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CIBSE Case Study Olympic Velodrome

--CIBSE 15:32, 29 July 2014 (BST)

Article from the October 2012 edition of the CIBSE Journal written by Andy Pearson.


[edit] Introduction

Creating the now-iconic velodrome for the London 2012 Games was something of a mean feat, with designers trying to break the laws of physics and create completely still air. Andy Pearson looks at how they made it work.

If you take everything you do and improve it by 1%, then that will add up to a big increase in performance,’ said Dave Brailsford, performance director of British cycling, in a recent television interview. He was explaining the approach adopted by the British Olympic cycling team that helped them win seven out of a possible 10 track cycling gold medals. Team GB’s Paralympic team topped the cycling medals table too, winning 15 medals at London 2012. That same philosophy of obsessive attention to detail is also evident in the design of the building where Team GB was so successful.

The Olympic Velodrome is situated at the northern end of the Olympic Park. The building’s curving red cedar timber walls and swooping oval roof are a tribute to the design team’s panache and engineering ingenuity. However, it is inside this iconic oval building that the real attention to detail starts to become apparent.

At the request of Team GB’s cyclists, the designers set out to create a facility that would enable them to ride fast enough to break world records. Surprisingly, it was not on the design of the track where the designers focused their efforts – instead, it was on the building’s environmental design. The cyclists wanted the designer to ensure the air temperature in velodrome was a tropical 28°C, because at this raised temperature, the air is slightly thinner. Thinner air means less aerodynamic drag, so the cyclists can move faster to shave milliseconds off their times. The fact that four world records were broken in the venue during the Games is proof of their success.

[edit] Refining a concept

Environmental engineer BDSP was part of the integrated design team responsible for the venue. Also onboard was Hopkins Architects, structural engineers Expedition Engineering and landscape designers Grant Associates. Since the team won the competition to design the velodrome in 2007, the designers worked with Team GB’s cyclists to refine their concept into the finished product. ‘We talked quite a bit with British Cycling at their Manchester HQ about what they needed and what they wanted from from the building,’ says Gustavo Brunelli, an associate at BDSP.

However, in addition to creating the ultimate cycling venue for the Olympics and Paralympic games, the Olympic Delivery Authority (ODA) wanted the facility to be able to be used by local schools and cycling clubs as part of the Olympic legacy. It was a difficult balancing act for the environmental engineers.

This meant that in addition to coping with 6,000 spectators and TV lighting during the Olympics, the services would also need to be capable of providing suitable legacy conditions for when the velodrome would become the centerpiece of the new cycle park, which will also include a new road track, a mountain bike course and the modified Olympic BMX track. At such times, as few as 50 people might be using the facility.

In addition to the legacy, the ODA also set demanding sustainability criteria. The building had a target of BREEAM Very Good; it also had to meet the requirement for all permanent venues to have a Target Emission Rate of 15% better than Building Regulations Part L 2006. The design team’s approach was to design the building for legacy use, knowing that it would need to be capable of being used in a different way for four weeks of Olympic and Paralympic competition. ‘We designed for legacy use and then we said: “How do we need to change the design to accommodate these major events?”’ says Brunelli.

[edit] Tracking a curve

The oval, 250m long Siberian pine cycle track is at the scheme’s heart. It was designed by Ron Webb, the man responsible for designing most of the world’s velodrome tracks; although its banked curves are slightly steeper than most, its geometry is fairly standard. The track is encircled by a lower tier of 3,000 spectator seating. Two additional banks of 1,500 seats form the upper seating tiers; these face the two straight sections of track. The main concourse separates the upper and lower seating tiers. Suspended above the arena is the cable-net roof structure.

The double-curved form of the roof is key to the building’s environmental strategy. The roof dips in its centre to shrink-wrap the building around the track, minimising the building’s volume. ‘The larger the volume, the more air you have to heat,’ explains Brunelli. This will be particularly important during the legacy period, when the temperatures may have to be raised to 28°C for competitions regardless of whether or not the auditorium is full. Under normal, non-competition conditions, a temperature of 18°C will be suffi cient. ‘The need to accommodate these two extremes shaped the building and the services design.’

