Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 137,276 pages of information and 220,134 images on early companies, their products and the people who designed and built them.
The bridge carries a minor road across the River Severn, between Newtown and Llanidloes. The bridge is very close to the A470 in Llandinam, but is doubtless unobserved by most passing motorists.
A good description of the bridge may be found here.
The bridge was built in 1846 for industrial entrepreneur David Davies (1818-90) to the design of Thomas Penson. The ironwork was produced by the Hawarden Iron Works, and Edward Jones of Gellidywyll constructed the masonry abutments.
The elegant design of the bridge owes much to the earlier work of Thomas Telford and William Hazledine. In fact it closely echoes a number of Telford's large iron bridges, as first seen at Craigellachie. The similarities of design and construction can be studied by comparing the photos of Llandinam and Mythe Bridge.
Llandinam bridge was neither 'state of the art' in 1846, nor a model of economic construction, but the detail design is clearly the work of a highly competent draughtsman. The quality of the foundrywork and of other visible aspects of construction is impressive. However, the tendency to favour sharp internal and external corners for the castings would later be frowned upon, although they were typical of the era.
The bridge appears not to have undergone much alteration, no doubt helped by the 3 ton weight restriction. As such, it provides a rare opportunity to study an iron bridge of the 'Telford/Hazledine' type in its original form. The larger Mythe bridge is another fine example of this type, although access for viewing is not so good, and it has undergone some modifications to the deck and to some of the spandrel castings.
The key elements of this very rigid structure are described below and shown in the photographs.
The cast iron arch ribs are sprung from sloping masonry abutments via cast iron abutment plates.
Each of the three arch ribs is assembled from five iron castings, bolted together, with transverse plates (diaphragms) sandwiched between the mating ends of the five castings.
On top of the arch is fixed a series of lattice cross plates. These cover the full width and are all bolted together via flanges at their long edges. These plates do not continue over the whole span of the arch. It appears that towards the crown of the arch, their role is taken over by unpierced plates which constitute basis of the deck.
Above the arch ribs are the cast iron spandrel struts, which support the deck. They are of slender cruciform section. Above these are the deck bearers, slender shallow cast iron beams. Although not readily apparent, the various spandrel castings will have tenons which engage in mortices at the top of the arch ribs and the underside of the deck bearers. These would be a loose fit, to be filled on assembly with packers and lead or 'rust cement'.
The deck is probably assembled from cast iron plates, joined at their four edges by bolted flanges. These plates will be connected to the deck beams by mortices protruding from the top of the deck beams engaging with sockets in the plates. The iron railings and kicking strips are bolted to the deck plates.
Returning to the arch ribs and spandrels, additional rigidity is provided by tie bars and by diagonal bracing.
Note: For some of the areas where the mode of assembly cannot be established, interpretation of the probable details of construction has been based on excellent published sketches by Geoffrey D. Hay showing the details of construction of the ironwork of Telford and Hazledine's Craigellachie Bridge.
Penson's Caerhowel Bridge, built 12 years later, has two spans which bear a superficial resemblance to those at Llandinam. However, the design has been simplified to allow more economical construction.