Construction Technology of Roof Membrane for World Cup Main Stadium

ZHANG Jian, SHEN Jiayong, LI Bai

(China Railway Construction International Group Co., Ltd., Beijing 100039, China)

Abstract: Based on the installation of the roof membrane of Lusail Stadium which is the main stadium of Qatar World Cup in 2022, a series of key technical challenges in the design and construction of the roof PTFE membrane of large stadiums are studied. Based on the analysis of the roof cable net system, the study formulated the overall sequence of membrane installation, and defined the construction method of membrane lifting and unfolding. Focusing on membrane fixing techniques with the tension rings, compression rings, and horizontal cable nodes, it also optimized membrane connection methods with arch and horizontal cable, and provided waterproof construction methods. According to the findings, the construction of stadium roof membranes should follow a logical sequence. The membranes’ fixing, connection and waterproof construction will have an important impact on the final quality.

Keywords: roof membrane；World Cup stadium；roof cable net；membrane construction；Lusail stadium

0 Introduction

The full name of PTFE is Poly tetra fluoro ethylene, which is a high molecular polymer created by polymerizing tetra fluoro ethylene as a monomer[1]. Because of the excellent properties such as high temperature resistance, corrosion resistance, and aging resistance, PTFE membranes are increasingly being employed in the construction industry[2]. The roof structure of the World Cup stadium has a vast span and high-altitude operation risks. Many challenges exist in the construction method and deformation control of the roof membrane. Since China has never hosted World Cup and has never undertaken the construction of a World Cup stadium project, it still lacks sufficient experience in the construction of the roof membrane structure of the super-large World Cup stadium, and a series of major technical problems must be solved urgently[3].

1 Project profile

1.1 Project overview

The Lusail Stadium is the main stadium for the 2022 Qatar World Cup. It will undertake the major events such as the World Cup group stage, semi-final, final and closing ceremony[4]. The stadium is located in Doha, the capital of Qatar, and spans a total area around 1 million m2with a construction area of about 190,000 m2. The stadium can accommodate 92,100 spectators, which is 1,000 more than the “Bird’s Nest” stadium[5]. It is currently under construction as the world’s largest and most technologically equipped professional stadium. The project is undertaken by a joint venture formed by CRCC and local enterprises in Qatar[6]. It is the world’s first top World Cup main stadium built by a Chinese company. Fig.1 depect a rendering of the stadium and the actual construction scene.

Fig.1 The stadium scene

1.2 Roof cable net system

The main structure of the Lusail Stadium is mainly composed of Curved V steel frame columns, compression ring and roof cable nets[7]. For secondary cable structure, the projection of compression ring upper chord center point is a circle with a diameter of 274m, while the projection of tension ring cable center point is a circle with a diameter of 122m[8]. The stadium resembles a bowl with a saddle-shaped edge. The elevation of west and east exterior edges is 76.600m, while the elevation of the north and south outer edges is 61.035m. The pitch is at a height of 5.000m. The plan view of the roof and the sectional elevation view of Lusail Stadium are shown in Fig.2 and 3.

Fig.2 The plan of the roof

Fig.3 The sectional elevation of Lusail Stadium

The roof structure of the Lusail Stadium has a spoke-type radial main cable with a total of 48 spokes, which is a cross-fishtail structure. The spoke wheel type cable net roof system, consisting of two inner tension rings and one outer compression ring truss connected by radial spokes, is the lightest, the most structurally efficient, and the most cost-effective system for large span roofs. A complete form-finding analysis and optimisation were provided for the revised dimension of the stadium roof structure. The Lusail Stadium cable net roof system consists of a range of primary element types as shown in Fig.4. The roof exploded view and the component weight is shown in Fig.5.

Fig.4 Roof structure components

Fig.5 Roof exploded view and component weight

1.3 Main roof PTFE membrane system

A PTFE membrane cladding system is adopted as the roof which covers all the seats and provides shade from rain and sunlight to the spectators in the seating bowl. Fig.6 shows an exploded view of the membrane system. The roof secondary arches, which operate as the membrane skeleton and provide the final phyllotaxis type pattern to the membrane surface, supported and formed the membrane cladding system.

Fig.6 The exploded view of the membrane system

The Main roof membrane covers 56,300m2and is divided into 96 panels along the radial direction of the roof, as shown in Fig.7. Each panel is about 586m2in size and weights approximately 1,065kg. It also includes about 3,500m2of canopy membrane, bringing the total area of the membrane close to 60,000m2.

Fig.7 The roof membrane panels

The membrane installation requires aluminum profile and some accessories such as T-connector and stainless stain fastener. Tab.1 lists the required aluminum profile and accessories.

