ENGINEERS BUILDING London's 35 storey Broadgate Tower were this week battling to get the project back on schedule after losing about four weeks because of site difficulties and poor weather, NCE has learned.
Construction and erection of six, 25m tall, steel A-frames supporting the eastern base of the tower ran over by three weeks at the end of last year due to the work being more complex than first thought.
Coupled with this, high winds over the past two months have put tower cranes working on the site out of action for a week.
Labourers for contractor Bovis Lend Lease and steelwork contractor William Hare have been working extended shifts to reclaim the lost time on the project.
Designed by structural engineer Skidmore Owings & Merrill for developer British Land, Broadgate Tower is being built in tandem with the 12 storey 201 Bishopsgate as part of a £292M extension to the Broadgate Estate in the City of London (NCE 20 July 2006).
The difficulties erecting the steel A-frames occurred when engineers attempted to connect them to a 2.4m deep, twin-deck, steel and concrete sandwich raft that sits above the railway lines running into Liverpool Street station.
The A-frames do not distribute load evenly. Three-quarters of the loading transfers through the A-frames on to eight new ground-bearing piles on the west side of the building. Only a quarter of the load is transferred into the raft that extends over the railway line (see diagram).
Ensuring this distribution was accurate, and connecting the A-frames to the raft, took engineers longer than expected.
Winds reaching 160kph at the end of January and during February and March rendered the tower cranes inoperable, losing the equivalent of one week's work.
All six A-frames are now in place and both Broadgate Tower and 201 Bishopsgate are nearing full height (NCE last week).
A British Land spokesman said: 'The programme has had minor delays, largely due to the weather, but also due to the complexity of the job. We are looking at ways of mitigating the effects for follow-on trades [for example mechanical and electrical contractors]. We are still on target for completion in mid-2008.'