15 August 2012

Scottish Bridges: 40. Craigellachie Bridge

I guess that of all the bridges we visited in Scotland, this was by far the best known.

As the "further information" section at the end of this post suggests, this is a very well-documented bridge, so I will say as little as possible about its history, and concentrate on how it looks today.

The facts, in brief: spanning 150 feet across the River Spey, Craigellachie Bridge was designed by Thomas Telford. It was his second cast iron arch road bridge of this type, the first being the now-gone Bonar Bridge. The bridge at Craigellachie was completed in 1814 at a cost of £8,200. It was extensively refurbished in 1964, and closed to all traffic in 1972. It's a Listed Building, Scheduled Monument, and an ASCE International Historic Civil Engineering Landmark.

On the day that we visited, which as with most of our trip was particularly wet, it was hard to appreciate the bridge at its best. Even in these conditions, it is a remarkable bridge. Perhaps the swollen river below just helps to emphasise why such a bold structural feat was attempted.

Compare Carron Bridge, which we had visited just over an hour beforehand. Both bridges are 150 foot span cast iron arches across the same river, with the ribs surmounted by X-braced spandrels. Craigellachie has four ribs and Carron has three. Carron was completed in 1863, almost half-a-century after its forebear, but visually it is a far poorer bridge.

The arch ribs on Carron are flat-faced girders, whereas Telford's bridge used pierced-web girders. They make the bridge look lighter and add visual texture. They are also less deep than those at Carron, although to be fair it has to be remembered that Craigellachie was designed only for highway traffic, while Carron Bridge had to carry steam railway locomotives.

This factor also accounts for the noticeably heavier spandrel members at Carron. Those at Craigellachie are amazingly slender, especially when viewed from close at hand. It's almost impossible to believe a modern highway bridge could be designed with such slender struts (and indeed, Craigellachie Bridge could not carry modern highway loads).

The pattern of the X-bracing is also different on the two bridges. At Craigellachie the spandrel members change in angle across the span such that the "diamonds" vary in angle to be roughly perpendicular to the arch rib. At Carron, they are oriented so that the "diamonds" are always vertical. That seems more rational, as their main function is to carry vertical loads from the deck down to the arch ribs, but it seems to me to be less attractive visually.

Another feature forced upon Carron by the nature of what it carries is that the deck is level, giving the bridge an appearance which rather stiff. Craigellachie Bridge has a vertical curvature which is much nicer.

While many of these features are consequences of either the loads supported or changes in engineering understanding, the last key feature which differentiates the bridges seems primarily a matter of choice. The abutment faces at Craigellachie are inclined perpendicular to the arch ribs, creating the sense that the bridge springs across the river, and also that the abutments are working properly to resist the thrust of the arch. The vertical abutments at Carron give a more precarious impression - it appears far from impossible that both the arch and the deck could just slide vertically downwards into the river.

The abutments at Craigellachie were never uncontroversial, however, because of the masonry turrets which punctuate the end of the span. These looked archaic even in Telford's day. I think they're rather pleasant, they frame the bridge visually, and look less obtrusive because they are dwarfed by the cliff face on the north-west end of the bridge.

Further information:

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