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Mace on target with steel frame

SITE REPORT - An iconic new structure is under construction over the road from the Lloyds building in London. Adrian Greeman explains why, just like Lloyds, it will also have a steel frame

WHEN Mace was appointed construction manager for the new Lime Street insurance office building opposite Lord Rogers' iconic Lloyd's of London headquarters, it was given the option of steel or concrete for the two building frames. The choice, for a main tower and a second lower block behind it, was steel.

'My own preference in fact is for concrete, ' says Mace operat ions director Nick Moore.

'In the main it is much more flexible, and less weather-sensitive. With steel you get a squall of rain or a bit of wind and you have to stop.' He also knew concrete well f rom his exper ience of working in south-east Asia and felt happy with the material ? including the difficulties of the post-tensioning that would be needed for this tall and elegant structure.

The Foster and Partners design may not the largest planned for London but at 125 m it is sufficiently tall and complex, with a stepped rear frontage, to present a visual challenge that complements the Lloyds building.

Mace is nothing if not methodical and it set tled down to a detailed and serious assessment, helped by the contract process for the job, which emphasised early contractor involvement.

The manager therefore met potential steel contractor William Hare, concrete contractor Laing O'Rourke ? already on board for the concrete building core ? st ructural engineer Whitbybird and others, including foundations firm Expanded Piling, for a series of brainstorming meetings.

It soon became clear that steel had the edge, and overall would work out cheaper than a concrete alternative. A key reason was the weight of concrete needed which, in such a high building, would impose significantly larger loads on the piling than steel.

'We were already talking very large piles, with a 2.4 m diameter, for more than 30 bored piles under the main tower, ' says Mr Moore. There was little space to add more piles, so to support a concrete frame the only solution would have been to dig deeper, adding between 10 m and 15 m to the various pile lengths.

While this solut ion would have cost more it also would have brought the piles down through the site's London clay into underlying Thanet sands.

'Once you are down that far, into permeable sand and gravels, you have potential water problems as well, ' says Mr Moore. Despite some new techniques being explored by Expanded Piling for ribbed piles it was clear the cost and additional time would be a problem.

Other factors weighed in as well. Concrete would have caused additional delivery problems for the site, which is in a typically tightly hemmed in City location among the old Saxon pattern roads.

After working through a logistics exercise the team realised concrete would involve more vehicle movements than steel, 'although a ready mix wagon would be smaller and usually away a little quicker'. Steel requires vehicles to sit in place because very often it goes straight into posit ion.

Because the site's free space is extremely tight, limited laydown areas would have affected both methods. So the winning material, steel, is lif ted directly from the t rucks.

One of Mr Moore's objections to steel was more telling: the difficulties of dealing with steel decking for the composite floor slabs, which would require cutting and shaping. As well as being awkward this would be noisy and there are significant restrictions on decibel levels, especially dur ing office hours. The overall impact would be to limit working times severely for the slabs.

'The problem would not be so great on a normal building, ' says Nick Day, William Hare's director, because the slabs are mostly square and the cutting is reduced. However, the architect's design involves a complicated step back at three points on the building along with a curved frontage.

So during the early talks Hare proposed a system of cutting the steel off site, with the prepared sheets carefully labelled and measured for precise positions in the structure. The slight additional cost was well wor thwhile and work is much quieter.

'It has to be drawn and done piece by piece, ' says Mr Day. 'It adds an additional operat ion.' The steel frame itself is also precisely tailored, and about as lean a structure as can be achieved.

'It used to be said that the conservat ism of structural engineering meant if one was to design a car it would be a solid Volvo at least, ' says Whitbybird Associate Peter Chipchase.

'But we think we have got somewhere towards the Formula One car for this building.' A new type of tubular composite column devised by Hare together with Whitbybird and christened the Lancashire column was another component that helped save cost and weight. It is built around an H section with plates welded to the open sides and the whole surrounded by a tubular casing. Once installed, the outer spaces are filled with concrete and the resulting column has an inherent fire resistance.

'It means you don't need cladding or intumescent paint, which saves costs and also increases the floor space, ' says Mr Chipchase. And it saves on the cascade of follow-on work, which suits a time conscious Mace.

Whitbybird has been charged by the client to reduce the response of the 150 mm thick concrete composite f loors, which requires stiffening some of the beams. These are up to 11 m long and sit onto other beams rather than columns.

'Response characteristics these days are the defining issue with beams rather than strength or frequency, ' says Mr Ch ipchase.

'It is not that difficult to fabricate, ' says Mr Day. Hare uses a digitally controlled machining system for cutting and fabrication work.

'It automatically does the detailing and machining once the design is signed off, ' says Mr Day. As a result the team sits in together to go through the design and sign it off, after which it is committed.

Hare has factories at Bury and Scarborough to manufacture the 6,000 tonnes of steel elements required for the two frames, one 29 and the other 12 storeys. Progress has been reasonably good so far, according to Mr Moore, who says the concrete building core is 'back on track' making an average of one floor in four to five days. The core is being made with a jump form system, easier to adjust for any deviat ions f rom the line.

Mr Moore says the concrete structure is useful for the steel erection, providing a good solid starting point for the fixings.

So far the steel is going well too for pace, on the 38 week schedule. 'We have been picking 26 pieces a day ? that's a good figure, ' says Mr Day.

It will not be long before the frame becomes a new icon on the 21st century London skyline.