Asphalt laying on the Tamar Bridge, South West England
Major bridge maintenance and replacement projects across the world are extending the life of many impressive historic landmarks as Guy Woodford reports
The Tamar Bridge, part of the main A38 trunk road linking Saltash in Cornwall with Plymouth in Devon, south west England, marked its 50th anniversary with a steel deck resurfacing project involvingJointly owned by Plymouth City Council and Cornwall Council, the Tamar has a suspended length of 642m and a main span of 335m and was the longest suspension bridge in the UK when it opened in October 1961. Fifty years on, the bridge has carried some 450 million vehicles to date and remains the fifth busiest fixed crossing (bridge and tunnel) in Britain.
Growing traffic volumes and increased vehicle weights have taken their toll on the original structure, but the latest resurfacing project is the first major work on the bridge for ten years. Between 1999 and 2001, the Tamar became the first suspension bridge in the world to undergo a complex strengthening and widening scheme while remaining open to traffic.
At that time the work also involved the replacement of the original concrete deck supporting the carriageway with a stronger, lighter steel deck, and resurfacing with hand laid mastic asphalt.
The latest project is said by Stirling Lloyd to follow a noticeable deterioration in the quality of the bridge's Plymouth side span.
Onerous loading conditions arising from heavy, slow moving traffic close to the toll plaza had contributed to the failure of the deck surfacing in the nearside eastbound lane of the Plymouth side span.
Working for the joint Cornwall and Plymouth councils, alongside main contractor, Cormac, Stirling Lloyd has spray-applied its Eliminator rapid cure, cold liquid resin waterproofing as part of the surfacing solution on sections of the bridge carriageway.
Following the removal of the old asphalt surfacing by Cormac, Stirling Lloyd Construction, one of the authorised contractors for the application of the Eliminator system, carried out high pressure water-blasting to remove corrosion and the residue of the previous waterproofing system before applying the ZED S94 anti-corrosive primer to the steel substrate.
Once the primer had cured, the Eliminator membrane was spray-applied in two colour contrasting coats. The first, yellow coat was applied to provide a minimum dry film thickness of 2mm.
Wet film gauge thickness tests were taken throughout application to confirm the requisite thickness was being achieved. When the first coat had cured and been electronically tested, to confirm 100% waterproofing integrity, a second 1mm layer of Eliminator pigmented white was applied.
As this second layer of Eliminator was curing, a flintag overscatter was propagated into the wet membrane.
The flintag is said to give an enhanced shear key interface between the Eliminator waterproofing layer and the subsequent thin surfacing.
Finally, a hot melt adhesive tack coat was used to enhance the bond between the membrane and the specified Gussasphalt surfacing.
The strong ZED S94 enhanced primer bond that the tack coat helps achieve between the membrane and the surfacing, together with the membrane’s high bond strength to the deck, enables the deck/membrane/surfacing to act as a composite, significantly reducing surface stresses.
Through the use of the Eliminator, Stirling Lloyd predicts that not only will ongoing maintenance costs be seriously reduced but the service life of the Tamar Bridge bridge deck surfacing could be extended up to 30 years.
Italian firm Barin has supplied one of its ABC 60/L skid unit machine’s for inspections of bridges in China.
The supplied machine has been mounted on a two-axle
With a maximum underbridge working length of 6m, the underbridge platform type machine being deployed in China has been completely built with aluminium alloy. It is said by Barin to be composed for a fixed element and a sliding manually extendable element. The supplied ABC 60/L model has a 1m platform width and a total platform payload of 300kg.
Barin says other features of the machine include the ability to overcome sidewalks up to a maximum width of 2m and wind barriers up to 1.8m high. The machine is also said by Barin to be capable of working parallel to the bridge axis, with an underbridge rotation of 135°.
During its operation, Barin says the ABC 60/L occupies only one traffic lane, allowing traffic to flow in the other lane.
In the U.S., the first form travellers have been erected at the Antlers Bridge Replacement on I-5 in Shasta County, California.
The new Caltrans owned US$125 million cast-in-place prestressed segmental box girder superstructure consists of 5 spans - 592m long and 31.8m wide. The structure is being built on a new parallel alignment over a lake which involves seasonal water level fluctuations of around 1.27m-1.78m.
