The unique waveform roof at Southern Cross Station is an eye-catching design solution that creates natural ventilation for trains below and a visual and physical link between two precincts of Melbourne.
This article was originally published in Blue 02: Systems and Structure in 2010. You can see the article in its original format, and other articles, online.
Southern Cross Station is Melbourne’s main transport interchange, straddling the edge of the city grid, and the newly regenerated Docklands precinct to the west. In 2001 the Victorian Government created a brief for the redevelopment of the existing Spencer Street Station, a terminus and interchange that was failing to provide the users of the public transport network with an experience appropriate for Victoria’s premier station and gateway to the regions. A landmark world-class facility was sought to accommodate the existing 15 million passengers that pass through the station each year and support the projected 35 million per year by 2050.
From the outset, the design team at Grimshaw were acutely
aware of the responsibility to meet the aspirations of the public transport
user by providing a well-designed interchange, whilst also understanding the
need to promote connection between the two disparate precincts on either side
of the rail corridor. The new station improves this by lining the three major
streets that it bounds, two of which are elevated across the tracks, with
pedestrian concourses. Glazed façades allow the inner life of the station –
both people and trains – to be exposed to the city from long vistas in most
directions. By removing the existing underground concourses and tunnels and
providing these street level concourses, the design team were able to introduce
airport-style customer information facilities and create a large waiting hall
with bars, food outlets and shops spread amongst the more traditional transport
services. This is all laid out under a single expansive roof that helps provide
the station with a coherent identity and the precinct with a landmark.
Equally important to Grimshaw was the ambition to generate the design from performance driven sustainability principles in parallel to the civic needs. The Government brief, whilst completely pragmatic in its transport and urban aspirations, did not embody any significant environmental agenda, requiring only that the “mechanical, electrical and hydraulic services of the Facility....minimise energy consumption without compromising the reliability, service delivery and specified accommodation standards”. In essence, the brief assumed that the designers would provide a traditional solution using mechanical services to ventilate public areas.
Victoria’s railway network continues to use diesel as well as electric engine locomotives. Trains are of different types and have exhaust points either in front-end locomotives or in multiple units along their length. Previous experience had taught us that the collection of diesel fumes in fireproofed ducts above each possible engine location, then filtering and discharging the exhaust, is both obtrusive and highly energy consumptive. These systems demand not only ongoing maintenance and substantial running costs but would also require additional visually intrusive building structure to support such services.
As an alternative the design team proposed that the 36,000 sq m station roof as well as the perimeter façades be designed as a true ‘skin’ for the station, protecting the inside from wind and rain while allowing it to breathe and ‘sweat’ – naturally exhausting contaminated air and aiding cooling during the warmer months.
Grimshaw worked closely with Advanced Environmental Concepts (recently renamed Built Ecology) to develop a natural ventilation strategy that would greatly reduce the impact of the station’s design on the external environment, lowering greenhouse gas emissions, greatly reducing the station’s dependence on carbon dioxide producing energy resources, and reducing plant maintenance. The roof form and detail subsequently evolved as a consequence, emerging not from any architectural agenda but pragmatically and purposefully from these environmental and performance-driven principles. The solution is a reinterpretation of the great historic rail ‘sheds’ of the nineteenth century, which provided high vaulting internal spaces able to collect rising smoke and steam in the apex of their arches before discharging them high above the station through clear-storey vents.
Continuous arches or barrel-vaulted roof structures similar to those seen in historic station halls, were quickly discounted for use on Southern Cross Station due to the proximity of the raised public concourses relative to the roof’s springing points at either end of the station. Instead, a series of offset vaulted domes are located above each track bed acting as reservoirs to the rising diesel fumes. At their apex, louvred vents are introduced, designed to maximise air pressure from the seasonal prevailing winds, and through the stack effect help draw the contaminants out of the space. When combined across the site a beautifully seductive undulating blanket is formed – like a desert landscape, or a mogul run.
The station’s undulating roof form also contains a cavity between the profiled aluminium top sheet and the soffit panels below. Warm and contaminated air enters this cavity through openings between the soffit panels, meaning that air flow is accelerated, taking advantage of the temperature differential as the space is heated by the external top sheet. This hot air then flows up and out through louvred vents at the apex of each dome.
The station ventilation design was developed through computer simulation techniques that enabled an accurate prediction of the air quality and behaviour within the station supported by best practice environmental design. Thermal Analysis Software and Computational fluid dynamics (CFD) were used to explicitly ascertain the potential for buoyancy ventilation and to calculate the behavioural properties of fluid.
CFD was used extensively to inform the design at all stages. Initially, it was used to demonstrate the merit and the thinking behind a mogul-like roof design. A simple 2D model can show that the wave shapes create areas of negative pressure at the top of the mogul. This model illustrated that local external wind conditions would favourably influence the capacity of natural ventilation exhaust volumes through the moguls with each vent having the potential for air flow rates of between 16,000L/s and 43,500L/s; compared with an alternative mechanical ventilation scheme, which was designed to achieve 15,000L/s.
The design was then taken further to test the indoor air quality of the station whilst diesel locomotives are running. Where bulk air flow analysis shows that the space can relieve heat build-up in the space through the stack effect, the CFD model shows that the temperature and properties of the exhaust alone is enough to drive the ventilation without simulating internal heat loads or solar heat loads that would further assist the ventilation flow rate.