An additional challenge from BDSP’s perspective was that the temperature of 28°C was required at track level, while at the same time it needed to be cooler in the auditorium where the spectators were seated. ‘The conditions demanded a solution that went against the laws of physics,’ explains Brunelli.

Radiant heating was not an option; the cyclists wanted the air to be hot. Blasting air downwards with powerful fans was out; the cyclists were insistent that they should not be able to feel air movement. In addition, the lightweight roof construction necessary to provide a column-free environment below, coupled with a fear of water leakage onto the precious track, meant that no plant could be mounted on the roof and no ventilation openings could be punched through it.

Despite their best efforts, BDSP were unable to break the laws of physics. Instead they developed a heating strategy for the velodrome based on three main elements: underfloor heating, under-seat heat and over-track heating using jet nozzles. Underfloor heating warms the in-field area in the centre of the track oval and the concourse. The system has been sized for legacy day-to-day heating to keep the auditorium at a set point of 18°C on a cold winter’s day.

For competition events, when the temperature needs to be at its peak, additional heat is provided through grilles beneath the spectator’s seating. Adding warm air beneath the seating helps distribute it homogeneously to avoid drafts and pressure fluctuations. ‘This displacement system will help create a small amount of air movement, to help spectators feel a little more comfortable, even though the air will be warm,’ says Brunelli. In addition to providing extra heat, the grilles also provide fresh air if called for by CO2 sensors.

[edit] Jet powered

The final element in the heating solution is the jet nozzles. The nozzles are set around the base of the upper tier of seating to supply heated air upwards, at high velocity, over the track. This helps mix the air preventing it from stratifying and making conditions even more uncomfortable for spectators. The jets will also be used to bring the venue up to temperature quickly between competition sessions. Extensive CFD modelling was used to ensure the jets did not create too much air movement at track level.

But what is too much air movement? The cyclists did not want to feel the air moving, but according to Brunelli, there is no metric to say what the maximum air velocity should be. BDSP turned to other sports for guidance; the most severe requirements were for air movement above badminton courts. BDSP used the same criteria that enables the flight of a lightweight shuttlecock to remain unaffected to define maximum track level air movement.

Twenty relatively small air handling units (AHUs) supply the auditorium with heated air. The AHUs are hidden in the curving, wedgeshaped plant space beneath the upper banks of seating and the external wall. Fresh air is supplied to the AHUs through wooden louvres set into the façade behind, just above concourse level. The AHUs are sized to deliver fresh air for 6,000 people.

They are arranged alternately so they either supply air to grilles beneath the spectator seating or to the jet nozzles. Rather than have all the AHUs operating all of the time, the system is matched to the heating and fresh air loads by simply switching individual units on and off in much the same way as the modules turn on and off in a modular boiler. To help with the air distribution, the units are generally brought into use as matched pairs either side of the auditorium.

Air is extracted from the auditorium through a slot just beneath the roof, at the high point of the auditorium’s wall above the spectators. Depending on demand, it is either heated and returned to the auditorium, or exhausted through louvres integrated into the building’s red cedar façade. A thermal wheel removes heat from the waste air and transfers to the incoming air before it is exhausted.

To ensure heat is retained in the building in winter, the four main sets of entrance doors leading onto the underfloor heated concourse are lobbied with double doors, synched so that inner and outer doors are never open at the same time. The lobbies have top-up heat provided by air curtains.

[edit] Breath of fresh air

In summer and midseason, the building will be ventilated using natural ventilation alone – it does not need heating. The natural ventilation system follows a similar pattern to the mechanical ventilation. Insulated dampers will open to allow fresh air to enter through the lower set of louvres set into the façade. From here, it will enter the plenum beneath the seating before finding its way out into the auditorium through the grilles beneath the seats. As the air is warmed, it will rise and pass out of the auditorium through slots beneath the high points of the roof. Finally, it will make its way back outside through the set of louvres at high level in the façade.

