ISG’s ripping yarn in central London

‘Tearing the guts out’ of 20 Old Bailey was undertaken with minimal structural work and limited impact to the outside, reports Stephen Cousins.

In a gory dialogue worthy of a murder case, David Levy, project director at ISG, describes a “cut and carve” job that involves “tearing the guts out” of an office he is reconstructing in east London.

It is fitting, then, that the 20 Old Bailey project stands directly opposite the most famous law court in Britain, and that its complex onsite delivery has all the excitement and intricacy of a criminal case.

The £59m conversion of the existing 1980s-built office into a contemporary high-spec commercial workplace had to be delivered in just 21 months, from demolition to practical completion. This resulted in the controversial decision to carry out all demolition at the same time as construction.

The logistically complex intersection of material deliveries and demolition waste removal is taking place alongside numerous trades that would never normally come into contact — as you read this two stair cores are being knocked down while, 20ft away, new plasterboard ceilings and lights are being installed. Work started in September 2015 and will be completed in June 2017.

A forensic investigation of the existing building enabled the project team, led by main contractor ISG, architect BGY and structural and M&E engineer Arup, to push the structure to its limits, adding two new floors on the roof and increasing net lettable space by 25%, without the need for any major strengthening works.

However, some anomalies remained concealed until demolition was underway resulting in hasty redesign work, which was a particular concern given the large amount of offsite prefabrication involved.

Organising the project required a grasp of logic even Sherlock Holmes would be proud of, comments Levy: “We have 20 staff on site, including three M&E specialists and two design managers, there has been a lot of hard work sat around the table examining the programme and the 3D model. You can’t beat getting a lot of brains together in a room, it gets things solved.”

Limited performance

20 Old Bailey will provide approximately 240,000 sq ft of office accommodation. The existing eight-storey office block, built in 1989, was structurally sound but very limited in terms of functional performance.

The post-modern stone facade looked confused and dated compared to the classical-inspired Old Bailey opposite (built in 1902) with its grand columned facade and rooftop dome capped with a bronze statue of Lady Justice.

Thermal performance was poor, the internal layout was inefficient, with two entrances, four perimeter stair cores, two lift cores and poor access to natural daylight.

Above: The exterior of 20 Old Bailey belies the complex remodelling behind the facade. Below: Looking up the atrium from the ground floor with Simon Featherstone, ISG’s project manager

Studies were carried out by Arup to determine whether to retain and adapt the existing concrete structure, or simply demolish and start again. The huge 60m-long by 50m-wide floorplates, three-storey basement, and robust columns and foundation piles offered great potential for repurposing.

Cost and sustainability concerns were key drivers behind the decision to work with the old structure, says Levy: “Knocking down and rebuilding would have required an awful lot of waste, lorry movements and related fumes. If you can retain as much of the building as possible you save time on construction, which ultimately cuts costs for the client as they get their rents in sooner.”

The remodelling involves infilling the existing square atrium, at the centre of the building, and knocking through a new longer rectangular atrium, aligned west-east towards the front facade, that will extend down to ground floor level to maximise natural light.

Eva-Katharina Barile, senior architect at BGY, told CM: “We stripped it back to create a bold new organisational diagram with the atrium acting as an internal street connecting entrances in the east and west facades. When people enter the building they will immediately see the heart of the building.”

Spacious and open floorplates were made possible by infilling two existing cores, a staircase and service risers and pulling together seven new banks of lifts, a stairway, toilets and services, into a single large main core.

The facade was removed and replaced with a plainer, unadorned limestone elevation with squared off double-glazed windows. The Moleanos limestone, quarried in Portugal, was treated to achieve a close match to regular Portland stone.

Additional lettable space was created on the sixth, seventh and eighth floors by extending the stepped back floor plates in line with the new facade.

A new two-storey steel frame extension on the roof, also stepped back from the main facade to create balconies, was built on top of the existing strengthened columns and comes close to the maximum permissible by planners to maintain the viewing corridor to St Paul’s Cathedral

The new layout adds 25% net lettable space, compared to the existing building, but required almost no additional strengthening to the original structure.

That is remarkable for a refurbishment of this type, says Liam Duff, lead structural engineer at Arup: “We did a lot of ferro scanning to show lines of reinforcement and other site investigations to see how we could really push the structure and how much load we could add, targeting areas where there was spare capacity.”

