ARCH
How to make light foot suspension bridges with unstiffened decks more rigid? John Harper’s solution, based on his knowledge of the properties of wire, was to enhance rigidity by tensioning curves of opposite curvature, viz the main cables against the deck cables. Marc Brunel had employed this technique in his bridge in Reunion in 1832, although this was the only known instance of its application. John Harper employed it in almost all his bridges, as did Louis after him. A notable exception was the Birkhall bridge which had no arch or camber; presumably HRH Prince of Wales preferred the horizontal. Various others, including bridges at Falkirk and Feugh Lodge were subsequently modified to the horizontal over the years, increasing their mobility.
In the illustration above, the deck is seen to rise well above the horizontal, usually 1 foot rise for every 50 feet span. In order to create this upward arching deck, the hangers at the mid point must be connected first, then the pairs progressively outwards towards the bank. This is well seen under Building a Harper Suspension Bridge in the Technical section.
This became the basis of all suspension bridges in Nepal following the introduction of Harper bridges in 1900, even to the present day. Below is the bridge at Dodhara Chandani in southwest Nepal near the Indian border, 1.2km long.
The concept of the arch to enhance rigidity was Harper's main contribution to the design of unstiffened light wire suspension bridges.
