Brabyns Park Iron Bridge
Brabyns (or Brabyn's) Park iron bridge crosses the River Goyt in Marple, Cheshire.
It was built in 1813 for the Nathaniel Wright's carriage drive, and is now used as a footbridge.
It is the only known bridge built by Salford Iron Works.
This historically important bridge was rescued from decay and oblivion by the considerable efforts of members of the local community and council. Thanks to their work, it has been carefully restored, with very little alteration to the iron structure, which remains largely as built over 200 years ago.
A walkway has been superimposed on the old bridge, and this is designed to remove any possibility of risk in crossing the river. Unfortunately, the barriers make it very difficult to study the structure of the old bridge. Fortunately, a platform has been built to allow visitors to view its elegant design. Unfortunately, nature is doing its best to screen the view. Fortunately the restoration process was recorded in great detail, and there is an excellent website which provides a wealth of information on the history and construction, with numerous photos showing the details revealed during the project [1]
Details of Construction
Superficially the bridge resembles a number of cast iron bridges made in the early 19th C. The pattern of diminishing circles in the spandrels is familiar in cast iron bridges of this era, although an unusual aspect here will be referred to later. (Spandrels are the filling between the deck and the arch).
The deck was supported by three cast iron deck beams, the outer pair having the name Salford Iron Works cast in. The 'font' has a surprisingly modern appearance for a Georgian bridge. Each is cast in two halves, bolted together at mid span.
The thrust of the arch is transmitted to the masonry abutments via cast iron plates attached to the face of the abutments.
The middle rib is mostly hidden from view. Each half is a single casting comprising the deck beam, arch rib, and diminishing rings. This was quite a common arrangement in small cast iron bridges. The photos on the Marple website taken after the structure was shot blasted show severe porosity and large craters on one face of these castings. The presence of such defects here is typical of castings produced in an open mould in this era, although the severity is perhaps unusual. The rough surface represents the top surface, the gas bubbles, debris, etc having risen to the surface of the molten iron.
In contract, the outer ribs were cast in closed moulds. This involved more work for the foundry, but gives a better result. The extra effort was dictated by the shape of the cross-section having mouldings and lettering on the outer face, and flanges on the inner face.
At the crown of the bridge, the ribs are joined together by a bolted flange. Then the deck beams and the ribs are united by being bolted to the '1813' date plate. When the bridge was grit-blasted during restoration, some details were found which give an insight into some minutiae of Georgian engineering practice. There are 10 bolts through the 1813 plate. Their heads are square, discretely countersunk in the plate for the sake of appearance. This would have required a lot of individual hand work when the bridge was trial-assembled in the works. To ensure that they were refitted in the right hole and orientation, each bolt head and hole were marked with an identification number. Nowadays this would be done with number stamps, and prior to that Roman numerals might have been used. Here a round-nosed punch was used to produce between 1 and 10 depressions. This can be clearly seen in one of the Marple website photos (File 158).
Having seen that the method of casting was different, we also find that the construction of the outer ribs is completely different to the middle rib. Instead of each half being a single casting, the outer halves are each assembled from three castings. The arch rib and the deck beam are separate castings, and the diminishing circles are cast as a group, and inserted in the space between them, and retained in place by small bolts and nuts. It appears that the largest ring of the group is not in contact with the casting attached to the masonry abutment (although this has not been confirmed). It therefore seems that the rings here are largely decorative, unlike those in the middle rib, which are an integral part of the structure.
Diagonal braces are fitted between the three arch ribs, an unusual feature for such an early bridge.