Steel Frame: Salford's test of tolerance
While delivering the University of Salford's new arts facility, BAM acted as both bridge-builder and steel contractor. Tom Ravenscroft learns how the team rose to the challenge.
Forming a new gateway to the University of Salford’s campus, BAM Construction’s New Adelphi Building, a £55m multifunctional arts facility, is in its final stages. Unusually, the building straddles a major route across the campus, with the blocks forming its lower segment positioned on either side of the road, while a steel-framed podium layer supports the upper storeys. This spans across the lower blocks and a 28.8 metre gap that accommodates the thoroughfare below.
In effect, the gateway is a bridge, so to build it BAM became a bridge-builder. But as this bridge was part of a building, it had to be clad, which meant creating an extremely rigid structure – even slight levels of deflection could damage the glazing.
This could only be achieved by using huge volumes of steel in the structural frame, formed from thick steel members that proved extremely troublesome. As the steelwork contractor struggled with the tolerances, there was an unexpectedly large volume of rework on site, including £400,000 of welding.
When the steelwork contractor went into administration, BAM then had to step in and take over the package. Little wonder that Simon Atkinson, BAM’s project manager, ended up learning more about steel than he thought possible. “I know more about welding that I ever wanted to. After this project I’m hoping for a nice simple concrete-framed building,” he says.
The challenging design of the New Adelphi was the result of its place in the wider strategy to densify the university by bringing the majority of its teaching facilities on to one site. This gave it bulk, as the building had to consolidate several departments in the arts and media faculty.
The building was earmarked for an irregular-shaped site alongside the main thoroughfare into the campus. But architect Stride Treglown struggled to pack all the university’s requirements into a single building – it needed a multitude of specialist spaces, including a theatre, recording and television studios, with a stipulation for large open-plan flexible spaces. The thoroughfare that ran alongside the site was simply in the way.
As the site was developed (from top down) the original separate concrete-framed blocks saw a steel frame introduced to span across the route. Once glazed and clad, the complex structure presents as a single gateway unit.
However, this walkway, which connects a new square at the heart of the campus and a block of recently completed student homes with Salford Crescent station, is an integral part of the university’s masterplan, and couldn’t be relocated.
Jon Healiss, divisional director at Stride Treglown, explains that this diagonal route was what eventually determined the overall form of the building. “The building was either too tall or took up too much public space if we packed it into the triangular site,” he says. “Then came the ‘eureka’ moment and I thought: why not just float the large rectangular studio floor over the walkway? After that the design fell into place.”
Clad to appear as one unified building, the lower levels consist of four concrete blocks. Three are clustered around an atrium on the building’s original planned site, while another triangular block stands alone on the other side of the walkway. Supported on these blocks is a rectangular steel-framed structure that connects the building on the upper two storeys. “Think of it as two separate buildings up to the third storey,” explains BAM’s Atkinson. “Above that we dropped a continuous floorplate across both buildings.”
The concrete lower levels, along with a basement, contain all the cutting-edge facilities that the university demanded, to attract students in a competitive higher education market. Over 40 separate acoustically protected box-in-box spaces (see box, p34) contain two television studios, six industry-standard recording studios, 12 amplified performance studios, 14 instrumental tuition rooms, a 100 sq m band-room and a fully operational 350-seat flexible theatre.
A double-height steel truss defines the perimeter of the building
Atkinson says the concrete elements were “more complex than usual, but doable”, and construction went as planned. “We were bang on programme with the concrete frame,” he says. “It needed to be precise to support the steel frame, but that fitted well and we were well within our tolerances.” However, the smooth running stopped abruptly with the commencement of the on-site assembly of the steel-framed upper two storeys.
Spanning between the two lower buildings in effect meant building a bridge – but an extremely challenging one. “Bridges are designed to be flexible,” says Atkinson. “You can drive cars and lorries over them all day and they will flex under the weight. When you put glazing on a bridge it becomes problematic. There is a much lower level of permissible deflection.”
BAM worked with structural engineer Ramboll to develop the design for the double-height steel truss that rings the perimeter of the upper two storeys. To achieve the stiffness required at all points around the facade, steels of different thicknesses were used dependent on the loads the beams had to carry. The steels varied in thickness from 10 mm to 55 mm, with the largest pieces, unsurprisingly, situated above the bridge on the north facade of the building and the cantilever at the building’s south-west corner.
