Windsor Railway Bridge
Windsor Railway Bridge is a wrought iron bowstring bridge in Windsor, Berkshire, designed by Isambard Kingdom Brunel. The ironwork contractor was George Hennet.
The bridge carries the ex-GWR branch line from Slough into Windsor and Eton Central station. It crosses the River Thames on the reach between Romney Lock and Boveney Lock.
The bridge is a single-span structure comprising three bowstring trusses which provided two bays for the original two GWR broad gauge tracks. The track on the upstream side was removed when the line was rationalised in the 1960s.
The line opened in 1849. The construction of the line had been delayed and could not be included in the original Parliamentary Act because of objections from the Provost of Eton College.
The brick approach viaducts were constructed between 1861-65 to replace the original wooden trestle structures.
The three bowstring girders were each supported by a pair of 6 ft diameter cast iron cylinders at each end. The clear span between the columns was 187 ft. The span was subsequently reduced by the iron columns being augmented by brickwork.
Note: Best viewed from the footpath on the north (Eton) side of the Thames. Windsor Town Bridge is a short distance downstream.
Design and Construction Details
See here for William Humber's description on the bridge, and here for detail drawings of the bridge [1].
The bowstring truss girders were constructed entirely from wrought iron plate.
The three arch ribs are of triangular cross section with a central internal vertical web. The centre arch rib is stiffer than the outer ribs. Fig. 4 shows both an outer rib (nearest camera) and the centre rib, the centre rib having double thickneses of 1/2" plates, indicated by the additional rivets.
The main deck beams, which provide the 'string' for the 'bow', have a curved (dished) top flange, while the lower flange has slight curvature. Fig. 7 shows how the top flange and the web are joined by angle iron. The deck beam plates are 1/2" thick, each being about 10 ft long. The rolled plate width was limited to about 2 ft, so three rows of plates were required to give the required height of the web. William Humber states that the web plates for the centre beam are doubled, but spaced 3" apart. Vertical angle iron web stiffeners are provided on both sides of the central deck beam, but only on the inboard side of the outer beams.
The vertical members (known as suspenders) supporting the deck from the arch ribs can be seen in Figs. 3 & 4. The inner suspenders are fabricated from angle iron and flat plate, while the outer ones are of rolled H-section. The suspenders are riveted to wrought iron gusset plates of 'arrowhead' shape (painted white), which also accommodate the fitted bolts (painted red) for the diagonal braces.
Each diagonal brace comprises a pair of flat chain links (eyebars) connected in the middle by tapered cotters. The cotters allow the tension in the eyebars to be adjusted. The links are not simply overlapped at the cottered joints, as this would cause eccentric loading. Instead, in most cases one end of the bar is bifurcated to straddle the plain end of its partner, no easy job for the blacksmith. In other cases there are two bars connecting to a single bar. Examples of both arrangements can be seen in Fig. 6. Note that the double links here are of extra thickness, and have larger fitted bolts and cotters, reflecting the greater tension required here.
It is not clear whether the adoption of two-piece diagonal braces with cottered joints was determined by the ability to pre-tension the braces, thereby stiffening the structure and reducing the tension in the suspenders, or simply to preclude the need for precise location of the holes for the fitted bolts.
Shaping the curves on the 'arrowhead' plates would have been a labour-intensive process. Nowadays it would be a simple matter to cut the profile using flame-cutting, plasma, laser, or water jet, but in 1849 it was necessary to punch a series of close-pitched holes and finish by chipping and filing.
The large number of riveted joints partly reflects the limited size of wrought iron plates available at the time. The original rivets are of a type favoured by Brunel, having a combination of flat heads and conical heads (Fig. 7).
Humber notes that the beams which supported the wooden deck, ballast and track were aligned on the skew, and pitched at 5 ft intervals. However the structure below decks has been completely altered. The transverse beams have been replaced, their numbers halved, and longitudinal beams have been installed. The transverse beams now being aligned normal to the axis of the bridge, rather than skewed. Fig. 8 shows that the position of the left and right transverse beams is staggered. It will be seen that where the transverse beams are supported on the curved bottom flange of the deck beam, the local thickness of the flange is doubled by riveting on small plates. These plates served the original transverse beams. Fig.9 shows how the longitudinal beams are supported by the transverse beams. The replacement beams are of I-section, some being fabricated by riveting, others being rolled steel joists.
