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Conder rises to a new challenge in Dartford

STEEL - When distribution firm ProLogis took on Fitzpatrick to deliver a major new warehouse next to the Dartford Crossing, speed of construction was key. Alasdair Reisner explains how Conder Steel Structures is helping to achieve this

WHAT a difference a few months make. The last time Construction News visited The Bridge, one of the first major regeneration projects in the Thames Gateway to get the goahead, the Dartford site was little more than a mucky expanse with the odd dump truck and piling rig to show that anything was happening.

Yet just a few short months later the first major structure on site - a 650,000 sq ft regional distribution centre for retail giant Sainsbury's, has stamped its authority on a skyline dominated by the nearby Dartford Crossing and Littlebrook Power Station, a testament to the speed of delivery that is offered by steel construction.

'When we first bid to ProLogis they had a completion date that they needed to achieve because they had Sainsbury's signed up to a lease. Its business is based on turnover so it needs to be complete as quickly as possible. The only way you can achieve that is with steel, ' says David Saunders, area manager at Fitzpatrick, main contractor for the scheme.

Overall Fitzpatrick's contract is a design and build deal for the regional distribution centre, resource recovery centre and a vehicle maintenance unit along with a 450-space car park.

Of these it has picked Conder to build the frames for the main distribution centre and the car park with fellow steel specialist ACL picking up the smaller buildings. Conder's works alone will involve the use of nearly 4,000 tonnes of steel.

'The reason we chose Conder for the main building was its capability both in terms of size and programme. It is quite a tight timetable, ' says Mr Saunders.

In order to get through planning the main distribution centre has a wave-like curved roof with steps in it, said to represent the waves of the nearby Thames. But while this may make the building more visually interesting and increases its appeal to local planners, it throws a bit of a curveball in terms of erection.

If the project had been a normal portal frame warehouse the roof would be pitched and would be formed of straight sections.

But the curves and steps in the roof meant that different solutions would be needed. Portal frame action cannot be used at the steps in the roof where internal columns are to be omitted because there is nothing to resist the thrust of the rafters as they are not balanced as in a traditional arrangement.

The requirement to omit columns arose from the demand from the client that there would be as much space as possible both for storage and forklift movement in the warehouse with minimal encumbrance. Additionally the use of part of the building for cold storage meant that the def lection in the beams needed to be minimised to ensure the integrity of the suspended cold store ceiling.

The solution to the client's spatial requirement was a 'hit-andmiss' frame. Instead of using internal columns on every grid line of the warehouse to hold up the roof, they are placed on alternate grids. The 'hit' column takes the entire load from the 33/35 m clear adjacent spans and over two 8 m bays. The hit frame is the stiff part of the structure that takes the full lateral load.

The hit columns are erected first; these are followed by the hit frame rafters. These are made up of plain section 762 mm deep universal beams that are cranked between splices with extended cleats to form the required cu rve of the distribution centre roof.

With the hit frames in place the team on site can then move forward, erecting spine beams between the hit columns from which the miss frames are supported.

The miss frame rafters are made up of two 18 m, 1,090 mm deep cellular beams that have been spliced together to form a 35 m span. Cellular beams are much deeper than standard beams and are formed by taking a standard beam, profiling the required cellular geometry, splitting the section into two halves, offsetting the halves by the cell spacing before re-attaching to form a beam with holes running along its length. The resulting beams are much lighter than an equivalent depth plain beam and much stiffer than the parent beam from which it was made.

All this results in less def lection. As a bonus the cellular beam can also be curved during cell formation to match the form of the roof, thus not requiring the extended purlin cleats.

'Our solution to the stepped miss frame was to design the rafters as simple beams, utilising cellular beams to achieve the deflection lim its economically, rather than trying to brace out the thrust from the frame action that would result from the m issing columns' says Conder Structures design manager Gerrard Cox.

Where the step occurs in the roof at the miss frame the lower roof is supported by the use of vertical hanger from a high-level edge beam. This 305 UC section in tension carries in the region of a 30 tonne loading.

Given the pace at which the building had to be erected it comes as no surprise to find that one of the major challenges for the Fitzpatrick/Conder team was in terms of logisitics. No sooner had the piling and foundation teams finished than they would be replaced by steelworkers. In this environment it was vital to make sure everyone knew exactly when they had access and what would be expected of them.

'You are going down the building in sections with each of the processes following the other in tandem down the site. If one person stops, everybody stops. We agreed dates with Conder so they can get access to a section of the site, so we had to make sure we kept ahead of them because steelwork will always go faster than piling and foundat ions, ' says Mr Saunders.

For Conder's part the firm had to make sure that when it did get access it was ready to mobilise its resources immediately. It spent a long time planning deliveries to make sure the right sizes and sections were sent to the site and there was enough for the gangs to be working with.

'The columns weigh 7.5 tonnes so you can only fit two on the back of a wagon. We spent the first few days just delivering to site. All the steel arrives in lots that fit on a single wagon and can be fully erected once they arrive on site without waiting for further deliveries, ' says Mr Cox.

Work on the main structure is now closing in on completion, allowing Fitzpatrick to continue the work required before the scheme is handed over to the client. With further similar developments in the pipeline nearby in Erith and Kent, Mr Saunders hopes that it will be possible to roll on the same team to further jobs.

'Conder worked with us on a job we did at Ashford. Burks Green was the designers there as well. That means we have had three-way working with them and I'm sure we will work together again. We know the sort of detail that Conder will come up with for things like tie-ins, fixings and the sort of size of members in its designs. It means you are not trying to reinvent the wheel every time, ' says Mr Saunders.

So maybe this will not be the last time that the team from The Bridge site get a chance to prove themselves as the Thames Gateway development takes shape.

Project details

Value: £35 million Client: ProLogis End user: Sainsbury Main contractor: Fitzpatrick Steel: Conder/ACL Architect: Burks Green