Multi-storey car parks

Text: Tommaso Tirelli
Photos: ArcelorMittal Europe - Long Products, Azéma Architectes

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 saftety. 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.

Related link(s)

Detail of the car park facade

Drawing of facade and detail of the shelter in front of the car park

Argoulets Car Park: ramp and inner cladding with Mascaret profile

Argoulets Car Park at the Toulouse Metro interchange

Text: Tommaso Tirelli
Photos: ArcelorMittal Europe - Long Products, Azéma Architectes

The 'open front system guarantees abundant natural ventilation, which is fundamental to safety in the event of fire. Records in Europe show that with the advanced structural calculation methods set forth by Eurocodes, it is possible to guarantee structural safety in the event of a fire, thus eliminating or reducing the need for expensive fire safety systems for metal structures.

The concept of multi-storey car parks is often associated with cases of 'anti-architectural' design, producing constructions in which the focus is placed on optimising the use of economic resources available in compatibility with safety levels and functional accessibility.

Fortunately, the international panorama offers cases of multi-level parking in which the designer's freedom takes shape and expression in the form of light steel framework as the load-bearing structure along with various other materials in the construction of the outer skin.

The use of steel technologies in multi-storey car park structures is becoming increasingly widespread in Europe, proving an economically advantageous solution which not only reduces manufacturing time, but also allows the car park to be seen as a genuine architectural form rather than considered in its purely functional nature. Given the growing need for car parking in our towns and cities, particularly in connection with public transportation systems and commercial centres, the steel multi-storey car parking solution proves particularly interesting compared with more traditional underground parking.

The abundant natural ventilation typical of the construction of these buildings and the specific structural planning to withstand fire conditions allow for safe solutions, even without the use of specific fire protection systems. This is proven by various installations in Europe, whose steel structures have been calculated according to the Natural Fire Safety Concept, as set forth by European safety standards.

Steel structural parking typically consists of 16-metre bays: a section between two pillars made up of 5 m x 2.5 m parking spaces alongside a 6 m track. This solution is highly practical as it allows those parking their cars great manoeuvrability without the obstruction of the pillars. Building open-plan above-ground structures comes with the benefit of natural ventilation and light. The former is essential when calculating fire resistance, while the latter is a factor that improves users' comfort and perception of space, while simultaneously reducing the building's energy use.

Moreover, being a 'light' material, steel helps reduce the impact of foundations and facilitates construction on sites or areas of limited size (e.g. airports, town centres, railways, etc.). Its prefabricated construction allows for fast installation and, at the end of its useful life, the steel structure may be dismantled and its components reused and recycled. Construction steel is in fact already recycled, with 80% of its production deriving from scrap metal put through an electric furnace.

The architecture of steel car parks

The car park, considered a construction genre all of its own, dates back around 100 years to the age of the development and diffusion of the automobile. The first car park was built in 1905 in Rue de Ponthieu, Paris, and its external design was similar to those of the adjoining residential homes. In 1940, American Richard Neutra undertook the first multi-storey parking project, for which he selected a steel structure and chose not to incorporate any form of facades into the design so as to reveal the function of the construction. Although the project was never carried out, it did provide a model for others to follow. In his projects for Philadelphia (1947-1962), Louis Kahn placed great strategic importance and architectural decorum on above-ground multi-storey car parks.

Today, the decision to make above-ground car parks an urban object with their own aesthetic value is justified for different reasons: those who park in the city centre, or close to an area of interchange with public transport links, airports, or hospitals, must be able to park their car without difficulty; the car park must be an easily identifiable object; for shopping centres, 'pretty' and functional car parks are an element of appeal for customers and for the diversification of competition.

The distributed layout of a steel multi-storey car park typically consists of a central track 5 or 6 m in length, with parking spaces placed perpendicularly on each side. The open space between the main pillars is generally around 15.5 m to 16 m in length. However, if the parking spaces are arranged at an angle to the track (for example 45°), the open space is smaller and the total length of the building may also be limited to 14 m; this layout offers greater ease of parking and vehicle flow within the car park, but it requires a larger parking space surface area than the 'comb' type. The minimum height of each floor is generally 2.1 m to the underside of the beam.

Access ramps can be built in different configurations (spiral or straight, external or internal) while their optimum inclination is between 8% and 10%, and generally does not exceed 12-15%. One solution for reducing ramp length while maintaining optimum inclination is a car park design with a staggered semi-level layout. For all ramp types, the ideal solution is to access each parking deck via a track that is longer coming in (to search for free spaces) and shorter going out and, where possible, to separate incoming and outgoing traffic flows. The pedestrian track must not cross those designed for vehicles. Pedestrian tracks, decks, and the division of sectors within the car park are identified by bright colours.

Related link(s)

Argoulets Car Park

Detail of the facade of Argoulets Car Park

Internal view of Argoulets Car Park

Detail of the openings of Tolosa Argoulets Car Park