Revolutionising the world of military bridging (with the Bailey) once was a tall order, to pull it off a second time would be nothing short of amazing, but MEXE did it with the Medium Girder Bridge (MGB)
A number of design drivers informed the requirement for the MGB, an increasing desire for air portability for example, but what made it possible was a rapid advance in aluminium alloy composition and aluminium welding techniques. Aluminium had of course been used extensively in aerospace industry but this was largely confined to cast or sheet parts, the new aluminium zinc magnesium alloys had excellent heat recovery properties and could therefore be welded. Different types of alloy were experimented with including Hiduminium 48, Impalco 720 and the French Superalumag 75 but none of these proved to be completely satisfactory so MEXE decided to create its own formulation, the DGFVE Specification 232. DGVFE Specification 232 contained 4% zinc, 2% magnesium and 0.35% manganese and the latest version is still widely used, DEFSTAN 95-31 contains 0.15% of copper to improve resistance to stress corrosion.
Not content with developing new material MEXE also pioneered the welding thick sections of aluminium alloy using the Metal Inert Gas (MIG) and Tungsten Inert gas (TIG) systems.
Design work for the MGB, a replacement for the HGB and EWBB except on lines of communication, started in the early sixties in response to a requirement for a hand built bridge that could carry a Class 60 load over a 100 foot span and be used on battle group supply routes, not in the direct fire zone.
At about the same time Sir Donald Bailey left to become the Dean of the Royal Military College of Science at Shrivenham and was replaced by Brigadier Jarret-Kerr (mentioned in the post on Bailey bridges). Leading the MGB design project was the former Royal Engineers Major, Eric Longbottom, who had helped to design the Mobile Bailey, the Heavy Girder Bridge and, yes, the MEXEFLOTE!
The original single storey proposal was changed to a double storey to enable greater flexibility and the design specification called for a Class 60 load in single storey construction at 30 feet span and the same Class 60 load at 100 feet span but in double storey construction. The concept is similar to the Small Box Girder bridge, that is two longitudinal girders with decking panels, that of course is where the similarity ends.
Basic Design and Features
One of the key aspects of the MGB’s success is its simplicity, this does not mean it was simple to design, far from it, but users have to contend with a mere 7 major components and a number of supplementary components like curbs and footways.
Only 4 of these major components are used in the single storey configuration.
The longitudinal girders are built connecting multiple top panels and deck units used to connect them. Bankseat beams connect the girders at either end of the bridge and ramps are hooked on to these. For a single storey bridge, that is all that is needed.
Adding the triangular shaped bottom panel, junction panel and end taper panels turn the bridge into a double storey configuration.
A great deal of thought was also given the speed of construction; there is even a special trailer where pallets can pulled onto the ground, the same roller bearings are used for single or double storey construction and when building the frame, jacks allow it to accommodate uneven ground.
A building frame is used to assemble and launch the sections
All bridges are launched ‘undecked’ and a launching nose used with long and double storey bridges in a similar manner the Small Box Girder bridge, this is a specially designed component available in light and heavy flavours.
The panels themselves are designed to be very quick to fit together and the outer jaw of the panel will fail first, thus any stress or fatigue failures will be detected early.
The MGB is shot through with clever design features.
In comparison with the Extra Widened Bailey Bridge, at a Class 60 100 foot span, the MGB weighs less than a third, needs just over a quarter of the manpower and less than 20% of the time.
To say the MGB was a significant advance is rather an understatement.
The MGB was fast, teams from the UK, Holland and the USA would compete for the Fairey Engineering Silver Cup at annual ‘Sapper Games’ and the World Record is (I think) just over 6 minutes for a 5 bay single storey by a Royal Engineer regiment, 4 Squadron 21 Engineer regiment I think, although the rules changed often and with modern safety regulations unlikely to ever be bettered even though it will provide hours of debate and discussion about rules, whether teams could use fewer (big strapping) men on the build to reduce timing etc.
What does the manual say?
Wikipedia has some good pictures of the construction sequence, click here
The component parts on their pallets are air portable by Chinook and in some configurations a complete bridge without decking panels can be placed into position by air although there would still need to be time allowed to prepare the bridge site and finish it off with other components either driven or flown in separately.
