Innovative steel solutions for the Enovos car park in Luxembourg

Detailed information

A ventilated car park with a steel-composite structure was planned for the construction of the new headquarters of Enovos, Luxembourg's energy provider, on a former industrial area in Esch-sur-Alzette in the south of the country. This multi-storey car park, designed with a split-level layout, offers around 400 parking spaces on five levels and is accessible through three exterior concrete stairwell cores.

In order to avoid constraining forces - temperature stress for example - the staircase cores were arranged outside of the main steel structure. With floor plan dimensions of 32 m x 60 m and a typical grid size of 15.75 m x 5 m, the car park complies with established concepts and benefits from the flexible design possibilities of steel-composite construction.

As the re-design of the area was not finished when construction of the project began, many boundary conditions and future options had to be considered in the planning process. The construction site required a pile foundation due to the difficult soil conditions. Thanks to a beam distance of 5 m, the total number of pillars could be significantly reduced.

At an early planning stage, an additional entrance to the car park on level 1 was foreseen at the south end. This option was considered in the organisation of the inner ramps from half level to half level.

In the area of the project, an old high-voltage power line crosses the construction site reducing the final building height to only 3 levels. However, this line will be put out of service and dismantled in the near future, so a fourth and fifth level can easily be added afterwards thanks to the steel structure.

New flooring system and cellular beams with sinusoidal openings

The innovation of this car park project lies in the use of the new flooring system Cofraplus 220® in combination with Angelina® beams. The distance of the web openings of the beams were adjusted to the cover the width of the flooring deck in order to create an aesthetically pleasing slender structure. In contrast to traditional floor beams, the large, sinusoidal openings allow a visual connection throughout the various floor slabs.

The transparency gained in the floor beams and the reduced shadows grant the car park a very open and bright interior space, despite the limited height of 2.2 m (lower end of the beam) which improves the users' perception of comfort and security.

Reduced steel tonnage and accurate cambering

The fabrication of Angelina® beams is similar to that of traditional castellated and cellular beams by cutting a hot-rolled steel section along its web and subsequent welding of the two resulting T sections in their web post. In the case of the Angelina® beam, the cut follows a continuous sinusoidal line that is adapted individually to the requirements of the project. In this process, the resulting height of the beams can be augmented to approximately 1.5 times its original girder height. Furthermore, material loss is reduced by the cutting of the beam ends, which makes Angelina® a highly optimised castellated beam.

The Angelina® beams used in the Enovos project are based on IPEO 400 in S460M grade for the longer spans and IPEA 330 in S460M for the shorter beams connected to the ramp columns. The cutting line, which ultimately decides the geometry of the beam's openings and its new height, considers the flooring profile's width, a distance of 750 mm between its ribs, as well as the structural design requirements and the constructive height of the entire floor. The final beam height of 575 mm can be compared with the load bearing capacity of an IPE 550. The legally stipulated drainage on both sides required, in addition, a cambering of 210 mm which, after pouring the concrete, was reduced to 110 mm.

Long spans and low crack widths

The technically approved flooring system Cofraplus® 220 (German Technical Approval Z-26.1-55) is based on the traditional additive design concept. According to this concept, after its assembly the 220 mm profile provides a working platform between the beams and takes the loads of the reinforcements and the concrete during construction.

Depending on its thickness of 1.0 mm, 1.13 mm, or 1.25 mm and the height of the concrete, spans of more than 5.5 m can be covered without additional propping. Spans of up to 10 m can be achieved using a specially designed support system. The Enovos car park required no props for the spans of 5 m. The height of the floor above the ribs is 80 mm after authorisation despite the Zulassung technique and the utilisation of 'Wing 20' that normally results in a height of 100 mm.
For assembly, the profiles were inserted into these so-called wings of the beams and subsequently fixed with self-drilling shear studs and bolted from below in the longitudinal overlap at the bottom of the ribs. This type of support allows for a continuous slab across the beam. The concrete ribs are in direct contact with the beam web, which significantly increases the negative moment capacity at this point. Due to this type of profile support, the connectors can be freely arranged. The wings are simple formed parts made of 3 mm steel plates in conventional S235JR steel grade that were welded to the beam during the finishing process at the workshop and exposed to the same corrosion protection as the beam itself. The whole structure of the Enovos car park was hot-dip galvanised, as is common in car park construction.

The load bearing characteristics of the continuous flooring system that, despite a concrete cover of  Cnom = 50 mm, does not require additional stainless steel reinforcements, convinces with its optimised crack behaviour - especially in the area of the hogging moment above the beams. Cofraplus® 220's load bearing characteristics are similar to those of a continuous ribbed slab. In non-cracked state, the concrete presents approximately the same stiffness in the floor slab area and above the supports.

Regarding durability, it was the client's wish to limit the calculated average crack width above the beams to 0.1 mm. These additional requirements were realised with simple reinforcement above the beams thanks to the continuous effect of the flooring system. This way, after the shot blasting of the concrete surface in preparation for the OS8 coating, no cracks appeared above the beams and no post treatment (sealing) was necessary.

The concrete composition for the strength class C35/45 included a maximum grain size of 16 mm and 400 kg/m3 cement without further additives. Superplasticizers provided the concrete with the necessary consistency to pump it through the 50 m pipe.