IMAGINE a giant Ikea-style racking system but, instead of bamboo furniture it is holding bundles of heavy steel sections on 24 levels of supports until an automated crane system plucks the bundles off and loads them onto trucks or rail. That gives you some idea of the massive Automated Distribution Centre under construction for Corus at its Scunthorpe works.
'It's not a building, it's more like a giant machine, ' says Corus senior project engineer Ian Clayton. 'The structure has been designed around the need for very tight tolerances that will allow the building's automated stacking system to pick up and set down bundles of steel that will weigh up to 6 tonnes.'
When complete, the ADC will be able to store up to 17,000 tonnes of steel sections completely under cover and protected from the elements, enhancing the quality of sections, minimizing handling time and increasing load flexibility, while also improving safety.
The ADC is a substantial building, housed within a steel-framed warehouse, which measures 159 m by 25.7 m and is just over 30 m high. It has four aisles running the length of the structure and each of these lanes comprises 24 levels for storing stock.
An extremely rigid structure had to be guaranteed, as the facility is being built to exacting tolerances to cope with the need to support the stock contained within it and enable the cranes to function correctly. In addition, the infinite array of load combinations meant that foundations had to guarantee the required rigidity of the structure.
'We thought about a concrete raft for the foundations but, after surveying the site, we knew it couldn't adequately support the building, ' explains Mr Clayton. 'We needed extremely rigid foundations without running the risk of differential settlement. The only solution was piling.'
The ADC was built on a backfilled area adjacent to the existing Medium Section Mill, where no building work had previously been undertaken, as it had been used as an open storage area.
A site investigation indicated underlying limestone at 22 m, on which it was intended to found the piles to achieve their initial designed working loads of 2,000 kN.
'It was at this point we chose to use steel bearing piles for the project, ' says Paul Saddington, Corus project manager for the ADC. 'Driven steel piles are best in these conditions, they're quicker and there's no resulting spoil to deal with.'
Clugston was the civils contractor and its work included the piling and installation of the concrete slab.
'The required capacity was an important driver in the whole construction process, ' says Neil Webster, Clugston's site manager. 'A total of 316 piles were driven, one for each of the ADC structure's columns.'
Beginning in February, the piling operation was completed in a 14-week programme by two piling rigs. All the piles were Corus Advance 305 x 305 x 186 UKBP units manufactured at Corus' Teesside Beam Mill and were installed on a 3 m by 6 m grid to match the column spacing of the ADC. That particular pile was used as it is a very 'driveable' pile and gives a high load capacity.
Finding obstructions was likely as the piles were being driven through backfilled material made up largely of unknown material and clay.
'Steel piles meant that if we encountered any obstructions during driving, the pile wouldn't be damaged. Hitting obstructions with a precast concrete pile could have resulted in cracking below ground, which might have gone unnoticed until it was tested, when replacement would be difficult, ' says Mr Webster.
As the superstructure design progressed, the loads on individual piles increased from 2,000 KN to 2,500 KN. To meet the tight construction programme pile installation was already in progress and it was found that to achieve the required set for the higher load, the piles had to be founded at a depth of 28 m on ironstone rather than the limestone at 22 m.
Using steel bearing piles proved advantageous as it meant this last-minute change could be accommodated by simply extending the piles.
The piles were designed to be installed in a 10 m-long length, with a second 15 m-long section welded on and then driven into the ground. 'Fortunately, steel bearing piles lend themselves to extension, so welding on a third section wasn't a problem, ' says Mr Webster.
Once the piles had been installed, each one had a steel plate welded to the top, followed by a concrete pile cap with holding down bolts cast in to accept the steel columns. All of this was done as the surrounding concrete slab was cast and once this was complete the bolts were surveyed to ensure their exact positioning before being grouted into place.
'This ensured that the columns were in the right position and it also made the subsequent erection of the main steel frame quicker, ' says Mr Clayton. 'All the steelwork contractor had to do was bolt on the columns, knowing that the positioning had been taken care of.'