load.
An arch is totally in compression.
Arches in buildings
Arches in buildings were used often in
ancient times.
The Romans used arches to build
aqueducts.
Arches are also used in houses, halls
and theatres for decoration.
The arch designs bring more air and
light to these venues.
See pages 18 & 19 of Shuter’s TOP CLASS Technology textbook Grade 8
Arch bridges
An arch bridge is a complicated structure which should be purely in
compression and should not bend.
It is often made from materials such as masonry (stone), cast iron and
concrete.
The two most important components of an arch bridge are the keystone
and abutment.
The keystone is the main
piece of the arch bridge and
is assembled last.
lt holds the whole bridge
together.
The abutment is the contact
point of the arch and river
bank.
See pages 21of Shuter’s TOP CLASS Technology textbook Grade 8
Advantages of arches
The entire arch is in compression.
Arch bridges can be very long because there is no tension in
the bridge.
Disadvantages of arches
An arch cannot stand until it is complete.
The two halves must be cantilevered using cables.
So, a cable-stayed structure must be built to hold both ends of
the arch before it is joined.
This is building two structures.
Mehmed Paša Sokolović Bridge inVišegrad Bosnia and Herzegovina
Stone arch bridge in Shaharah, Yemen
Arches in dam walls
An arch dam has a dam wall that is curved and made from concrete.
The dam wall curves upstream towards the flow of water.
As the water presses against the arch in the dam wall the compression
is spread to the ends of the dam wall and strengthens it.
Cantilevers
A cantilever is a structure that is supported at one end only, while the
other end projects out into space.
See pages 23&24 of Shuter’s TOP CLASS Technology textbook Grade 8
Cantilever bridges
In the arch and beam we saw that the bridges were
supported at two places, that is at both ends.
A cantilever differs from the arch and the beam in that
the attachment points are not at opposite ends.
The cantilever projects out into space and the support is
at one end only.
Most cantilever bridges have two cantilevers, with a
beam suspended between their free ends.
You have already learnt about cable-stayed cantilever
bridges on pages 18-19.
The Pierre Pflimlin bridge is a balanced cantilever made of concrete,
shown here under construction.jpg
Advantages of cantilever bridges
Building out from each end enables construction to be done with little
disruption to traffic below.
The span can be greater than that of a simple beam, because a beam
can be added to the cantilever arms
The supports can be simple columns.
Disadvantages of cantilever bridges
Like beams, they maintain their opposition of large tensile and
compressive forces, as well as shear, and therefore are very large and
heavy
Structural failure
Structural failure is the inability of a structure or structural member
to perform its intended function.
It is caused by collapse or excessive deformation.
The three most likely ways structures fail are:
fracture of a member,
bending
and
toppling over.
Fracture
Fractures often occur due to a lack of strength in the structure.
When designing a structure the load that the structure must withstand
needs to be considered.
A structure cannot withstand forces continuously for extended
periods of time.
Most structures get brittle and lose their strength with time.
This is a natural phenomenon.
These structures either have to be replaced before they fail or need to
be "patched up".
Most structures are built with a life span.
This means that we calculate how long a structure will last before it
fails.
This gives us plenty of time to either replace it or fix it.
Bending
Bending occurs due to the flexing and buckling of a structure due to
a lack of stiffness.
Sometimes structures are not made of the appropriate material,
resulting in failure.
Toppling over
Toppling over occurs due to a lack of stability in the structure.
The structure is either top heavy or has a base that is too narrow.
Important considerations
When building a bridge, a designer needs to
calculate the following:
The maximum amount of traffic the bridge can hold at one
time.
The weight of the traffic.
Strength of winds that pass through the area .
Weight of the bridge
END OF SESSION