ZHA's magic gets Riyadh Metro off to a flying start

Can an act of construction immediately change the culture of a city? The 176km-long, six-line Riyadh Metro project that links 85 stations might. Integrated with a new 85km three-line rapid bus system, the metro project in the Saudi Arabian capital is a major milestone in the city’s history. Not only will it create a huge shift in transport offerings in a city that relies on private cars and motorways, it will increase the quantity and quality of outdoor public space in an urban condition that has lacked it. Masterplans for the rapidly expanding Saudi capital which have been delivered since the metro went on site in 2012 provide for pedestrian-friendly public space. Here, many still work in high-rise towers with basement parking and live in walled compounds, along highways.

Four larger interchanges on the system – Snøhetta’s Qasr Al Hokm Downtown station, Gerber Architekten’s Olaya station, Omrania’s Western station and Zaha Hadid Architects’ King Abdullah Financial District (KAFD) – are icons for the whole. KAFD station achieves this in much more straitened conditions than the other three. Despite its importance as a terminus for Lines 6 and 4 (which runs to the airport) and as an interchange to Line 1, its site is limited. According to Filippo Innocenti, ZHA project director for the KAFD station: ‘When they designed the masterplan for the financial district, there wasn’t a plan for the metro.'

They found space for it. KAFD station stands at the edge of the Henning Larsen-designed Financial District: a dense group of faceted towers amounting to 1.6million m² which had begun by the time the metro was planned. A monorail loop, a network of fully contained and air-conditioned footbridges and a yet to be completed system of submerged walkways left no room for the station in the tightly planned district. When the transformational plans for the city-wide metro came in 2012, a long narrow plot was found at the very perimeter of the district alongside the King Fahd Road. 

The building turns this elongation into a virtue. The initial competition entry was defined by several lattices of glazing in the centre, with flat and canted sections at either end like the bow and stern of a boat. It addressed the King Fahd Road as a relatively flat object. However, the practice realised early on that the north and south sections of the street facade would be subject to high wind loads. Working iteratively and parametrically, the sequence of opposing sine waves that originally defined the lattice on the surface was carried into three dimensions to create an undulating self-supporting facade and roof envelope.

This creates not just a more robust structure but a more coherent one. The facade, clad in ultra-high-performance concrete (UHPC) panels, is supported by a series of primary structural arches, their longitudinal steel tubes forming a frame that spans the building’s cross-section. These arches are separate to the structure supporting the trains and are vertically supported from the ground-floor slab. The visual effect is simultaneously unprecedented and familiar. The latticed ‘mashrabiya’ is often overused as a concept for Western architecture practices working in the Middle East. Here, though, it is more complex, with the primary structural arches supported by a secondary frame designed to resist lateral loads and provide restraint for the primary arches. 

Between these, a steel diagrid supports both the curtain-wall system and glazing. Starting from outside, the build-up consists of individual UHPC cladding, a flat glass curtain wall, the steel structure, and glass-fibre-reinforced gypsum (GFRG) lining. Their size is dictated by the diagrid spacing – 1.4m-2.0m – and steel section sizes from 200mm by 100mm to 300mm by 150mm. This permits the use of flat glazing which can be protected from direct exposure to the sun by individualised panels that either accentuate the undulating form of the arched structure beneath or, depending on position, provide specific levels of shading.

The exterior’s combination of both complex and intelligible continues inside. A different structural system supports elevated platforms with a series of Y-shaped columns that carries the viaduct for Line 1 and to which the lower floors are attached. Escalators from the mezzanine level, on which Lines 4 and 6 rest, to the concourse level, sit on the upper, cranked sections of these columns, as do those from the concourse level to the Line 1 platform. The way in which this circulation is overlaid on the structure is aesthetically compelling and provides clear, legible routes to the users of the building. 

Another of the building’s successes lies in the way in which these two structural systems – the undulating sculpted envelope and the orthogonal structure supporting the train lines – interact: for example, on the south facade, where the panelled curves from the east and west facades are resolved in diagonals; or the concourse on the second floor, where the billowing form of the exterior is traced on the ceiling in perforated metal detailing adjacent to the transversal columns. Nor is this simply an exercise in ‘parametricism’ – the digitally-derived style championed by ZHA – but is a means of creating public amenity. The southern atrium uses the fact that Lines 4 and 6 terminate at KAFD to show off the full relationship between biomorphic exoskeleton and the muscular supports of the viaducts. It’s all the more impressive since the building is, at the extreme, just 45m wide and the viaducts of Lines 4 and 6 together are 26m wide.

Some previous ZHA buildings have been compromised by the way in which the smooth curves created in software have been delivered in reality. In Riyadh a huge amount of work was done to ensure the UHPC panels on the exterior and the GFRG on the interior were hung perfectly. The analysis of the whole facade envelope and the structural response of each panel went through pre-processing in Rhino and Grasshopper, structural analysis in Sofistik and post-processing in Matlab, which supplies the range of stresses for sizing the bracket connections. 

Exterior panels are supported at four points by stainless steel brackets, connected to a frame below, which also comprises the insulation layer. This fastidious computational process creates not only the sumptuous, elegant curves of the panelling but also the meticulous shadow-gap detail, which is particularly impressive where the station’s external envelope folds to accept the financial district’s distinctive faceted bridges at first-floor level. 

The building has little time for contemporary mores on energy use: 8,000 tonnes of steel (more than the Eiffel Tower) were used, including 4,200 tonnes for the exoskeleton alone.

The entire building is air-conditioned to a temperature of 21°C in the ticket hall and 24°C on the platform. These demands require sliding door panels on every platform across the system – tough to integrate in the design, particularly as the specifications arrived late. Air-conditioning units sit behind gridded vents at key junctures throughout the building – such as beneath escalators or platform benches – each controlled according to passenger volumes. Large spaces such as the atrium are cooled only at floor level, providing optimum comfort with lower energy costs. 

ZHA is still working on eight projects for which Zaha Hadid herself led the design team before she died in 2016. KAFD station is one such project and it is a stunner: its curved repeating geometries achieve that rare thing in architecture where music is the only real analogy. The building is, on its limited site, an example of how transport systems can change radically how we think of and use cities. This is Riyadh’s first explicitly public utility offering. It is a means of catalysing not just urban expansion – the city’s population was 5.7 million in 2012 and is predicted to increase to 8 million by 2030 – but also urbanity itself: that sense of different people sharing the same space for similar reasons. Along with the other 84 stations, the impact of KAFD station on a rapidly growing city will surely be huge. 

Tim Abrahams is a critic and contributing editor to Architectural Record

IN NUMBERS

GIA 49,080m²
Building footprint 7937m²
Area of green softscape 1050m²
Total envelope area 30,000m²