Stadia roofs are complex structures which require significant architectural and engineering effort. Many modern stadium roofs are retractable because this creates maximum flexibility; however retractable roofs present their own set of unique challenges. In this article, Aurecon explores the successful design of these roofs in order to maximise both fan experience and all-important return on investment.
In the past, many popular, multi-purpose stadia roofs were simply domed. Today, retractable roofs – the next-generation of roofing systems – are convertible and can be opened up to let the sun shine in on good weather days, and closed during bad weather. This provides flexibility in creating the level of required level of weather protection while maintaining optimum play conditions, fan experience, and conditions for natural grass to grow.
Retractable roofs require clever design because they need to be able to move hundreds of thousands of kilograms of steel, glass, cladding and ceiling material, and mechanical and electrical services in a matter of minutes. Very few retractable roof stadiums are alike, and each one has their own support, traction and control systems that enable the roofing panels to safely and reliably fold back, retract or slide off the top of each other in order to expose the area they cover.
Margaret Court Arena, Australia
Building a solid business case for a retractable roof is key. Clients and developers need to carefully weigh in on the legacy of a stadium – in other words, what will the stadium be used for after a prestigious event has passed?
Retractable roofs ensure that a stadium can be used for purposes besides sporting events, such as concerts, exhibitions and shows. This flexibility in stadium usage helps build a business case for investing in a retractable roof.
Another advantage is ensuring that games, events and exhibitions aren’t cancelled on account of bad weather.
Fans and supporters don’t attend events if they know they are going to get wet. Thanks to retractable roofs, events can proceed as planned regardless of the weather conditions. The fan experience and the environment in which fans experience sporting events is far better when it involves a retractable roof. This is part of the financial business case for choosing a retractable roof.
Many of the popular domed stadiums that were built during the 1960s and 1970s required artificial turf as the enclosed structures wouldn’t allow adequate sunshine for natural grass to grow. Retractable roofs have been a game changer as they create a better environment for natural grass in the open conditions.
In projects like Wembley Stadium in London retractable roof panels open over stands on three sides facilitating grass growth as well as improving the quality of outside broadcasting due to elimination of large shadows cast on the playing field. These are other facets of the business case that compels sports stadium developers to invest in retractable roofs.
Wembley Stadium, England
In many instances, a stadium roof makes sound business sense for developers, but these roofs are also a double edged sword. In large stadiums, where they are mainly used for both sporting and non-sporting events, roofs protect the spectators from environmental effects such as sun, rain and wind.
But while the roof provides shelter and a comfortable environment for spectators, it creates a large shadow effect that can impact, for instance, in a football ground, turf and grass growth. This creates a catch-22 for the owners and operators because protecting spectators from harsh environmental effects essentially hinders the air and sunlight that needs to get to the grass in order to ensure its growth and longevity.
Overcoming these to arrive at an effective, efficient and dynamic retractable roof solution requires considerable input from architects, engineers, owners and operators. Just some of the solutions Aurecon offers developers include the following:
Adding translucency to a retractable roof provides a level of lighting to the grass that can facilitate growth. However, while the roof sheeting and panels can be translucent, the effect is limited due to the fact that a structure still needs to be created to carry the roofing. Forsyth Barr Stadium in Dunedin, New Zealand, is the world’s only permanently covered stadium with a natural turf playing field.
It was officially opened in August 2011 in time for the 2011 Rugby World Cup. It has been heralded with widespread praise for its great atmosphere and functionality. This was a visionary project. Throughout the entire planning stages of the stadium, the client (Carisbrook Charitable Stadium Trust) and the project team ensured that the turf remained at the forefront of the design in this ‘whole stadium’ solution. The stadium was built around the turf rather than the other way round. The key challenge for the design team was growing strong, healthy turf under a fixed roof made of ethylene tetrafluoroethylene (ETFE).
Aurecon engineers and modellers teamed up with turf experts the stadium architect and undertook rigorous studies using a test rig over a two year period to understand the effects of ETFE on grass growth under contrasting seasonal conditions in order to ensure the pitch would be of the high standard required for international sporting events.
Another possible solution to facilitating a healthy pitch is to retract the roof over the stands when the weather is good. Wembley Stadium, for example, has retractable roof panels that open over stands on three sides exposing the grass to sunlight when needed. Aurecon undertook the structural engineering of the arch and the roof, which required the use of advanced structural engineering to achieve the 315-metre span arch and elegant proportioned roof structures design. With the north side covered by the fixed roof, seven independently driven roof panels move in a parallel motion to the south to open over the eastern, western and southern stands. This provides an almost shadow-free playing field.
All-weather arenas provide an ideal solution because they can be used for entertainment purposes when they are fully closed. The acoustics are good and the necessary degree of shelter is provided for people attending the event. In this case, the retractable roofs create the flexibility that most developers need in terms of providing shelter. A striking example of a fully retractable roof structure is Melbourne’s 55 000 seat Etihad Stadium. The natural turf is kept in a healthy condition through the large opening of the roof, but there are portions of the grass that remain shaded when the roof is open. As part of the turf management process, localised “grow lights” are used to supplement the daylight.
TEDA Soccer Stadium, China
While stadium design is often driven by a short-term need to set the stage for a specific event such as the Olympics, the sustainable legacy of the structure needs to be considered. Future proofing a stadium means thinking ahead about the upkeep that stadiums require and how the investment can be maximised for future events.
Careful planning and out-of-the-box thinking helps architects and engineers design beyond prestige events, meet client’s requirements and simultaneously create a structure that will maximise the stadium’s usage in the future. For example, designing a base structure that can accommodate a future retractable roof would be a lot more economical than strengthening the stadium structure to be able to carry the loads of a retractable roof. Adding more area or changing or strengthening the roof to accommodate heavier wind loads at a later stage can be very costly, so the primary structure needs to be designed to handle potential future needs.
Aurecon advises clients to consider their holistic future needs and design for the necessary provisions to accommodate future needs in the base structure. The effect on fixed roof cladding, waterproofing issues, access, safety and future wind load capacity all need to be accounted for during the initial design.
Finding the right solution in terms of lightweight roofing design that can handle heavy wind loads as well as accommodate future expansion requires careful analysis and collaboration between the clients, architects and engineers.
Constructability is another key consideration because stadiums need to be built safely, economically and in the least time possible. Any temporary construction to the stadium or supporting construction of the roof will create a level of disruption and risk. In order to minimise the risk involved, the sequence of construction needs to be planned and the temporary works that are chosen need to be carefully considered in order to ensure that they are safe and economical.
To achieve a ‘prop-free’, fast and safe method of construction for RAC Arena (formerly Perth Arena), the roof, inclusive of all fixed and movable steelwork, and all the cladding and services where constructed fully at the ground level and winched in position in one quick 2 hourr operation. Approximately 2 500 tonnes (2 500 000 kg) of fully clad and serviced steel roof, larger than the size of a full soccer pitch, was lifted 23m in height.
RAC Arena, Australia
The knowledge of constructability issues is critical in design of long-span roofs of all types. The roof of the Express Rail Link West Kowloon Terminus in Hong Kong, for example is very complex. The terminus is a major cross boundary transport multi-platform project that includes a train station, rail link and below ground terminal, which serves as a gateway to Hong Kong. Aurecon was tasked with the erection analysis and geometry control of the steel roof structure for the development and the team of consultants had to ensure the roof steelwork was not over-stressed during erection.
In order to achieve this, both the curved roof structure and the inclined columns had to be temporarily supported during the various stages of roof construction. However, the temporary works needed to be removed as soon as feasible to allow other construction activities to proceed. This required a very detailed analysis and design of the construction sequence.