Fire safety engineering for Rue de Hollerich mixed-use building

Detailed information

Fire engineering approach

This residential building project located at 17 Rue de Hollerich in Luxembourg City is the first multi-functional building in the Grand Duchy of Luxembourg designed by the Fonds du Logement to have a metal structure. The choice of a metal structure was dictated by the intrinsic qualities of steel: flexibility, strength, durability, aesthetics, infinite recyclability, and competitiveness.  Furthermore, the use of a metal structural frame enables a reduction in the duration of work on site, an important factor for a site located in an urban area. Steel makes it possible to fulfil the wishes of the architect and blends easily with other materials. Moreover, steel offers an extensive range of fire-resistant solutions that enable compliance with safety requirements both aesthetically and economically.

In particular, this building illustrates various solutions with regard to fire resistance: partially encased columns and beams, incorporation of columns into the walls, visible steel columns, slim floor system, and elements protected by intumescent paint. These solutions, developed by R&D, were implemented in this building with the collaboration of R&D, the technical advisory team (TECOM), Eurostructures - who completed the entire frame - and BEST, the Luxembourg consulting engineers.

The Rue de Hollerich building was an opportunity for the BEST consultancy to learn the techniques of fire engineering, supported by R&D in discussions with the architect and the Luxembourg City fire service, and finally to incorporate fire engineering in its offer to its clients. Following this collaboration, this consultancy joined the SecureWithSteel network. This network comprises consulting engineers who are partners of ArcelorMittal (two or three practices per country) who have expertise in the field of fire engineering and use it to develop economic fire-resistant steel solutions.  It constitutes one of the avenues for the transfer of know-how from R&D to consulting engineers.

The car park

The structure comprises steel sections partially encased between the flanges. This system (the AF system developed and patented by ARBED in the 1980s) is in widespread use. It makes it possible to attain the two-hour fire resistance required for underground car parks, which are dangerous places for firefighters who often have difficulty in locating and reaching the burning car or cars in the event of a fire. The beams, with a 16 m span, are HEA500 connected to the concrete slab by means of studs welded onto the upper flange. The columns are HEA450 for the bottom level and the section is reduced to HEB300 on the first car park level. The steel solution and its reduced weight enabled optimisation of the use of cranes and their position on the site located in an urban area.

The commercial premises

To ensure that the steel structure is completely and directly visible when approaching the building, the rows of columns on the ground floor along the front and rear facades consist of steel alone. 

At the rear, the columns are located on the exterior and were designed using a specific method in the Eurocode (Annex B to EN 1993-1-2 dedicated to external structures). 

At the front, the fire resistance of the columns was achieved by oversizing (HEM220 instead of HEB220) and was substantiated by means of a fire engineering study that determines the actual rise in temperature of the section subjected to the various possible fire scenarios (localised fire, flashover). In this way, the passer-by on the street sees the steel sections located behind the glass facade directly and immediately takes on board the steel character of the structure.

The apartments

AF columns were used in all the apartments. For the floors, the slim floor system (see diagram opposite) made it possible to provide a structure without downstand beams and to satisfy the architect, who wanted a level ceiling surface in the apartments. 

These columns and beams were optimised for fire using SAFIR fire-resistant structure design software and the natural fire approach using OZONE software. As the majority of the columns are totally or partially incorporated into the walls, this beneficial protective effect was also taken into account in the design calculations, which has resulted in very slender columns (HEB220).

The interior corridor

On the second floor, the first level of apartments, an interior walkway located on the facade enables residents to make their way to the peripheral lift shafts and to leave the building without passing through the shops. 

Given the low fire load of this interior walkway and its separation from the apartments by fire-resistant walls, doors, and windows, the steel columns are also visible further reinforcing the steel structure character of this building.

Overall stability

Horizontal stability is provided by a combination of concrete walls and windbracing panels.

In general, there are at least three windbracing panels per storey, which are located in different fire compartments. These braces were designed ignoring, in the event of fire, the windbracing located in the compartment containing the fire. In this way, the horizontal loads are taken up by the other windbracing located in compartments that are protected from the fire. Thus the metal latticework is also bare steel without the addition of passive protection material.

In a few places, this approach could not be followed, given the small number of windbracing panels.  These braces were protected with intumescent paint. This type of paint, which looks like conventional paint but is slightly thicker, swells if heated and in the event of fire forms a foam several centimetres thick that greatly slows the heating of the steel elements.

In short, steel is the material par excellence for construction. It is consistent with the execution of both prestigious buildings such as the Chamber of Commerce and cost-sensitive residential buildings such as the one in Rue de Hollerich. Moreover, the fire-resistance of steel buildings can be ensured in an economic and flexible manner, enabling compliance with safety, aesthetic, construction, and cost requirements.