Menai Suspension Bridge Anniversary: 200 Years of Telford’s Engineering

Suspension bridges function by transforming the vertical loads caused by traffic and the weight of the bridge itself into tension within the cables or chains. This is a straightforward example of a principle taught at A-level Physics, namely the resolution of forces along a curved member. Although the chains take on a catenary curve when hanging under their own weight, when the load on the deck is spread out evenly the curve becomes approximately parabolic. Telford's engineers would not have put it that way, but the mathematics involved is the same.

The way the load is carried is as follows: the traffic exerts a downward force on the deck; the deck then passes this load on to the vertical hangers; the hangers in turn pull down on the chains; since the chains are under tension, they pull inward and downward on the towers; the towers, being in a state of compression, push down on their foundations; at the anchorages located behind the towers, the chain tension is transmitted into the rock via embedded anchor plates; all the forces are balanced, and if you draw a free-body diagram of any one of the elements, the total of the forces will be zero, which is the equilibrium condition that A-level Mechanics is based on.

He decided to use wrought iron for the chains rather than cast iron and this choice most likely prevented the bridge from collapsing. Cast iron is strong when being compressed but is brittle when under tension; if a cast-iron chain link is overloaded it will break suddenly. Wrought iron, on the other hand, is ductile and deforms before it breaks, providing a clear warning and enabling the loads to be redistributed to the adjacent links. The difference between brittle and ductile failure is a basic principle in structural engineering and is included in the A-level materials section.

The eye-bar chain design is worth looking at. The links consist of a flat piece of wrought iron with a large circular eye formed at each end. A pin goes through the eyes of neighbouring links to join them together. Because of its geometry, the load is carried along the central axis of each bar, which reduces the bending stresses. The pin joints also permit a certain amount of rotation, so the chain can alter its shape as the loads shift across the deck. This is in contrast to modern suspension bridges, which make use of wire cables composed of thousands of separate steel wires bundled together. Wire cables are stronger for their weight, but the eye-bar chain was the best technology available in the 1820s and it worked.

The force of the wind was the one that Telford failed to take into account. In January 1839 a severe storm caused damage to the deck, lifting and twisting the timber roadway. The original flat deck lacked torsional stiffness and therefore was unable to resist the twisting forces produced by asymmetric wind loads. It was then rebuilt using deeper timber beams and a stiffer structure, which was an early example of the principles of aeroelastic behaviour a lesson that would be rediscovered in a more dramatic way at the Tacoma Narrows in 1940.

Since then, the bridge has undergone a number of modifications. In the 1930s the original wrought-iron chains were substituted by steel chains, in the 1940s the timber deck was replaced with a steel one, and substantial restoration work has gone on into the 2020s. With each change the bridge's function has been kept while its materials have been updated. The stone towers, though, are still original and have carried a load continuously for 200 years.

For anyone who is interested in civil or structural engineering, the Menai Bridge provides a perfect example when it comes to force resolution, material selection, and the need to take into account all possible load cases, including those which were not anticipated. Although structural analysis at that time was mainly based on intuition rather than calculation, Telford managed to get the main points correct. The bridge's continued use for modern traffic is not due to chance; it is the result of good engineering.