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Method behind the ma adness

STEEL

The design for Skanska's £67.5 million Palestra office scheme in London could never be described as conventional.But, as Alasdair Reisner discovers, the eye-catching scheme brings with it a number of technical challenges for the project team to get to grips with IF A BUILDING site could have a motto, then surely for Skanska's Palestra site in Southwark, south London, it would be 'you don't have to be mad to work here, but it helps'.

Let us start with the scheme's architect, Will Alsop.Not a man renowned for sticking to rigid convention, Alsop has come up trumps again with a wonky-looking multicoloured box that is a world away from the average staid city office scheme.Add to this the facts that the project started construction as other London developers were hurrying for cover in a falling office market; that it is being built speculatively and that it has a steel frame at a time when the global price of steel has shot through the ceiling and the project begins to look, well, daft.

But John Crawley, project director for main contractor Skanska, claims there is a method to this madness.

'When we first looked at the job the client wanted us to build it quickly.

With the downturn in the market, that changed, so we now have a less intensive site that will take longer to build, at a lower price.Hopefully the building will be ready just in time for the upswing in the office market, ' he says.

But there was a danger that the Palestra scheme could be hostage to wildly inflating costs if contractors and subcontractors raised their price as the project went on.

'We were signed up in October as a fixed-price design and build lump sum.All of our subcontractors were tied into their price as well.That made the project viable, as no one had the opportunity to go back and look at their price again.That was one of the key factors in making the project a goer, ' explains Mr Crawley.

But there must be some regret that, having signed up to a price ceiling, they then had to watch the price of steel rocket.

'It's right to say that the steel prices have caught a lot of people, including ourselves and our steel subcontractor, William Hare, ' says Mr Crawley.'Some of the price hikes have been incredible because of the amount of steel that is going to China. But the fact that we both signed up to a guaranteed maximum price is one of those things that you have to accept in contracting.There is no way back.'

But the decision to build with steel seems strange given the fact concrete seems to be making huge gains in popularity in the office building sector. So why was steel chosen?

'When you look at how flexible the building needs to be and the floorplates involved, I think it would have been very hard to achieve as lightweight a result and to do it as quickly with concrete, ' says Mr Crawley.

The steel-decked floors and thin concrete covering allow holes to be cut easily in the floors, which is an advantage given that, as there is a possibility of multiple tenants occupying different floors of the finished Palestra building, tenants may want to move between floors without using common areas.

'Steel lends itself more readily than concrete to a building that is being built with floor flexibility in mind.The moment frame [see page 38] would have been fairly difficult to produce in concrete in terms of the amount of formwork and falsework that would have been required, 'he adds.

In order to gain the maximum benefit from the use of steel, the team had to work with William Hare and engineer Buro Happold to bring in the most innovative methods of construction.

For example, the steel beams and columns used for the Palestra project are prefabricated by Hares. By binding two beams together, the team reduces the amount of crane time required to place them and, in addition, fewer secondary beams are required, allowing better options for future tenants to knock out panels in the future.

On first view the plans for the Palestra development might well lead many to think that the scheme is the work of a madman. But when the office is completed in the summer of 2006 and joins Skanska's other recent eye-catching additions to the capital - the Swiss Re tower and Moorhouse office project - it will prove that the only crazy thing about it is that other contractors are not delivering such innovative solutions themselves.

How Skanska will 'F-up' a perfect moment PERHAPS the toughest technical challenge associated with the Palestra project is the massive overhang nine storeys up the structure.As part of Will Alsop's bold vision for the development, the whole floorplate of the building shifts up to 7.5 m out above Blackfriars Road for the top three storeys. But, while it may make the development stand out among its neighbours, it is a severe engineering challenge for Skanska.

The team's first solution was to build the cantilevering section by hanging it from 2.5 m-deep beams.Unsatisfied by this solution and its requirement for such deep beams, the team then developed the design and came up with a moment frame, or 'Vierendeel truss'.

This involves concreting the top floors of the steel frame to provide stiffness to the floorplates.The cantilevering section is then made up of a series of 14.5-tonne, Fshaped steel frames, with the long legs connecting horizontally back to the main frame using full penetration butt welds, leaving the short legs pointing vertically down to form the remainder of the cantilevered structure.

But these connections provided a further ordeal for Skanska.With such huge forces expected to be acting on them, the joints would have been a welder's nightmare.

To allow the welders to work, the sections would first have to be preheated.This would involve placing a booth around them, warming up the sections and carrying out the welding, and then leaving the booth in place while the weld tempered to prevent it failing due to rapid cooling. In all, the process would have taken days, rather than hours, for each weld.

'So what we did was invert the F-sections, so that the short legs faced upwards rather than downwards.We call it F-up, ' says Mr Field.

'Not f*** up, ' interjects Mr Crawley.

With the F-sections inverted, the end plate would fix in the service void rather than the floor void.This also allowed the team to move from a welded connection to a bolted one, making major time savings on the project in a stroke.

'That said, they are huge m48 grade 10.9 bolts - they look like dumbbells, ' says Mr Field.

The team then realised that it did not need to actually concrete in the top floors of the development in order to provide the stiffness needed to attach the moment frame. Instead temporary diagonal cross-bracing was added to the steel frame, allowing the load of the moment frame to be tied back to the core.

This one innovation saved the team as much as a month on the programming. But, in order to take advantage of this timesaving, the team is currently in a race against time to get the main frame up to a height that will allow the moment frame to be installed.This has to be achieved by January to allow 10 days for installation.

'If we get a lot of bad weather between now and then, we could miss that period.

You've then got the Olympic bid between February 14 and February 21, ' says Mr Crawley That is when the International Olympic Committee comes to London to determine whether the city should win out over its rivals in the competition for the 2012 Olympic Games. In order to prevent the dignitaries from getting caught up in any gridlock challenges, all road closures have been forbidden throughout their visit.

'London has to run like clockwork that week, ' explains Mr Crawley.