Tour D2: optimised steel structure featuring innovative ArcelorMittal solutions

Detailed information

With a height of 171 m and 37 floors, the Tour D2 is Paris' newest landmark and offers a surface area of more than 54 000 m2 for office spaces. The emblematic office building, created by the French architect Anthony Béchu and his American colleague Tom Sheehan, is part of an extensive urban renewal project and is located on the former site of the Veritas Building which was demolished in 2011.

The Tour D2's characteristic oval shape and unique facade design defined by an external diagrid structure is reminiscent of a diamond that sparkles in the sunlight. On its top floor, an open roof garden offers a 500 m2 green oasis in the heart of the business centre. Protected from the winds with 2 m high glass windows, the so called 'cloud garden' also comprises a small golf course. The external diagrid continues around the garden, and an impressive, open steel dome structure crowns the tower's top.

Steel structure

Due to its design, the choice of the material for the D2's structure was clear from the beginning: steel was chosen for its flexibility and its advantages as far as sustainability is concerned. Thanks to the steel structure, a material reduction of around 30% could be achieved and speedy construction was guaranteed. Furthermore, floor spaces could be optimised and flexible, while modern working spaces
were created. All these aspects contribute positively to the building's environmental footprint.

The characteristic external steel structure, which carries half of the horizontal and vertical loads, is connected to a central concrete core through cellular floor beams. Thanks to the use of a composite flooring system and the long spans of the cellular floor beams, the total usable floor area could be increased. Both structural steel and composite flooring deck for this project were supplied by ArcelorMittal

ArcelorMittal steel

The facade diagrid consists of large, hot rolled steel sections, mainly HD 400, in high yield steel grade HISTAR® 460 MPa. These sections of 12 m length were assembled on site in shape of a V, each weighing around 14 tonnes. Due to the tower's oval shape, the radius of the curvature varies and the inclination angles measure between 3º and 15º. Each V, protected with intumescent coating and enveloped in aluminium casing, covers the height of three floors and is integrated into the curtain wall of different sized and shaped glazed elements.

ArcelorMittal Commercial Sections supplied a total of 4000 tn of structural steel, 3000tn of HISTAR® 460 sections for the external diagrid, and 1200 tn of ACB® beams. With their favourable strength-weight ratio, these cellular floor beams based on IPE 450 and HEA 450 sections allow longer spans at a lighter weight than conventional steel beams and guarantee open, column free spaces. Additionally, the ACB® beams' circular openings allow for the passage of technical installations, thus gaining usable floor height.

The ACB® floor beams connect the external structure to the concrete core and support the Cofraplus® 60 flooring deck. Supplied by ArcelorMittal Construction France, Cofraplus® 60 is composite flooring system that was chosen as well for its light weight and high strength. The solution combines the performance of a galvanised steel profile with that of reinforced concrete. Furthermore, Cofraplus® 60 is easy to install which presented a significant advantage on the construction site in La Défense's tightly knit urban environment. In total, 50 000 m2 Cofraplus® 60 were used for the flooring of the Tour D2. The entire flooring structure was protected against fire with fireproofing plaster.

Construction could be carried out at a steady rate of three floors in three weeks, according to time schedule and budget. The reduction of weight and material of 30% was achieved thanks to the massive use of steel in general and high yield steel in particular.

Sustainability

Sustainability also plays a crucial role in the tower's operational phase. All technical installations comply with the highest criteria of energy efficiency and the Tour D2 received the French certification Haute Qualité Environnementale (HQE®) and BREEAM.

Apart from office spaces, the Tour D2 offers a fitness centre, a panorama restaurant, a cafeteria, and meeting rooms with the aim of creating a comfortable, high-quality working environment.