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Take the A-frame

Broadgate Tower - London's latest skyscraper is nearing its peak. Martin Cooper reports.

Towering over Liverpool Street Station, London's latest high-rise structures are progressing on schedule. Steelwork for 201 Bishopsgate and Broadgate Tower development is close to completion.

To begin with some heavy structural engineering had to be employed at the base of both buildings to allow them to be built over four mainline rail lines (NCE 20 July 2006). A 2.4m deep concrete and steel-framed raft, built in 1999 to straddle the railway for a project which was then shelved, was strengthened and extended by adding another 800t of steel.

The smaller 201 Bishopsgate steel frame ts on the original raft structure, but Broadgate Tower reaches further out, which necessitated the extension to the raft and the six massive 25m tall steel A-frames that distribute the tower loads evenly over the raft and down into its piled foundations.

SOM is the architect and structural engineer. The first five floors of each building are suspended from the A-frames, which extend from the raft to level five. Each of these frames consists of two 20t box girders of 40mm plate steel.

The buildings are nearing full height - 35 storeys for Broadgate Tower and 12 for the adjacent 201 Bishopsgate. When steel work contractor William Hare's work is completed a total of 13,000t of steel will have been fabricated, transported to central London and erected.

At peak production late last year, the site team was taking delivery of 120t of steelwork every day. 'It feels not long ago since we were planning the delivery programme and doing trial runs to check the logistics would work with the size of the lorry loads, ' says William Hare's London director Nick Day.

'Routes into the City and trafc arrangements were all agreed with the Corporation of London and Transport for London and overall everything has gone well.

Loads were seldom late and we have generally matched or exceeded our planned piece rate [of steel erection].' William Hare was given the £34M steelwork contract for the buildings - about 8,000t in Broadgate Tower and 5,000t in 201 Bishopsgate - in December 2005, two months after starting work on strengthening the steel raft. Erection of the main steelwork got under way in May last year, with three tower cranes per building, including two jump cranes inside the tower.

Work has progressed smoothly, Day says, helped by planning that 'modularised' the steel elements where possible to reduce pressure on the cranes. Also, secondary steel was engineered in the early days of the project with service holes and lift shaft and cladding brackets prefixed to the steel when it arrived on site.

Columns have been fabricated at three storeys high to further reduce the number of crane movements and each of the main signature diagonal bracing members in Broadgate Tower consists of two 13.5m welded sections.

Clear oor spans in both the Tower and 201 Bishopsgate have been achieved with 20m spans and 12m bays, and beam depths of 500mm-600mm are typical - Fabsec beams in 201 Bishopsgate and plated girders in the Tower. It all adds up to about 300 pieces of steel per floor in the tower and about twice that number in each floor of 201 Bishopsgate.

'It has all fitted together well.

We have had to work closely with SOM and the cladding contractor Josef Gartner to make sure that all of the steelwork and fixings fit, ' says Day. 'Overall this is a very integrated project.'

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