Max headroom: How floor jacking could give old buildings a new lease of life

One of the biggest justifications for demolishing rather than refurbishing old buildings is constricted floor-to-ceiling heights. It is much easier to knock down a building than reconfigure the floors to meet modern standards. This is also particularly carbon intensive as most of a building’s upfront carbon footprint resides in the structural frame.

However, a team refurbishing a steel-framed building in the City of London has pioneered a solution to this conundrum which has the potential to give a new lease of life to buildings currently destined for demolition.

The building is part of the Bank of America’s campus in the City and was built for Merrill Lynch in 1999. It has the opposite, but related, floor-spacing problem with overly generous floor-to-ceiling heights.

Two of these floors were once the largest trading floors in the world, with a 40m by 45m footprint and slab-to-slab heights of 5m with a 1.5m deep service zone. The 25-year-old building was ripe for refurbishment, so engineer Elliott Wood was tasked with finding a way to squeeze more space out of the building.

Extending up was not an option as it sits next to St Paul’s Cathedral and was hard up against the protected viewing corridor ceiling. Going down was not possible either as the building was constructed over a section of the Roman Londinium wall which is a scheduled ancient monument. There are also various rail tunnels under the site.

The site is hemmed in by St Bartholomew’s hospital to the north and a pub to the south, and the street and a courtyard to the west and east. So, maximising the existing site boundaries was the only option.

Utilising some of the excessive floor-to-ceiling space by inserting a new floor made sense, but it meant reconfiguring the first and second floors and adding a new one to create three evenly spaced floors. Daniel Bassett, the Elliott Wood associate on the project, says the existing column locations were fixed, so the new floors had to follow the same grid.

“It seemed such a shame to basically replicate the existing floors,” he says. Jacking up the existing floors as one large unit to achieve the desired spacing was one option, but as far as Bassett was aware, this had never been done before.

The client was sceptical about severing floors, moving then reconnecting them. They were also sceptical about how the market would react to something that had not been done before

Daniel Bassett, associate, Elliott Wood

Rather than dismissing the idea as impossible, he sounded out the Mace team responsible for the Rising Factory, a large protective enclosure around the top of a residential tower on the Olympic Park, which was jacked up as each floor was built in 2019. They were very positive. “It gave us the confidence that it was technically feasible,” Bassett says.

Understandably, the client was reluctant. “The client was sceptical about severing floors, moving then reconnecting them,” he says. “They were also sceptical about how the market would react to something that had not been done before.”

A decision was made to develop and tender a conventional solution, where the first and second floors were demolished and rebuilt in new locations along with an additional floor. In a bid to reduce carbon, CLT slabs would be used over steel beams rather than replicating the composite metal deck and concrete slab of the existing floors.

A second “wildcard” option was also developed, where the first and second floors would be severed from the columns and jacked up in their entirety, level one by 2.3m to become the new level two and the existing level two by 1.2m to become the new level three.

The existing floors would be reconnected to the columns in their new locations and a new floor would be built at level one to give a clear space of at least 2.7m on each floor.

It was proven that it was half the carbon of the other approach, it saved about a third of the time on programme and was cheaper

Daniel Bassett, associate, Elliott Wood

Interested contractors had the option to bid for the second option as well as the first, with two compliant bids being returned. The second option turned out to be very appealing.

“It was proven that it was half the carbon of the other approach, it saved about a third of the time on programme and was cheaper,” Bassett says. This was due to the site constraints and the amount of work involved in demolition and reconstruction.

“We would have had to demolish the floors, carefully take the steel work out, refabricate it and bring it back and install it and the CLT,” explains Stephen Dorer, operations manager at specialist enabling works contractor Deconstruct, which won the job. “The biggest issue we had was getting a CLT contractor to price it because of the lack of hook availability.

“We went to six or seven CLT contractors in the market at that time and we only got one price – and even that was non-compliant.”

Bassett and Dorer worked together over a couple of years to reassure the client that the floor jacking procedure was going to work. This included taking them to other Deconstruct jobs where jacking was being used. These reassurances, coupled with the time, cost and carbon savings, eventually persuaded the client to give the green light to the floor jacking option.

As part of the reassurance process, Deconstruct took the risk on the structural frame in the event of anything going wrong. “This was a far diminished risk to us on the basis that the building had been stripped and you could clearly see all the existing frame and we’d walked the floors probably 10 to 15 times by that point,” Dorer says. “So we were quite well versed in what that building looked like and the way it was behaving.”

A point cloud survey of the frame was commissioned so the team had a full 3D model of the frame in the “as-built” condition.

The job was programmed to ensure that the floor slab could be connected as quickly as possible to the columns once it had been jacked up into its new position. The building is all steel, with lateral stability provided by bracing in the core area. The bracing had to be removed and reinstated to correspond with the new floor locations.

Perimeter beams were installed in advance of the floor jacking complete with studs ready to receive the L-shaped brackets needed to support the primary floor beams of the jacked-up floors.