As built, the bridge was supported on American oak baulks sitting on concrete-filled cast iron cylindrical piers. Rollers were incorporated at one end to accommodate expansion. At some point the piers were augmented by brick abutments, and large cast iron pedestals replaced the oak blocks (Fig. 10).
Fig.11 shows how the arch rib merges with the deck beam at the end of the girder. Overlapping sections of angle iron were 'joggled' by an angle smith.
Note: William Humber also describes and illustrates Shannon Bridge (Athlone Railway Bridge), whose bowstring girders are contemporary with Windsor and are similar in size and function. The constructional details correspond more to what became conventional practice, although the arch ribs are unusual, and could perhaps be described as 'semi-box' section. They are basically of riveted I-section with wide flanges, but angle iron is riveted to the outer edges of flanges, and intermittent vertical plates are riveted to the angle iron. See photograph here. The suspenders and diagonal braces are rigidly connected to the arch ribs and to the deck beams. [2]
Reports
1849 'Exquisitely-wrought Iron Bridge.— Mr. Hennet, our renowned contractor, has lately been engaged in constructing an iron bridge, by the order of the Great Western Railway Company. The bridge, which is of wrought iron, and was this week completed, is perfectly unique in all its contrivances. It weighs 450 tons, 9000 cwt, or 1,008,000 lbs., and is intended to cross the Thames, at Slough, from a branch line of the Great Weatern Railway to Windsor. In its removal from Mr. Hennet's yard, near the New River, to the terminus of the Great Western Railway (by which it was conveyed to Slough), and which was performed by piecemeal, averaging about 15 tons each upon exceedingly strong, and heavy carriages, it had to pass over the wooden bridge at Bathurst Basin. Here, however the Dock Company interposed by informing Mr. Hennet that he must be answerable for any evil consequences which might happen to the bridge by so great a weight passing over it. Supporters were, therefore, applied to the bridge, previous to any portion of the iron work travelling across it. The first part of the iron work thus conveyed over Bathurst Bridge, together with the carriage, weighed nearly 24 tons; and it became manifest to those who witnessed the progress, and the very great trembling and vibration of the bridge, that but for the precaution exercised, the whole would have been precipitated into the water ; we are, however, happy in being enabled state that the several parts of the iron bridge have found their way perfectly safe to Slough, via the Great Weatern Railway, and the progress of its erection we understand, will commence on Monday morning next — Bristol Mirror. '[3]
Related Bridges
Brunel designed a wrought iron bridge with girders similar to those at Windsor, but of shorter span (100 ft), to replace a timber span over the River Usk at Newport, destroyed by fire in 1848. The line was finally opened in June 1850.
Brunel's 1859 Royal Albert Bridge at Saltash, which may be described as a combination of an arch and a suspension bridge, has some features in common with Windsor Bridge. The wrought iron plate deck beams have rounded (semi-circular) top flanges, and slightly dished bottom flanges. Two different arrangements are used to suspend the deck beams. Each of the four deck beams are suspended from the chains (the main eyebars) by ten pin-jointed standards (suspenders), and by eleven rigid standards which cross the chains and extend upwards to connect (by riveted joints) to the large wrought iron compression tubes which accommodate the thrust from the force applied by chain tension. As at Windsor, diagonal braces are connected by pinned joints near the tops of the suspenders, but the lower end is connected at chain level, rather than close to the deck. As constructed, the diagonal braces were in two parts, joined by tapered cotters. However, whereas each brace at Windsor is independent of its neighbour, those at Saltash share a common connection at a central X-shaped gusset plate. Illustrations provided by William Humber show that the original arrangement at these X-shaped plates was appreciably different to the present arrangement. In the old arrangement each piece of the diagonal brace was a single eye-bar, connected to the X-plate by tapered cotters. Now each diagonal eye-bar is assembled from several pieces bolted together, and the connections to the X-plates have wedged pins and a series of six bolts through the X-plates and the eye-bars.
See Also
Sources of Information
- [3] Wikipedia