Bristol’s leading green light

The superstructure was built around a central atrium stretching the building’s full height

The first BREEAM Outstanding office outside London is currently under construction in Bristol – the Aurora. James Kenny met up with the Willmott Dixon site team and developer Cubex. 

Office developments which push the envelope environmentally have, to date, been chiefly found in the UK’s capital. But that’s changing, and West Country property firm Cubex is one of the developers leading the charge.

Its flagship site is the 1.9 ha Finzels Reach mixed-use regeneration scheme in the heart of Bristol, where the jewel in the crown is Aurora, a new 8,825 sq m Cat A office building – and the first office outside London to achieve a rating of BREEAM Outstanding.

Bristol was the first UK city to win European Green Capital in 2015, and the seven-storey development was originally designed to comply with the local council’s requirement to achieve BREEAM Excellent. But developer Cubex made the decision early to push on and target Outstanding – demonstrating a performance level that is reached by less than 1% of the UK’s new non-domestic buildings.

Gavin Bridge, director at Cubex, explains why: “Being a regionally focused developer, we are looking to continually improve the design and efficiency of our buildings to deliver London-quality developments in the region.

“Increasingly, businesses are looking to relocate all or part of their staff from central London and, so to promote cities like Bristol to those businesses as a place to relocate to, we have to offer the very best space.”

To this end, as well as BREEAM Outstanding, the design targeted Leadership in Energy and Environmental Design (LEED) gold accreditation, and full compliance with the British Council of Offices (BCO) 2014 specification.

Bridge adds: “Sustainability through BREEAM, healthy buildings through the Well standard and smart-connected buildings through the Wired certification [a commercial property rating system for digital infrastructure] are the three benchmarks that are needed to stand out from the competition.”

Designed by local architect Bush Consultancy, the seven-storey development has floor plates of just over 1,400 sq m and a central glass atrium.  As well as basement car and bicycle parking, it offers unusual features which have an eye on emerging technology, such as a rapid electric car charging facility on the ground level and a drone landing pad on the roof.

The £17.5m construction contract was awarded to Willmott Dixon in July 2016. Operations manager Gareth Williams says the BREEAM Outstanding requirement “focused minds” early in the design stage.

Installing exterior glazing was complicated by the need to safeguard protected plane trees

One of the earliest activities was a life-cycle assessment of the building. The BREEAM process includes a functional adaptability assessment which considers the ability of the structure to accommodate future change of use. Both concrete and steel framing options were considered and, after a carbon assessment was carried out, the concrete option came out on top.

Williams explains: “We carried out the assessment and it verified our decision to replace the steel frame with a concrete frame – it demonstrated that around a 20% carbon saving would be achieved over the whole life cycle of the building.”

The energy strategy was also addressed in the early stages of the project. Energy-efficiency measures include a rooftop-mounted array of photovoltaics which will generate 29,800kWh per year of renewable energy, a sophisticated building management system to manage plant and building performance, and intelligent LED lighting with daylight and sensor controls. The thermal mass of the concrete frame will provide daily and seasonal temperature regulation, minimising peak loads.

Water-saving technology, including low-flow sanitaryware, leak detection and flow control devices, means a 59% reduction in water consumption is forecast for the Aurora, compared to a typical office building.

This combination of energy performance measures has enabled the building to provide a 37% energy saving against Part L Building Regulations, saving on operating cost and carbon.

The glass and aluminium facade incorporates vertical fins for solar shading

In addition, a climate change adaptation and resilience strategy was developed by evaluating the potential effects of climate change and the impact of extreme weather conditions.

“This was probably the biggest design challenge,” says Jonathan James, architect from Bush Consultancy. “Consideration was given early in the design phase as to possible uses the office could later provide, such as multi-tenant occupancy, change of use to hotel or residential units.”

Throughout the design and construction process, the health and wellbeing of the office’s eventual occupants has been a central concern.

As Alex Roberts, sustainability manager for Willmott Dixon, explains: “When people think of sustainability, they mainly consider energy or ecology. But health and wellbeing are as important as energy. We know that health and wellbeing play a huge role in the success of a company now and that a happier workforce means increased productivity and happier customers.”

In order to achieve this, a healthy building environment is being promoted, with optimised thermal comfort, high acoustic performance and glare control. Natural light connects the building’s users with the outside.

“A key factor in the design of the building was light levels,” James says. “With triple glazing-like performance, a near 100% glazed facade allows significant penetration of daylight on to the floor plate, thus improving daylight autonomy – the ability to operate without the lights on. The solar control glazing, with external fins, allows visible light in while blocking the entry of infrared and UV light. Low U-values hold the heat in during winter.”

The glazed facade allows natural light to penetrate the workspace, while the concrete frame regulates temperature

James says his design approach used a “fabric-first” ethos, meaning that material selection has been based on best performance, such as insulation values, recycled content and life-cycle costing. Products and materials have followed responsible sourcing and chain of custody policies, including FSC and BES 6001 certification.

However, even the most carefully executed design comes up against unexpected complexities on site and Willmott Dixon’s construction methodology has been complicated by the Aurora’s city centre location.

The brownfield site, previously a brewery, was remediated prior to the redevelopment. Made ground runs 4m below ground level, overlying 9m of alluvium above sandstone rock. After excavation of the basement, some 194 CFA piles were installed, 600mm in diameter and typically 12m long.

Work on the superstructure started in February 2017. It is designed around the central glass atrium which extends the full height of the building, and is accessed through a double-height reception. This lobby area was perhaps the most significant structural engineering challenge on the scheme.

Dave Berry, technical director at structural engineer Hydrock, explains: “The entrance lobby has a double-height, column-free zone, so all the structure above this area had to span 16.5m. This isn’t practical with concrete flat slabs, so we introduced a 3m-wide downstand beam at each floor level from second to seventh.

“However, due to height restrictions and ceiling zones, the overall depth of the downstand beams needed to be restricted to 750mm. Because of this relatively shallow depth, we introduced several interventions: the adjacent slabs became ribbed slabs to reduce the self-weight of the structure; a back-span was introduced for the downstand; and the beam and adjacent slabs were pre-cambered due to the instantaneous dead load deflection.

“Finally, columns were reintroduced above the second floor, so that all floors above could load share.

He continues: “By introducing these higher-level columns, a temporary column (and jack) was required below the second floor to transfer loads to foundations during construction. This prevented a build-up of stress in the second floor downstand beam as the floors above were constructed.”

Construction of the glazed facade, which also features vertical fins serving as solar shading, was complicated by the proximity of London plane trees.

Williams says: “The trees are protected by tree preservation orders, so we had to take great care to avoid coming into contact with them. There simply wasn’t enough room to use a mast-climbing work platform, for installing the large glazed panels, so cherry-pickers were used to complete the task instead, without having a detrimental impact on the programme.”

Achieving a score of 87.6%, the building received its BRE design-stage certification for BREEAM Outstanding in February 2017, and completion is scheduled for this June. Aurora was developed speculatively but is already about 45% pre-let – a clear sign of the growing appetite for office developments with strong environmental credentials.