Enovos Car Park
At first glance the Enovos Car Park seems to be a classical, ventilated steel car park. But at closer observation a special open and transparent aesthetics becomes apparent, achieved thanks to the use of the new flooring system Cofraplus 220 in combination with AngelinaTM beams. Furthermore, the car park convinces with various technical performance features like reduced weight of the beams, precise precambering and crack width control, which could be easily accomplished with Cofraplus 220.
In course of the construction of the new headquarters of Luxembourg's energy provider Enovos on a former industrial area in Esch-sur-Alzette in the south of the country, a ventilated car park in steel composite structure was planned. The multistorey car park, designed in split-level layout, offers around 400 parking spaces arranged on 5 levels and is accessible through 3 exterior concrete stairwell cores.
In order to avoid constraining forces, e.g. due to temperature stress, the staircase cores were arranged outside of the main steel structure. With floor plan dimensions of 32m x60m and a typical grid size of 15,75m x 5m, the car park generally complies with established concepts and benefits from the flexible design possibilities of steel composite construction.
Since the re-design of the area was not finished when the construction of the project started, 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 the beam distance of 5m, the total number of pillars could be reduced significantly.
In an early planning stage an additional entrance to the car park on level 1 was forseen at the south end. Therefore, this option was considered in the organization of the inner ramps from half level to half level. In this area, an old high voltage power line is crossing the construction site reducing the final building height to only 3 levels. But since this line will be put out of service and dismantled in the near future, a 4th and 5th level can easily be added afterwards thanks to the steel structure.
New flooring system and cellular beams with sinusoidal openings
The innovation in this car park project lies in the use of the new flooring system Cofraplus 220® in combination with AngelinaTM beams. The distance of the web openings of the beams are adjusted to the cover width of the flooring deck to create a slender structure with new aesthetics. In contrast to traditional floor beams, the large, sinusoidal openings allow a visual connection throughout 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 of the limited height of only 2,20 m (lower end of the beam) which improves the users' perception of comfort and security.
Reduced steel tonnage and accurate cambering
The fabrication of AngelinaTM 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. But in the case of the AngelinaTM 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 ca. 1,5 times its original girder height. Furthermore, material loss is reduced to the cutting of the beam ends, which makes AngelinaTM a highly optimized castellated beam.
The AngelinaTM 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, the distance of 750mm between its ribs as well as the structural design requirements and the constructive height of the entire floor. The final beam height of 575mm 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 210mm which, after pouring the concrete, was reduced to 110mm.
Wide 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 220mm 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 1,0mm, 1,13mm or 1,25mm and the height of the concrete, spans of more than 5,50m can be covered without additional propping. Spans of up to 10m can be achieved using a specially designed support system. The Enovos Car Park required no props for the spans of 5m. The height of the floor above the ribs was 80mm, although according to the approval an offset of 20mm had to be considered due to the use of the „wing 20“, resulting in a total concrete height of 100mm over the beam.
For assembly the profiles were inserted into these so-called wings of the beams and subsequently fixed with selfdrilling shear studs and bolted from below in the longitudinal overlap at the bottom of the ribs. This type of support allows to achieve 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 the profile's support the connectors can be freely arranged. The wings are simple formed parts made of 3,00mm steel plates in conventional S235JR steel grade, that were welded to the beam in course of 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 galvanized as common in car park construction.
The load bearing characteristics of the continuous flooring system, which despite a concrete cover of Cnom = 50 mm does not require additional stainless steel reinforcements, convinces with its optimized crack behaviour, especially in the area of the hogging moment above the beams. Cofraplus 220's load bearing characteristics are similar to these 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 0,1mm. These additional requirements could be realized by simple reinforcement above the beams, amongst others thanks to the continuous effect of the flooring system. This way, also after shot blasting of the concrete surface in preparation fo 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 16mm and 400kg/m3 cement without further additives. Superplasticizers provided the concrete with the necessary consistency to pump it through the 50 m pipe.
The Enovos Car Park has already been opened although the façade has not yet been completely assembled. Thanks to the steel structure, the client can make use of the building even before it is completely finished.
Text: Dipl.-Ing./MBA Christoph Radermacher, ArcelorMittal Construction, and Dipl.-Ing. Marc May, ArcelorMittal Europe – Long Products