An added complication was the fact that a Vierendeel frame had been employed on the east elevation. The underground infrastructure meant that only four columns on that side of the building are supported from the ground, with the loads from the other five columns transferred through the Vierendeel structure.

“This was probably the trickiest part of the job because we had to relocate two levels of the existing Vierendeel at the new floor levels while it was still carrying all the load of the building,” Bassett says.

With new structure ready for connection, the existing floor was prepared for jacking. The first job was to saw through the slab adjacent to the building perimeter.

A slot 50mm wide was cut – the tolerance needed to ensure that the floor did not snag on any obstructions as it was lifted.

The next job was to install some 50 jacks. These pull the floor up from above rather than pushing it up from below because there was nowhere to place the supports for the jacks at ground level thanks to all the underground infrastructure. And there was plenty of capacity in the frame.

“By the time the building had been stripped out so there was virtually no live load on the frame, there was more than enough capacity on the floor above to hang the floor below,” Bassett says. The jacks were located by the primary beams near the columns, with a metal cradle placed under the beams and connected to the jacks.

“The load was still taking exactly the same path – it’s just jumped up a level. But the loads are still fundamentally coming back down the same way through the columns,” Bassett explains.

The jacks were tensioned to take the dead load of the floor. Then the primary and secondary beams were cut where these joined the perimeter beam and columns.

Laser levels combined with sensors could detect any differential movement, with an alarm programmed to sound if this exceeded 5mm.

Bassett and Dorer were surprised how flexible the slab was as the jacks pulled up the lightly loaded parts of the floor more quickly than the heavier areas. This meant constantly stopping work and adjusting the jacks individually to maintain a level lift.

“We found ourselves shutting the jacks off a lot because we were getting differential movements that were slightly out of kilter. So we were isolating certain jacks – and that’s probably why it took us three days when we probably could have done it in two,” Dorer says.

The team modified the procedure for the second floor by weighing down the lightly loaded corners with water bowsers and managing how a huge, 21m-long transfer beam, integrated into the floor, was handled differently.

Each floor features the same type of transfer beam to span the section of protected Londinium wall below. These beams are 750mm deep, 150mm more than the primary beams supporting the rest of the slab.

The transfer beams were buried in the deep service zone of the original building, but this has been cut back to the minimum to accommodate the extra floor. The transfer beams have been lifted up by an extra 150mm, with the section projecting above the slab covered by the raised access floor. This meant separating the floor from the transfer beam.

The slab was cut next to the beam for the first lift, which made the slab more flexible, contributing to the differential movement. For the second lift the slab was cut next to the beam after the floor had been lifted into its new position to minimise differential movement. Then the beam was lifted independently by another 150mm.

These measures meant the second lift was much quicker – Dorer says it took a day and a half. He adds that the ability to adjust each jack meant Deconstruct could even take up the intolerances from when the slab was first laid.

With the slab raised to its new position, the L-shaped brackets were fixed onto the new edge beam and connected to the primary beams with the 50mm gap made good.

The new level one floor was installed – this features the same composite slab as the existing floors with difference the beams protrude above the slab to claw back some valuable floor to ceiling height.

The 400mm deep raised access floor will act as a plenum for air distribution,  which eliminates the need for space-hungry ductwork in the ceiling area.

Overall, all the floors will feature 2.7m of clear space between the raised access floor and the ceiling service zone. Extra space has also been gained by extending level five – which was stepped back to create a terrace – to the building perimeter.

 

The original plantroom at level six has been rebuilt and is now partially office space thanks to more modern, compact plant needing less space, getting rid of the building maintenance unit and incorporating some of the services in the basement. The stairs, risers and toilet positions have also been reconfigured to make more efficient use of the space.

It is such an environmentally friendly and socially beneficial idea that I think it’s got legs going forward

Stephen Dorer, operations manager, Deconstruct

The combination of the additions and the new floor have resulted in some impressive space gains and carbon savings. “In terms of high-level numbers, what it has meant for the project is that we’ve reused 80% of the steel work in the structural frame.” Bassett says.

“And we’ve upped the gross area in the building by 30% and the net area by 41% by having these much more efficient floor plates.”

Would the team repeat the exercise? The answer is a resounding “yes”; Dorer says Deconstruct are already considering using the technique on three other buildings.

“It is such an environmentally friendly and socially beneficial idea that I think it’s got legs going forward,” he says. “I don’t think this will be the last one that we do – there will be many more to come.”

Steel frame with composite metal deck and concrete floors are widely used for office construction and the shear studs which connect the steelwork to the concrete make reusing the steel very difficult, with the concrete being reduced to aggregate. Dorer says Deconstruct are looking at using the technique on concrete-framed buildings, but he concedes that breaking and making the connections between the columns and slab make this is more difficult. “It’s not impossible,” he says.

If moving floor slabs in existing buildings to create decent floor-to-ceiling heights does become the norm, then that is one less excuse to demolish and rebuild. And that can only be a good thing.