In Birmingham a two-block residential building that tops out at 22 storeys has emerged from the middle of an old subterranean car park.
Project: Arena Central – Project Dandara
Client: Dandara Living
Contract value: £40m
Contract type: JCT 2011 Design and Build
Main contractor: Galliford Try
Start date: January 2017
Completion date: May 2019
Nowhere underlines the resurgence in Birmingham more than the Arena Central development.
A collection of high and low-rise shops, offices and residential buildings are emerging around the existing Alpha Tower, a Grade II-listed office built in the 1970s and one of Birmingham’s most well-known architectural landmarks.
It is this development, as well as the Paradise Birmingham scheme a stone’s throw away on the other side of Paradise Circus Queensway, that will help cement the city’s status as a focal point for commerce, technology start-ups and new media platforms.
Amid this bustle of activity, main contractor Galliford Try is focused on a scheme that will offer housing for those looking to live in the heart of this regeneration area.
Galliford is two-thirds of the way through a £40m 324-apartment build-to-rent scheme for client Dandara Living, one that will significantly boost the number of homes available in close proximity to Birmingham’s city centre.
Comprising two blocks of 17 and 22 storeys linked by bridges at each level, the development will feature studio, one-bed and two-bed apartments with balconies across all elevations.
Convenient parking space
Galliford Try project manager Aidan Smith is overseeing the building, which is located within the footprint of a subterranean car park that used to serve the Alpha Tower and hotel behind.
That car park was a two-level facility built below ground with an open central section. The new building has been launched largely through that central void, with the reinforced concrete car park broken away in areas to accommodate the new development. The building ties into the remaining sections of car park to provide spaces for its residents.
galliford try arena central 1
“Some of the car park has been incorporated into the design for the development,” Mr Smith says. “Our temporary works department had to look closely at the design of the car park to accommodate the different loading on the structure during the demolition process, as well as tying it back into the new building as it came up from the ground.”
It is the nature of the site itself that has dictated the building’s design and construction, with the car park not the only subterranean test for the project team.
Keeping Network Rail happy
Railway approach tunnels serving Birmingham New Street station dive beneath the site at its south-eastern edge.
With sky-high Network Rail fines for work that impacts the safe running of those tunnels, the development team was keen to stay as far away from them as possible.
“We needed to show Network Rail that our design would not influence their asset,” Mr Smith explains. “We needed to develop the design so that the analysis of the structure and ground conditions proved there would be no adverse impact on the tunnels.”
The rather more mundane presence of major service bundles running down the footway and alongside the site has also influenced the design.
With the structure being built within the footprint of a car park that served the hotel and Alpha Tower office, the team has been careful not to disturb its neighbours – and the railway tunnel that runs beneath the site.
Throughout the project, monitoring stations have kept kept track of vibrations and noise to avoid any disturbances, particularly during the demolition phase. This saw specialist subcontractor Armac Group broke out part of the structure of the reinforced concrete subterranean car park before it was tied back into the final design.
“We needed a car park for the final development and by tying parts of the existing structure into that of the new, we avoided disturbing the railway tunnel as much as possible,” Galliford’s Mr Smith explains.
Initial plans were that the building would feature an undercroft at ground-floor level thanks to a row of columns supporting the structure above. Mr Smith admits that the risk involved in working around the crowded services pushed the design team to make a few changes.
“I wanted to avoid working in that area as much as possible,” he explains. “We managed to change the design from column-supported to a cantilevered solution, which reduced much of the risk to those services.”
The frame for the dual-block building is largely precast concrete with a cast in-situ reinforced concrete basement and ground floor featuring a 1.8 m-thick transfer slab – cantilevered across the south-east corner. A precast concrete frame springs from this slab from levels 1-22 on block A and 1-17 on block B.
All that concrete makes the building relatively heavy, but the geology around the site has allowed the team to avoid a piled solution for its foundations. Instead, it relies on the Bromsgrove Sandstone bedrock to bed its 3 m-thick reinforced concrete raft foundations.
This was broken out to depths of 1.5 m to enable the team to place and tie the 40 mm-diameter steel reinforcement bars at 125 mm centres and cast the C50 design mix concrete.
“We had to break out around 1-1.5 m into the sandstone to accommodate the 3 m rafts,” Mr Smith explains. “There is a lot of steel reinforcement within those rafts. It is very congested.”
Concrete design to start with
The building’s design features one stair and lift core in the taller block, which provides access and emergency access for both blocks. Like the rest of the building, this is formed of cast in-situ concrete to the first level and then precast concrete beyond.
Precast concrete had always been the material of choice for the frame construction and cladding (see box below), according to Mr Smith. The project team looked at various different options for the structure and its subcontractors, before settling on the team at Creagh Concrete to deliver and install the frame panels, cladding sections, balconies and floor slab.
The hollowcore floor slabs are lifted into place with reinforcing steel rods used to stitch the sections together, before a final screed is placed to address any level differences and deliver the final slab level.
galliford try arena central 7
“We knew we wanted the robustness that precast concrete offers and spent a lot of time researching our subcontractors,” Mr Smith says. “Creagh has the capability and experience to deliver both the hollowcore floor slab and the architectural precast concrete that we wanted. The screed that we place on top of the floor slab is there just to iron out any changes in level. It gives us a little wriggle room.”
The placing of each level is based on a turnaround time of just 10 days. Prefabricated bathroom pods, manufactured in Hull, are lifted directly into position at each level as the building edges further towards completion.
It is important that these are delivered between days five to seven – any later runs the risk of disrupting the project programme.
The apartments themselves are served by a sprinkler system that also extends out into the communal areas. They are electrically heated, while the size and shape of the floor slabs did not afford the team the opportunity to develop a prefabricated M&E solution.
With a turnaround time of just 10 days to place each level, the Galliford Try team is running the following trades just two floors behind.
That speed and efficiency of construction should see the team hit the handover date of May 2019 with room to spare.
Precast proves its worth
The decision to use precast concrete systems for the bulk of the building’s structural frame, cladding and balcony units was taken at an early stage on the project.
The developer wanted a robust finish on the building, which would limit the amount of ongoing maintenance required.
With Creagh’s precasting facilities in Northern Ireland able to provide both the structural units and the acid-etched finished architectural cladding units for the facade, the Galliford Try team was quick to bring the firm in as main supplier on the project.
“We did lots of due diligence,” Galliford’s Mr Smith says. “We felt Creagh could deliver and complete the work on time and we liked its in-house design capability. The team there spent months perfecting the mixes to make sure it could manage to consistently produce the dark grey of the facade.”
The bulk of the precast panels measure 8 m in length, with the floorplan based on a similar 8 m grid. They arrive on site from a holding point just outside the city and are lifted directly into position. The inner wall panels are formed from 180 mm-thick solid concrete and arrive with a perfect surface that just requires a few coats of paint.
The insulated structural facade panels are 300 mm thick and require no further treatment on site once placed, other than sealant between the panels. The balcony units weigh as much as 12 tonnes and are cantilevered from the structural frame. They arrive without the final glazing barrier, which is installed on site before the units are lifted into place.
“It is almost as finished as it can be when it is installed,” Mr Smith adds. “There is no treatment required to the outer face other than sealing between the units. It means we reduce the exposure to working at height and therefore the risk on the project.”