The engineering legacy of the FIFA World Cup Qatar 2022^TM: Al Thumama Stadium

Al Thumama Stadium is located in the southern Al Thumama district of Doha, near Hamad International Airport. The district derives its name from a plant that grows abundantly in the area. Known locally as thumam, this perennial grass is adapted to desert climates. The stadium shown in Figure 1 was designed by Qatari architect Ibrahim M. Jaidah in collaboration with Mark Fenwick of Fenwick Iribarren Architects and the team of schlaich bergermann partner, who were invited to develop a design reflecting Qatari culture. The gahfiya – a traditional woven cap worn by men and boys across the Arab world and representing a symbol of dignity and independence – was chosen as a symbol for this landmark stadium. The preliminary design was then passed to the Turkish design and built contractor Tekfen Construction, who formed an international design team including Heerim Architects from South Korea, structural engineering firm Thornton Tomasetti from the USA and Jain MEP Consultants from Canada.

In tournament mode, it was planned that the 40 000 seat stadium would host FIFA World Cup Qatar 2022 matches through to the quarterfinals. In legacy mode, after the tournament, the stadium's seating capacity was to be reduced to 20 000 by offering 20 000 seats to countries in need of sporting infrastructure. The stadium was to be transformed into a sports city, complemented by other mixed-use developments, including a sports clinic, a three-storey boutique hotel inside the stadium and an indoor multi-purpose hall and aquatic centre (sbp, 2019).

The structure of the stadium is sub-divided into several building components – the reinforced concrete (RC) bowl, the dismountable upper tier structure, the roof and the façade structure. The bowl includes one basement and five elevated concourse levels, as shown in Figure 2. The basement is at the same elevation as the pitch level and houses players’ dressing rooms, entrance tunnels to the field and the press conference room. The recirculation plenum is also located at the basement to provide state-of-the-art cooling technology for the lower tier bleachers. The ground level serves as the main entrance for fans, VIP, staff and the media. Level 1 houses the lower seating bowl and level 2 consists of the premium suite level and VIP boxes. Level 3 provides the upper concourse seating bowl and level 4 functions as a plant room to house MEP (mechanical, electrical and plumbing) equipment, mainly required for cooling of the upper tier.

The roof and façade cladding consists of two different materials – solid and highly translucent mesh membrane – with the effect of maximising sunlight on the pitch for grass growth while protecting spectators from direct sun through the roof. The gahfiya pattern runs through the entire façade, creating very appealing lighting effects. During the day, sunrays pierce through the façade into the interior while, at night, the stadium glows from the inside and emphasises the pattern.

The bowl structure consists of a reinforced in-situ concrete framework of floor slabs, columns and beams below level 3. Columns are placed on a radial grid around the playfield, spaced 8–10 m apart. The beams running between the columns create a moment frame system, which is the main lateral resisting structure of the concrete bowl. These elements carry and transfer vertical and horizontal loads to the foundation. Staircases and elevators are enclosed by gravity load bearing concrete masonry unit staircase walls, supported by RC framing beams at every level and detailed such that they do not work as a part of the lateral load resisting system. The RC beam depths were optimised to provide the required headroom for architectural and mechanical needs at critical bays. Haunched beams with deeper sections at the column faces and reduced depth at mid-span were used.

The upper tier was designed for a 20 000 seating capacity, using structural steel as the construction material. Steel raker beams were sized to carry bleacher loading and serve as a part of the lateral system in the radial direction. Braced frames are provided for lateral stability in the circumferential direction. Level 4 of the stadium (Figure 2) was also considered part of the upper tier. A composite steel framing was designed to carry the required MEP equipment. In addition, a roof was provided as an MEP requirement to create an space. Permanent loads in combination with live loads considered the guiding loads for the stands. For assumptions of dynamic loads, guidelines published by the Institution of Structural Engineers (IStructE) were used (IStructE, 2008).

The design of the steel rakers and the precast units was governed by the vibration criteria set by the IStructE (IStructE, 2008). Frequency analyses were carried out for the rakers and the precast units in each gridline as the span of the precast units varied from 7 m at the sideline to 10 m at the corner of the bowl. The interface between structural steel and RC was detailed by using embedded plates with anchors at level 3 to transfer both gravity and lateral imposed loads to the lower tier.