The image below shows an Australian trial conducted in 2002 with a 22m single span MGB.
To summarise, the MGB can span 9.8m in single storey configuration, 31.1m in double storey, 76m using multi span equipment and 49.4m using the link reinforcement set, all built by hand and in very short times.
The MGB entered service in 1971.
Although the Medium Girder Bridge was officially replaced by the General Support Bridge it has been retained in use for training and has also been deployed on recent operations a number of times, it seems the Mean Green Bridge just won’t go away!
In UK service, the modern form of the MGB is the medium Girder Overbridge, or MGOB.
An ‘overbridge’ is a method of reinforcing an existing bridge that might have been damaged or simply not have the required carrying capacity, covering craters or reinforcing culverts for example. A series of wedges are used to create a distinctive hump which limits deflection when carrying heavy vehicles.
This is a good example from US forces in Afghanistan of the MGB being used an overbridge although not using the overbridge kit.
To accommodate longer gaps a set of multi span equipment was designed and introduced. The multi span equipment consists of two major components, the span junction set and portable pier set. Any length bridge can theoretically be built from MGB components but for more than three spans the practical launching procedure is complex and places a great deal of stress on the bridge so for these reasons, a limit of 3 spans is enforced, 76 metres.
Bridge sections are joined together using the span junction set, it being essentially a linking adapter for double span bridges and the bridge is supported on multiple piers, not dissimilar in concept to the pontoon piers I looked at earlier in the series.
The pier has a maximum height of 18m and can be used in wet or dry gaps, built up in sections with articulators being jacked into position to adjust position. Improvised or existing piers can also be used.
For longer spans the MGB can be to create a floating or pontoon bridge, at Class 70. It can be used in single storey or double storey configuration depending on the distance between pontoons. Where a large rise and fall of the river is expected due to flooding or tides double storey construction allows the landing span to be longer and thus accommodate this variation. A double storey floating bridge can accommodate bank heights of up to 5m.
Any type of pontoon can be used but WFEL produce a dedicated MGB Pontoon.
The same span junction set used in multi span bridges is also used to connect multiple bridge sections to provide articulation when being supported on pontoons or other floats.
MGB’s can also be used to create floating ferries at Class 90.
To increase the load capacity of single span MGB’s a Link Reinforcement Set was introduced. The LRS uses pinned reinforcing links that are attached underneath the bridge and tensioned using steel cables and Tirfor jacks although the tension is kept to a minimum, enough to remove slack from the links and move them into a vertical position.
The LRS does not increase the load carrying capacity but maintains it over a longer distance, up to 49.4m.
This all sounds very simple but have a read of this manual to dispel any of those thoughts!
Mechanically Aided Construction
In order to reduce the personnel needed to construct MGB’s the MACH MGB (Mechanically Aided Construction by Hand) is used. MGB modules, a 3 bay double storey panel for example, are built away from the bridge site and built on site using suitable cranes or hydraulic jibs.
Fairey Engineering Limited were contracted to manufacturer the MGB and still do to this day. Fairey Engineering are now called WFEL Limited and for a comprehensive history of WFEL, their web site has the details, click here to view.
WFEL have applied their expertise to the design and construction of a demountable ski jump for the Joint Strike Fighter programme and were recently awarded a contract to supply an additional MGB for the British Army and associated repair services.
MGB was increased to Class 70 at some reduction in lifespan with the introduction of the heavier Challenger tank in the eighties.
The MGB is still in service with the British Army, US Army, USMC and many more.
The video below shows 29 Armoured Engineer Squadron, part of 35 Engineer Regiment, in preparation for Afghanistan building an MGB
and in Afghanistan
Demonstrating their versatility, MGB’s have been used in Afghanistan as footbridges, joining together undecked top panels to form a narrow bridge.
They are still widely used and there is a great example of one being used in the aftermath of the recent tsunami in Japan (anyone have a clue what they are saying?)
To summarise, the MGB was a significant step forward in military bridging, still sold today and still in wide service today, yet another testament to MEXE, WFEL and the Royal Engineers.
OTHER POSTS IN THIS SERIES