Structures, facades, and fire safety in multi-storey car parks
The use of high strength steel for columns permits a reduction of their size and cladding has to ensure optimum ventilation, an essential fact when calculating fire safety. Natural light is a key element for the users' comfort and perception of space but also crucial for the building's energy consumption. A car park is subjected to many functional requirements, but at the same time architectural aesthetics are becoming more and more important.
Structures
Car park columns may consist of hot-rolled "H" or tubular profile steel. The combined solution, with concrete between the H profile projections or as a filler in tubular instructions, may be adopted depending on the case. In principle, concrete between the projections does not contribute to the mechanical stability of the column, but provides heat protection at the core of the H.
In order to optimise the size of the pillars, it is recommendable to use grade S355 or S460. Columns are laid out every 5, 7.5, or 10 metres (every 2, 3, or 4 parking spaces respectively) and placed along the perimeter of the facades or at the back of the rows of stalls in a comb layout. The columns, which run continuously from the ground to the top of the car park, are anchored at the base and secured to the horizontal fixtures, in line with each floor, by a system of braces. This bracing action is also provided in the stairwells and lift shafts by reinforced concrete. The primary and secondary beams are hinged at the ends: secondary beams are type IPE, IPEA, or HEAA, with a span of 7.5 m to 16 m and spaced 2.5 m to 5 m apart.
Primary beams are typically produced from the ranges IPE400 and HEAA650, with spans of 5 m to 10 m and are generally oversized to guarantee fire resistance without protection. Beams and columns are subjected to galvanisation. Decks, connected with the beams to provide combined strength, may be of different types: combined load-bearing systems with fretted metal sheeting, beam-sheet connectors, structural reinforcement, and concrete casting. It is preferable to design the thickness and dimensions of the sheeting in such a way that it does not necessitate the use of casting supports. The capacity of the load-bearing sheet may be 3.3 m to 3.5 m (without supports) or 5 m depending on the layout of the columns and the primary and secondary beams. The intrados of the deck consist of galvanised or pre-coated steel. Another possible type of decking is pre-stressed slabs, adapted for 5 m loads. These may be prefabricated ready-to-use elements that do not require a final pavement coating. However, paving normally consists of a protective layer of resin, which is lighter than asphalt.
The weight of the steel structure, including access ramps, is typically between 38 and 45 kg/m2.
Facades
The above-ground steel open-plan multi-storey car park, thanks to the versatility of its metal structure, allows different options in terms of the facades, provided it offers a guaranteed free flow of air.
Ventilation must be ensured over at least 50% of the total surface of the outer shell and 5% of the deck surface area for each level. Depending on the location of the car park and its intended image, designers can choose from different materials such as metal grills or glass to convey an image of transparency and lightness, or use more substantive solutions with brick, cement, or wood.
The galvanised metal grill, as used in Aix-en-Provence, has proven to be a clean, simple, and economically appealing choice.
Elsewhere, architectural research has led to the adoption of 'added value' solutions, such as stainless steel mesh (Cologne airport car park) or straight or perforated steel sheeting. The Argoulets car park at the Toulouse Metro interchange uses a facade system consisting of micro-bored sheeting, which changes appearance in different lighting conditions: by night, lit from inside, the building's outer skin appears transparent; by day, the metal surfaces reflect the exterior surroundings.
The roof surface is used as a parking deck, and the principle of the roof as a 'fifth facade' is being increasingly diffused by architects. In the Aix-en-Provence car park, the roof is adorned by a crown of metal pergolas supported by a single central pillar, housing external lighting units. Other projects use roofs with photovoltaic panels to make the building energy self-sufficient or green roofs to facilitate the construction's incorporation into the urban context.
Structural fire resistance and standards
The evolution of technical standards for the structural design of buildings affords the possibility of evaluating fire safety by means of a complete analysis of structural behaviour in fire conditions. Recently, calculation models for evaluating structural safety in fire conditions have been calibrated by means of the analysis of a widespread campaign of experimental tests on full-scale open-structure car parks. In particular, results from the EU-funded research project 'Demonstration of Real Fire Tests in Car Parks and High Buildings' conducted over the period from 1998 to 2000, showed that this type of building may be designed with a steel structure, satisfying the minimum fire safety levels without the need to apply protective coatings.
The test in Vernon, France in particular began with analyses for selecting realistic fire scenarios (based on real fire statistics) in order to determine the thermal actions generated by burning cars and to establish the sections of structural elements to be subject to testing. During the tests, three cars were set alight; the resistance and deformity of the structure were then tested and the thermal release rate determined. The average temperature inside the car park remained relatively low; it reached higher levels in the vicinity of the burning cars (with a distribution over the deck section of between 600°C at the exposed intrados and 100°C at the extrados) and fell significantly moving away from the flames. There was no structural damage: any deformation of the beams was not significant enough to present a risk of collapse and the beams were restored to normal by cooling the metal frame.
The main hazard in this situation is the smoke that is released during the fire. With the results from the Vernon test, conducted on a complete, full scale open-plan parking structure, the advanced calculation models proposed by Eurocodes were validated and results were obtained from numerous simulations. In France, this has led to an evolution of fire safety standards: in addition to the 'traditional' approach, which uses the conventional ISO fire curve, it is possible to justify the fire resistance of a ventilated open-plan car park structure with the fire engineering method. The heat acting on the structure and the temperatures are calculated based on fire scenarios (approved by the competent authorities) following advanced or more simplified methods of calculation.
The fire resistance calculation of all elements must first obtain a positive review by an accredited laboratory before they can be installed on-site. Since there are numerous design parameters for a car park (structural grid, height, etc.), designers can use a guide edited by CTICM which gives general recommendations on project conception.
The introduction of specific standards into French legislature in 2004 and 2006 in fact paved the way for the creation of non-protected steel car parks, which demonstrate economic and feasible convenience compared to solutions using traditional materials. A study conducted on the first installations revealed that the cost of metal framework and decking makes up 50% of the total cost and that the average construction time is 4-5 months.
In Germany, the market for steel-structured above-ground car parks has been flourishing for many years, with no special fire resistance R necessary for structures.
In Italy, as in other EU member states, fire safety regulations today are in evolution and oriented, in accordance with Directive 106/89/CEE (6), towards European regulations, where "prescription" matches "performance". Three new ministerial decrees, of 16 February 2007, 9 March 2007, and 9 May 2007, among other provisions, identified Eurocodes as the reference regulations for fire resistance tests. These decrees also introduced the concept of 'project fire load' and calculation according to an engineering and performance-based approach in conformity with the Eurocodes. Further studies into the performance-based approach for calculating above-ground parking structures are today underway within the scope of projects by the Commission for the Fire Protection of Structural Steel, founded based on the initiative by the Fondazione Promozione Acciaio with participation from the Italian Interior Ministry.
Text: Tommaso Tirelli
Photos: ArcelorMittal Europe - Long Products, Azéma Architectes