The project is situated in a high seismic area and incorporates complex earthquake resistant design details. Furthermore, a 0.7km long section of highway south of the bridge will be realigned to improve roadway safety; the existing 33.78m long concrete decked steel truss structure will be demolished and removed.
The West Gate Bridge strengthening project in Melbourne, Australia is said to be one of the most complex and high risk civil engineering projects undertaken in the country during recent years.
A ‘perfect storm’ of interrelated challenges included the Bridge’s intricate and varied array of steelwork due to the structure’s infamous collapse during construction in 1970. Add to this a project completion timeframe never previously achieved on comparable bridge strengthening works; the need to ensure absolute safety and structural integrity of the operating bridge throughout; the lack of as-built drawings; and highly restricted access conditions and onerous construction sequencing and logistical constraints; made it one tough assignment.
The 2.6km West Gate Bridge comprises the 850m long cable stayed steel box girder bridge over the Yarra River and segmental prestressed concrete box girder approach viaducts. The bridge carries over 160,000 vehicles per day, including 24,000 trucks and heavy vehicles, and this had to be maintained safely throughout with no traffic lane reductions during peak hours. This single factor dominated the technical, safety and logistical challenges at every level.
Faced with the need to widen the bridge from four to five lanes in each direction to cope with rising traffic demand, owner VicRoads sought international expertise and experience when assembling a project Alliance to undertake the task.
British firm Flint & Neill (F&N) was selected for their expertise in steel box girders and their experience with strengthening West Gate’s sister structures in the UK: the Severn, Erskine, Wye and Cleddau Bridges. F&N were appointed together with local consultants SKM to carry out the structural assessment and design of strengthening, and
F&N said the structural assessment and strengthening design was highly complex, often requiring a first principles approach in order to verify adequacy. The works involved extensive stiffening and reinforcement of the steel bridge, propping of the cantilevers to accommodate the extra traffic lane, post-tensioning and carbon fibre reinforcement of the concrete viaducts and many other aspects.
Following the renovation, delivered a custom-designed under-bridge access unit to enable regular inspection and maintenance work on the underside of the Westgate Bridge deck.
The bespoke model MBI 200-1.5/S, mounted on a
A crossing of 4m over the safety fence, which protrudes outward on the sides, can be reached. The maximum horizontal range of the MOOG platform is 20m under the bridge, and the maximum lowering depth is 8.4m. With a load capacity of 800kg, the spacious platform is said by MOOG to offer enough area for several people at the same time. Therefore, all surfaces of the West Gate Bridge above and under the road can be reached flexibly. With these ranges, the MBI 200-1.5/S is said by MOOG to the biggest bridge inspection unit in Australia.
Additional equipment requested were a pneumatic lift, an air compressor, a special camera and lighting system, a platform extension and a diesel generator. The additional equipment is said by MOOG to increase the platform’s fields of operation and offers more comfort for the user.
Meanwhile MOOG’s latest engineering design development, a prototype of the crawler-mounted MBI 50, is said by the German firm to have undergone successful testing.
The MBI 50 is a small bridge inspection unit crawler-mounted with a low payload. It is said to be ideal for operations on a narrow sidewalk, cycle-way bridges, or sidewalks with low load capacity restrictions. Just 1.6m of space is required on the bridge for the MBI 50, and its maximum point load of 70 kN/m² is said by MOOG to be ‘underbidding’ all comparable units on the market.
Its total load of 3tonnes is equally distributed on both crawler tracks, so that the pressure per square centimetre during operation is minimal. A transport height of nearly 2.9m is said to allow for standard transport on a common car trailer, so the maximum trailer load of 3.5tonnes will not be exceeded.
The compactness of the unit’s upper structure means the working platform can be stored smartly around the tower to keep the width down to 1.6m during transportation. According to MOOG, most requirements for low capacity bridges are met sufficiently with a horizontal working range of 5m under the bridge, a 300kg working load on the platform, and a lowering depth of 4m during operation.
An additional portable scaffold completes the overall scope of the equipment, with different installation variations allowing the platform to be rotated under narrow bridges. A 12kVA diesel generator is said to provide the necessary power for the electric pump and the electrical sockets located on the platform.
To allow their customers to get to know and experience the new MBI 50 technology, the prototype unit will be hired out by MOOG to interested customers.