Table 1 The required aluminum profile and accessories

2 Membrane lifting and unfolding

2.1 The overall sequence of membrane installation

The installation of the membrane is mainly divided into the following:①membrane lifting;②membrane unfolding;③fix membrane with tension ring;④fix membrane with compression ring;⑤fix membrane with horizontal cable nodes;⑥membrane connect with arch;⑦membrane connect with horizontal cable;⑧bridging membrane installation.Before starting membrane installation, the rope net access and life line must be ensured to have been installed by a professional trade according to the rope net installation approved method statement.

2.2Membrane lifting

After being manufactured in a factory in China, the membrane must be folded as shown in Fig.8. The roof membrane will be installed on a proprietary lifting frame and then will be lifted from the ground onto the standing seamed roof (in the specified zone) by a crane of suitable capacity, where it will be disconnected from the crane by the rigger in place. All lifting operations will be overseen by the lifting supervisor.

Fig.8 The dimension of membrane panel

For each 1/96 bay, all material will be divided into 3 components (membrane, aluminum profile, accessories) and lifted separately. Each portion will be lifted by a tower crane, and unloaded near the upper inner chord of the compression ring and temporarily tied to the roof with a steel frame before unfolding. According to calculations, the force acting on a single belt by the heaviest component is about 13kN. The safety is enhanced by using a 3t bearing capacity belt.

Membrane and accessories will be stored on the metal roof, and the contractor will double-check the bearing capacity of metal roof and take the necessary measurements to protect the metal roof based on the material’s self-weight and unloaded layout. According tostructuralsteeldesignreportforLusailStadium(Ref. SC-C01-CAG-HBC-RED-ST-00075), the live load has been assessed and combined with other loads for standing seam roof system design, as shown in Fig.9. The weight of material kept on standing seam roof should not exceed 1kN/m2.

Fig.9 Roof uniform live load

2.3 Membrane unfolding

2.3.1The measurement for keeping arch stabilization

When only one membrane panel is attached with arch and horizontal cable system before the membrane is unfolded, the tension belt should be taken prior to the membrane installation to keep the arch stabilization. The temporary belt is subjected to a maximum force of 34kN (to keep the arch stabilization with one membrane panel attached).

2.3.2Membrane unfolding method

The membrane will be unfolded using one of the following two methods: Method 1 will be selected for unfolding the membrane on the rope net access, and Method 2 will unfold the membrane on the installed membrane panel.

Unfolding the membrane above the horizontal cable is Method 1.The tension belt mentioned above will be adjusted as shown in Fig.10, for arch stabilization prior to membrane unfolding.

Fig.10 Membrane unfolding method 1

Method 2 unfolds the membrane on the adjacent installed membrane panel and moves it to the installation area.

2.3.3Membrane unfolding equipment

The machine for unfolding the membrane (hand winch) will be attached to a gutter frame which installed above the tension ring. The maximum load of the hand winch is 1t. The feasibility of unfolding method has been proved during the mock up installation process in the factory. According to the winch frame installation procedure, the canopy bridging membrane and handrail should be erected when the related roof membrane has unfolded completely.

3 Membrane fixing

3.1 Fix membrane with tension ring

When the membrane has been unfolded and pulled towards the inner tension ring, the operatives will then maneuver the membrane to be attached to the inner tension ring. An aluminum profile is inserted into the membrane to facilitate the process. Then connection points are installed, and a ratchet strap is used from the membrane to the tension ring, in order to pull it to the designed position. Once at this position, a secondary profile will be installed on the tension ring, where a bottle connection will be achieved and the ratchet strap is removed.

To facilitate the membrane connect to the tension ring, the membrane edge that is fixed with tension ring should be dragged to reach the cable. During the membrane unfolding process, the membrane will be connected to the tension ring by the following three steps shown in Fig.11.

Fig.11 The steps of fixing membrane with tension ring

3.2 Fix membrane with compression ring

To facilitate the connection to the compression ring, the same process as connecting to the tension ring will be followed. This can be achieved by walking on the safety net. Considering the effect of the membrane self-weight, membrane boundary cannot reach the compression ring at the initial status, about 1.6m far away from the upper inner chord of compression ring. A puller belt (capacity: 2t) will connect the membrane with the roof steel structure, and drag the membrane to reach the compression ring. The fixing details of the membrane and the compression ring are shown in Fig.12.

Fig.12 The fixing details of the membrane with the compression ring

3.3 Fix membrane with horizontal cable nodes

The membrane will then be maneuvered by the operatives and then pulled towards the bottoms of the horizontal cable node and the strut node, positioned within the primary and secondary arches respectively. The aluminum profile will then be inserted into an open clap connector and attached to the structure by threading a bolt through the clamp and aluminum profile and into the threaded connection of the structure. All of this can be achieved by working on the safety nets, at this point, operatives will work under the membrane. For each membrane panel, a total of 11 points must be fixed. The fixing details of the membrane and the horizontal cable nodes are shown in Fig.13.