Stages of construction
How the building structure was developed
The walkway that runs from the station (top) to a new square (bottom) defines the site’s organisation
At ground floor level the building is divided into four distinct blocks
The four blocks are connected at the upper levels by a steel-framed box bridging over the walkway
On the north of the facade the two concrete blocks stand 43.2 metres apart, and it was here that the frame was subject to the greatest stress and subject to the highest levels of deflection. Originally the architect wanted to cross the entire gap with one span, but this proved unfeasible due to the stiffness requirement, so a 10-tonne column was inserted as a support.
This still left a 28.8 metre maximum span that would require large amounts of steel to reach the required stiffness. Furthermore, a bespoke cladding system had to be developed that could cope with the higher levels of deflection. BAM worked with curtain wall contractor Alucraft and glazing manufacturer Schüco to develop a system that could perform under these conditions. This high-spec cladding was then used across the whole building as it worked out cheaper than two separate systems.
Due to the complexity of the structure as well as its critical relationship with the cladding, Atkinson decided to refer the work to BAM’s in-house technical services team to check all the calculations. “We wanted to make sure that the acceptable deflections wouldn’t be exceeded and the solutions put forward were acceptable,” he says. “This is something we often do when there are several parties involved, to check the co-ordination. If something goes wrong, there can be a lot of finger-pointing, which can delay the project.”
Although all the calculations proved correct, the thickness of the steel required to achieve the stiffness needed to support the cladding over the main span would prove to be problematic, including the complexity of working with such thick steel members.
For instance, among the key elements of the build were the welded joints linking six steel elements. Here the heat needed to weld the joints caused the beams to distort, sometimes up to 10 mm.
As the steelwork contractor had difficulty producing the elements to the required tolerances in the factory, a large amount of additional site work was required. To deal with misalignments as great as 25 mm, in many cases the steel beams had to be cut and replacement pieces welded in place onsite. The thickness of the steel made this an extremely daunting task, with some connections requiring 36 rounds of welding.
As we walk the site, Atkinson points out a joint that took over a week to weld. “For every member that had to be cut, two sets of welds were needed to attach the new piece, one at each end of the replacement member,” he says. “The thickness of the steel means that we needed a huge number of rounds of welding. This took a long time as the joint needs to cool between rounds.”
The amount of onsite welding was considerably higher than foreseen
In total, the bill for site welding reached £400,000. For every piece of steelwork that was altered, the structural integrity of the frame and the required stiffness needed recalculating, at further cost.
When the steelwork contractor went into administration, BAM took over and directly employed the onsite fabricators and welders. “In effect we became a steelwork contractor,” says Atkinson. “We directly employed some of the supply chain to maintain the warranties and paid the welders directly. Ultimately this was good as we ended up with a great deal of control.”
Almost inevitably the project is behind schedule. The original completion date was November 2015, however the issues with the steel have delayed the building. The university intends to occupy it for the start of the 2016/17 academic year.
“It would have been a tough job for anyone,” Atkinson reflects. “One steelwork contractor we spoke to called it the impossible job. We probably needed an extremely specialist bridge-builder that was used to welding such thick pieces of steel.”
Although the project has been a challenge, the steelwork is now complete and soon students will be entering the University of Salford campus through the “gateway”, unaware of the issues that the building caused. For Atkinson and the rest of the BAM team, this job will not be one they forget anytime soon.
Studio spaces demand acoustic separation
The university’s arts and media department crams a huge number of specialist facilities into a compact 16,000 sq m building.
With two television studios, six recording studios, 12 amplified performance studios, 14 instrumental tuition rooms and a 350-seat flexible theatre, it was essential that all the spaces were acoustically separated from each other.
To achieve this BAM built more than 40 separate acoustically protected box-in-box constructions, where the floor, walls and ceilings of the rooms are separated from the structure with an air gap, to significantly reduce vibration and avoid flanking transmission.
Weighted Standardised Level Difference (DnTw) is used to measure the difference in sound between two rooms – the higher the number, the better the sound insulation.
Building Bulletin 93 (BB93), which explains the minimum performance standards for the acoustics of school buildings, specifies that sound insulation between a music classroom and any other room is recommended to be a minimum of DnTw 55-60 dB. The facilities in the New Adelphi Building will exceed this, with levels of DnTw 70 dB.
To maximise the usable floor area, architect Stride Treglown developed a narrower “standard” 70 dB wall, with the acoustic design provided by Sandy Brown Associates. Each wallis 200 mm thinner than the standard construction, which created an extra 90 sq m of space within the building.
The final wall build-up consisted of 140mm concrete blockwork, a 160mm cavity filled with 100mm mineral fibre insulation, and three layers of 15mm plasterboard plus two layers of plaster. On site, BAM worked with Hoare Lea Acoustics.