The design also needed to determine a ‘worst-case’ scenario which was difficult to surmise. This could be a situation where trains are running at full throttle, creating more plentiful exhaust contaminants (however it could be argued that the full throttle scenario also creates extra heat and exhaust flow, thereby artificially enhancing ventilation quantities). Thus two adjacent trains were tested under this full throttle scenario along with a more typical train idling condition. Both events provided acceptable air quality results, with the CFD analysis confirming that the roof geometry would enhance the efficiency of the ventilation and that the passive design not only enables exhaust quantities to be achieved by virtue of the hot exhaust gases but also did not rely on solar gain or internal heat loads.
Using Grimshaw’s detailed 3D roof model of the moguls, which vary from 24 metres to 6 metres above the ground plane below, Advanced Environmental were able to assess the complete station geometry, local wind effects on actual ventilation flows, specific removal of train exhaust gases and particulates and determine the necessary apex louvre and façade openings.
CFD modelling enabled the design team to confirm with confidence that air quality standards were met, and predicted that both carbon monoxide and nitrogen dioxide levels complied with standards and health and safety requirements. It was also utilised to assess the design in a fire simulation. The same basic principles apply to the fire scenario as apply to the ventilation of diesel exhaust fumes. A 20 Mega-Watt “ultra-fast” train fire was tested over a time period of 8 minutes using the CFD model, with the results suggesting that the heat from the fire would drive the fire smoke toward the station roof cavity through natural buoyancy.
As well as exhausting contaminated air, the design supports the thermal performance requirements that needed to be met for the space, as the mogul design draws fresh air past the occupants, even on days where there is little or no wind. The roof and façades protect the whole station from wind, rain and excessive direct sun, and help minimise radiant temperatures within the space whilst maintaining the visual transparency through the façades, an important urban response.
Public facilities have been laid out within the station to maximise the benefits provided by the envelope. The concourse waiting areas are located away from the entrances and in general are not enclosed. Where necessary the waiting areas are heated using slab heating systems. Where additional intervention is required to provide an acceptable indoor thermal environment, supplementary heating and cooling is provided. As an example, an enclosed waiting area beneath the Collins St concourse has been provided with a dedicated air-conditioning system, a solution appropriate to such a space.
The installation of double skin ETFE fabric skylights directly above the platforms provide natural light to the passengers below and the insulating properties of the air within the ETFE pillow protects from excessive radiant heat. A light-diffusing frit has been applied to the skylight fabric, providing a scattering effect to the natural light and reducing the potential for daylight to act as a glare source. This design negates the need to operate electric lights during the day which would otherwise be required. The station remained in operation throughout its redevelopment, even as its roof was being assembled above the platforms. The ventilation solution also had to be functional throughout construction and this relied upon a balancing of free area around the station façades. The delayed completion of the Western façade created imbalance in the free area locations of each façade. This created some performance issues as diesel was not being evacuated as quickly as had been hoped. Thorough testing at that time, and at completion of the façades soon after, proved that the design met, and is exceeding all, performance targets. Roof air quality sensors continue to provide readings for the station operator. The installation of double skin ETFE fabric skylights directly above the platforms provide natural light to the passengers below and the insulating properties of the air within the ETFE pillow protects from excessive radiant heat. A light-diffusing frit has been applied to the skylight fabric, providing a scattering effect to the natural light and reducing the potential for daylight to act as a glare source. This design negates the need to operate electric lights during the day which would otherwise be required.
The station remained in operation throughout its redevelopment, even as its roof was being assembled above the platforms. The ventilation solution also had to be functional throughout construction and this relied upon a balancing of free area around the station façades. The delayed completion of the Western façade created imbalance in the free area locations of each façade. This created some performance issues as diesel was not being evacuated as quickly as had been hoped. Thorough testing at that time, and at completion of the façades soon after, proved that the design met, and is exceeding all, performance targets. Roof air quality sensors continue to provide readings for the station operator.
The success of the Southern Cross Station project stems from the complete alignment of aspirations within the design team. Working to a shared vision, the complex environmental solution was made easier to achieve. The support given to the design team by the contractor in finding the right design solution was imperative and should not be underestimated. While the roof was designed to ensure that fans could be retrofitted as a fall back scenario, Leighton Contractors allowed the design to adequately develop and be optimised through the early stages of the contract. The performance testing has reconfirmed that there is no need for any additional mechanical plant to aid the ventilation of the space.
Grimshaw continue to develop designs that confront the environmental responsibilities of the building and of the site which can often create the architectural form and provide a more responsible building. Southern Cross Station is highly energy efficient in comparison to a traditional building and has fulfilled the aspiration for both lighting and air conditioning tending towards a zero energy consumption. This is important – as energy costs increase, the project must not become a financial burden on the public transport network and take funding away from other transport developments.
The transformation of a maligned urban site into Southern Cross Station was an ambitious attempt to improve the nature of transport architecture and reinstate a level of amenity commensurate with the status of the station, while working to stimulate development throughout the precinct and towards the Docklands. If the station’s true architecture is internal, its exterior will become its symbol. The fifth elevation – the rolling landscape of the performative roof, when viewed from the high buildings surrounding it, becomes the beacon for this corner of Melbourne. It also creates a clear statement of intent that such public buildings must be more responsible to the environment in which they sit – both in terms of the civic and the sustainable.