From a bridge engineer's point of view, the ideal access to a bridge soffit is from directly below. It is a then a simple task to park a truck-mounted or self-propelled platform underneath a bridge. However if the structure is very tall or spans a river, this is impossible.
In such situations there are two options. Usually, the most economic is some sort of scaffold tower. However, this is not practical where work has to be carried out during a short night time possession when erection and dismantling would take too long, or over deep rivers or ravines. One popular alternative is a truck-mounted underbridge platform.
These mobile stages can be lowered from the top of a bridge to provide access below. Launched hydraulically from the deck, the platform reaches out and down over the side of the bridge. Once deployed, it swivels round to provide a safe working platform underneath.
Platforms can be long gantries that provide work space for several people with equipment, or baskets carrying just two or three people. Those working below the bridge stay in constant communication with the operator above in the lorry cab, via a headset radio link.
On the right job mobile platforms can provide great savings over scaffolding. WS Atkins recently used a Moog MBI 150 2/S underbridge platform to survey Blackfriars Bridge in London. The machine allowed a 15m gantry platform to be lowered down over the bridge parapet to provide safe access for up to five people.
'The mobile platform was very manoeuvrable and much more cost effective than fixed scaffolding,' says WS Atkins project engineer John Burke. 'It allowed quick access to carry out spot checks over a large area under the bridge. An extensive amount of scaffolding would be needed to give such freedom of access and would require considerable advanced works for erection and dismantling.
'We were able to complete the inspection within two days, considerably quicker than would have been possible using scaffolding.'
'The underbridge platform was the only feasible option,' explains WS Atkins associate Robin Self. 'We needed to get under the bridge as far as possible. Scaffolding would be fine for a longer duration but for a job that only required a quick in and out, the mobile platform was ideal.'
He adds: 'However this was only a preparatory survey on the condition of the existing coatings. We will have to erect scaffolding to carry out the redecorating work, as blasting needs to be done from a fixed, enclosed platform.'
Where a large work area is not required, baskets are quicker and more flexible than platforms and are ideal where the engineer wants to carry out spot checks rather than a whole survey.
One of the most common scenarios for mobile platform is working on a bridge over a railway line. 'We use underbridge platforms for rail bridges because we are limited for time on the track,' says Nottinghamshire County Council principal project engineer Andy Milne. 'Typically a night time possession is four and a half hours. Therefore time is the issue. A platform is much quicker than a scaffold.'
However, using the mobile units can raise other issues. 'There is a limit to the extension of the platform.' explains Carl Bro senior bridge engineer Matthew Carroll. 'This sometimes means you have to change sides of the road and may involve blocking lanes. This raises a number of traffic management issues.'
A possible option here is a rail mounted platform. Specialist rail plant manufacturer Rexquote has just launched a new road/rail access platform.
Based on a Thwaites TD18 articulated dumptruck the platform can carry four people with hand tools to a height 9m and an outreach of 7m. 'This is a new concept in a growing market,' says Rexquote's Andy Hersee. 'The TD18 is ideal for tunnel and bridge inspection because of its all terrain ability and sizeable payload.'