Tight scheduling and coordination is the bread and butter of any construction project. But what happens when you have just 90 minutes to lift and install primary steel columns and clear the street before being swamped by 90,000 football supporters? That was the challenge facing main contractor Skanska during construction of the £100m Brent Civic Centre building currently nearing the end of a 29-month programme in Wembley, north London.
The site’s proximity to Wembley Stadium, which is less than 200m away, meant the team had to take advantage of the adjacent road being closed during last summer’s FA Cup final and erect two of the building’s external steel columns supporting the ETFE roof during the game. In a slickly coordinated operation, steel contractor Bourne Construction Engineering deployed its mobile cranes into the street outside the stadium as the match started. The columns were erected and fixed to newly-delivered steel roof elements during the game, then plant was cleared as the match finished.
Seven months on, Brent Civic Centre is now close to Practical Completion, with the fit-out due to progress until 1 March. Confirmed in September 2012 as the UK’s greenest public building and boasting several innovative green technologies, the 40,000m2 structure consists of a steel and glass-clad office block to accommodate council staff, and an iconic timber-slatted “civic drum” housing a public library, community hall and civic chamber. It’s also a tour de force of design and construction challenges, including a post-tensioned concrete frame, challenging structural steelwork and 34 different cladding systems.
The roofscape in Hopkins Architects’ design is also a little unusual. The civic chamber at the top of the drum features a cone-shaped ceiling with a roof lantern at its centre, flooding the space below with natural daylight. Then, the full-height atrium features an ETFE roof supported by about 430 tonnes of steelwork, extending to form a canopy over the civic drum.
The rear of the building showcases cladding by China’s Yuanda
Consolidated functions
Standing in the shadow of the established icons of Wembley Stadium and Wembley Arena, Brent Civic Centre is designed to consolidate all the council’s civic, public and administrative functions under one roof. The project’s £100m price tag will be met through the sale of up to 17 of the council’s administrative buildings, including Brent Town Hall, plus additional borrowing. The centre is expected to save the council up to £4m a year through reduced operational costs such as heating, lighting and maintenance, as well as rents on leased offices.
Skanska won its £85m design & build contract through competitive tender, with Hopkins retained as design consultant through a novation agreement. “We won the contract based on our track record for delivering green buildings. The bid team also clearly demonstrated what Brent was looking for in terms of technical requirements and our understanding of the contract,” says Paul Davies, project director at Skanska.
A key aspiration for client and contractor was achieving the coveted BREEAM “outstanding” rating, which meant developing several innovative sustainable technologies including air handling units with integrated air source heat pumps, a system designed to halve the amount of fresh water used during construction, four green roofs, and a combined cooling, heating and power (CHP) unit that runs on waste fish oil.
“We work-shopped with the BRE to identify the various engineering innovations that would help us bring the energy performance certificate up to an A and therefore secure us the BREEAM ‘outstanding’ rating,” explains Doug Cooke, design manager for building services at Skanska. “As a public building our energy performance will be monitored yearly post-construction and sustainable design targets must be met in practice, so there is nowhere to hide on this job,” he says.
Brent Civic Centre is a building of two halves. A nine-storey L-shaped office block, accommodating up to 2,300 council staff, encloses a steel-framed, glass-fronted entrance atrium that occupies the full height of the structure and connects the council offices to the civic building. This is a five-storey drum with a public library on the ground floor, a double-height community hall and garden terrace on the first floor, and a civic chamber hall and meeting rooms above.
The atrium features the five-storey drum with public library on the ground floor
Steel cross-bracing
Hopkins’ office block has a pared back aesthetic and features the architect’s trademark exposed steel cross-bracing on the south and east elevations. But its exterior avoids a monolithic feel by including 18 different steel cladding systems, supplied by Chinese cladding giant Yuanda. In total, the centre uses 950 tonnes of structural steel, almost half of it for the ETFE roof.
Exposed fair-faced concrete also defines the look of the building. It is used in the minimalist office ceiling soffits, carpet-free stairwells and the circular structural columns. “It sometimes makes me nervous when I walk around the office floors because the soffits only have a few lights and sprinklers and it looks like we have a lot of work to do,” confesses Davies. “But that is the minimal design Hopkins went for, most of the services are concealed under the floor.”
The concrete incorporates 50% ground granulated blast furnace slag (GGBS) cement replacement, which boosted the building’s green credentials by reducing embodied energy. Local supplier London Concrete was also chosen to cut delivery vehicle journeys and therefore reduce emissions.
However, the GGBS hurt the programme when post-tensioning the building’s frame, says Davies: “The slab had to reach a certain strength before we could bring it into tension, and although GGBS looks as good as cement it takes longer to
cure, especially in the winter months. We didn’t fully appreciate this and as a result it imposed constraints on the construction programme in terms of falsework striking times and the amount of falsework on site,” he says. Fortunately, he was able to make back the lost time through other efficiencies.
