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Sail of the century

Structures Spinnaker Tower

Mowlem's construction crew is preparing to hoist an 1,100t steel sail up the mast of Portsmouth's Spinnaker Tower.

Andrew Mylius reports.

Burger King is weighing on Jon Neale's mind.

Not because he has a yen for flame grilled Whoppers, shakes and fries, but because in a month's time he'll be working very hard to ensure 560t of structural steelwork does not swing through the fast-food joint's plate glass window and smash the place to smithereens.

Neale is contractor Mowlem's project manager for construction of Spinnaker Tower, a 165m tall, 21st century 'folly' on Portsmouth's Gun Wharf Quay - he describes the £21M landmark as 'more a sculpture than a functional structure'. The tower's dagger-like concrete A-frame mast was completed in September (see below). In the first week on January, wind and weather permitting, Mowlem, steelwork fabricator Butterley and lifting subcontractor Faggioli/PSC will hoist the lower 56m of the structure's steel 'sail' into position.

The steel erection strategy has been shaped by site constraints.

'We're a very busy offshore site, ' Neale states. 'We're working on a 50m by 30m platform built out into Portsmouth Harbour (NCE 8 November 2001) and we're trying to build an extremely tall tower in middle of it.'

Ship berths crowd the site on its seaward side. A bustling new shopping centre and restaurants hem it in to landward - the site's Perspex hoarding is less than 10m from the nearest frontage.

Accordingly, the spinnaker's backstay legs and lower part of the bows - close to half of the steel structure - will be put together on the ground and jacked into place in a two day lifting operation. But from there on the steelwork will have to be craned up piece by piece.

'Stage one lift takes us to 56m above the pilecap, ' says Neale.

'That's as much as we can build without putting the steelwork through Burger King's window.'

On sailing boats, spinnakers - the huge sails that billow in front of the mast - are ultra-light.

Here, though, the spinnaker will be made up of 1,100t of steel, its curved edges formed by two massive box-section bows crossing between the A-frame's legs and projecting behind them down to the ground. Tubular steel ribs will describe the front of the sail and 'spacer tubes' will tie back from the bows to the mast. 'The spinnaker is designed almost like a bowstring bridge, with pin bearings at top and bottom, ' Neale says.

Butterley has made up all the steelwork components in its Ripley fabricating yard and delivered them south, fully painted, by low loader. For ease of transportation and manoeuvrability on the tightly constrained site the structure has been broken down into 50 box sections ranging in weight from about 9t to 24t. Bolted connections will be used throughout. A 'crow's nest' of three viewing decks contained within the triangle of space between bows and mast at the very top of the spinnaker will be built using I-section beams with composite floors.

Steelwork elements began to arrive in Portsmouth the week before last. The junction where the two box section bows cross - a colossal 94t cruciform made up of 50mm-100mm thick plate - has been positioned at ground level between the A-frame's feet. From here 11 box section elements per bow, numbered two to 12, are being built out to complete the spinnaker's backstay legs and lower section.

A temporary lifting eye on each element is located so the box hangs in perfect equilibrium at the angle required to mate with its predecessor. Heavy 'military' trestles prop the structure while it takes shape, with each element spoke, angled hardwood blocks mounted on rubber bearings.

Lifting will be carried out using strand jacks. Faggioli/PSC must not only raise the structure off the ground but also rotate it through 90 O, up between the Aframe's legs. Two 600t capacity main jacks will lift from points on the A-frame's trunnion, 30m up, hauling the steel up from connections just forward of its balancing point.

'There'll also be two tail jacks with 180t capacity each plus six secondary jacks controlling movements, ' Neale says. These will be anchored to the pile cap and provide restraint from forward, aft and either side.

As soon as the delicate lifting operation has been performed the cruciform will be bolted and welded to pin bearings designed and fabricated by specialist Ekspan. The trunnion bearings sit in heavily reinforced and locally thickened housings, constructed by Mowlem after the A-frame was completed.

They have not yet been grouted solid as some fine positioning will be required to connect them to the cruciform.

Once the connection is made, Faggioli/PSC will partially transfer load from the jacks to the structure - 10%-15% - to stabilise it. 'At that point we can get holding down bolts into the feet, ' Neale continues.

Subsequently, load transfer will be carried out incrementally.

The last operations will be to weld the backstay legs to base plates - a precaution against vandalism - and then concrete them up to 8m height to protect against bomb blast.

Hopes were that the lift could be executed before Christmas, but foul weather has hit assembly of the box-sections. 'If we start the big lift we have to finish it, ' Neale says. 'We will need one or two days to lift and five to seven days to fix the feet and grout up the trunnion bearings.'

Phase two erection will be more akin to building a conventional steel framed high rise building, says Neale, who before arriving in Portsmouth worked on London's Canary Wharf towers. 'We'll do it traditionally, floor by floor, using a Wolfe 320 luffing jib tower crane.'

In June next year the last two lifts of all will complete the tower's 40m tall raked spire, a 30t cone of steel clad with fibreglass.

Concrete

It came as a shock to Neale when concrete subcontractor Bierrum went into receivership just as work on the slipformed A-frame came to a close, and with some complex follow on work still to do. 'Beirrum went bust before the trunnion thickening had taken place. It was very complicated to build, involving shoring 30m above the ground with very little to brace off. We took on people who had been involved to keep knowledge on site - for consistency, ' he says. The upset meant that Mowlem had to manage more work than anticipated, and the contractor ran the job using night shifts for a short while to keep the project on schedule.

Beirrum's sudden demise was particularly sad, as Spinnaker Tower was a real test of and testament to its skills.

Starting life at ground level as two inclined hexagonal Aframe shafts, the tower rose by up to 3m every day, gradually converging to form a single mast 76m up. In addition to an inward 2.3deg tilt, the densely reinforced walls vary in shape over their height.

Yet the greatest challenge was undoubtedly the metamorphosis of two legs into one. Over a 34m length of the slipform, the 60t rig had to be virtually stripped and rebuilt in mid air as it was converted from a twin column shutter to one forming a single mast.

Bierrum's £250,000 rig, designed and built by Austrian specialist Gleitbau, had to be large and adaptable. It began life last March as a 20m wide, four level platform creating twin columns 7m apart. Four months and 138m later, it was less than a third the width and had shed two of its platform levels as it completed the slipformed section of the by now single 3.4m wide mast.

From the client came the demand for a durable, high strength mix to protect the tower from the aggressive marine environment. This was hard to reconcile with the needs of the slip forming team for a flowable, easily worked mix with relatively late initial set. The solution was a critical balance between the mix's partial cement replacement, granulated blast furnace slag, and a plasticiser. The slag, with its lower heat of hydration, helped increase strength and durability, though its smooth 'creamier' finish also aided workability and encouraged a slower set. The plasticiser ensured a trouble free flow into complex shutters.

David Hayward

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