The amazing world of bridges

Andrea Centeno

Introduction to bridges

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.

Definition

Types of bridges

The main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. Arch BridgesThe 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 BridgesThe 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 bridgesBeam 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 bridgeA 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

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..

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.

 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.

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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-operate-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.