The use of a Hover-Platform in Bridge Construction

The use of a Hover-Platform in Bridge Construction

I was going through some old files recently and found the photograph below of one of the Avonmouth Bridge site hover-platforms.

There were two of these devices used to transport box girders from the assembly areas to their erection points on the site in the early 1970’s. This was the first time that the hover principle had been used in the construction of a major bridge

There had been a great deal of development work carried out on high-speed marine versions of the hovercraft through the 1960’s. These were quite complex self-propelled vehicles that required skirt systems, and airflow patterns, that would resist washout of the air cushion due to wave impact.

Carrying a typical steel box girder section the photograph shows one of the two Avonmouth hover-platforms made from; a simple structural frame; roofing sheets; a 250hp diesel engine; a 5ft centrifugal fan; an in-house designed and manufactured modular skirt system. Overall the platform measured 15m x 17m, and weighed around 30 tonnes including counter balance for the engine and fan. An air-cushion pressure of less than 1psi would lift an 80 tonne payload, 400mm clear of the ground.

 

The Avonmouth machines used only a peripheral skirt system that would contain a ‘mono-chamber” of pressurized air. Moving slowly over smooth ground these machines could carry relatively large payloads very efficiently.

When Fairfield Mabey were awarded the main contract for the construction of Avonmouth Bridge in 1969, the normal method for moving large items around a site was rail track and bogies.  For the Avonmouth project around a mile and a half of track would have been necessary to provide a main spine route and the various spurs and transfer tracks needed to go with it. On the north side of the river the line of the new bridge crossed a trunk road; a docks access road; and a regional railway line. A semi permanent system of rail tracks would have presented difficulties. The south side of the structure didn’t have any obstructions of a similar nature, but it was a flood plain, and the ground was very soft. The hover principle would deal with all of these problems.

Generally it was a safe machine to operate. In the event of an engine failure the large volume of air contained by the skirt system, acted as a cushion, and the platform came to ground quite gently. Damage to the skirts was common, but they were of such simple construction that all repairs could be carried out on site with minimal delay. Dust generation was certainly an issue, and a constant irritation to the housewives of Avonmouth on washday. Bowsers were in regular operation damping down the ground surface before movements were undertaken. But of course that’s not unusual on civil engineering construction sites.

New equipment brings new problems, and in this case the most unexpected was when the operator of one of the two machines decided that he was actually operating a flying machine, and should therefore be paid as a pilot and not as a steel erector. Work stopped and the labour force assembled in the mess cabin to debate the case with their shop stewards, who subsequently called for a meeting with the management to discuss the claim.

In terms of piloting it wasn’t exactly the most demanding job. Once started the only control was the throttle for the diesel engine. An increase in engine revs would lift the load and a reduction would lower it. The upshot of the claim was that, as the skirts never actually lose contact with the ground it could not be claimed to be a flying machine, and that pilots who neither steer their craft or get them clear of the ground don’t get paid so much.

Ridiculous as this may sound (never a dull moment in those days), Sir Christopher Cockerill had a similar problem with HM Government when developing the original hovercraft concept in the 1960’s. His initial development work was classified as it was considered to be of strategic importance. However, the chiefs the Army, Navy and Air Force could not agree on how the hovercraft should be categorized. After an extended period of indecision the project was declassified, leading Cockerell to make the following observation, “the Navy said it was a plane not a boat; the Air Force said it was a boat not a plane; and the Army was plain not interested”. Nevertheless, the view of the British military subsequently changed as other counties around the world enthusiastically adopted the technology, not least being the USA.

In all there were around 200 box movements required on the Avonmouth site taking into account some necessary double handling. The Fairfield Mabey hover-platforms were certainly not going to win any design awards for their sleek lines, but they worked, and they were very cheap to build and operate.

By modern standards the box sections of the Avonmouth Bridge were relatively light. Today such components would probably be moved around a site using self-propelled hydraulic modular trailers, after some degree of ground preparation. But these devices didn’t exist in 1969. At that time, given the newness of the technology, I think it was a brave move to adopt the hover principle, and I think it paid off.

I played no part in the conceptual or detailed design of the hover-platforms. My only contribution was to oversee the construction of one the two machines and its operation for the duration of the project. Since then I have been involved in large number of bridge projects, but I can’t think of one where a hover-platform would have been the ideal solution for moving large structural components. Nevertheless, I don’t discount the idea that somewhere a bridge site may exist where the hover principle could be used to advantage once again, especially in developing countries given the low-tech simplicity and economy of a basic hover-platform.

If there is anyone out there who has experience of using hover-platforms to move large lumps of bridgework around a site I would very much like to hear from you.

 

Geoff

23rd Feb 2013

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