Iron Arch Bridges
In order to describe an Iron Arch Bridge, we need to look at the Ironbridge in Shropshire, which was the very first of this type. It was built in 1779 by Abraham Darby III, owner of the Coalbrookedale Iron works. Whilst the form that it takes is of an arch, as with Stone Arch Bridges, it bears very little resemblance to those structures, and is in fact a variant of Truss Bridges.
Essentially, the Iron Arches used by Abraham Darby were composed of relatively small pieces of iron put together in the same way that a Timber Arch Bridge would have been at the time. The arched members were designed to concentrate the load towards the abutments, so supporting the deck without the need for a central pier or structure that would impede boats on the river below.
After the original Ironbridge was completed, it became an instant attraction in what was a heavily industrialised and rather unpleasant piece of the Severn Valley. It was readily copied elsewhere, not least by Thomas Telford, who used the basic principles of an Iron Arch in many places across the British Isles, slowly refining it into the graceful structures seen at Craigellachie Bridge amongst others. No longer was the arch semi-circular, but as with Stone Arches of the era, a flat arch was being used, supported on tapering abutments.
In the following decades, many stone bridges over the larger rivers would be rebuilt with Iron spans. Some were due to destruction by floodwater as at Fochabers Bridge, others to improve the volume of traffic that the bridge could handle. However, unlike many of the other bridge designs favoured by the Victorian engineers, the Iron Arch never seems to have caught on in the Railway Industry.
By the end of the Victorian era, a vast number of different bridges had been constructed using Iron Arches, some were multi-spanned, some had arches so slender that they hardly showed, but all owed their origin to Abraham Darby. Since the dawn of the twentieth century, the number of iron arched bridges constructed has been very small, as first steel took over, and then different styles of truss found favour, with arched trusses as parapets being the most closely related design in use today.