Inglis Bridges
During the First World War Charles Edward Inglis, as director of the bridging department of the Royal Engineers, designed and developed an ingenious and well thought-out series of transportable steel bridges that could be erected rapidly, and dismantled for re-use. Versions were used by the British Army throughout the First World War and the inter-war period, until superseded by the higher capacity Bailey Bridge.
Inglis had come up with the idea before the war, when he was a member of the Officers' Training Corps at Cambridge University. As a member of the Royal Engineers branch of the OTC, he was engaged in war exercises which involved constructing trestle bridges.
In 1919 Inglis, by then Professor, later Sir, C. E. Inglis, O.B.E. (Major, R.E.), presented a fascinating Paper describing the development, production, and operational use of the bridges, reproduced in The Engineer under the title 'Portable Military Bridges'.[2]
The first form of the bridge - the 'light bridge' - used modules of pyramid form, with a square base frame and four tubular legs, 8 ft square and 8 ft high. The legs had hinged joints at the top, and were connected together in a junction box, allowing them to be folded together for transport. Individual pyramid modules were connected together at the base, and also at the apex via a tubular compression strut. The maximum length was 96 ft (12 modules), based on the requirement to carry a single line of infantry crowded closely together. The first example was supplied for test purposes in October 1914, and was produced by Dick, Kerr and Co, free of charge.
An improved design was produced and submitted for testing at Chatham in the summer of 1915. The first bridge to be assembled overseas was erected over a canal in France in December 1915. After unloading the parts from two lorries, the order to assemble was given, and the bridge was ready for traffic 11 minutes later.
Each individual part could be readily manhandled. The bridge was built on blocks in skeleton form with a counterbalance arm and jacked up onto a wheeled trolley (Fig. 7). It was then pushed over the gap, the counterbalance removed, the bridge supported on the abutment, and the decking laid. The design had to take account of the fact that when supported on the carriage for installation, the stresses in the horizontal members are reversed.
The production versions, at least in the period covered by Inglis's Paper, were all made by Kryn and Lahy, and the tubes were made by Stewarts and Lloyds. The tubes were lap-welded and the ends swelled by upsetting.
In order to transport vehicles, a pair of 'pyramid' bridges could be used, with a deck fixed between them, giving a central roadway 10 ft wide. However, it had a major drawback in that the total width was excessive in the typical situation where the bridge was replacing a existing bridge which had been destroyed, and needed to use the exiting abutments.
As a result a larger version was next produced, having modules 12 ft square and 12 ft high. (Fig. 5).
The larger version of the 'pyramid' type was suitable for a variety of equipment, including field guns, but a new version was required for lorries. This had vertical panels joined by horizontal members. The first example was produced and submitted for testing by Kryn and Lahy at their own expense.
An example of the new version can be seen at Monmouth. It was built in 1931. Instead of using the 'pyramid' design, it has pairs of 'flat' girders are of the Warren truss type, joined by transverse members in the form of rolled steel joists lightened by holes cut through the webs. The tubes are joined by cast steel nodes, as for the 'pyramid' type. The wall thickness of the horizontal tubes is 1/8" greater than for the diagonal tubes, the outside diameter being the same.
Referring to the photographs of the Monmouth bridge, the principle of the joints is by no means obvious. In fact they are stiffened pinned joints. During initial assembly the pins are easily inserted in slots in the node and in the attachment fitted to the tube ends. The slots in the tube end attachments and in the nodes are accurately machined, the load-bearing ends of the slots being machined to a radius corresponding to that of the pins.
It will be seen in Figs 2 & 3 that at the ends of the tubes there are collars with integral lugs. These collars are nuts, with 11 threads per inch. The ends of the tubes are 'swelled' (upset) and threaded correspondingly. See Fig. 6. When tightened up against the machined faces of the nodes, the tubes are put into compression, and the asssembly is considerably stiffened. Hence the term 'stiffened pinned joints'.
Inglis bridges were widely used on the Western Front in 1916, and further afield in Europe in 1918, replacing destroyed road bridges. Another significant use in 1918 was the Allenby Bridge across the Jordan River.
A heavier duty version was developed in the 1920s. Some sources assign Mark Numbers to the various designs. There may be scope for confusion between versions designated Mk I and II, so they are not used here, but we will refer to the final version as Mk. III. Extra load capacity was obtained by doubling or trebling the number of side members, using cast steel nodes having 8 or 12 attachment points instead of 4. To avoid height restrictions, overhead transverse bracing and overhead diagonal tie rods could not be used. Instead, external bracing was applied, using diagonal braces fixed to brackets extending from the deck cross beams. The groups of 2 or 3 tubes could be clamped together at mid length by 'batten plates'. See Fig. 8. The additional components greatly increased the cost.
In addition to the Inglis Bridge, Monmouth, another Inglis bridge survives across the Basingstoke Canal at Aldershot, but it is not in public use. A 50 ft bridge, formerly at RAF Sandtoft, was removed and partly reassembled in South Yorkshire, with a smaller section going to the Royal Engineers Museum at Gillingham, Kent [3] A replica Inglis Bridge of the 'pyramid' type was erected in a park in Leyland, Lancashire in 2016. Other Inglis bridges survive in Canada, Germany, Pakistan, and there is Mk. III examplen the Simpson Reserve in New Zealand [4].
See here[5] for an excellent source of information and illustrations.
Note: Patent specification for the 'pyramid type' bridge (Charles Edward Inglis) here, and for Mk III bridge (Charles Edward Inglis and Kryn and Lahy) here.
See Also
Sources of Information
- ↑ [1] USA War Office: TM 30-410 Handbook on the British Army with Supplements on the Royal Air Force and Civil Defense Organizations
- ↑ [2] The Engineer, 26 September 1919, pp.310-2.
- ↑ [3] Doncaster Free Press, 9th January 2018: Historic military bridge makes final journey
- ↑ [4] AN INGLIS PORTABLE BRIDGE SURVIVOR by Paul Mahoney and Kate Zwartz, 4th Australasian Engineering Heritage Conference, Lincoln University, Canterbury, 24-26 November 2014
- ↑ [5] THINK DEFENCE website: The Inglis Bridges