Above: The project viewed along the ground floor ‘street’ from the main entrance looking towards the Fleet place entrance. The scaffolding is at the bottom of the atrium

No new foundations have been added, there was no column strengthening and very little beam strengthening. Loading on the frame was kept to a minimum by focusing on modern construction techniques and lightweight structures and finishes: the rooftop extension is a lightweight steel frame; the majority of plant was installed in the basement; all infilled areas have lightweight metal decking; and there is no new internal blockwork above ground floor level, just studwork and plasterboard.

The structure was even able to withstand loads from tower cranes mounted onto the steel roof — a space-saving alternative to running conventional tower cranes through the old lift shafts.

Levy comments: “Conventionally, you would want to run tower cranes through lift shafts, but we needed them to extract our demolition waste. People don’t normally put tower cranes on roofs but we decided that was the answer so we had to find the right solution.”

Temporary works supplier, Lucking & Clark, worked closely with Arup to devise an effective structure for the cranes to stand on that involves a gridded temporary works system rising off the permanent steelwork stood on the existing concrete columns.

The plan to run demolition works alongside construction is estimated to have saved six months’ time on site, compared to demolition and doing a new build. The project is due to hand over at the end of June.

Slice an imaginary section through the building and a bustling “antfarm” of trades is revealed working side by side on every level, from demolition workers to electricians, dry liners and steel workers.

Waste management

The two cranes on the roof work in tandem, one lifts and distributes materials, the other helps with construction. Lorry deliveries come in via a narrow side road and demolition waste is channelled down through lift and service riser holes to the basement level where it is driven out on skid steer loaders and lorries.

Total demolition rubble, to date, is 5,860 tonnes of concrete, 100% of which is being recycled, and 1,025 tonnes of mixed rubbish, 98% recycled. The high levels of recycling contribute to the building’s BREEAM “Excellent” certification.

Above: Looking down the demolished south west staircase

Unbelieveable as it sounds, the north-east corner of the building, from ground floor to basement level, is occupied by a fully functioning pub, the Magpie and Stump, which has remained open throughout construction. All the pub’s deliveries come in through the existing office, its kitchen extract vents had to be removed and replaced with new prefab ducts, rerouted to roof level for the duration of the works.

“It was quite surreal at Christmas seeing fairy lights through the pub windows through the dust of a  construction site,” says Levy.

Demolition to create the new main core was carried out in a stepped sequence, to avoid having to install a mountain of temporary works and risking a potential collapse.

Every other floor was demolished, new steelwork was installed to tie the structure back together again and support the new floors, then the intermediate floors were demolished.

“We used the permanent steel works as temporary works, it meant lot of toing and froing, crash decks out of normal sequence, but it got the job built quicker and cheaper,” says Levy.

The decision to prefabricate all the building services, including generators, pump sets, boilers and electrical
risers, was critical to slashing the programme time but required a focus on upfront design, and a strong grasp of the existing building’s structural eccentricities. It also reduced the ability to make many changes once construction was underway.

A 3D model of the entire project was created by Arup, embedded with a 3D model containing data from all 12 services suppliers, coordinated by lead M&E contractor Imperial Ductwork Services.

The M&E is being handed over in 14 separate packages – a result of ISG’s decision to unbundle the M&E into separate subcontract and/or supply only packages – with priority given to areas on the critical path.

The electrical service riser snakes around existing columns and, to ensure a tight fit, was installed in a series of straight and bent sections. The last two connections were site measured to take up any discrepancies/tolerances.

“The secret of jobs like this is to think about the final finishes when you are starting demolition so you can identify any potential problems well in advance,” says Levy.

Unexpected problems

Not everything went to plan. Unexpected issues, uncovered during ongoing surveys of the existing structure, resulted in late stage design changes and construction rescheduling.

Steel decks that infill the existing lift shafts and atrium rely on hefty steel bracket connections to the existing concrete columns. Scans of the columns revealed that much of the reinforcement was in unexpected locations, so the brackets had to be redesigned as bespoke to fit. As a result, work had to be reprogrammed to allow for the delay caused by the discovery of the rebar deficiencies. It meant the whole of the main core works, including lifts, toilets, a staircase and associated risers, was built a lot later than originally programmed.

The engineers were shocked when they learnt that columns had only 35mm of fire resistance cover, inadequate for current Building Regs. But boxing out the columns would have significantly extended the programme and radically reduced net lettable space. Fortunately, ISG was able to identify a very thin fireboard that could be plastered over to minimise the impact on space and time.

Levy comments: “On this project there have been lots of smart solutions that required a little bit more time on site, which was a small inconvenience compared to the huge overall programme savings achieved by moving key areas off the critical path.”