A new bridge for the Shanklin Cliff Lift shows how good communication between erectors and designers can make challenging sites safe to work on.
Project: Shanklin Cliff Lift Bridge
Client: Isle of Wight Council
Project value: £1m
Main contractor: MCM Construction
Principal design consultant: REIDsteel
Start date: November 2016
Completion date: July 2017
Shanklin on the Isle of Wight is a popular seaside resort.
The beach, once used for the PLUTO undersea oil pipeline as part of D-Day in the Second World War, is at the bottom of a substantial sandstone cliff.
The many activities available in the parish are concentrated down at beach level, while most of the hotels are up on the cliffside. The resort is popular with elderly visitors, but the slopes and stairs down to the beach can prove difficult to traverse.
As a result, back in 1891 a lightweight steel lift was built at the foot of the cliff with a bridge connecting it to the clifftop.
The lift was damaged beyond practical repair during the Second World War due to its proximity to the PLUTO project, but was rebuilt as a more solid concrete structure that opened to the public in 1958.
Need for replacement
While the heavy concrete lift tower has stood strong over the last 60 years, the bridge to the deteriorating clifftop has had to be replaced a number of times.
Attempts were made to make a bridge that was easier to maintain, but the health and safety implications of working over the cliff edge meant that in 2015 the old bridge had to be removed, and a temporary structure put in its place.
REIDsteel first began work in late 2016, by which time the clifftop road had been closed to cars because of the unstable nature of the cliff. The team was given an extremely tight weight limit and an ambitious architect’s design to realise. This featured 4 m-tall panes of glass running down the sides of a warren truss-style bridge.
Shanklin lift 2
Recognising the logistical implications of manoeuvring such heavy panes of glass, REIDsteel wanted to opt for a Vierendeel truss (without diagonals), as it would allow the glazing to be fitted from the inside.
Vierendeel trusses work without diagonals but are heavier than most other truss types, which on the sandstone cliff was a problem, so weight savings needed to be found. On every face of the bridge, the cladding needed to lose weight.
The original design featured walls that dropped below the floor to provide some protection from sea spray. The original REID design also featured a soffit sheet to keep the steelwork internal.
Both of these had to go, leaving the members and flooring exposed on the underside.
To reduce weight further and ensure the floor’s corrosion resistance, a lightweight GRP flooring panel from Fiberline Composites was used, bearing onto galvanised steel transoms.
Even with the weight so reduced, the new bridge was still very flexible when assessed using linear element analysis, so a nonlinear finite element analysis had to be employed to prove that the bridge would not suffer from harmonic response. This was achieved using LUSAS FE software.
“Despite the bridge’s relatively small size it has been a huge success in a situation that few other contractors would have felt comfortable operating in”
With the bridge now light enough for the cliff, the challenge lay in how to erect the structure using a relatively light mobile crane sat on a sandstone cliff. It was possible to get one piece of access equipment simultaneously on the clifftop, but its range of motion was greatly restricted. This meant that nearly all fixing had to be done from the inside of the lift shaft or from the cliff side.
By attaching a galvanised landing frame to the lift shaft before the bridge, it made it possible to fix the Vierendeel trusses top and bottom from within the lift shaft. The same result was achieved at the cliff end by a landing frame and temporary bracing. Next, transoms could be worked out from the cliff edge and fixed with oversized bolts to allow for corrosion in their exposed condition.
Good working relationship
Now the deck had to be laid without access to its underside and no fixings on the surface.
This was achieved with a Sikadur adhesive which was able to be applied entirely from above. Unlike steel fixings, the adhesive will not suffer from corrosion in the saline environment, and can handle all of the loads on the deck in only 5 per cent of its design capacity.
Once the deck and the temporary edge protection lifted out with the trusses had created a safe working platform, the roof could be easily installed, as all connections had been stubbed off the trusses to bring them inboard of the sides of the bridge.
“Because each lift was carefully ordered and fixing positions were moved into accessible positions, the project was completed with no damage to the cliff”
The roof was lifted on in two segments already largely pre-clad, which enabled all the remaining roof work to be carried out from the access equipment.
A running line system was installed beneath the roof, allowing up to five people to work safely within the bridge even when all the edge protection was removed. This allowed all the glazing panels to be put in place from within the bridge without any plant whatsoever.
The project was a classic example of how a good working relationship between erectors and designers can produce something safe to install in even the most difficult circumstances.
Because each lift was carefully ordered and fixing positions were moved into accessible positions, the project was completed with no damage to the cliff, no risk to persons and to a very high standard of quality.
The bridge may be small, but its installation represents a huge success in a very demanding situation.
John Harrison is a senior structural engineer at REIDsteel and headed up the design team on the Shanklin Cliff Lift Bridge