The Soccer City Stadium and its structural design

Detailed information

The existing stadium, which was first constructed in 1987, consisted of a playing field surrounded by embankment seating, two levels of corporate hospitality suites, and an elevated seating tier on the western side only.

The architects for this prestigious project, Boogertman + Partners, in partnership with Populous, have created, an 'African Pot' which will in future be recognised instantly by spectators in every corner of the world. To achieve this unique look, a structure, circular in shape on plan and section, was created to envelope the upgraded triple-tiered concrete seating bowl.

The circular plan format of the pot, which encircles the rectangular seating bowl and field, was selected to ensure that all facade detailing could be consistent in plan and section, thus ensuring an easier detailing, manufacture, and installation process. This furthermore ensured that the 120 concrete facade columns would be consistent in shape and form. Given that the existing concrete structure was limited in its ability to carry the additional roof load imposed on it, it was decided by the design team to remove the roof structure from the seating bowl structure and place the roof structure on 12 off-shutter concrete shafts. These shafts required an ingenious piling solution.

Piling

Approximately 1350 piles have been installed at Soccer City. The forces generated by the concrete structure and roof mean that exceptionally high loads are transferred to the foundations, which resulted in the design and construction of some of the most extreme piles ever installed in South Africa. All the piles and lateral support were designed by ARQ and Verdicon and installed by GEL.

Whilst many of the piles carry large compressive loads, many piles are also subjected to exceptionally high tension loads. The calabash-shaped facade and the roof transfer the loads from the roof down 12 reinforced concrete shafts and 120 inclined perimeter facade columns to the piled foundations. Some shaft foundations are required to resist tension loads up to 13000kN in combination with sheer loads of 6000kN and a bending moment of 125000kNm. Due to the limited space, it was only possible to install a maximum of 12 piles per shaft foundation, resulting in some piles being subjected to tension loads of 5800kN (580 tons). In order to accommodate the massive loads, the designers decided to anchor the piles 6m into the sandstone bedrock using dowel bars installed through the base of the pile.

In some cases, the 1500 diameter piles were installed up to a depth of 33m, necessitating almost 60m³ of concrete in a single pile.

Shafts

The roof is supported by 12 large, 40m-high rectangular concrete shafts, each of which was designed to withstand large horizontal and vertical loads. The shafts vary in plan from 3.5m x 5.0m to 3.5m x 14.0m, with an average wall thickness of 600mm. A huge reinforcing steel content of 460kg/m³ (approximately three times more than normal reinforced concrete) made the placing and compaction of the concrete extremely difficult. The stiffness of the shafts under varying load combinations had to be determined accurately as this affected the forces in the structural steel roof structure. The design and stiffness of the shafts was further complicated by various openings which had to be provided for electrical, mechanical, fire, domestic, and storm water services. All these services were designed early on in the process and modelled in 3D by the architects to ensure that all penetrations were fully co-ordinated before the reinforcing was designed by the engineers. This was required as it was impossible to entertain any late requests for penetrations by the services engineers. These shafts are founded on the piled foundations described above with some piles subjected to downward loads of 1100 tons and upward loads of 580 tons. In addition to the shafts, 16 circular columns of 1 metre diameter support the roof.

Shaft bases

In order to transfer the large loads from the roof via the concrete shafts into the tension and compression piles, large pile caps with depths in excess of 4m were required. The construction of these large bases required careful planning as heat of hydration had to be controlled, and the safety of construction workers, who were often required to work beneath heavy reinforcement, had to be ensured. Varying soil conditions on site often required that the engineers had to adapt the design of these bases.

Lateral support

To allow access into the completed stadium bowl, three deep tunnels had to be cut with vertical excavation. The western players’ tunnel runs below the existing stadium structure and has permanent support with a maximum vertical height of up to 10.0m. The players' tunnel was designed as a sloping mine shaft as a reference back to Joburg’s rich gold mining history.

The south-west and north-east tunnels have been constructed through new portions of the stadium. Due to programming constraints, it was decided to construct permanent lateral support in these tunnels, consisting of soil nails, shotcrete, and mesh. Due to the construction of deep pile caps at the base of the north eastern tunnel, vertical lateral support heights of up to 13.0m have been constructed.

Raking beams

In order to vastly improve the sightlines of the existing stadium, the rake of the existing western upper tier has been increased to a maximum 34 degrees, and an additional raised seating tier has been introduced on the upper portion of the embankment seating. Two levels of hospitality suites and the upper tier have been completed on the northern, eastern, and southern pavilions, thereby creating a classic triple–tiered bowl. All raking beams were constructed using purpose-made formwork. The existing pre-cast steppings to the western upper tier were removed, crushed, and recycled as base layers for the bulk earthworks. All new seating steppings are constructed from pre-cast concrete - all made on site.

Pre-cast yard and pre-cast elements

To reduce the handling time and damage to precast units, as well as to achieve a value-for-money product, the main contractor GLTA/Interbeton elected to establish an on-site batching plant and pre-cast yard. This allowed for easy inspection and control of the units, which were all unique in dimension due the existing geometry of the old stadium bowl, allowing them to be placed correctly with the minimum amount of handling.

Facade columns

One of the most challenging elements of the concrete structure was the design and construction of the facade columns. The facade structure is supported on 120 inclined concrete columns enveloping the stadium. The columns are 16.3m high, and the top of each of these columns has a horizontal eccentricity of 6.5 metres in relation to its base, resulting in large moments and upward loads on the piled foundations. Due to the large moments and forces in these slender columns, the reinforcing steel is extremely dense (860 kg/m³), which made the use of a vibration poker extremely difficult. GLTA/Interbeton opted to use self-compacting concrete to construct these columns. All facade columns are connected with tie beams which act in ring tension so as to limit long-term deflection of the columns and facade structure. The design and construction of the facade columns was planned and executed very carefully with temporary propping and bracing, so as to prevent deflection during construction.

Facade cladding

The final selection of the facade material came about after an extensive search by the architects to select a product that would ultimately reflect the nature of the concept of the calabash. Having discarded ideas of composite aluminium, steel, and various roof-sheeting options, the architects were coincidently introduced to an extruded fibre reinforced concrete panel called Fibre C, from Rieder Elements in Austria. The product was supplied in panels with varying surface finishes, honed and sandblasted, in combination with a variety of eight earthy colours, to create the unique variegated facade cladding. The panels, which are light-weight and only 13mm in thickness, were supplied in 1200 x 1800mm typical panel sizes and were fixed to a galvanised steel sub frame. The panels, furthermore, have excellent thermal properties and have been subjected to rigorous testing, including hail impact, water penetration, and discolouration tests.

Ramps

Eight large pedestrian ramps, designed for the efficient ingress and egress of spectators to the upper levels of the stadium, have been provided. These ramps, which also provide vehicular access to all levels, follow the shape of the facade bowl and consequently change position in plan from one level to the next. In addition to the sloped facade columns, the other columns supporting the ramps are inclined and required intricate design analysis and construction techniques. The three dimensional ramp structures form massive atria that visually and physically connect all the concourse levels of the stadium.