Fig.13 The fixing details of the membrane with the horizontal cable nodes

Before connecting to the cable node, the membrane edge should be clamped by the specific Al Profile. The combined profile will use M12 Bolt to drag the membrane panel to cable node.

4 Membrane connection and waterproofing

4.1 Membrane connect with arch

The membrane will have an aluminum profile attached, with bolts located in specified locations on the profile. Ratchet straps will then be used to pull towards the final position and achieve alignment. Once the membrane is located in the final fix position, the bolt will be inserted through a connection hole and then a retaining nut installed, and the ratchet straps will then be removed.

4.1.1Tools preparation for lift membrane up

A new tool is designed for lifting membrane and shifting the membrane edge on the plane. Two winches (each capacity: 1t) are fixed on the tools, one fixed on the top of steel pipe to pull the membrane up, and the other to adjust the membrane edge on the plane. According to the test results from the mock-up installation process, the lifting force is 115kg/m .

4.1.2Work platform preparation

1)Fabricated ladder access To reach the top of arch to fix the membrane with arch, the Al ladder and panel will be assembled as the working platform. The ladder will be fabricated in accordance with the design and calculations, and will consist of a two ladders with an intermediate platform in between. As a mobile system, a mobile ladder with a hooking system will be used to provide access to the associated access platforms. The system connection point to the arches will be easier by the arch’s alternate connection holes that are not in use. The system will be inspected weekly, according to theProjectTemporaryWorksProcedure.A life line will be installed to the top of the arch, so that when transferring from the ladder to the platform, a life line can be attached.

2)Soft ladder access Once the membrane panel is installed completely at one side of the arch, the soft ladder will be mounted on the panel for the opposite side panel fixing with arch. Carabiner will be installed each 1.5m along the arch to connect the soft ladder with arch. Soft ladder access is shown in Fig.14.

Fig.14 The soft ladder access

4.1.3Connection design details

The Al profile will be attached with membrane along the arch from both ends on the rope net access, and the length of profile will be set as 1.5～2m/pcs based on the feasibility of operation. As shown in Fig.15, the membrane and Al profile will be dragged by winch fixed on the lifting tool in the plane, and an M12 stainless steel bolt will lock the profile on the arch according to the design details.

Fig.15 Connection design details

4.2 Membrane connect with horizontal cable

Each membrane panel should have 12 pieces of membrane edge fixed with horizontal cable , as illustrated in Fig.16. These edges will be fixed from the upper tension ring to the compression ring. The membrane will then be attached to the horizontal cabling system with an aluminum profile and pulled down.

Fig.16 The membrane connection with horizontal cable

According to the practice of the mock up installation in factory, the membrane edge that connects to the horizontal cable will be lifted up when the membrane panel is attached to the arch. The distance between membrane valley edge and horizontal cable is greater than 1m. The space between the membrane and the rope net access is enough for the operation to connect the membrane to the horizontal cable.

Before the membrane is connected to the arch, the Al profile fixed the membrane valley edge with the horizontal cable should be attached to the membrane. The length of the profile will be set about 2m each piece. The hand winch (1t) and puller (1t) will be used to pull the membrane valley edge down. The final distance between the membrane valley edge and the horizontal cable must be adjusted to match the 3D geometry of the roof membrane. According to the analysis, the force acting on the strip under the dead load and pretension load combination is about 1.6～2.1kN/400mm. The final stress when the membrane is connected to the horizontal cable is shown in Fig.17.

Fig.17 The final stress after membrane and horizontal cable connection

4.3 Waterproofing installation

Considering the arch will shift during the installation of the roof membrane units, the waterproofing will be installed on side after 6 continuous bays have been completed. The scope of waterproofing includes membrane connect with arch, membrane connect with compression ring, membrane connect with gutter installed on the upper tension ring, and membrane connect with cable node.

Bridging membrane welding at 380 degrees is essential to create a fully sealed, weather tight and free draining roof. The actual temperature will be shown on the monitor and automatically controlled by welding machine. For bridging membrane installation, a rope ladder will be installed from both ends of an arch to the middle, with a carabiner used to fix rope ladder with stainless steel bolts. During the bridging membrane welding process, the bridging membrane will be welded from the middle point to both ends for each arch and the rope ladder will be dismantled step by step. The waterproofing details are depicted in Fig.18.

Fig.18 The waterproofing details

5 Conclusion

Lusail Stadium is one of the buildings with the largest roof membrane area in the world. This project solved a series of technical problems in the design and construction of large-span roof membranes.

The construction of stadium roof membranes should be completed in a systematic manner. The fixing, connection and waterproof construction of the membranes will have an important impact on the final quality of the membranes. The roofing membrane construction sequence and fixed connection method developed in this project have accumulated valuable experience for Chinese companies to conduct similar projects in the future.