Above the spacious reception hall are six partially staggered parking decks for a total of 850 vehicles. From almost any point on the upper decks, unique views open up across the old town and the surrounding countryside. This all-round vista is made possible by dispensing with a solid facade and by the use of a delicately proportioned load-bearing structure of elegant precast concrete units in conjunction with a vehicle impact barrier made from slender posts and prestressed steel cables.
Load-bearing structure
The objective of the design was to incorporate a railway station hall and shops into an unclad concrete car parking garage. The structural concrete components also form the ceiling inside the shops and the reception hall. The multi-storey parking garage extending along a residential street was designed to allow the penetration of daylight through its volume.
The designers strove to achieve the thinnest possible slab edge while assuring the highest quality surface finish. The confluence of these objectives quickly led to a prefabricated solution, with high-density concrete reaching strengths of at least 70 MPa. Standardisation allowed the design of elaborated shapes and profiles, the cost being absorbed by the large quantity of modular elements. To avoid the appearance of a structure completely assembled from individual parts, in situ concrete was used to connect separate components together to give the appearance of a monolithic structure.
The structural grid measures 7.80 × 9.90 m. The floors extend beyond the perimeter column line with a cantilever of 3.80 m. Hollowcore floor slab units, efficient and cost effective, are used for the middle bays spanning 9.90 m. These, in turn, are surrounded by shallow in situ concrete beams aligned with columns every 7.80 m. The in situ beams create belts connecting the columns together and increase earthquake resistance.
The edges of the building are constructed of 3.80 × 7.80 m precast concrete slab panels tapering in depth from 55 to 11 cm. These elements are shaped to incorporate the trapezoidal section of the longitudinal beams at the leading edge of the element and support the hollowcore units before they are concreted in. Incorporated into each element are ribs designed to connect to the columns and a thickened edge that forms the joint between elements. This area allows the integration of a prestressing cable, which prevents micro cracking and eliminates long-term deformations. The in situ concrete reinforcement continues into the 5 cm structural in situ concrete screed cast on top of the hollowcore units to transfer the positive bending moments into the vertical supports.
40 cm diameter columns, originally designed as prefabricated concrete components, were changed to in situ concrete elements to simplify the construction. Some columns include stainless steel drainage pipes connected to floor gutters. The exterior safety stairs are isolated from fire by the concrete walls used to increase horizontal stiffness. Steel K bracings with their ends cast into the concrete floors provide stiffness in the vertical plane.
Access ramp
A semi-circular helicoid ramp connects all the levels. The ramp is supported by a two-dimensional grid formed by inclined steel posts. These posts connect the upper and lower floors as diagonal truss members. A cable vehicular impact barrier is fixed to these posts. The posts provide structural redundancy by preventing the damage or collapse of the entire construction in the event of a severe vehicular impact.
Vehicular impact protection
The vehicle impact barrier is provided by two 15 mm cables, tensioned between steel posts at 7.80 m centres bolted down into the prefabricated edge ribs. The cable deformation absorbs the kinetic energy of the vehicle, which reduces the force applied on the structure by a factor of five. The steel post bases are constructed from a single folded 12 mm plate, without welds, improving resistance to metal fatigue under the daily random loading due to vehicles touching the cables. Three 10 mm diameter cables are added to support stainless steel wire netting, transforming the car barrier into a pedestrian railing. This system, entirely designed by our team was used for the first time in France, and has been tested under the supervision of Centre Technique et Industriel de la Construction Métallique (CTICM).
