UK Military Bridging – Equipment (Pre WWII Equipment Bridging)

The title for this post should really be ‘everything up to the Bailey Bridge’ because equipment bridging took a leap forward with the Bailey that made all before it more or less redundant although as I will show, the Bailey would not have been possible without the innovations introduced in the bridges described below. Most of the designs here saw service during WWII but were designed prior to it, hence inclusion.

Hopkins Bridges

The earliest forms of equipment bridging were based on pontoons and trestles which I have covered in a previous post on floating equipment. The first British equipment bridge was called the Inglis Portable Military Bridge (Light Type) introduced during WWI, although a number of Stock Spans were introduced earlier, but these were used in construction bridging rather than as a self-contained portable bridge.

From Canada in the Great War;

A number of standard-span portable bridges, varying in span from sixteen to eighty-five feet, were stored at the base depots. These bridges consisted of loose members and were bolted with machine-turned bolts. They were, however, very cumbersome, and this rendered their erection slow. The weight was another disadvantage, e.g. the eighty-five-foot span was a single-way bridge and weighed sixty-three tons.

A cheaper alternative was the Hopkins Bridge that used conventional steel girder forms bolted together, but this was much slower to construct than the Inglis. The Hopkins Bridge was used to cross the Canal du Nord at Havringcourt, I covered this in an earlier post, click here

There are also a couple of Hopkins bridges over the Basingstoke Canal in Aldershot, one between the Army Cricket Ground and Army Golf Club.

It is a shame that such important military industrial heritage is not better preserved.

Calender-Hamilton Bridge

In the immediate pre-war period most bridging designs came out of Christchurch but the Calender-Hamilton was an exception. Archibald Milne Hamilton was a New Zealand born civil engineer responsible among many things for a strategically important road through Kurdistan between Northern Iraq and the Iranian border, described in his book, here it was an incredible feat of civil engineering.

During the construction of the road many bridges were needed and most of them were built with surplus British military bridges such as the Stock Spans and Hopkins Bridges. Hamilton came up with the idea of a set of standard parts that could be fitted together to form different lengths and loan carrying capacities. The strength was varied by bolting together multiple angle sections, much like the concept for the later Inglis and Bailey bridges.

The concept was demonstrated to the Royal Engineers and Signals Board.

The Calender-Hamilton Bridge was genuinely innovative and manufactured in two variants. Type A used 5 feet bays and 5 feet trusses with the Type using 10 feet trusses and bays. The Type A was mainly used by local authorities to repair bomb damage and the Type B was adopted by the Army and called the Unit Construction Bridge, this combined single and double trusses to create a 140-foot-long bridge that could support tracked vehicles up to 30 tonnes, although this was varied as needs dictated.

The CH bridge was not rapidly built, but it was strong and robust; thus, they were used not for assault bridging but as follows on semi-permanent bridges on lines of communication.

Many wartime and post-war examples remain in service. The longest lasting temporary bridge in the UK is over the River Eden Langwathby, click here to view and the Walton Bridge over the Thames used a CH type from 1950 to 1983. The Indian Railways make extensive use of CH Bridges and have an excellent guide on their deployment (including a nifty picture of Christchurch Cribs), click here to read. A number have been used on the trans Labrador Railway (click here), for any metallurgists in the house and an interesting advert here.

Painter Brothers are also still in business today as part of the Balfour Beatty Group, manufacturing Calender-Hamilton bridges for use worldwide, click here to visit their website. Modern CH bridges are of course an improved version and can span lengths up to 150 meters long and 3 lanes wide, but the fundamental design remains unchanged. Acrow Canada distributes the Painter Brothers CH Bridge in North America.

Large Box Girder Bridge

Another important step on the way to the Bailey Bridge was the Large Box Girder Bridge, the first military equipment bridge in the world to use prefabricated box shaped sections.

Once Major Gifford Martel had taken command of the newly formed Experimental Bridging Establishment his first task was to continue trials with the Canal Lock Assault and Inglis bridges, but he also started work on what was to be his most important contribution to military bridging.

He identified that with the Inglis Bridge and Stock Spans a considerable proportion of the weight and build time was taken up with providing the roadway, they lacked adaptability and in the case of the Inglis Bridge, expensive.

He therefore developed an entirely new concept that would use common building blocks to form different length bridges, each section pinned together as necessary depending on the length and load carrying requirement. It was this modularity that would later be developed in the Bailey Bridge.

Launching was normally carried out using gin derricks and shear legs as described in the previous post on construction bridging but cantilevering the bridge across a gap was also carried out. Once to two girders were in place they were adjusted to the correct spacing the decking planks placed across them with ever fifth being oversize to accommodate a footway. Angle iron was pinned in place to form a curb.

Adopted by the Army in 1925 the Large Box Girder Bridge was adaptable and relatively cheap, able to carry loads of up to 40 tonnes, it remained in service until replaced by the Bailey.

Click here to see an image.

The Mk II was introduced in 1938 with a number of changes; a larger main box unit, a new hornbeam, higher strength steels and new chrome molybdenum pins. The new bridge in 4 girder form could accommodate Class 24 loads at a span of 130 feet.

As can be seen from the image below, Mark II Large Box Girder panels were not small.

Small Box Girder Bridge

I looked at the role played by the Small Box Girder Bridge when combined with the Churchill tank during D-Day and beyond in the previous post on assault bridging but before being used in this manner it was also used more conventionally.

The maximum vehicle load at the time apart from tanks was less than 10 tonnes and with the introduction the medium new weight tanks it was thought a bridge that was easier to transport and faster to build than the Large Box Girder bridge would be a valuable addition.

The Small Box Girder bridge was in effect, a scaled down Large Box Girder bridge and after a series of trials and modifications was formally introduced in 1933.  The 8 feet 6 inches fixed width timber decking was laid on two or four girder sections up to 64 feet long.

Launching was completed using a lightweight launching nose that had a small wheel fixed to the end. Construction times were very low, a well-trained team could bridge a 60-foot gap in less than 20 minutes. Donald Bailey, who was by now working at the EBE, recommended the use of high tensile strength steel and new welding techniques, and it was this that allowed such a light structure to carry the required loads.

Clips from British Pathé show the Small Box Girder Bridge under construction, here, here, here and here

Its ultimate form, the Mark III SBG bridge, was Class 12 at 64 feet in 2 girder configuration and Class 24 at the same span in 4 girder configuration.

The Small Box Girder bridge was copied and used by many other nations including Germany (where it was called the Kastenrager-Great or K-Gerat) and the United States where it was called the H-20

So we can see that the Inglis, Calender-Hamilton, Large Box Girder and Small Box Girder bridges all contributed ideas and techniques (modular panels, welding, steel selection, launching nose and combining structural members) that would be utilised on the Bailey bridge.