Angers suspension bridge

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Angers suspension bridge before 1850
Collapsed Angers suspension bridge
The stone bridge from 1856
Today's Angers Bridge

The Angers suspension bridge ( French : Pont de la Basse-Chaîne ) was a suspension bridge over the Maine in Angers , Maine-et-Loire , France , which became famous in 1850 for its collapse, often due to resonance vibrations.

Location, name

The bridge Pont de la Basse-Chaîne at the southwest end of the old town immediately below the castle of Angers connected the boulevards that had been laid out in place of the city fortifications. Upstream at the other end of the old town, the Pont de la Haute-Chaîne bridge was built around the same time . The names of the bridges (translated as bridge at the lower chain and bridge at the upper chain ) remind us that the city walls of Roi René (Le bon Roi René, René the Good) that once surrounded the city were replaced by two in the years 1446-48 Chains across the river were added to repel intruders.

The suspension bridge

The city of Angers commissioned Joseph Chaley (1795–1861) with the planning and construction of the bridge, who the year before had completed the construction of the Grand Pont Suspendu in Freiburg , Switzerland , which at that time was the largest bridge in the world with a span of 273 m was. The civil engineer Théodore Bordillon was appointed to support him.

Construction of the bridge began in 1835 on the basis of the royal decree of June 12, 1835 and was completed in September 1838. It was a back-anchored suspension bridge with a span of 102 m that crossed the Maine at the foot of Angers Castle . The lower part of the pylons was made of natural stone, while the upper part consisted of cast iron pillars that carried the cable saddles. The load-bearing cables each consisted of 1,067 iron wire ropes and were manufactured directly on the bridge using the improved method developed by the Frenchman Louis Vicat (1786–1861). The significantly stronger cables for the back anchoring were fastened in heavy, masonry anchor blocks. An attempt was made to protect them from corrosion with a fat lime paste in the cable ducts. The 7.20 m wide roadway was made of wood.

Operation of the bridge

The Pont de la Basse-Chaîne was in accident-free operation for twelve years. The soldiers of a large barracks crossed the bridge regularly on the way to the firing range and other facilities on the other side of the river. Since it has been common knowledge since the collapse of the Broughton Suspension Bridge , England in 1831, that bridges can be vibrated and finally collapsed by the lock step of a column of soldiers, the bridge in Angers is said to have usually been crossed without stepping or singing, too if there was no specific instruction to do so.

Collapse on April 16, 1850

On April 16, 1850 two battalions had marched across the bridge without any problems. A third column with about 730 soldiers is said to have crossed the bridge without stepping. At that time there was a strong wind that caused the bridge to vibrate slightly. On top of that, there was a heavy rain shower, which apparently pushed the back rows in faster. The vibrations were amplified by the fact that the soldiers unwittingly braced themselves against them. Suddenly the ropes on the right bank broke, the two pylons fell from their bases and the roadway fell diagonally into the river on the right side, while it was still held on the other side by the ropes and the intact pylons. A total of 226 people died in this accident; it is one of the worst bridge accidents in history.

Determination of causes

The subsequent investigation of the causes, during which the anchoring of the suspension ropes in the anchor blocks was also exposed, came to the conclusion that the accident was probably not only caused by the vibrations generated by the strong wind, their amplification by the soldiers and the increased traffic load. Rather, the decisive factor was that the sealing of the ropes in the anchor blocks with the fatty lime slurry did not provide complete protection against corrosion, and therefore the ropes were partly heavily corroded and their tensile strength was considerably reduced. The additional stresses caused by the external circumstances should therefore inevitably have led to the collapse of the bridge.

consequences

A direct consequence of the accident was an increase in the usual load assumptions. The previous test loads on a bridge with large weights were obviously not sufficient to take into account the dynamic forces of a moving traffic load with simultaneous cross winds.

More worrisome, however, were the corrosion problems in the anchor blocks, which presumably had to be present on all comparable bridges, but could neither be checked nor eliminated due to the design. This also led to concerns in America and Switzerland about the use of suspension bridges with wire ropes, which were intensified by several collapses of bridges of this type in the following years, namely by the collapse of the suspension bridge in Roche-Bernard during a storm in 1852 The collapse of a suspension bridge over the Swiss Rhône as early as the test load in 1853 and the collapse of the Wheeling Suspension Bridge in the USA in 1854. Nevertheless, wire rope suspension bridges finally prevailed over chain bridges, in particular due to the work of John August Roebling .

New bridge

The suspension bridge was replaced by a stone arch bridge built in the years 1851-1856, which was in turn replaced by a reinforced concrete girder bridge in 1960.

See also

Web links

Coordinates: 47 ° 28 ′ 13.6 ″  N , 0 ° 33 ′ 48 ″  W.