At a new car park in Calgary, the designers have reduced the traditional ramp to a barely perceptible 1-2% gradient, making the structure adaptable to future uses
The Canadian city of Calgary, Alberta, badly needed a new multistorey car park – not least so other parking spaces could be freed for development. But how to justify the carbon cost of building a large new parking structure, while at the same time looking to a future with far fewer cars?
Calgary’s answer is a 26,500m2 convertible parking garage: one that accommodates 510 cars today, but is designed to be easily adaptable to residential or commercial uses in the future. It has been done before, but the Calgary design is particularly ingenious.
“A key problem with converting car parks is that they have deep floor plates,” says Joanne Sparkes, senior project architect with designer Kasian. “It means that the centre of the floors struggle to have any natural light. Our solution is to have the parking surfaces arranged around a long central courtyard, so you have floors only 12m deep and naturally lit from two sides. That’s much better if the building is eventually converted into apartments, for example.”
At 4m floor-to-ceiling, the space is also much higher than a standard car park to allow for any future service requirements. But by far most innovative feature is that the parking levels are not separated by ramps, or accessed by the traditional car-park spiral.
“We were very focused on minimising waste in the future,” says Sparkes. “These features are difficult and wasteful to remove. So we have designed without them.”
Kasian’s solution is to have just one floor, but one that gently and continuously slopes up the 100m length of the car park, returning at the end via a semicircle and then back up the other side of the central courtyard. The continuous 1-2% gradient creates an elongated coil right up to what is effectively a seventh, top storey.
“So we do away with floor-to-floor ramps and spirals, meaning no waste in the future. When the time comes to convert the space, the floor’s gentle slope is easily made horizontal by terracing. This can be done by adding low concrete toppings, or by raised flooring.”
In fact, the future has arrived early in Calgary, with the first two floors of the structure already occupied by a range of small businesses. “This change happened during the design process,” explains Sparkes. “But it enabled us to test our concept. It’s really worked well, and we have used the lower levels of central courtyard space for a single steel ramp to take cars direct to the first parking floor.”
This steel ramp is easily removable, hanging as it does off the main structure, which is constructed from reinforced concrete cast in situ. The gently sloping slab is 180mm thick and spans the 12m width of the parking surface. It is supported at 5.51m centres by beams 500mm wide and 800mm deep, though these are integrated into the slab, so the down-stand is just 620mm.
“Being single parked, and just 12m wide, the slabs are quite conventional,” says Ian Washbrook, principal with structural engineer Entuitive. “There was no need to consider prestressing. The beams connect with columns located at the perimeters that are also 500mm wide, but these are almost walls, being 900mm deep. On the first floors they are deeper still, up to 3m-long shear walls, and they help give the structure its distinctive bell-bottom shape.”
This substantial arrangement of wall-like columns and deep beams also helps provide the building with the stiffness necessary to resist seismic events and wind loading. “We are in a seismic area so there is also a three-way movement joint near the centre of the building which effectively splits the structure into two ‘C’ shapes on plan,” says Washbrook. “In addition, we had to bear in mind that the current cladding, an arrangement of vertical aluminium elements, creates relatively little wind resistance. Once fully enclosed, wind loading would be higher, and dead-loads would also be higher, because of extra cladding, fittings and toppings necessary to convert the space for human occupation.”
The design team had to consider the harsh conditions to which the concrete would be exposed too. “We get a lot of snow here, and use salt on the roads,” explains Washbrook. “So although the rebar is conventional and not coated with anything, we use a dense, exposure class C1 concrete with increased cover. Where you might normally have 25mm cover, we have 45mm. This protects against free-thaw and shields the reinforcement from chloride attack resulting from clumps of salty snow falling off vehicles and onto the slabs.”
Despite the eventual expected conversion of the structure, no attempt was made to create a visual concrete type-finish. “It will be up to future occupiers what they want to do,” says Sparkes. “The existing commercial occupiers on the ground floor have embraced the industrial, bare concrete look, but of course the concrete could be painted or lined in the future.”
Sparkes has ensured that the structure’s sizing will facilitate easy conversion into other uses: “The 12m by 5.5m bays are about the right size for a studio; two bays for a two bedroom apartment and so on,” she says. “The columns are at the edges of the building, so the interior spaces remain column-free to help with future flexibility. When interior walls are added, these can naturally rise to meet the downstanding beams.”
At nearly C$80m (£48m), Calgary’s new car park has not come cheap. But the project was never about cheap fixes: “It’s an investment in the future,” says Washbrook. “If it’s looked after, it will last well over 100 years. And the environmental value of not demolishing and rebuilding? You just can’t beat that.