The bowl was separated into four separate structures with expansion joints in between. Joints at the RC floors are provided by means of incorporating a corbel detail. A sliding pad is provided at the interface to allow the segmented beam over the corbel to move independently. The expansion joint at the upper tier was formed by double rakers.

Lateral and gravity analyses for each bowl segment were conducted in the three-dimensional analysis software SAP (from CSI). The upper tier structure was included in the analysis together with the lower bowl to gain better understanding of the load path and stiffness distribution, especially for earthquake loading. Two node frame elements were used for the columns and beams, while the slabs were defined as single-sided membrane elements with no out-of-plane stiffness to ensure that the slab did not contribute to the lateral stiffness of the structure.

The utilisation of moment frames for the bowl allowed a more uniform load carrying/distributing system. In addition to that, the reasonable bearing capacity of the soil and the low ground water table allowed for a shallow foundation system with isolated footings under the columns and continuous footings for the walls.

The legacy mode consists of three-storey, 12 m tall steel structures erected on top of the east and west side lines. The west sideline was intended to serve as a clinic/clubhouse, whereas a hotel was planned for the east side line. The vertical and lateral loads of the legacy mode were taken into account when designing the lower bowl structure. In addition, single-storey service rooms to enable the legacy mode at the north and south endzones, were also included. Almost the entire RC structure of the lower bowl was to be preserved in legacy mode, with minor additions and retrofits done after the tournament.

The legacy conversion process would involve the dismantling of a majority of the upper tier bowl, while keeping the roof and façade systems in place. Therefore, the dismantling process of the steel structure was already considered in the design by using removable and bolted connections, meaning that the individual steel girders could be dismantled as easily as possible. This condition precluded any crane access from outside the stadium. The sequence of the dismantling procedure was planned in detail, as crane access could only be achieved from the field side. When planning the stadium, sustainability was taken into account in accordance with the Global Sustainability Assessment System (GSAS). The venue achieved a five-star GSAS rating. In terms of structure, the sustainable goal was to achieve an optimised lightweight structure with a minimum use of material.

The stadium roof (Figure 3) is circular and covers an area of around 35 000 m². Its outer diameter is 230 m and the oculus opening over the pitch measures 88 m, resulting in large radial roof spans of approximately 70 m. The roof is a spoked wheel structure characterised by three circular primary rings – two inner tension rings (upper and lower tension ring) are connected by 40 radial trusses to an outer compression ring.

The structural height of the roof between the two tension rings was set at 11.25 m to protect and stabilise the structure against internal loads from self-weight and pre-tension as well as imposed loads such as wind, sand, temperature and maintenance, or also event live loads. Vertical loads are transmitted from the tension rings through the radial cables and beams to the compression ring. The difference in prestress level in the lower and upper tension rings creates a moment that counteracts the moment created by the eccentrically applied external loads (relative to point A in the schematic load transfer diagram in Figure 4) (Bögle et al., 2011).

The upper tension ring sits 4 m above the outer compression ring, providing a slope of 5.7% and thus, allowing for sufficient rainwater drainage towards the gutter along the outer ring. Vertical steel columns on top of the concrete superstructure on level 3 support the roof. The roof is a closed structural system on its own; thus, under permanent load conditions, no horizontal reaction forces occur. Horizontal stiffness of the spoke wheel is provided by column bracing at eight bays. Vertical bracing, together with the lower and upper tension rings, form a stiff truss along the inner perimeter of the roof. This truss acts as a load-bearing system against unbalanced loads acting on the roof, such as unbalanced wind loads and unbalanced live loads, considering wind loads as the guiding load case for the roof. In the roof plane, every second in-plane bay is braced with in-plane bracing beams. They provide horizontal stiffness, especially for wind loads acting on the façade or through friction over the entire roof cladding. Horizontal loads are transferred to the concrete superstructure through the vertical roof column bracing.

In-plane bracing beams are located in every second radial bay. The other bays are spanned by in-plane secondary beams, which do not participate in the global structural response of the roof due to being axially released. In-plane tertiary beams spanning to the half point of the primary and secondary beams were added to form a symmetric system and to provide additional support points for the membrane framing.

