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Korea ready for kick off

COVER STORY - Daegu: Some top quality football will be needed if justice is to be done to Korea's new Daegu Stadium, reports Andrew Mylius.

In a play on 'world cup', Korean architect Chul-Hee Kang's 1996 concept sketch for Daegu Stadium showed the globe cupped in a colossal pair of hands. Translating his one-world vision into concrete and steel produced a structure of two distinct halves.

Kang's firm, Idea Image Institute, designed stands around the stadium's pitch and running track in reinforced concrete.

Finished in dazzling colour, it is the perfect arena for an international multitude of fans. Meanwhile, the roof - developed by consultant WS Atkins' structural engineering division - describes Kang's enveloping hands: left and right sides are mirror images, with inclined main arches suggesting thumbs and horizontal arches enclosing the stands at either side taking up the position of fingers.

Daegu Stadium's all-encompassing twin roofs are simultaneously gargantuan and highly refined. Spanning 273m, they could leapfrog any existing UK premier league venue, says Atkins head of structures Mike Otlet. Such feats of agility call for muscle. But visually and structurally, they are feather light.

The principal element in each roof is a main truss arch. This is inclined back at 30'from the vertical, propped by 13 secondary trusses, which channel thrust forces into a near horizontal tension arch - described by Otlet as the 'back arch'. The truss itself is propped by columns which are raked for aesthetic effect, but support only axial load. While the main arch works hardest, it is the back arch that provides the roof with stability, Otlet says.

Both arches spring from the same point - a mounting fabricated in 100mm steel plate atop hollow reinforced concrete towers that punch up through four tiers of stands at either end of the stadium. Constructed by main contractor Samsung on modest 1m thick, 30m 2reinforced concrete pad foundations, these cantilevered structures must resist enormous forces:

To provide spectators with adequate protection from weather, the main arches are relatively flat, rising from end to mid-point by just 28.7m. Huge compressive force is generated by this shallow geometry and transmitted to the towers as 3,200t of horizontal thrust, 'the equivalent of 30 Boeing 747s on take off', says Otlet.

Ground anchors bonded into fractured rock underlying the stadium have been used to root the towers firmly in place.

A small amount of flexure in the towers was needed to dissipate compression in the bottom chords of the main arch trusses.

But too much movement and they would be put in tension, requiring remedial engineering.

Otlet anticipates no problems, but a year after erecting the main arch trusses, his team continues to monitor the towers closely.

Mountings are fixed to the tower heads by shear keys and tied firmly down with 28 Macalloy bars apiece. In the scheme of the overall stadium, these 6.5m by 4m ovals of heavily constructed steel are relatively invisible. But all of the main arch compressive force and back arch tension is channelled via bearings welded to the mounting's central spine, a steel plate stiffened and reinforced by four transverse steel diaphragms.

Bearings themselves are spherical, silicone-impregnated polyester resin assemblies - the most durable and slippery available. In addition to deflections in the main and back trusses under expansion and contraction, wind and seismic loading, the bearings had to accommodate movement during construction.

Samsung fabricated the stadium's steelwork on site, supporting the main arch truss in sections on four giant scaffolding towers during erection. The main arch sagged 200mm when the falsework was struck.

Compression keeps the ends of the main arch engaged with their bearings. Only four bolts have been fitted to ensure location. By contrast, 72 bolts are required on each of the back arch bearings to prevent tension pulling them apart.

Despite the high forces at play in Daegu Stadium's roof structure, it is wonderfully economical, maintains Otlet. Because it is fully welded, the roof is 25% lighter than it would have been if fabricated off site and bolted together in the manner of another Atkins stadium triumph - Cardiff's Millennium Stadium.

And other savings have been made: in total, 4,350t of steelwork has been used to deliver Daegu's roof.

The main arch truss is relatively beefy, with a 6m 2section, 762mm diameter principal chords and 273mm diameter diagonal bracing. A good deal more slender, the diamond-shaped back 2section. But the real lightness is found under the roof's translucent, PTFE coated glass reinforced fabric canopy, between the two arches.

Spanning from the main to back arches, the prismatic secondary trusses have a 406mm diameter bottom chord, with two 200mm by 300mm box sections to the top and 141mm diameter cross bracing. At the ends of the stadium, where they are spanning the shortest distance - 25m - the secondary trusses are relatively shallow.

Moving towards the centre of the structure, though, the trusses lengthen, reaching 67m. At the same time, their section deepens to a sharp V, providing an impression of almost gothic delicacy.

Rainwater drainage channels are incorporated into the secondary trusses' top chords, minimising visual clutter.

Delicate curved purlins at 8m centres support the roof fabric between secondary trusses. The fabric is stretched between the secondary trusses but also fixed to the purlins, functioning as an integral structural element. Normally, a series of longitudinal beams would have been required beneath the bottom chord of the secondary trusses to 'restrain' them from flexing and, in a worst case scenario, buckling. After analysing performance of the roof and the stiffening role played by the fabric, however, Atkins decided it could do without the beams, cutting weight and lightening the roof aesthetically further still.

Wind tests carried out on a 1:250 scale model at test house BMT showed that the roof could cope fine with positive pressure.

Uplift, which threatened to suck the fabric off its supporting steelwork like a loosely tethered tent off a mountainside, has been countered by lashing it down with cables midway between and parallel to the purlins.

Daegu Stadium is the product of one of the most exhaustive exercises in computer modelling Atkins' structural engineering team has ever carried out, says Otlet. 'We decided very early on we would model every detail, every connection, ' he recalls.

This was, in part, to optimise the structure, aesthetically as well as structurally, but equally to ensure that the structure could be built. Tolerances were exact and, particularly in tight areas like the mounting/bearing assembly, it had to be proved everything would fit.

Modelling was also used to make life easier for the contractor. Three-dimensional renderings and story-board sequences were used alongside conventional engineering drawings, with the result that Samsung came back to the design team with fewer fabrication questions than any job Otlet had previously worked on. The project also came in, exactly a year ago, on time and within budget.

In the heat of the World Cup play-off on 29 June, Daegu Stadium's engineering finesse may not matter much to the 70,000 plus strong crowd. But it will count towards the atmosphere.

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