Bridge Road was closed overnight on 22-25 May this week in order to allow the installation of the remaining steelwork structures of the new Countess Wear footbridge.
This 160m long, 3m wide footbridge is the major new structure in the Bridge Road project. It is being installed adjacent to and within the existing Grade 2 Listed stone arch bridge, and has been designed to complement the existing bridge.
The new bridge has a thin profile to ensure that little of the stone bridge is obscured, and via a system of hidden supports it will appear to “float” alongside the old bridge.
This report will give a glimpse into some of these hidden details and show some of the heavy lifting taking place.
On Monday night and Wednesday night we installed the remaining three stainless steel cantilevers onto the previously installed piles.
These piles were installed in 2014 down through the stone piers of the existing bridge, through the riverbed and into the bedrock below, giving a strong support for the new structure.
The cantilever beams each weigh around 2.4 Tonnes and are built robustly from special “duplex” stainless steel which has a very high structural strength and improved corrosion resistance; most often used in offshore oil rigs.
This material was selected for this application as the cantilevers will be buried under the road surface and we don’t want to be digging up the road again to maintain them! The buried connection will be cast into a large block of fibre reinforced concrete to provide additional protection.
On Tuesday night and Thursday night we installed the four remaining deck spans, out of nine in total.
These deck spans are made of more conventional painted structural steel, however to keep the profile as thin and stiff as possible the longer spans have specially fabricated asymmetric plate girders, with the design taking into account the deck plate itself to form a compound section and thus improving stiffness to weight and depth ratios.
The lengths and weights of each span vary significantly (up to 18 Tonnes), however each one is lifted by four eyelets connected to built-in lifting points. Handling these heavy pieces of steelwork is a skilled job and was done with great care by specialist Contractors Taziker Industrial and South West Crane Hire.
The positioning of the cranes was a key consideration as we needed to distribute the weight in the right places to avoid the risk of any damage to the stone arch bridge below. As you can see from the photos, the crane deploys legs or “outriggers” which are required to keep it stable, but unfortunately do not leave enough room for traffic to pass, only pedestrians and cyclists.
The installation of span 6 was the most technical lift; this is one of the longest spans and is positioned alongside the largest arch in the stone bridge; it would not be possible to position a single large crane at the middle of this arch as the weight would be too high.
Instead two cranes were used in what is termed a “tandem lift”, the advantage of this is that the weight of the cranes can be better distributed onto the stone bridge, and placed at stronger locations on the arches.
By the end of Thursday night all nine deck spans were installed.
Next steps include:
• fine adjustment of the decks and supports for alignment and level
• installation and grouting of the bridge bearings
• measurement, fabrication and installation of various pieces of trimming steel
• fabrication and installation of the bespoke stainless steel parapet which will be a key architectural feature
• Installation of the weathering steel fascia panels which will hide the side of the bridge beams and complete the architectural design.