An isometric view of the structural components of the roof is shown in Figure 5.

The compression ring forms the outermost stiff member of the roof and, together with the two tension rings, form the main components of the spoked wheel structural system. It is 722 m long and is composed of 40 segments. Its cross-section is a welded box section, that's 1.2 m deep and 1.8 m wide. Internal trapezoidal stiffeners prevent the large and slender plates from buckling. The box sections were assembled through bolted head plates which were machined to the foreseen compensated element length and connection angle as well as surface smoothness with highest precision. A main head plate of 80 mm thickness sits above the global radial axis and provides pin connections to the radial truss and the columns, while an eccentrically located secondary head plate (60 mm) closes the ring connection of every second ring element. All the steel beams and column or strut elements were made from S355 steel.

The radial trusses are composed of bending-stiff upper beams, lower radial cables and slender separating struts. The radial beams are welded box sections, 600 mm high and 400 mm wide. The radial cables are fully locked coil cables of 125 mm diameter with cast-steel end-socket connections. The vertical struts are slender hollow tubes, 168 mm in diameter, fixed to the cable with clamps and hinges connected on both ends; they support the radial beam at the connections of the diagrid cladding elements. From the lower tension ring, the struts form a V linking to the upper tension ring at the inner stadium tip and supporting the radial beams at their inner quarter points. The diameter of these longer and higher loaded elements was increased to 299 mm.

The lower tension ring is 339 m long and consists of a bundle of eight fully locked coil cables of 110 mm diameter arranged in two layers. For each cable, three connectors were required for fabrication and transportation purposes. The cables are connected at each radial axis to welded joints, clamped with machined brackets. Each connection detail weighs approximately 4 t.

The upper tension ring sits further to the inside by 9.9 m and is 277 m long. It is composed of two fully locked cables of 110 mm diameter, also assembled from three segments. Its main purpose is to provide stability against uplift from wind loads and limit roof deflections, while the lower tension ring acts against gravity and downward loads and has a higher force level. Vertical cross-bracing rods between the two tension rings using high-strength steel of 64 mm diameter assure their combined action without relative lateral displacements and provide stability of the roof for unbalanced roof loads.

Diagonal beams between the radial trusses form either the in-plane bracing, secondary or tertiary beams. While their outer dimensions are similar, with box sections of 400 × 200 mm, their plate thicknesses and end connections were optimised according to their structural function. In-plane bracing elements are positioned at every second bay in a zig-zag layout and span between the radial beams; the tertiary elements complete the pattern from the mid-spans to the radial beams, therefore spanning only half the distance. These are the primary bays and correspond to the pre-assemblies for lifting into position in one module. On the other bays, the secondary or infill bays, diagonal elements between the radial beams are designed with an axial released end connection allowing for construction tolerances. Stainless steel connection plates allow for movement and easy assembly directly on the roof.

The columns are box sections, that are 600 mm wide and 700 mm deep, oriented in the radial direction. The columns are pinned on their lower ends and fixed in the radial direction on the compression ring at their upper ends with double pin connections. The base plates below the pin connections are anchored 2 m deep into the RC columns below with threaded rods. Twisting and horizontal displacement of the compression ring is restraint by eight cross-braced bays spread along the perimeter using pairs of fully locked coil cables of 70 mm diameter in each direction, slightly prestressed between the roof columns.

The technical equipment in the roof is of course of major importance for events and is placed on two gantry rings. The inner ring sits above the lower tension ring between the inclined struts (Figures 5 and 6) and is 2.1 m wide. The outer gantry ring is at half-span of the radial trusses and is 530 m long and 2.5 m wide. Access is provided through six radial structures of 2.1 m width, spanning approximately 60 m from the compression ring to the inner gantry. The gantry structures are suspended from the radial trusses above with slender tension rods and are independent from the global structural behaviour of the roof. All gantry segments have two floors – the lower floor with service racks and an upper floor with grating for maintenance access.

