Weymouth Lifting Bridge, UK
Bridgemaster system eases congestion feared by locals during refurbishment works
The Weymouth Lifting Bridge in Dorset is a double leaf lifting bascule bridge which was built in 1930 and carries the D91518 over the inner harbour, linking the Town Centre with the North Quay.
The distinctive bridge which lies at the entrance to Weymouth Harbour opens to allow the passage of boats between the inner and outer harbour. The bridge has a clear span of 26.8 metres and is 12.2 metres wide, while the deck construction comprises two riveted steel plate girders which vary in depth between 3.118m at the pivots and 1.443m at midspan, and a cantilevered footway from the external beams.
Dorset County Council
Stirling Lloyd Construction Ltd
- Weymouth Lifting Bridge
Dorset County Council inspects all its bridges at least every two years and after the most recent inspection highlighted “serious wear and tear” the Council commissioned a complete refurbishment. The concern over the breaking up of the surfacing, which had created unsafe slippery patches, was heightened by the problems encountered with the thin surfacing on the Twin Sail steel lifting bridge in Poole. Investigations confirmed that there was significant deterioration of both the deck plate and the supporting Steel Hollow Box Section (SHS) members.
There was local failure in a number of the SHS members and a substantial loss of section in the SHS members that had been penetrated by the bespoke fasteners used. Each SHS member that had been penetrated was at least three-quarters full of silt and water. The deck plate damage comprised the loss of section due to sea water corroding the plate, exposing the box section which was also rusting. There was also cracking in almost every plate and there appeared to be transverse weld failures between the 1800mm and 1460mm deck plates.
Parsons Brinkerhoff were appointed as design consultant for the project and instigated a deck replacement strategy that would meet the working life requirement of 120 years in “BD16/82 Design of Composite Bridges Use of BS 5400: Part 5:1979. The entire deck plate and all SHS members were to be replaced, with the new deck structure design taking into account the lifting capacity required and the necessary weight distribution.
To enhance durability a 6mm thick deck plate was to be used, supported on 230 transverse members which were to be welded to the existing rolled steel joists at 150mm centres. For the actual running surface, which would also provide the protection to the new deck, a combined waterproofing and surface course providing a minimum PSV of 55 was required.
Not only did the refurbishment need to meet strict performance criteria, but it also had to be carried out in a timely manner as the bridge had to be re-opened ready for the 2012 Olympic sailing events that were going to take place on the coastal waters around the town.
The requirement for a speedy turnaround was compounded by the fact the bridge would have to be closed for the duration of the refurbishment, with the need to divert traffic and local residents feared major congestion in the town. With this spectre and the inevitable press interest the pressure was on the Council to close the bridge for the minimum amount of time.
Once the deck and steel plate had been finalised Parsons Brinkerhoff turned their attention to the protection of the deck and the running surface. For this important element of the work they specified the Bridgemaster system, based on the long-term performance it could offer and the SRV it would provide. Designed for use on orthotropic steel decks Bridgemaster consists of a primer, an advanced resin based screed combined with an aggregate overscatter and a final sealer coat.
The system provides a waterproof, skid-resistant wearing course, all in a single layer. This lightweight system enables structures to be lighter by design and increases a structures dynamic load. The contract to apply the Bridgemaster system was awarded to Stirling Lloyd Construction Limited (SLCL). Once the deck had been prepared to a SA2.5 Swedish Standard finish by vacuum blasting, tensile adhesion tests were carried out to confirm that a strong bond would be achieved between the deck and the Bridgemaster system – a key factor in the systems durability. Results well in excess of 2.5MPa were achieved, validating Bridgemaster claim to act as a composite with the deck, reducing weld stresses. Once these tests had been completed, the first element of the Bridgemaster system, the ZED S94 anti-corrosive primer was applied.
This would enhance the bond of the subsequent screed to the deck as well as providing protection for the recently exposed clean steel. Once the primer had dried, the SLCL application team set about applying the Bridgemaster ‘HD’ screed. The pre-weighed components of the kit; resin, fillers and catalyst were mixed together thoroughly, ensuring all the fillers were wetted out until a homogenous mixed slurry was achieved. This was then poured onto the primed deck and distributed using a rake with stops set to achieve the specified thickness of 6mm. This binder was immediately back rolled using a spiked roller to enable any trapped air to escape and consolidate the binder.
The binder was then overscattered with 1 – 3mm aggregate. Once the binder had cured the excess aggregate was removed and a final sealer coat applied. The sealer not only enhanced aggregate retention but will also help to maintain a clean running surface on the lifting bridge. Once the sealer had cured, in less than an hour, the bridge was re-opened to traffic.
A weight has been lifted
SLCL completed the application of the Bridgemaster system to the 380m² deck in just three days, ahead of programme. The on-site quality assurance programme utilised before and during application, together with the installation teams experience helped ensure a high quality application and an aesthetically pleasing finish.
The use of the site friendly, easily installed Bridgemaster system enabled the refurbishment of the Weymouth Lifting Bridge to be carried out speedily, ahead of the harbour’s impending Olympic commitments and successfully while also ensuring long-term in-situ performance benefits that will enable the local residents and Dorset Council to benefit from a structure that will be well protected for many years to come.