Sizing the ventilation openings was critical for the system to work, and interaction between the engineers and architect was essential in securing the right size and number of openings in the façade and beneath the seats to allow sufficient air flow. ‘We generated charts for the architect with different combinations of the number and size of the slots so that the architect could decide on the most appropriate combination,’ says Brunelli.

BDSP also carried out extensive CFD modelling to check that the natural ventilation would perform under different wind speeds and directions. The design team had set themselves a nominal target that the interior temperature would be no more than 3°C above outside ambient in summer. The modelling showed that on windless summer days with a full house, for those spectators in the higher seats, this target would be exceeded with temperatures ‘at the high end of what is acceptable’ says Brunelli. ‘Our priority was always the cyclists. It will get pretty warm and a bit uncomfortable if you put 6,000 spectators in the space at the peak of summer – as, in fact, it did during the Olympics.’

The natural ventilation strategy is helped by the exposed concrete in the in-field area and the concourse. The building has no cooling, so the thermal mass contained in these areas helps to moderate temperature fluctuations.

Brunelli says BDSP would have liked to test the NV strategy in the summer, but couldn’t ‘because the Olympics were taking place.’ Instead, they made do with feedback from February’s World Cup cycling event at the velodrome. ‘We tried different ventilation permutations, but conditions were not ideal because it was the wrong time of year to be operating in natural ventilation mode.’

Natural ventilation will also keep the support rooms beneath the lower tier of seating comfortable. These spaces include a gym, changing rooms, offices, meeting rooms, storage, a shop, workshop and a bicycle hire facility. The gym and meeting rooms will also have a mixed mode option with local district expansion cooling if needed. The main mechanical plantroom containing the heat exchanger from the site’s district heating, the calorifiers and cold water storage are also tucked under the lower bank of seating.

These rooms even have a night cooling mode to keep running costs to a minimum when Lea Valley Regional Park Authority takes over the venue – further proof that the design team has enshrined Dave Brailford’s philosophy of obsessive attention to detail throughout this iconic structure.

Modelling was used to ensure cyclists and audience were comfortable at all times. The simulations show cross sections through the arena to show distribution in both summer and winter.

[edit] Let there be light

Most velodromes are dark boxes that hide the cyclists from the light of day, but this velodrome is different. It features a large area of glazed rooflights to flood the space with daylight. The rooflights are supplemented by a band of glazing surrounding the concourse.

The areas of glazing reduce the need for artificial light during legacy operation where a uniform light level of 300 lux on the track and in-field areas will be sufficient for general use and the area of rooflights was optimised on that basis.

It was important, however, that the rooflights did not create contrasting light levels on the track and under no circumstances should they permit direct sunlight to land on the track because this would be distracting and could result in an accident. In addition, the architect didn’t want too many openings in the roof to compromise its insulation or to create too many perforations, which could be a potential source of water leakage.

BDSP worked with architects to look at different rooflight confi gurations. The best option was found to be strips of rooflights. This configuration was also easier to integrate into the roof’s supporting cable net structure. To prevent sunlight entering, the inside pane of the double glazing has two PVB interlayers, set at 90 degrees to each other, to diffuse the light.

The rooflights are supplemented by 356 artificial lights – Philips 1kW ArenaVision – designed to give a level of 2,000 lux for TV, but also to have the capability to give lower light levels of 1400, 700 and 300 lux for the different events.

For most of the year, the artificial lighting will be used in the 300 lux setting. The lights have been configured so that 300 lux can be achieved with four different sets of fittings and the controls are designed to use each configuration in turn to equalise lamp life. ‘It was a very complicated aiming exercise,’ says Brunelli.

A far higher level of power resilience was needed for the Olympics than would be needed during the velodrome’s normal operation, even during legacy events. As a result, the building has been designed to allow a back-up power plant to be trailered in by lorry and hooked up to the electrics.

Simulation of lux levels from the rooflights. Designers were looking to achive 300 lux of natural light distributed evenly on the
track to minimise need for atificial light during legacy use

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