By their very nature large bridges pose a significant problem when it comes to their inspection and maintenance. They are often very high and span across large rivers making access from the ground all but impossible. In these situations it is necessary to install some means of access on to the bridge, whether it be a permanent or temporary platform.
Most bridges require some form of access below the deck. Suspension bridges have the added complexity of also requiring access above deck level to the main cables, which can be up to 200m above sea level. One thing they all share in common is that access is extremely hazardous and must be executed in a safe manner that reduces the risks to a minimum.
Eadon Consulting are specialists in the design and analysis of bridge access gantries and platforms and have experience of working on many different types.
Under Deck Gantries
Underdeck gantries usually consist of a main structure spanning transversely under the deck and supported by wheel bogies on runway beams or tracks. The gantry can span the whole width of the bridge as in the case of the new Forth Road Bridge Underdeck Gantries designed by Eadon Consulting.
Alternatively the gantry can be made up in sections which run alongside each other in parallel such as the split Queensferry Crossing Welding Gantries.
As a general rule it is always considered favourable to keep complexity of a movable gantry to a minimum. This has the effect of simplifying the operation and also reducing the maintenance and inspection requirements of the gantry itself. Often however the shape of the bridge means that complexity is required in the form of additional mechanisms for example to keep the gantry level or provide higher level access by means of an elevated work platform. This can be seen in figure 3 below showing the Kessock bridge gantry, the replacement for which Eadon have carried out a Category 3 (CAT3) check on.
Suspension Bridge Main Cable Inspection Platform
In order to inspect the internal wires of a large suspension bridge main cable a large bespoke platform with full encapsulation is required, with enough load capacity to cater for the cable compaction and wrapping machines required to seal the cable after the inspection has taken place.
Eadon designed such a platform for the Severn Bridge high level cable inspection.
This is a relatively simple type of platform which is not able to traverse along the main cable due to its sheer size and weight. Instead the platform is mounted on the bridge deck and substantial hoisting equipment is installed to lift the platform up to the main cable where it is secured.
Suspension Bridge Main Cable Access Gantry – ‘Cable-Crawler’ Gantry
To access the full length of a suspension bridge main cable Eadon Consulting have designed several cable crawler gantries for bridges around the world.
These gantries have a walkway on each side of the main cable which consist of two trusses joined by a flat stiffened plate forming a U shape. The two walkways are then joined at the ends by a series of ‘goal posts’. This configuration allows the platform to traverse along the main cable while avoiding the cable bands and hand strands as well as the deck hanger cables.
The platform is motorised and moves by hoisting itself along using winches anchored to the cable bands.
Modular Temporary Access Platforms
All of the above gantry types are manufactured to suit the particular bridge they will be installed on. Often temporary access is required to a bridge but it is uneconomical to provide a bespoke gantry. Eadon have worked extensively with ALPS and provide independent Category 3 checking of their modular temporary access platforms. These platforms consist of aluminium or steel space frames and can be fixed together in whatever configuration is required to suit the bridge. Rather than have the expense of purchasing and maintaining a platform which will be rarely used, the ALPS equipment can be hired for the duration necessary. The platforms can either be a static platform which is just used in one location on a bridge, or they can be fixed to wheel bogies that allow the platform to move along the runways fixed to the bridge.
Considerations in Bridge Access Gantry Design
The design of bridge access gantries is a complex process encompassing structural, mechanical, and electrical engineering. To completely cover topic would require a large text book of its own. Rather than attempt to explain in any detail the design process here are a few of the most key items to consider when designing a gantry.
- Bridge support and gantry structural capacity
- Gantry selfweight and rated load
- Wind loading, both horizontal and vertical
- Crabbing loads
- Runway tolerances
- Powered or manual operation
- Redundancy in primary supports
- Failsafe systems
- Anti-crabbing system
- Emergency access/egress
- Inspection and ongoing maintenance, replacement components
- Installation / removal
- CE marking and the Machinery Directive 2006/42/EC
Eadon Consulting have built up vast experience in the design, manufacture, testing, installation, and use of bridge access gantries. For more information or to discuss your gantry requirements please telephone 0114 3090400 or contact us.