Harlech Castle footbridge, UK

Harlech Castle, in Snowdonia National Park, is one of the great castles built by King Edward I to enforce his rule over the Welsh. It is built on an outcrop of rock standing some 30m above Tremadog Bay. Construction began in 1283 and was completed in only seven years at a cost of £8,190.

To improve visitor access a spectacular ‘floating’ bridge has been installed, as part of a £6M project that is transforming the castle, which forms part of the Castles and Town Walls of King Edward in Gwynedd World Heritage Site. The work has been funded through the Heritage Tourism Project, part-funded by the European Regional Development Fund and the Welsh Government, and led by Cadw, the Welsh Government’s historic environment service.

The bridge links a new visitor centre in the refurbished Castle Hotel, opposite the castle, replacing long flights of timber steps. It occupies the position of the castle’s original drawbridge. Mott MacDonald developed the outline design of the new footbridge working closely with Cadw and other stakeholders from a set of early requirements. Katalin Andrasi, our project manager said: "Cadw had a clear vision for the footbridge and we listened very carefully. We've worked very closely and collaboratively with Cadw over months of design development to arrive at a truly magnificent structure."

The 46m footbridge is an underslung triangular Vierendeel truss tapering in elevation and braced diagonally in the deck’s plane. The three spans are angled relative to one another at approximately 166 and 143 degrees.

This structural form was chosen as it avoids the use of bulky beams and keeps the superstructure beneath deck level while minimising interference with the views of Snowdonia from the visitor centre car park. Constructed of structural steel, the bridge was built in three sections by SH Structures in Yorkshire. The deck is formed from hardwood Ekki timber planks spanning between the secondary steel angles.

The structure has four supports, all of them with moment connections to the foundations. At the castle end, the bridge is supported on a new rectangular hollow section frame erected in the original drawbridge pit behind the castle wall. The end transverse member of the deck joins this frame via two bolted connections. The visitor centre end is supported on a reinforced concrete abutment which forms part of the new visitor centre terrace.

The two intermediate supports are elliptical steel columns infilled with concrete, one of which is founded on a shallow reinforced concrete pad within the remains of the original outer tower whilst the other is supported on reinforced concrete piles at the end of the car park. Construction techniques had to be carefully considered to protect the historical fabric of the castle.

Cadw had a clear vision for the footbridge and we listened very carefully.

Katalin Andrasi

Project Manager at Mott MacDonald

Bearings are located at the abutment wall at the visitor centre end allowing rotation and sliding along the line of the deck but providing simple restraint in other directions. The support frame in the drawbridge pit and the elliptical columns flex under thermal and vertical loads to allow the structure to ‘breathe’. Expansion joints are located at each end of the bridge.

Piers were fabricated separately and lifted in to place prior to the superstructure arriving to site. Each column carried a section of the Vierendeel truss and the top and bottom chords of the deck structure were fixed to these members on site.

The Vierendeel truss tapers in elevation, ranging from 650mm at the two ends to 1500mm at the piers. The minimum width of the walkway is 2m which increases to 3m above the pier sat in the moat to form a viewing area.

Roles

• Footbridge structural engineer (feasibility, concept and outline design, technical advisor to Cadw) and footbridge architecture: Mott MacDonald
• Visitor Centre structural engineer: Mott MacDonald
• Footbridge superstructure fabricator: SH Structures
• Main contractor (all works): RL Davies and Sons Ltd
• Footbridge superstructure detailed designer: David Dexter Associates