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Savings in the frame

TECHNICAL FEATURE: 20-32 Baker Street: Transforming a block of Georgian buildings in the heart of London into a modern office is requiring unusual construction techniques. Mark Hansford reports from Baker Street.

Sherlock Holmes was always fond of an analytical challenge. But even Baker Street's most famous resident would have struggled with the engineering challenge being tackled just down the road from number 221b.

For 25 years the block of 300 year old buildings that make up 20-32 Baker Street have been awaiting development. But with City of Westminster Council determined to preserve the Georgian facade, winning planning permission has proved a lengthy business for the building owner, Loftus Family Property.

Two years ago, however, came a breakthrough. Georgian architecture specialist Erith & Terry finally found a solution that would satisfy both the planners and Loftus' desire for uninterrupted office space, and consultant Whitby Bird & Partners was recruited to carry out the detailed design.

The craftsmanship meets new technology solution has a steel framed structure spanning on to traditional load-bearing masonry facades, constructed off a concrete ground floor slab.

A central north-south corridor of the building houses stairs, lifts, braced stability frames and lightwell, from which long span 'Cellform' beams radiate to the masonry facades along the east, south and west elevations.

These beams support metal decking which is then topped with a 130mm lightweight concrete slab.

The traditional load-bearing facade is constructed using four different brick types and Portland stone, to give the impression from the street of three separate buildings. The facade also provides lateral stability to the structure Elementary? Not quite, explains Whitby Bird project associate Piers Greenan.

'Because we are maintaining the Georgian appearance, floor to floor heights are very shallow. So we have some very long span shallow beams, which services have to go through.'

To span the 13m plus from facade to central core, 420mm deep beams with 300mm diameter web openings at 450mm centres are sufficient. But not without significant deflections and rotation at the beam ends.

'The long spans produce deflections between 15mm and 20mm in the permanent imposed load case, ' explains Greenan.

'And this is enough to induce rotation.'

Were the beams to be set solidly into the facade, cracking of the load-bearing masonry would be inevitable. So an innovative pivoted connection was devised, within precast bearing blocks dubbed 'concrete doughnuts' on site, which are then set into the masonry.

Innovation in design, however, pales in comparison to the innovation in construction, which has slashed up to 10 weeks off the now 85 construction period.

Erection of the steelwork and internal elements of the superstructure is clearly a potentially faster operation than the construction and curing of the load-bearing masonry facade. A conventional construction sequence would have seen the steelwork subcontractor waiting at every floor level for the brick-laying to proceed.

Instead, contractor Skanska - appointed under an amended JCT 80 contract - worked with Whitby Bird to develop a construction method that would allow the steel frame to be erected independently of the facade.

The solution finally chosen by Skanska was a temporary and sacrificial dead shoring system.

This allows steel frame erection, concreting of floor slabs up to the inside face of the facade, and roof works all to be completed ahead of facade construction. It also offers programme flexibility, in that floors can be fitted out with services, raised floors and false ceilings while still in their temporary position.

The system works by suspending the permanent beams from a series of stub columns, the lengths of which are varied to allow for deflections of the permanent beams. Steel packing shims of varying thicknesses are installed between the top flange of the permanent beams and the underside of the stub columns to give a final level adjustment.

The precast concrete bearing blocks are then wedged up against the end of the permanent beams. As construction of facade progresses at a rate of around one floor/month, the beam levels are finally adjusted and the 'concrete doughnuts' are grouted into place.

Once the facade has achieved its required design strength - each beam extends 125kN on the masonry and lime-based mortar - the connection at the stub column is released, allowing the load to be transferred into the facade.

While this approach is not the cheapest - Whitby Bird estimates that the 150t of temporary steel has added 20% to the cost of the building - the client is happy with a eight week reduction in construction period. And Skanska senior temporary works engineer Simpson feels it could actually be bettered.

'The key is that with this system the brickwork is off the critical path and we could get the roof in early, ' says Simpson.

It is also a system that steelwork subcontractor Bourne Steelwork is comfortable with.

'We're very familiar with load transfer, ' says Bourne contracts director Mike Jones. 'We have used it on several other projects, and it is a well designed system that everybody can work around.'

With an 85 week construction period and work due to finish by the end of September 2002, construction is now well advanced.

The steel frame was erected within 12 weeks, and bricklaying has reached the second storey, allowing load transfers to begin.

Each transfer is carefully monitored for signs of movement. It is a nervous moment, as the temporary columns are set 1m in from the building facade, and any undue beam deflection could push the construction outside acceptable tolerances. But so far, to the relief of the project team, the telemetry has revealed nothing.

Inspector Gregory once famously asked of Holmes: 'Is there any point to which you would wish to draw my attention?'

'The curious incident of the dog in the night-time, ' replied Holmes. 'The dog did nothing in the night-time, ' said Gregory.

'That was the curious incident, ' returned Holmes.

One hundred years on, inactivity is again the key. This time the key to a mystery of an engineering kind.

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