Steel focus: Steel distribution goes high-tech

There aren't many steel structures in the world that have been designed and constructed to such tight tolerances and complete structural rigidity as the Automated Distribution Centre (ADC), which will shortly be fully operational at the Corus Scunthorpe works.

This fully automated warehouse, which is 159m long, 25.7m wide and is just over 30m high, will eventually store up to 17,000t of steel sections completely under cover and protected from the elements. Four computer-operated cranes - one for each of the ADC's aisles and operating at speeds of up to 3m/second - will place and retrieve steel bundles, weighing up to 6t, from 24 stacking levels and load them onto trucks.

To allow this high-tech, computer-aided stacking equipment to operate correctly, an extremely rigid structure had to be guaranteed. As well as a maximum 17,000t of stock held within the structure, an infinite array of possible load combinations also had to be taken into account when deciding on which type of foundation would best suit the structure.

"We originally thought about a concrete raft for the foundations, but after a survey of the site, we knew it couldn't adequately support the building," explains Corus Northern Engineering Services (CNES) senior project engineer Ian Clayton. "We needed extreme rigidity without the risk of differential settlement, and steel bearing piles were the solution."

Piling

Piling contractor Clugston installed a total of 316, 25m-long steel bearing piles, one for each of the ADC's columns on a 3m x 6m grid pattern. 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, all the bolts were individually surveyed to ensure their exact positioning before being grouted into place.

This part of the project was the key to getting the entire structure right, says Brian Turton, senior project manager for steelwork contractor Billington Structures. "We got involved early on in the construction process and, together with CNES, we recommended that all the bolts were surveyed to ensure their exactness. This would guarantee the columns would be going up in the right position."

The structure is working to some exact tolerances and a slight deviation at the bottom of the column would mean a seriously out-of-line steel member nearer the top of the structure, preventing the stacking cranes working. Once the bolts were set, Billington began erecting the 4,350t of structural steelwork for the ADC's main frame. The columns were formed from Corus Advance 914 UKB sections and erected in two spliced 15m-long sections, to give the overall height of the building. There are five lines of columns and this required two 50t-capacity mobile cranes to work at either end of the building, while cherry pickers were positioned in the bays.

As the frame went up, it was continually surveyed to ensure it was plumb. "We left the connecting bolts at the splice un-tightened until the survey confirmed their alignment, as we had a couple of millimetres to play with if needed," explains Turton. Sequencing also played an integral role in the structure's erection programme.

Prongs

Once Billington had completed approximately one quarter of the building's 54 bays, it was able to begin placing the prongs that form the racking on which the steel will be stored. More than 9,500 of these 1m-long steel prongs were installed over a 30-week programme and once fitted they were individually surveyed to ensure they were in the correct position.

This work was carried out by two teams, allowing for a double-check to be incorporated. "The second check was to give us a 'vote of confidence' in the work," says Paul Saddington, Corus ADC project manager. "Exactness was paramount, because if any of the prongs were slightly out, then the entire stacking system wouldn't work. However, from the outset, the results were encouraging, with very little rectification work required."

Cladding

Following immediately behind the prong installation, Billington also began installing the ADC's single-skin steel cladding. More than 17,000m2 of cladding was put up, which equates to more than 124t of the material. The ADC is located on a confined site, with an internal live railway line on one side and the Corus Medium Section Mill (where the steel for the facility will come from) on the other. This meant there was no room to run a traditional scissor lift alongside the building for the cladding operation. Consequently, two mast climbers were attached to the two sides of the structure's frame. Working up to the eaves, each mast climber enabled six bays to be clad, before being dismantled and moved to the next section of the building.

"The programming and sequencing were key to the project," says Turton. "At one stage we had steel erectors at one end of the structure, the prongs being installed in the middle and cladders further down the frame."

Finishing touches

Billington finished the structural frame, the prongs and cladding - including the roof - by January this year. The company left an opening at one end of the building for the installation of the automated stacking system and cranes, and once this phase had been completed, the final areas of cladding were then finished in March. Summing up the steel construction, Turton says: "I've heard some people describe the structure as a machine because of the equipment inside. But when we were erecting it, we had to think of it along traditional construction lines. We weren't exactly working to building tolerances, but special tolerances."

It is unlikely another ADC will be built in the UK, but when asked if he'd work on another, Turton has no hesitation in replying yes. "We've done it once, it would be easier next time."