Central Plaza

One Central Plaza is an iconic landmark in the heart of Dublin, previously known as the Central Bank building and designed by brutalist architect Sam Stephenson in the late 1960’s. The concrete colossus has a strong presence and can be seen from multiple points throughout the city. 

The building is transformed by Hines in partnership with the Peterson Group, creating a dynamic new destination called Central Plaza, bringing together offices, retail and restaurants. 

Octatube engineered and built One Central Plaza’s two-storey glass-roofed structure to top off this city-center icon. Pursuing transparency, most of the roof as well as the façade is made of glass. The rooftop venue, with unrivalled 360-degree views of the city, is the crowning touch of this restoration project.

The roof was reimagined by architects Henry J. Lyons. The design, a contemporary geometric structure, follows the exposed suspensions of the building – with cantilevering triangles as a subtle nod to the original design of Stephenson. We built the roof on the existing roof structure which is special and challenging in terms of design and structural engineering.

The structure
The new steel rooftop structure measures 42.5 x 27.5 x 6 meters and weighs more than 200 tons. It consists of multiple steel RHS beams (400x200 mm and 200x100 mm) that are connected to each other by hidden bolted connections. The structure is a monolithic steel frame without expansion joints. Quite the challenge for our structural engineers in regard to thermal expansion. How can we make sure the roof structure is able to cope with variations in temperature and the coexisting deformations? 

Our solution was to apply fully glazed corners and disconnect the façade from the roof. If the temperature increases or decreases, and thermic expansion occurs, the roof can slide over the façade. The roof can expand and retract respectively 30 and 15 millimeters. We did meticulous detailing of the façade to roof connection to allow this movement. 

The glass
The steel roof construction is covered with 152 triangular, quadrangular or pentagonal glass units - celebrating lightness and transparency. The space offers optimal views. In order to achieve this we made the structure as sleek and clean as possible, facilitating maximum transparancy. It forms a counterpart to its brutalist carrier.

At the same time we have regulated the sun load. The glass is be screen-printed with a ceramic frit. It doesn’t feel like a greenhouse inside.

Challenges
The Central Plaza building is very unique; the structure comprises suspended floor slabs held up by perimeter Macalloy tension rods, that are supported back to the central core at roof level. A Brutalist characteristic: exposing the building’s structure.

Photograph by Infomatique (Wikimedia Commons)

Live loading on the floors (i.e. people moving around) results in elongation of the tension rods. Thus, the outer perimeter of the 10th floor has relatively large deformations. To reduce the impact on the roof structure, additional compact bearing rubbers are applied at the anchors. When the floor moves downward due to live loading on the lower floors, the influence on the column is negated, which results in the reduction of loads on the roofs.

Mock-Up
We put much attention into detailing, in close collaboration with the client Hines, the architect Henry J. Lyons and the contractor Stewart Construction, ensuring what has been imagined can be delivered.

For Central Plaza we produced a mock-up, a scale model of a fragment of the roof in which different elements come together. It’s a method of testing that we like to incorporate in the design and engineering process. Most designs have not been realized before and the mock-up offers the possibility to check the design for efficiency. It gives an idea of the quality that can be expected in the new building. After the mock-up, small and smart alterations can be made before proceeding with the total production and assembly. 

Assembly and logistics
We did the steel-production in our factory in Delft. The construction consists of more than 4000 parts such as bespoke columns, roof parts and (corner) beams.

The geometry of the building determined that the roof had to be build up from separate elements rather than larger weldments. This way, all elements got the best conservation and could be efficiently shipped to site. The steel structure and elements are connected to the main structure via different types of consoles and custom brackets. The elements were transported by truck from Delft to Dublin.

Standing on top of Central Plaza is a remarkable experience, taking in the abundance of light and vistas of Dublin. A revaluation of urban heritage and a breathe of new life in the heart of Dublin.