The smart engineering behind long-span stadia structures

2. What sets apart a stadium with an operable roof from other long-span roof projects?

Mark Sheldon:

There is an exponential growth in complexity. The structural design leader needs to fully understand the implications of issues that might be normally considered as ‘someone else’s problem’. He or she will need to work closely with the bogie designers to resolve forces into the structure emanating from issues such as emergency stops, friction in bearings, rail tolerances, panel skew, bogie skew, drive forces, buffer collisions, and operating wind speeds, to name a few.

One of the many stadiums we have designed with a significant operable roof is Marvel Stadium in Melbourne, Australia. Each of the two moving panels spans 165 metres and weighs in the order of 1-300 tonnes.

3. What engineering smarts are typically applied to ensure longevity and ease of maintenance of a stadium roof structure?

Garth Rowland:

Generally, a safety in design process is followed, whereby we work with the client and end-users and the design team to understand the type and frequency of maintenance required. The challenge, then, is to devise a maintenance process which can be safely, efficiently and expediently undertaken without compromising the project aesthetics and general function.

This includes consideration of materials, detailing, aesthetics, safety, access and egress. The facility maintenance needs to complement the stadium operations and not limit events which are increasingly 7 days a week, 52 weeks a year.

Peter Ayres:

Maintenance issues such as corrosion protection of steel elements need special attention. Elements that are not easily accessible should normally be provided with a paint system that has a longer maintenance free period, to reduce maintenance costs. Special attention must be given to elements and connections that are exposed to the weather, with inspection accessibility.

4. How does the choice of materials impact design and cost?

Garth Rowland:

Whole-of-life decisions are required; more durable materials can incur additional capital cost. If, however, the maintenance costs and disruption to stadia operations can be reduced for the project design life, the savings will dwarf the outlay with additionally improved performance for safety, and aesthetics.

There’s also an ever increasing importance for sustainable design and embodied carbon in the structure. This is a key project opportunity particularly given the scale of stadia projects and the public profile that can be harnessed to promote best practice to the community.

Peter Ayres:

The most common material used for long-span structures is steel, because of its lightweight, high strength-to-weight ratio, ease of fabrication and erection, and cost. The choice of the cladding material also plays an important role. In principle, this should be as lightweight as possible; hence, the use of tensile fabric in many of the new modern stadia.

5. What are the trends in long-span roof structure design that adds to patron comfort?

Garth Rowland:

The critical aspect of roof performance is coverage: shade cover in the summer to alleviate the hot conditions, and rain coverage in winter to avoid disruption.

Coupled with this are column-free solutions that facilitate unobstructed views of the playing surface. The trick is getting the balance between these factors, in close consultation with the client. That extra metre of roof coverage is the most expensive on the job but may well be critical to the stadium's performance. A recent example of where we have optimised this is for the Allianz Stadium roof that provides 100 per cent coverage for fans.

Mark Sheldon:

There is always a conflict for open-air arenas and those with operable roofs, to decide how big the opening should be. This is particularly relevant for projects with natural turf such as Marvel Stadium and Wembley Stadium. Too much roof and the grass will require a significant annual budget for turf repairs and replacement. Too little roof and the patrons get wet when it rains. The structural engineer tends to get drawn into things like new ideas for turf growth, and analysing crowd numbers and weather patterns on event days to help decide the optimum roof size.

Peter Ayres:

Providing a structure with a clear, unobstructed view from anywhere within the stadium is a must. Most new stadia make use of the roof structure to accommodate the stadium lighting, obviating the need for obvious lighting pylons. By ensuring that the natural frequency of the structure is such that no wind induced vibrations are possible, we can prevent any transference to the seating tiers below. Roof overhangs are becoming increasingly larger to facilitate spectator comfort, eg; rain and sun shading.

