The Menai Bridge

The Menai Bridge.

In the nineteenth century the Menai straight’s in Wales forms a formidable obstacle to people travelling to Ireland. The engineer given the job of spanning this traitorous waterway is Thomas Telford a 62 year old self taught Scott.

“When the time he arrived at Menai, Telford had the reputation of one of the finest civil engineer in Britain be given us a huge job to improve the main continuous route between Britain and Ireland.”

Telford considers constructing an arch bridge from cast iron. But he would have to use scaffoldings support the arch while it’s been built. This would block the movement of ships along the busy waterway. The planners don’t accept this so Telford goes back to the first principles of bridge design.

Rope bridges have the use to cross rivers since ancient times and mountaineers like Mark Handford still use them today.

“The most crucial thing in it is the anchor point. So we’ve got a tree as a solid anchor, without these anchors, the all bridge gonna collapse.”

Next, they pull the ropes taut. The bottom rope carries the load; the top ropes are unfolding onto. Tighten them together forms a rudimentary suspension bridge. It’s fine for soldiers but would hardly do for the London to Dublin stage coach.

Adding planks of wood to form a deck makes the bridge easier to cross. And iron chains allow it to carry heavier loads but the deck still sags. To build a truly modern suspension bridge, the engineers must find a way to flatten the sagging deck. The answer is to suspend it from stone towers, then extend the ropes downward to level the deck. Telford now has a design that can bridge the Menai straight’s. But he still has the problem of securing the chains at each end.

“Do you think that pulling down that chain is going to give …. unless it is really firmly anchored, now that’s Telford challenge.”

Somehow Telford must anchor the chains not to the trees but deep into the rocky banks on other side of the straight. To do so, workers blast their way through the rock to create an eighteen meter long tunnel. At the end of the tunnel, they position strong iron frames. The end of the chains threads into the tunnel and are secure to the frame with three meter long bolts. Metal braces wedge the bolts and frame firmly into the cavern. The only way this anchorage can fail is if the rock gives way.

Telford tries. The new bridge help reduce the journey time from London to Dublin by nine hours. His anchorages of held of hundred and seventy seven meter deck and place for over an hundred and eighty years. Today it still carries thousands of vehicles every week. As the first great modern suspension bridge its shine a light on a future.

Traduction :

[Ajouter des planches de bois pour former un plancher rend le pont plus facile à traverser. Et les chaînes en acier permettent de transporter des charges plus lourdes mais le tablier continue de bomber. Pour réellement construire un pont suspendu moderne les ingénieurs doivent trouver une solution pour aplanir la courbure du tablier. La solution est de le suspendre à partir de tours en pierre, ensuite de tendre les cordes vers le bas pour mettre au niveau le tablier. Telford à maintenant la structure qui peut enjamber le détroit de Ménai. Mais il reste le problème de sécuriser les chaînes à chaque bout. […]

De toute façon Telford doit ancrer les chaînes non aux arbres mais profondément dans la berge de part et d’autre du détroit. Pour ce faire, des ouvriers perforent leur chemin à travers la roche pour créer un tunnel de 18 mètres de long. À la fin du tunnel, ils placent de solides cadres métalliques. Le bout des chaînes se faufile dans le tunnel et sont arrimé à l’encadrement avec des écrous de trois mètres de long. Des attaches en métal coincent les écrous et le cadre fermement dans la caverne. La seule manière pour cet ancrage de céder et c’est si la roche lâche.]

http://channel.nationalgeographic.com/series/big-bigger-biggest/3310/Overview#tab-Videos/05271_00