Andrea Centeno
Introduction
to bridges
Definition
A bridge is a link or connection between two objects
usually places either side of an obstacle, such as a river,
chasm, or estuary. Across gulfs and rivers, between peoples
and countries, bridges break down separation and foster
connectedness.
Types of bridges
The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever
bridges and suspension bridges.
Arch Bridges
The essence of an arch is that ideally there should be no tendency for it to bend,
except under live loads. It should be purely in compression, and for that reason it can
be made of materials such as, masonry, cast iron and concrete, that perform poorly in
tension.
Cable-Stayed Bridges
The cable-stayed bridge is related to the cantilever 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 big difference between cantilever bridges and cable-stayed
bridges is that the former usually have a suspended span, and the latter do not.
Types of bridges
Cantilever Bridges
In the arch and beam we saw that the bridges were supported at two places - the ends.
In fact, if you want to hold any object in position in two dimensions, you always need
two points of attachment. In three dimensions you need three points. Two-legged
animals need feet in order to achieve stability in the three dimensions. This statement
is not strictly true, because animals can balance using their muscles to change their
position in response to perceived movement. Nevertheless, feet make the process
easier.
Beam bridges
Beam bridges are the most simple of structural forms being supported by an abutment
at each end of the deck. No moments are transferred throughout the support hence
their structural type is known as simply supported.
Truss bridge
A truss bridge is a bridge composed of connected elements (typically straight) which
may be stressed from tension, compression, or sometimes both in response to
dynamic loads. Truss bridges are one of the oldest types of modern bridges.
What are the benefits of
bridges?
Some of the benefits of bridges are obvious: supplies of
food and traded goods can get across an obstacle or
through difficult terrain in a shorter time
This means that, in economic terms, the cost of travel and
trade falls and the financial benefits of increased social
cohesion and sharing resources rise. Other longer-term
payoffs from easier travel, which is crucially dependent on
good bridges, come as a result of increased opportunities
to share ideas – intellectual, political and religious.
Today bridges allow easy travel across major rivers and
estuaries, over the new obstacles of motorways and
railway lines, and between neighbouring islands.
International trade and travel depend on shipping and air
routes, but efficient distribution networks depend on
bridges.
Vasco da
Gama bridge
Vasco da Gama
bridge
The Vasco da Gama Bridge is a cable-stayed bridge flanked
by viaducts and rangeviews that spans the Tagus
River near Lisbon, capital of Portugal.
It is the longest
bridge in Europe (including viaducts),
with a total length of 17.2 km
(10.7 mi), including 0.829 km (0.5 mi)
for the main bridge, 11.5 km (7.1 mi)
in viaducts, and 4.8 km (3.0 mi) in
dedicated access roads. Its purpose is
to alleviate the congestion on Lisbon's
other bridge (25 de Abril Bridge), and
to join previously unconnected
motorways radiating from Lisbon.
.
Purpose of the
bridge
Its purpose is to alleviate the congestion on Lisbon's other bridge, and to join
previously unconnected motorways radiating from Lisbon.
.
The bridge was opened to
traffic on 29 March 1998,
18 months after
construction first began,
just in time for Expo 98, the
World's Fair that celebrated
the 500th anniversary of
the discovery by Vasco da
Gama of the sea route from
Europe to India.
Materials and
construction
The materials used in the construction of the Vasco da Gama bridge were
reinforced concrete structure with main deck stayed to main pylons and central Hshaped pylons of 155m high.
For the construction the project was split in four parts, each one built by a
different company, and supervised by an independent consortium. There were up
to 3,300 workers simultaneously on the project, which took 18 months of
preparation and another 18 months of construction.
The cost of the bridge came at zero to the State, as it was built in the BOT (buildoperate-transfer) system by Lusoponte, a private consortium which got a 40-year
concession on the tolls of both Lisbon bridges. Lusoponte's capital is 50.4% from
Portuguese companies, 24.8% French and 24.8% British.
Advantages and
disadvantages
The bridge carries six road lanes, with a speed limit of
120 km/h (75 mph), the same as motorways, except on
one section which is limited to 100 km/h (60 mph). On
windy, rainy, and foggy days, the speed limit is reduced to
90 km/h (56 mph). The number of road lanes will be
enlarged to eight when traffic reaches a daily average of
52,000.
The bridge has a life expectancy of 120 years, having been
designed to withstand wind speeds of 250 km/h (155 mph)
and hold up to an earthquake 4.5 times stronger than the
historical 1755 Lisbon earthquake (estimated at 8.7 on
the Richter scale). Due to the bridge's length, it was
necessary to take the Earth's curvature into account to site
the piers correctly, otherwise a deviation of 80 cm (31 in)
would be apparent at either end of the bridge.
Environmental pressure throughout the project resulted in
the left-bank viaducts being extended inland to preserve
the marshes underneath, as well as the lamp posts
throughout the bridge being tilted inwards so as not to
cast light on the river below.