The site is close to Wembley Stadium
The community hall on the first floor of the civic drum features a 22m diameter column-free double-height space, made possible thanks to an elegant, exposed braced steel structure designed to support the level three floor slab. The slab was first cast on a deep-trough profiled metal deck and supported on temporary works before the braced steel structure was fixed. This steelwork comprises 12 cranked radial steel tubes, which project out from a perimeter ring-beam positioned at mid-height in the room. The micaceous iron oxide-coated tubes form a tension ring at the point where they crank.
Before the tubes could support the floor above, the slab had to be jacked up by 40mm, then the steelwork was tightened to the correct tension and locked into position and the jacks were released to transfer the weight of slab onto the steels and on down into the civic drum’s concrete structural columns.
“The space expertly brings together the latest construction technology with simply detailed concrete and intricate steelwork, in my view it’s one of the building’s most spectacular features,” says Davies.
Also impressive is the civic chamber, a triple-height timber-walled cylindrical space at the top of the civic drum, which features a coned ceiling and roof lantern and is overlooked on the fourth floor by an internal circular glass balcony. The curved walls and use of timber serve to soften the stark lines of steel and concrete elsewhere in the building, as does the use of timber cladding fins on the rounded walls of the civic drum. Hopkins originally specified red wood for the fins, but at 300mm deep and 50mm wide and with some sections likely to be exposed to direct sunlight there was a danger of warping, so a German engineered timber was installed.
Energy efficiency improvements are expected to help the building exceed Building Regulations by up to 65%, with total energy consumption estimated to be 93.5kWh/m2, compared to 220kWh/m2 for a conventional similar building.
Natural ventilation
The passive design makes extensive use of natural ventilation and a highly efficient building fabric to maximise daylight and increase solar shading.
Engineering innovations include a multi-fuel CHP system that will provide around 11-13% of the building’s heating requirement. Unusually, the system will primarily run on fish oil residue, recently accredited by the Department of Energy and Climate Change as having the lowest carbon footprint of all biofuels.
Reducing the amount of pipework in the building and therefore heat wastage was an important factor. Ten out of the building’s 28 air handling units feature a unique integrated version of an air source heat pump which made possible a 15% reduction in pipework and a reduction in pump power inputs by 23%.
Lighting was expected to generate a huge 35% of the building’s carbon load, so bespoke light fittings using the latest LED technology were developed to halve this figure. Among a range of other energy saving technologies, the building’s basement car park will also include 47 electric vehicle charging points designed to cut staff vehicle emissions.
These future-proofing systems are a bold move by Brent to set itself apart as one of the greenest councils in the UK. With a site so close to Wembley, it’s certainly going to be in the spotlight.
Raising the roof
Brent Civic Centre’s atrium and “drum” area are covered by a huge flat-roofed structure comprising a network of long steel I-beams and inflated, ETFE “pillows”.
Supplied and installed by ETFE specialist Novum Structures and steel contractor Bourne Steel, the 31m high roof is supported by a total of five 30m long I-beams, which at their ends stand on columns on the interior face of the office block and the civic drum.
Before delivery to site, the steel members were cut into 15m lengths. Beams were lifted using tower cranes specially designed for the weight and reach needed to install them. A critical time came last May, during the FA Cup final, when two of the supporting steel columns had to be installed onto the civic drum and then attached to roof beams in just 90 minutes before fans left the game.
Stool fixings run along the top of the I-beams and support an 8mm-thick steel plate-insulated gutter with a lip, onto which the ETFE pillows are stretched and fixed. The frame also incorporates air pumps designed to constantly top up the pillows and keep them inflated.
The semi-opaque pillow structure is an important part of the building’s environmental strategy, allowing natural daylight in without exposure to solar gain. It also forms an insulation layer designed to help remove the need to artificially heat the atrium.
“I’ve seen recent projects with large entrance halls, and it’s amazing how the reception staff still end up using a fan heater to keep them warm,” says Paul Davies, project director at Skanska. “It will be interesting to see if this huge space can stay warm.”
A steel and glass roof lantern structure rises from the fifth floor of the civic drum building, the top of which pokes out through one of the squares in the ETFE roof grid like the Shard poking through clouds. Construction of the lantern was tricky and access restrictions meant that a small spider mobile crane had first to be dismantled into sections, then lowered onto the civic drum’s third floor slab. Once reassembled, the crane was able to lift each steel section of the lantern from inside the building in a strict sequence until the structure was complete.
The lantern roof projects above the ETFE roof, mimicking the tip of the Shard on a cloudy day
