Angers Bridge

The suspension bridge of Angers (French: Pont de la Basse- Chaîne ) was a suspension bridge over the Maine at Angers, department of Maine -et -Loire, France, who became famous through her often returned to resonance vibrations collapse in 1850.

Location, name

The Pont de la Basse- Chaîne at the southwestern end of the old town just below the castle of Angers joined the boulevards that had been applied instead of the city fortifications. Upstream at the other end of the old town bridge Pont de la Haute- Chaîne was built around the same time. The names of the bridges ( roughly translated bridge at the lower chain and bridge at the upper chain) remember that the once surrounding the town ramparts of Roi René ( Le bon Roi René, René the Good ) in the years 1446-48 by two chains were added over the river to ward off intruders.

The suspension bridge

The city of Angers was Joseph Chaley (1795-1861) in charge of the planning and construction of the bridge, who had previously completed the construction of the Grand Pont Suspendu Fribourg in Switzerland in the year, which at that time with a span of 273 m the world's largest bridge had. To assist him, the civil engineer Théodore Bordillon was set aside.

The bridge's construction was started in 1835 based on 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, which crossed the Maine at the foot of the castle Angers. The pylons were bricked in the lower part of natural stone, the upper part they consisted of cast-iron columns supporting the cable saddles. The supporting cables each consisting of iron ropes in 1067 and were produced directly on the bridge by the improved process of the Frenchman Louis Vicat ( 1786-1861 ). The significantly stronger cable for the rear anchoring were mounted in heavy masonry anchor blocks. With a rich limy through the cable guides they tried to protect them from corrosion there. The 7.20 -meter-wide road was made of wood.

Operation of the bridge

The Pont de la Basse- Chaîne was twelve years accident-free in operation. 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. There was since the collapse of the Broughton Suspension Bridge, England in 1831 generally known that bridges through the lockstep of a soldier column to vibrate and eventually can be brought down, to the bridge at Angers have been usually crosses without impact and without vocals, also if this were no explicit instructions.

Collapse on April 16, 1850

On April 16, 1850 two battalions without problems were already marching across the bridge. A third column with about 730 soldiers to cross the bridge without impact. At that time there was a strong wind that caused a bridge to light vibrations. On top of that was added in a heavy rain shower that made -up apparently the back rows faster. The vibrations were reinforced by the soldiers that defied them unintentionally. Suddenly, the suspension ropes tore up the right bank, the two pylons fell from their pedestals and the road fell on the right side diagonally into the river while she was still being held on the other side of the supporting cables and the intact pylons. In this accident a total of 226 people died; it is thus one of the heaviest bridge disasters in history.

Cause investigation

The subsequent cause determination in which the anchoring of the cables was exposed in the anchor blocks, came to the conclusion that the accident probably the gain was caused by the soldiers and the increased traffic load not only by the generated by the strong wind vibrations. Decisive is rather been that the sealing of the cables have caused in the anchor blocks by the bold limy not provide complete protection against corrosion and therefore the ropes were partially corroded and considerably reduced in their tensile strength. The additional burden posed by external circumstances would therefore inevitably must lead to the collapse of the bridge.

Follow

An immediate consequence of the accident was an increase in the normal load assumptions. The previous sample loads of a bridge with large weights were clearly not sufficient to take into account the dynamic force of moving the traffic load at the same time winds.

More worrying, however were the corrosion problems in the anchor blocks, which had probably present at all comparable bridges, but could neither be verified nor removed during operation. This also resulted in the U.S. and Switzerland to concerns regarding the use of suspension bridges with steel cables that were reinforced by several collapses of bridges of this design in the following years, including 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 already at the load test in 1853 and the collapse of the Wheeling suspension Bridge in the United States in 1854. Nevertheless, wire rope suspension bridges finally prevailed over chain bridges, especially due to the work of John August Roebling.

New Bridge

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