Spanish Town Bridge (Jamaica)

in Spanish Town, Jamaica

1800 'The engineer of Sunderland bridge, Mr Wilson, has had cast on an improved plan, a bridge for the river Rio Cobre, in Jamacia [sic], 83 feet wide, which is now set up at Rotherham, in Yorkshire.'

1800 'On Friday a bridge of cast iron, the first seen in Hull, was put on board the ship Ellison, Captain Gatecliffe, to be fixed across a wide ferry over a river, near Kingston, in Jamaica. This expensive work was entered at the Custom-house for 1060l.; the mere freight of it is 100l. and the expense of its erection will be great ; a manufacturer from West Yorkshire will necessarily go over for the purpose. Its weight is 87 tons (the strongest horse can only drag two tons, so that its conveyance by land would require 43 horses and 43 carts). The span or rainbow-sweep of the arch is 80 feet. The cast iron rails to guard the foot passengers, will be placed at six inches distance, and of course amount to 320 in number. The pieces which cover the top of the bridge are 41, and are two feet broad. It is an imitation the two bridges over Colebrooke Dale and Sunderland river.'

Photographs of the bridge may be found on the World Monuments Fund website . Some excellent photos showing the details of the arches may be found here and here . Further information on the bridge, its location and history are available online

It is typical of Thomas Wilson's designs, having cast iron 'ladder'-like voussoirs, and rings of diminishing diameter in the spandrels. To try and clarify the terms: traditionally, voussoirs were the wedge-shaped blocks which form the arch in masonry bridges. In this case the voussoirs are cast iron frames or panels. Each panel is in the form of two concentric arched ribs connected by four radial arms. Each panel can be envisaged as a 'curved ladder' with four 'rungs' (although one panel, at the crown of each arch, has only two 'rungs'). More than 20 of these panels are butted together to form one arch rib. Each corner of the panels is bolted to the flanged end of a transverse tube (see photos in links).

The photos reveal a number of large blowholes in some of the arch ribs. To J. G. James, such blowholes gave the impression that the members are hollow .

A spandrel is the part of a bridge located above the arch and below the deck, in this case formed of iron castings having rings of diminishing diameter to suit the tapering gap. It appears that the spandrel castings sit on top of the arch panels, and that they are secured to the panels by a small number of radial bolts.

It will be seen that this impressive bridge was designed to be assembled from relatively small 'modules' designed to be fairly readily handled and transported. Of course, transporting iron castings across Yorkshire in winter was not quite the same as toiling under the Jamaican sun.

It is quite an economical design, requiring only a limited number of patterns from which to make the castings. The foundry would have needed to manually dress the abutting ends of the 'voussoir' castings to give good mating faces. The quality of contact of these ends is very important when we consider that the load on the bridge is taken by the arches in compression. Ideally, then, the ends should be perfectly square to ensure stability. As an analogy, consider a series of dominoes placed end to end, and then pressed together from the ends. Lack of squareness at the ends would soon cause the line to bow out and collapse. Of course, in the case of the bridge, additional stability comes from the adjacent panels being joined and tied transversely by the flanged cross-ties. A degree of constraint will also be provided by the spandrels bolted to the top of the panels. A feature apparent in photographs is that the lower abutting faces of the panels appear to be in close contact, whereas there appear to be discontinuities at many of the upper junctions.

Stratfield Saye Bridge in Berkshire is very similar in appearance, but is about half the size.

See Also

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