The lightweight cladding of the roof is the most visible part of the entire venue and is composed of two membrane materials with different translucencies; a glass fibre polytetrafluoroethylene (PTFE) mesh membrane type IV (solid membrane) combined with a highly translucent laminated glass fibre PTFE mesh at specific locations. The translucent meshes, when applicable, sit in the middle of the rhomboids and the solid membrane covers the rest of the panel space. In total, 680 cladding modules with six different types of material combinations were applied corresponding to the global pattern (Figure 7), and arranged in such a way that greater roof transparency is achieved on the southern side to provide the best compromise between light on the pitch, heat gain and spectators’ comfort. The numbers and types of panels on the roof cladding are shown in Table 1.

The solid (shown in orange in (Figure 7)) and translucent membranes (shown in blue) share slender frames that are supported by underslung cables and struts, pushing the frames out of the roof plane to form double curved surfaces, and allowing the membrane to transfer loads to the stiffer steel substructure (Kuhlmann et al., 2009).

Curved vertical steel beams (40 in total) define the bays and form the rigid supports of the façade structure, as shown in Figure 8(a). They are aligned with the roof grid and are pin-supported on the ground. An intermediate support is provided from the bowl on level 3 just below to the roof column foot points by vertical V-shaped struts (Figure 8(b)). On top, bi-articulated horizontal struts connect the vertical beams to the roof compression ring, providing lateral restraint. Like the roof structure, eight bays are braced for global lateral stiffness following a pattern of 1–3–1–5 (braced–not braced–braced–not braced).

Prestressed cable nets composed of sets of twin convex cable trusses (with slender struts separating the inner from the outer cable) span between the vertical frames in two diagonal directions, as shown in Figure 9. The geometry of the trusses is designed to follow the toroidal surface of the façade cladding with the outer ends of the struts. Top and bottom steel trusses (Figure 9) with underslung prestressed cables form the upper and lower frame sides of each cable net bay. The cable trusses span about 20 m with a length to depth ratio of only 1 : 22, making these a very slender supporting structure in line with the fabric pattern. The cable net was prestressed to resist wind loads without going slack in the serviceability limit state. The cables are open spelter socket cables of 24 mm diameter; along the top and bottom trusses, the vertical component of the prestressed cables is compensated by stronger cables with 36 mm in diameter, forming integrated underslung structures together with short vertical struts. On top of the upper horizontal truss, a parapet structure extends the façade structure slightly above the roof.

The 24 700 m² façade, which appears closed and seamless (as shown in Figure 1), is divided into 40 bays that are approximately 35.1 m high and 18.7 m wide. The façade is made up of fabric comprising two membrane materials with approximately 24 000 ‘openings’ to represent the gahfiya. These openings, which provide the membrane with its gahfiya pattern, are not fully open; their appearance was achieved by welding glass fibre mesh material (which allows light to pass through) to the solid glass fibre PTFE solid membrane of type III (Cremers, 2015). The cladding panels are prestressed along their perimeter connections. The perforated mesh was required to supplement the peripheral membrane reinforcement in order to maintain the internal localised stresses around openings at acceptable levels and to keep openings in their intended shape under the influence of pre-tensioning forces and wind forces (Göppert and Paech, 2015; Heinsdorff, 2014).

Due to the curved shape of the panels, the tensioned membrane pushes against the convex cable net. Additional aluminium extrusion profiles on the inside and outside of the membrane provide further support between the net struts and guarantee the uniform appearance. The façade membrane is sandwiched between external and internal aluminium frames, and is connected with screws through the mesh at a spacing of 250 mm (Paech, 2016).

Construction required the highest precision following a calculated fabrication geometry to eliminate construction tolerances on site. The splice connections of the compression ring were therefore machined, checked and surveyed using pre-assembled/trial assembled segments.

The cable roof structure was built using the ‘big lift’ methodology (Göppert et al., 2012; Paech et al., 2014). First, the columns and the compression ring segments were installed and braced, forming the closed and self-stabilised outer perimeter ring. In the second step, both tension rings and radial cables were assembled and placed on the ground and lower stands of the bowl. As the upper tension ring was attached to the connection nodes of the lower tension ring by temporary cables, both rings were pulled upwards simultaneously, using hydraulic jacks on the outside of the compression ring pulling on temporary strands, which were fixed to the outer ends of the radial cables as shown in Figure 10. The pulling sequence followed a defined scheme of multiple steps of 260 mm stroke length until the cable sockets reached their pinning positions at the compression ring.

