Script of the bridges
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.
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
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 H-shaped 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 (build-operatetransfer) 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.