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BEARING THE LOAD

Refurbishing a six storey steel formed building in central London has involved some meaty structural shuffling to create open retail and office spaces. Jessica Rowson reports.

Refurbishment gives life to an old building and inevitably requires some structural chopping and changing – floor beams may need to be trimmed back and mezzanines and balconies added. However when an architect has issues with the location of a heavily loaded column at ground level, fundamental alterations to the load path through the building need to be made.

(click her for diagram)

This is the case at 240 Regent Street, London the site of the former Dickens & Jones department store. Behind the 1920s Portland Stone façade will soon be two levels for retail, four floors of offices plus a roof-top restaurant. The office space features an atrium with an ETFE (ethylene tetrafluoroethylene) roof.

An existing column at ground floor that came down in front of the glazed lift doors definitely had to go. To remove this, the load from the columns from the first floor and above had to be transferred into two new beams put across either side of the existing column at first floor level (see diagram). Steelwork fabricator Bourne Steel introduced a jacking system to remove the column while still supporting the five floors above – think of a conjuror whipping away a tablecloth to leave the china dinner service intact.

To do this, two "slings", made from flat plate and I-beam stubs, were hung down from the new beams. Two arms were also bolted to the existing column so that they poked loosely through the "slings." Jacks were then placed in the slings and the arms of the existing column and load was applied, pushing the two apart. When the column moved upwards, the team knew that they had successfully transferred the load coming down the original column from the floors above, into the beams running across the room.

"Once there is enough energy in the beams, the beam stops going down and the column goes up," says Bourne Steel Engineering managing director Alan Pillinger. "As soon as the column moves, you've reached equilibrium point and got the load in the column. The movement can be very small and so it safeguards the existing structure from big movements. People expect fireworks but it's actually very quiet and very slow. It's controlled, slow and measured – which is how you want it."

This system was introduced because it is hard to know exactly what load you have going down a column, he explains.

"The load path is not always what you think it is. There can be a variation as much 200%," says Pillinger.

He explains that the normal approach is to calculate what a column might need; a 300t jacking load applied to achieve a certain lift, but here, the amount of load applied is not measured, but just applied gradually until the point of uplift is achieved.

This meant that the load transfer was gradual and that the capacity of the new structure could be monitored before the existing column was removed.

"The twin beams are designed to deflect," explains Bourne Engineering Technical Manager Ian Johnson. "Not only do you check movement down at the column joint but also of the twin beams. If the deflection of the twin beams is getting towards more than 15mm and there's no column movement, you know something is wrong with the beams."

Alterations were also needed to the columns either side of the main entrance. The existing columns around the edge which supported the façade did not run straight down the building. Down to first floor, the columns were set close to the façade but at first floor, the column loads were transferred to a column set back from the façade by about 700mm via a cantilever beam.

The new development required all the columns to be lined up close to the façade. Bourne Steel provided a two column frame with tie beams that could take the load from the first floor straight down. However installing the three storey frame into the confined area proved tricky.

"The problem was getting the steel into the tight space," says Johnson. "We knocked a hole in the slab where we could drop the column down."

One of the tie beams for the frame had to be installed in an operating transformer room with very limited access. The work took place over one weekend morning when the transformers could be shut down.

"It was a short operation," recalls Johnson. "There was one man with a key who watched you in and watched you out."

Once the steel frame was in, the load was transferred to the frame by a Harjack. Harjacks differ from conventional hydraulic jacks. They are
mechanical jacks, based on a pair of wedges otherwise known as folding wedges. As the wedges are pushed together, the vertical load they apply is increased (see diagram). The old fashioned way with mechanical jacks was to use a hammer to bring the wedges together. The process in a Harjack is mechanised and the wedges can be brought together by turning a big nut.

"It's amazing – five floors of steelwork, a couple of hundred tonnes, transferred by turning a little wrench," says Johnson.

The project started on site in summer 2006 and is due to be completed this autumn. However these complex operations took up only the tiniest fraction of the project's two and a quarter year programme.

As Johnson says: "It took six to eight months to plan, but only one day on site."

Who's who

Client Shearer Property Group & Delancey
Contractor Bovis Lend Lease
Steel contractor Bourne Steel
Structural engineer Cundall Johnson & Partners
Architect Rolfe Judd

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