Since time immemorial, bridges have been built out of stone. This was the only way to straddle larger expanses. Church domes e,g., and also those of other large buildings are also built using this basis. The crucial point is, a stone arch, which transforms the forces which would bend the bridge into compression (supporting) forces.

As far as bridge building is concernerned, the year 1779 was a crucial date. Over the river Severn in the heart of, at that time, Britains industrial production, the worlds first iron bridge was constructed. Apparently, only here was the industrial revolution so far advanced that cast iron could be produced which had the necessary quality and which was also not too heavy.

This was exactly the major drawback of stone bridges. Either one constructed several smaller arches, carried by pillars set in the water, or one large arch, which however, made the bridge, because of the imperative half- round form, in it's entirety heavier, thus requiring more building material.

In contrast, the Severn-bridge. Allegedly, with its length of 30 meters, it weighed only 380 tons (less than 15 truck loads of todays truck and trailer vehicles), indeed, it still needed gigantic stone structures on both river banks. It consists of pre-cast elements, which were mostly riveted together to form five arches carrying a straight section.

If the bridge is to span a greater expanse, the stability must be provided from above. The result is a suspension bridge, which can be recognised from afar by its high abutments. A scaffolding across the whole distance is then not necessary, the cables can be pulled, over the abutments, from one shore to the other. The first cable however, must be transported across by a ship or a helicopter.

In days gone by, one used ropes. Of course nowadays, for loads weighing several tons, steel ropes (cables) are used. They consist of single, rolled steel strands wound together to form a cable, these cables are again wound together until the necessary strength has been achieved. Bacause of their enormous weight they are only joined together at the construction site to form cables of up to, and over one meter in diameter.

The first suspension bridge of this type was spanned over the Menai-Straits, from England to Wales, from where one could reach the ferry points to Ireland. The problem however, was the anchoring of the cables, which at that time were in fact still chains, on the two shores. Since the shores were predominantly rock, tunnels were dug, through which the chains were threaded and secured by strong cross-bars.

What if there is no rocky ground anywhere near the river bank? This must then be artificially produced, using, among other things, hundreds of thousands of cubic meters of concrete. Of course, first of all, foundations have to be dug. Nowadays, dredgers are used for the foundations, e.g., of pylons in the middle of waterways. How indeed, was this done with the first pillars which were planted, for the famous Brooklyn-bridge over the East- River in New York?

The gigantic, square, open bottomed funnels called Caissons, were pulled by several tug-boats to the correct spot and then sunk. A large number of compressors were used to force the water out so that up to one hundred or more workers could dig the sand away until rock became visible.

The sufferings of the workers, who were mostly divers, has been comprehensively described, because of the necessary high pressure, they were working in an atmosphere that was anything but healthy for humans. Not only did they need air-locks up to the surface to keep the pressure constant, but had to go through decompression before they surfaced.

Since the erection of the Golden Gate Bridge, steel is used for the pillars instead of a stone structure. In this case, height is a special problem, even very high-built ships must be guaranteed a free passage under the bridge. Thus, they could avoid building a bascule bridge like, e.g., the Tower Bridge in London (see above picture), which was one of the first hydraulically operated and has the restriction of having overhead motor traffic crossing the Thames.

Nowadays, if one wants to get really high up, the more than 300 meter high pillars consist of several square profiles which are bolted together on site using screw-bolts as thick as an arm. This way one can achieve the maximum utilisation of the stronger sagging cables. The cables can now have a relatively smaller dimension.

If in the 1930's, the bridges were to compete with the highest buildings, the supporting cables for the road surface had to be considerably longer, which also made them more susceptible to wind. Have a look at the clip about the collapse of the famous Tacoma Narrows Bridge, how the wind and oscillations, caused a disastrous swaying of the road surface.

It seems hard to believe, but in the meantime, bridge constructions are tested, like motor car bodies, in wind tunnels. Whereas, in the beginning, the lateral shape of the road surface was designed to offer as little wind resistance as possible, nowadays, in newer projects, the stiffening of the construction under the road surface is favourised, this efficiently counteracts against twisting and also positively influences the air-flow around the bridge.

The maintenance of a bridge is also an incredible affair, for individual teams it's a lifetime job. The Japanese have already gone one step further, they are using robots, e.g., to scan the cables. Should it be necessary, there are other robots which repair the damage. From the beginning of construction, such bridges are made so that the whole thing can be inspected by people from the inside. Despite the application of robots, they have to be inspected for as long as they exist.

Of course, a bridge can not always be built so high that large ships can pass under them. If it has to be built lower, it is given segments which can be raised (bascule bridge). Nowadays, one should not imagine them to be only small examples. American bridge builders are proud of their latest achievement, a bridge near Washington with several, 1600 tons heavy raising segments carrying a six-lane highway. 04/11