6. Where and how do you see operable roofs really delivering value to patrons and the owner?

Mark Sheldon:

Margaret Court Arena is a classic example of delivering value. It was originally a 5 000-seat outdoor venue as part of the Melbourne Park tennis precinct. Its configuration meant it couldn't be used as a separate venue; so, for about 11 months of each year, it was rarely used. During January, one of Melbourne’s hottest months, it is used for the Australian Open tennis tournament. With its hardcourt surface and fully exposed concrete surfaces, the temperatures for patron comfort can be stifling.

The new design encapsulates the playing surface, providing shade to many of the patrons during the tennis match. Importantly, the arena will feature an operable roof; so now, for the other 11 months of the year it has become a basketball/ netball/ concert venue, generating significant additional income for the operators.

The major upgrade we engineered for Rod Laver Arena was also a game-changer for the client. Previously it took around 30mins to close, and as the tennis must be played with the roof open whenever possible, a storm event played havoc with the broadcasting whilst the roof closed and people manually dried the court. By designing a completely new drive system and software we were able to reduce the closing time to less than 5 minutes, which means now it just an extended ad-break for the broadcasters and then they are back into the action.

Peter Ayres:

Financial viability over a long-term is possibly one of the most critical aspects that impact large stadiums today; so a cost-effective structure that is flexible to accommodate different types of events, e.g. concerts, and not just sports events, with low-maintenance requirements, is what we should be targeting.

Other considerations include:

• The weather pattern (rain, snow, wind) in the area will probably play a large role in deciding whether to make use of an operable roof. Areas with long periods of bad weather may need this to improve the standard of play and to draw the spectators in, providing them with a better environment in which to watch the game. This will improve the owner’s revenue.
• The type of sport envisaged for the stadium or arena will also play a role.
• The provision of a retractable roof may, however, have a significant impact on construction, maintenance and operational costs, and could ultimately affect the viability.
• Operable roofs are likely more viable for smaller multi-use sports arenas, where clear spans are not as large.

7. Wind engineering is fundamental for long-span structures. How can engineers add value in this area?

Mark Sheldon:

A wind tunnel analysis is essential, to optimise the design of long-span structures. Using a normal code-based design approach could add over AUD1 million to the cost of a structure because of the lost opportunities to optimise the structure using reduced loads. This is critical when dealing with architectural structures, where we strive for elegance. In conjunction with some of the leading wind engineers in the world, we have developed and applied advanced load response correlation and area-averaging techniques to optimise designs without additional cost burdens.

Garth Rowland:

The Australian wind code was not designed for the variety of stadia roof forms that we see nowadays; and even where it can be applied, the results can be conservative. Wind engineering adds certainty in design loads, which improves the structural understanding and facilitates reductions in steel tonnages from those, based on code pressures. Wind engineering is a prerequisite for stadia design.

Peter Ayres:

Wind tunnel testing to reflect the realistic conditions can significantly impact the design of long-span structures and, ultimately, lead to significant cost savings.

8. World-class stadiums often have unique and distinctive forms. What are the key drivers in optimising the form of a long-span roof?

Mark Sheldon:

The concept design phase is the fun time. You start with a blank sheet, and try to understand the clients’ aspirations and the key drivers before launching into the design. There is a science in how a seating bowl should be configured, and this needs to be resolved, first. Issues such as sight lines, aisle spacings, and different seat offerings are the obvious ones. There is also proximity to the pitch for players, broadcasting requirements, loading docks, segmentation/separation of players, spectators, officials, media, enhanced experience offerings, etc which also influences the bowl shape.

Then, there is the business plan for the venue wherein revenue can be generated outside of game day. Issues such as conference facilities, parking, corporate areas, access to dining, access for kitchens/catering/muster rooms/security/control centres/bump-ins/green rooms, etc., also need to be resolved. These will all dictate what will happen behind the seating, which will start to generate a roof shape.

Once a shape starts to evolve, roof concepts that best address this form can also start to evolve. This reflected our approach for TEDA Soccer Stadium in Tianjin, China.