Kons Building: Innovative steel-composite solutions for a complex inner city location

Detailed information

Initial situation & planning

The building, known as Galerie Kons, was originally built as a hotel in the 1930s, expanded after WWII, and substantially transformed into a shopping centre in the 1990s including office spaces and underground car park levels.

Its demolition and replacement with a new construction was decided in 2013 as the building’s space concept (with large halls, mezzanines, etc.) no longer corresponded to the modern requirements of the real estate market.

The project’s time schedule was very tight - only two and a half years were foreseen for the demolition of the existing building and the construction of the new building. Planning started in spring 2013, while demolition started in May 2014 and took one year. The new building was completed at the end of 2016.

The close collaboration between the investor, architects, engineers, general contractor, and steel fabricator, inspired by the Dutch Bouwteam approach, and the comprehensive, integrated planning process were the keys to the success of meeting the ambitious specifications of the investor. At the same time, the requirements of the local authorities were met.

The new building: mixed-use, modern, & sustainable

The new building has up to seven storeys above ground and up to five underground levels.

With a base area of 4400 m2, it comprises 14 600 m2 of office space, 2400 m2 of commercial space on the ground floor, 8300 m2 of underground car park, and 3500 m2 dedicated to residential use at the back area of the plot.

The requirements of the design and construction of the new building were complex. Apart from the sustainability criteria to achieve BREEAM Excellent certification, the tight schedule and the inner-city construction site with difficult access and limited storage space, and a number of other technical and legal limitations created challenges that were overcome thanks to innovative construction products and processes, careful planning, and close collaboration among the parties involved.

The following measures were taken to meet the project’s requirements:

• The use of lightweight structural solutions reduced overall loads and avoided the costly reinforcement of the existing underground levels -5, -4, and -3.
• A recessed top floor ensured the maintaining of the limited facade height established in the local construction regulations.
• Despite the limitation of the building height, the clear room height could be set to 2.7 m to guarantee comfortable technical installation and acoustic requirements of R(W)= 53dB.
• Flooring height was reduced and optimised to meet fire safety requirements REI 90 and the integration of technical installations.
• A column grid of 5.4 m allows for flexible, moderns office spaces.
• The ventilated, high-quality curtain wall facade with natural stone cladding complies with material, acoustic, and local construction regulations to maintain uniformity at Place de la Gare.
• The construction process had to be thoroughly planned due to site limitation (limited storage space, noise, heavy traffic, road closure occasionally on weekends).

Flooring structure: steel composite vs concrete

The flooring structure for the whole building was originally calculated as pre-fabricated or in-situ concrete floor slabs on concrete floor beams and columns. Although weight limitation could be respected in this design, space for technical installations could not be guaranteed (in particular for office and commercial areas). In view of the limited space at the building site and difficult access, unloading and storage of the long, prefabricated flooring slabs would have created complications.

ArcelorMittal’s technical assistance team presented an alternative solution, calculated in steel composite design and proposing composite steel beams integrated into the floor system. Given the circumstances, this design was chosen for the main (above ground) part of the building and the office and commercial spaces.

ArcelorMittal composite flooring solution for offices: integrated SFB beams & Cofraplus® 220

The solution is based on the use of Slim floor beams (SFBs) - rolled H sections with a plate welded to their bottom flange to support the composite flooring profiles and shear connectors on the upper flange in order to ensure the composite action of the steel beam with the concrete floor.

As for the floor decking, Cofraplus® 220 profiles allow for a larger beam to beam distance. The profiles are based on a steel thickness of tN=1.25 mm, providing a height of 220 mm and a rib distance of 750 mm. With a span of over 5.40 m, they are assembled transversally to the beams and complemented by a top concrete layer of 10 cm. Thus, the system complies with all structural, acoustical, and fire safety related regulations.

This composite slim floor solution presents many advantages, both for the overall building design and construction site management.

Structural advantages: reduced flooring weight & height, increased spans

Thanks to the rib-shape of the flooring profiles, the weight of the whole flooring structure could be reduced (<360 kg/m2), which resulted in smaller structural elements and therefore in overall weight and material savings, positively influencing the construction logistics as well as the BREEAM assessment.

Although the height of 32 cm (+15 mm of SFB lower flange plate) does not differ a lot from the concrete flooring system, this steel-composite solution presents a major advantage regarding technical installations and supply lines. These can be placed inside the steel profile ribs which enables guaranteeing an overall floor construction height of 90 cm including 15 cm for the raised floor. Thanks to this slender construction, a comfortable clear room height of 2.7 m can be achieved to comply with the requirements of the real estate market and the future main tenant of the building.

Furthermore, SFBs allow for a larger column distance (5.4 m), which makes it possible to realise flexible office spaces.

Advantages at the construction site: reduced transport & storage, easy installation

Compared to the initially planned concrete flooring solution, a significant reduction of transport to the construction site was achieved: Up to 30 steel profiles can be stacked to a package allowing one lorry to transport 1500 m2 of flooring profiles.

Thanks to the limited size of the steel elements, they do not require a lot of on-site storage space, which is ideal for inner-city building sites.

The SFBs were prepared in the workshop and delivered to the site ready to install, so there was no need for further on-site fabrication processes.

Thanks to the low weight of the steel profiles (16 kg/m2), they can be installed manually and quickly without the need of cranes (up to 400 m2 per team, per day). Once assembled and bolted, the steel profiles act as working platforms and formwork and no additional supports are necessary.

This steel-composite flooring solution was used for all of the office spaces. The parking area in the underground levels was realised with in-situ concrete and reinforced where necessary with HEB 650 using S460 grade. Due to the irregular grid of the load-bearing walls and specific acoustic requirements, the flooring system in the residential part was also built with in-situ concrete.