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Christmas rush The days of sorting Britain's post by hand are sinking further into history with construction of an automated Royal Mail centre in Leeds capable of processing over two million letters e

As contractors and their construction sites battle to regain full productivity after the long seasonal break, Christmas 1998 seems a long way distant. But not so for engineers erecting a mammoth steel frame on the outskirts of Leeds, where the number of posting days to Christmas is a crucial factor in their contract's critical path.

To ensure that any embarrassment of Christmas cards arriving after 24 December this year is not the fault of Royal Mail North East, the company is investing £30M creating one of the world's most advanced computerised sorting offices. The new centre, planned to be fully operational by early December, will be capable of processing an impressive 15M letters every week - no matter how illegible the addresses - in a non stop 24-hour operation.

As late commissioning of the four £1M-plus state of the art sorting machines required to achieve this Christmas promise would - to say the least - defeat the object, the contractual risk for on time completion of the £15M building that will house them is considerable.

'The 48 weeks we have been given to complete the structure is not what I would call a generous programme,' is how contractor Ballast Wiltshier's contracts manager Peter Moran diplomatically sums up the fast track timetable. However, with a third of the contract now behind him, he adds: 'But it is achievable.'

Steelwork subcontractor Atlas Ward has enjoyed little festive respite in keeping to the punishing cycle of erecting two complete 92m wide, three span roof trusses a week. And on the ground beneath, preparations are under way to start equally fast track floor slab construction.

Use of fibre reinforced concrete to create one of the largest slabs of its type in Britain should, Moran estimates, cut construction time by over 60%. 'The slight increase in cost using steel fibres is more than compensated for by hopefully completing in roughly four weeks a slab that would, with conventional reinforcement, have taken 12,' he says.

Time savings are achieved not only through the elimination of steel fixing but also with faster concreting. Replacing rebar with a 30kg/m3 of concrete 'dosage' of fine 60mm long steel fibres allows better crack control and larger pours. Moran claims that in theory the 15,000m2 slab could be cast in a single operation, though in practice he hopes to form it with pours considerably larger than usual.

The 225mm thick floor is not strictly a ground slab, as weak fill beneath much of the 8.5ha site forces it to be suspended on the pile caps of over 2,000 concrete cast insitu piles that blanket the building's footprint.

Thirty years ago the site, alongside the M1 motorway at Stourton just south east of Leeds, was part of an opencast coal mine later reinstated with up to 8m deep fill. So the 14m long piles, driven to a tight 2.4m grid, must carry not only the floor but also the entire 1,200t superstructure.

This 162m long building shell is formed of 18 lateral steel frames. Each three-span bolted frame has been designed by structural consultant Curtins as three separate portal frames with universal beam columns topped by open truss roof girders.

The requirement for a conventional shallow pitched roof over such large spans forces the trusses to be up to 4m deep with even the central 51m span erected in one section using a double crane lift. Roof trusses incorporate an unusual lower boom member formed by a 600mm wide universal beam laid on its side.

'As the beam is required only for tension and not bending, positioning it flat provides added lateral stability across the frame against live loading,' explains Curtins' senior structural engineer Jim McLeod.

Now that the structural framework is nearing completion, attention switches to a stack of about 760 profiled aluminium roofing sheets lying alongside the building. With each narrow roof sheet some 46m in length, Moran answers the obvious question: How were such long panels transported to site?

'They weren't,' he says. 'We brought in large rolls of flat aluminium sheeting, then cut and profiled them into panels on site using a mobile rolling plant.'

Again time saving was crucial and, although such long lorry loads of completed panels were probably possible, obtaining the required police escort would have demanded three months notice each time.

The main purpose in designing the roof cladding as a single sheet for each slope was to minimise joints and - crucially - reduce maintenance. This low maintenance requirement is a client-ordered theme running through the entire design.

Royal Mail sees such assets as essential so it can, if necessary, more easily sell the building if the postal industry's fast changing technology ever renders the centre redundant.

For this same reason flexibility has also been a key design requirement. For instance, the high floor loading of 50kN/m2 exceeds anything Royal Mail is likely to place on the slab, but could be an attractive feature for a future purchaser.

'We have deliberately built maximum flexibility into both the structure and internal facilities,' says site architect for the William Saunders Partnership, Steve Wass. 'This should make the building attractive to a wide range of future clients.'

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