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Kons Building: innovative steel-composite solutions for a complex inner city location
The combination of ArcelorMittal’s composite floor decking Cofraplus® 220 and SFB Slim floor beams proved to be the ideal structural solution for the complex requirements of the mixed-use property KONS in Luxembourg City. A tight time frame, the need for a reduced building weight and height, difficult site conditions and limitations resulting from the local regulations presented challenges for the planning and construction processes.
Initial situation & planning
The building commonly known as “Galerie Kons” was originally built as a hotel in the 1930ies, 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 by a new construction was decided in 2013, since the building’s space concept (with large halls, mezzanines, etc.) did no longer correspond to the modern requirements of the real estate market.
The project’s time schedule was very tight, only 2,5 years were foreseen for the demolition of the existing and the construction of the new building: planning started in spring 2013, demolition started in May 2014 and took a year. The new building was completed at the end of 2016.
The close collaboration of investor, architects, engineers, general contractor and steel fabricator, inspired by the Dutch “Bouwteam” approach, and the comprehensive, integrated planning process were the key to the success to meet the ambitious specifications of the investor, fulfilling at the same time the requirements of the local authorities.
The new building: mixed-use, modern & sustainable
The new building has up-to 7 storeys above ground and up to 5 underground levels.
With a base area of 4.400 m2, it comprises 14.600 m2 office space, 2.400 m2 commercial space on the ground floor, 8.300 m2 underground car park and 3.500 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 the BREEAM Excellent certification, the tight schedule and the inner-city construction site with difficult access and limited storage space, a number of other technical and legal limitations created challenges that could be overcome thanks to innovative construction products and processes, careful planning and close collaboration of all the parties involved.
Following measures were taken to comply with the project’s requirements:
- The use of lightweight structural solutions reduces overall loads and avoids a costly reinforcement of the existing underground levels -5, -4 and -3,
- A recessed top floor allows to maintain the limited façade 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 comfort, technical installation and acoustic requirements of R(W)= 53dB,
- Flooring height was reduced and optimized to meet fire safety requirements REI 90 and integration of technical installations,
- A column grid of 5,4 m allows flexible, moderns office spaces,
- The ventilated, high quality curtain wall façade 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 the site limitation (limited storage space, noise, heavy traffic, road closure only occasionally on weekends).
Flooring structure: steel composite vs concrete
The flooring structure for the whole building was originally calculated as a 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) and 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, the office and commercial spaces.
ArcelorMittal composite flooring solution for offices: integrated SFB Beams & Cofraplus® 220
The solution is based on the use 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 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, 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
Due to the rib-shape of the flooring profiles, the weight of the whole flooring structure could be reduced (<360kg/m2), which resulted in smaller structural elements and therefore in overall weight and material savings, influencing positively the construction logistics as well as the BREEAM assessment.
Although the height of 32 cm (+15 mm of SFBs lower flange plate) does not differ a lot from the concrete flooring system, this steel-composite solution presents a major advantage regarding the technical installations and supply lines. These can be placed inside the steel profile ribs which enables to guarantee 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 could be achieved to comply with the requirements of the real estate market and the future main tenant of the building.
Furthermore, SFBs allow 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 could be achieved: Up to 30 steel profiles can be stacked to a package and this way one lorry can transport 1.500 m2 of flooring profiles.
Due 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 on 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 400m2 per team and day). Once assembled and bolted, the steel profiles act as working platforms and formwork, no additional supports are necessary.
This steel-composite flooring solution was used for all office spaces. The parking area in the underground levels was realized 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 the specific acoustic requirements, the flooring system in the residential part was also built with in-situ concrete.
- Luxembourg City
- Engineering Office:
Schroeder & Associés Ingènieurs -conseils
- Steel fabricator & assembly:
Stahbau Ziemann GmbH
- Assembly flooring deck:
ME-DA-WA System & Elementbau
- General Contractor:
CLE, Westside Village & LUX TP S.A.
AXA Real Estate
- Project Development:
PEF KONS Investment S.A. Luxembourg,
BPI Luxembourg S.A. Strasse,
Besix Red – Real Estate Development, Luxembourg
©ArcelorMittal Europe/C. Radermacher
C. Radermacher, M. Braun, Dr. M. Schäfer, R. Hettinger & Constructalia