Jacks of 200 t and 140 t capacity working in pairs were used with maximum radial forces of approximately 3 MN before connecting the radial cables. In addition, four diagonally spanning coupling cables of 36 mm were temporary installed to limit the compression ring ovalisation at the end of the pulling process. After pinning all the radial cables, the temporary stabilisation cables were removed.

The lower tension ring was put in position, approximately 3 m higher than the final position after all gravity and prestress loads would be applied. Before continuing to place pre-assembled steel elements on the cable structure, the lower tension ring was pulled down to its final position by temporary tie-down cables. The tie-down forces were around 630 kN to achieve the desired installation position for the steel modules. Counterweight was provided by stacked prefabricated concrete modules on steel support structures adapting to the bowl geometry and the inclined steps. By this operation, unbalanced deflections were eliminated when installing the steel structure above.

The pre-assembled steel structure of the first primary bay, consisting of two radial beams, in-plane beams and struts, was then lifted and installed onto the cable structure using a crawler crane. The lifting weight was about 70 t.

The next step was to install the following primary bay leaving one secondary bay free. The members of the secondary bay (diagonal in-plane beams) were then directly installed on the roof using standard cranes. This lifting method of a primary module followed by an infill module was continued until the entire steel structure of the roof was installed. Once the heavy lifting was completed, the upper tension ring cables which were hanging from the lower tension ring were pulled upwards into their position by small temporary derricks attached to the inner tips of the radial beams. The cables were simultaneously pulled outwards to apply the required prestress, using radial jacking with maximum forces of 1.4 MN at each tip. Once the cables were fixed into position, the roof structure reached its final position and was ready for the installation of the membrane cladding.

The façade structure was built as follows. Once the steel structure was erected using standard cranes, the outer and inner layers of the pre-assembled cable nets were installed and fixed to the stiff boundaries. Then, the horizontal struts between the two net layers were installed from top to bottom, creating the convex net geometry. The corresponding nodes were therefore pulled apart by a small installation device using a maximum force of approximately 3 kN before the struts were installed one at a time. Once the net was completed, the compensated membrane cladding was installed bay by bay and stressed against the outer stiff frames. Finally, aluminium cladding profiles were installed for local fixation.

The design of the stadium was based on Eurocodes (Mollaert and Forster, 2004), considering region-specific conditions as defined in the Qatari design code. Wind loads were considered as per wind tunnel tests, which were conducted in 2017 by RWDI. Analysis of the structure was based on a non-linear calculation method using bespoke global imperfections in vertical and horizontal directions. Sensitivity studies, by varying the cable Young's modulus and fabrication length tolerances, were considered in the design as well as robustness checks, such as failure of one steel element or a single cable.

For each of the two models (roof and façade models), detailed stand-alone models were analysed together with simplified global models to consider interaction effects. The simplified global model was calibrated to obtain similar structural stiffness; for example, for the façade, the number of cables was reduced by increasing their individual stiffness. For all load combinations, the maximum and minimum forces and moments were considered in every individual beam in the design process, allowing high optimisation of the design compared with an envelope force approach.

The first eigenmode was found to be a symmetric roof movement at 0.42 Hz and the second mode was vertical asymmetric at 0.67 Hz. The maximum deflection under variable loads at the inner roof tip was less than 900 mm downwards and 500 mm upwards from its installed state.

Al Thumama Stadium unites a progressive architectural vision with an efficient structural masterpiece. The pre-tensioned cables allow for a lightweight long-span roof combined with fast construction using the big lift method. Structural lightness is conveyed through the double-layer cable net façade and its cladding using membrane materials with different translucencies.

The authors would like to highlight fruitful collaborations over the years between the Supreme Committee for Legacy and Delivery (SCLD) and the design team, as well as the execution contractors, who managed to realise the challenging design intention in every detail. The authors are very grateful for the cooperation and support received from the SCLD throughout the process of writing this paper; the efforts and support of Eng. Hilal Al-Kuwari, Eng. Othman Zarzour, Eng. Tamim El-Abed and Dr Alexandra Kardara are particularly acknowledged.