Scaffolding design for the maintenance of the Runcorn Viaduct

The Runcorn viaduct spans over the river Mersey from Runcorn to Widnes.  We were commissioned to provide scaffolding access for spans 1 to 7.  Span 7 is located on the north shore and span 1 in the centre of the river.  The majority of the structure is made up of cut stone.  However, the primary support for the central span is formed of steel trusses.  The scaffolding provided access for Network Rail engineers to the stonework sections for inspection and to also conduct de-vegetation works.

The design arrived at anchored into the bridge using CL25 brackets with beams running from span to span.  This created a deck on which the independent scaffolding positioned along the façade and the birdcage scaffolding under each span were built from.  We did consider placing anchors at a level from which a span deck could be suspended underneath.  However, this was deemed unsuitable as operatives would need access from the top of the viaduct resulting in line blockages.

After making the initial proposal our client requested that the birdcage scaffold was constructed under each span only as it was required, as it was deemed an unnecessary use of equipment.  This meant we had to ensure that the independent scaffolding was strong enough to withstand wind load without the structural rigidity of the birdcage behind it.

 
Scaffolding design for maintenance of Runcorn Viaduct
 

Once we had arrived at a solution suitable for the client we conducted a series of calculations.  We quickly realised that using standard steel scaffolding components would impose a load that is too great onto the brackets, as the design required far more beams then was originally thought.  To rectify the problem a stack of aluminium tubes were used on spans 1 to 6.  This reduced the imposed load as it amounted to about a third of the weight of the steel tubes.  Span 7 continued to be constructed out of steel scaffolding components as it was fixed to the ground.  Further issues were encountered when we found that one CL25 bracket would not be adequate to support the loads from the independent scaffold on its own, so the brackets were grouped into threes.

To avoid any live loading from the curvature in the beams, we fabricated a load spreader using a UB section of steel, with a tube welded to the top to act as a puncheon.  This was arranged to ensure the load path would be equally shared across all three brackets.  This has proved to be a useful piece of kit to have as we have since used it elsewhere.

The initial design which included the structure to be completely decked out as a birdcage allowed complete access from each side and under each span.  After the design changes were made, the top two levels of the independent scaffolding became the main access route from side to side, with lower decks becoming restricted.

 
Scaffolding design for maintenance of Runcorn Viaduct
 

The erection of the scaffolding, specifically spans 1 to 5, took place using a barge.  Hop-up brackets were anchored into the façade just above water level.  This allowed temporary scaffolding to be built up to the level in which the CL25 brackets were to be installed.  The CL25 brackets were then installed and the deck began to form.  As the Mersey is a tidal river span 6 had a tendency of becoming dry.  This meant the scaffolding for this span could be erected as the tide was out.  Once all the beams were in place, an as-built wind load assessment was undertaken since certain elements of the structure seemed to be more susceptible to wind loading than first indicated.

Once the works were complete the scaffolding was dismantled in a similar manner as the erection.  For a job that took us under a week to turn around, network rail were more than complimentary.