Overlooking Cardiff Bay and the Bristol Channel, the National Assembly for Wales building embraces the desire for open and transparent government. Arup provided structural, civil, transportation, facade, geotechnical, wind and lift consultancy to this unique project.
The assembly building was designed to be an open and inviting public space, as well as providing accommodation and secure private areas for members of the new assembly, or Sennedd. Two of its three levels are open to the public, with a café and galleries overlooking three committee rooms and the concentric debating chamber of the 60 Assembly Members.
The building's striking sculpted roof was inspired by the natural flow of forces within a folding plate structure. By refining its unique and complex geometry, it was fabricated and constructed simply, without compromising the unusual design.
The assembly building also exemplifies sustainable design, incorporating durable finishes that will last throughout its 120-year design life. Arup's work was innovative, pragmatic and cost-effective. It included single-glazed blast-resistant facades, an exposed in situ reinforced concrete frame and slate walling and paving, constructed from the plentiful source of the North Wales roof slate industry.
Designing the undulating roof
The horizontal planes of the assembly building's design define and embody the concepts behind the Senedd. The slate plinth anchors and engages the public realm and waterfront, inviting and drawing people in. Over this, the undulating roof plane frames and articulates the extent of the building. It also folds and turns in on itself, holding in tension the funnel-like form that marks the building's centrepiece, the debating chamber.
The roof structure was designed to flow over a series of slender point supports, spanning some 26m. It cantilevers beyond the supports by a further 8m to provide essential shading for the glass facades. The unique structure was conceived as an undulating plate that developed inherent out-of-plane stiffness through folding.
The design team sculpted the roof form using the structural design software program Rhinoceros (Rhino), refining its profile to improve structural performance, repetition, buildability and visual impact. The final geometry had to ensure that the robust, yet delicate, form was preserved while its elemental framework was easily fabricated, transported and erected.
Using the Rhino model, the steel work contractor developed the Arup-X steelwork model to incorporate each nut, bolt, splice plate, end plate and stiffener, ensuring that the roof could be erected easily on site.