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Karachi Cracker

In Karachi, Pakistan's second city, a tall building is characterised as anything over 10 storeys. The loftiest have reached 20. So it is no overstatement to say that the 78 storey Karachi Port Tower, construction of which is scheduled to start this year, will transform the city's skyline.

Compared to the Burj Dubai, which will be the world's tallest building at 146 or more storeys, 78 storeys doesn't sound so remarkable. But Karachi Port Tower will be the tallest building on the Indian subcontinent.

Building on this scale in Pakistan is a one-off and poses some interesting challenges. Nobody in the country has ever carried out a site investigation for a building of this size before. Exceptionally deep and large foundations are required but local batching plants are not equipped to produce concrete in the volumes and strength required. The specialist falsework, formwork, cranage and concrete pumping equipment needed for ultra-high buildings does not yet exist in Pakistan.

"Construction will require international know-how, but with local knowledge. Three joint ventures of foreign main contractors with local firms have been shortlisted," reveals Mott MacDonald director Steve Gregson, who is leading structural, facade, mechanical and electrical, and fire engineering. "But whichever of the three is selected, they will be heavily reliant on local subcontractors."

Throughout the design process a close eye has been kept on buildability and making the structure suitable for local conditions and skills.

Client Karachi Port Trust is the port authority and operator and is also a major property and infrastructure owner. It is undertaking the project on a speculative basis. In addition to office space it also wants housing, a hotel and a conference centre, and it specified "something iconic". Mott MacDonald and architect Aedas won the design competition last year, and are taking the design to "detailed concept" stage.

Offices will occupy ground level up to floor 58, a hotel will take up floors 59 to 76 and the top two floors will be apartments and leisure facilities. The contractor will be appointed to deliver the £200M-plus scheme under a FIDIC design and build contract.

Steel construction is rare in Pakistan, so Karachi Port Tower will be built from concrete.

It will consist of a cylindrical core ringed by columns at the building's perimeter. Structurally, square cores are stiffer, Gregson notes. The cylinder was specified for architectural reasons and to achieve spatial efficiency within the tower's circular footprint. But lack of stiffness has been more than made up for by increasing the core's diameter to 31.5m and tying in the ring of perimeter columns.

The core size and other aspects of the structural design were dictated by the post-9/11 rethink of fire evacuation from tall buildings, driven by Mott MacDonald's fire specialists. "You used to be told 'if there's a fire, evacuate using the stairs'," says Justin Garman, one of
Mott MacDonald's fire engineers. "But the World Trade Center disaster showed that stair capacity wasn't enough, and that some people were physically incapable of descending tens of storeys by stair.

"So now, for very tall buildings, lifts are being looked on as integral to the fire evacuation strategy." Lift capacity has been designed for an office population density of one person per 11m2, so there will be a lot of them.

Karachi Port Tower will be equipped with a combination of express and local lifts. High-speed lifts, moving people over large numbers of floors, will be double deckers. Passengers will then catch local lifts from transition zones to their destined floor.

Over the height of the tower there will be three transition zones. Structurally these are very different to the tower's typical open plan floors. Floor slabs throughout the tower will be 260mm thick post-tensioned concrete, stiffened by a 400mm deep edge beam. Columns will be tied into the circular ring by an 850mm-deep downstand.

But the two-storey transition zones will be of far heavier construction, with thicker floor slabs and heavily reinforced concrete outrigger shear walls running from the core to the building's perimeter columns. Each zone will house a technical floor dedicated to building services, and a fire-proofed refuge. It is to these refuges that people will be led if fire breaks out. They will then be speeded to ground level in express lifts.

The transition zone shear walls play an important role in linking the core and columns. Gregson says that at the lowest of the technical floors the stiffening effect of the outrigger walls is minimal. "When we modelled the structure we found we don't need outrigger walls there, so we've omitted them and gained a fairly significant cost saving."

Design has had to deal with the age-old problem of differential axial shortening between core and columns under dead load. This occurs when a structural member is squashed by the weight of the structure above. The taller the column, the greater the degree of potential shortening.

Sized purely for structural efficiency, columns would have shortened by more than 75mm, Gregson notes. "You can allow for a degree of axial shortening by introducing a slight camber into the floor slab. That camber comes down as you build the structure up, and the floor ends up level." But a greater than 75mm correction was at the edge of technical feasibility.

Columns have therefore been sized to reduce stress and shortening. In plan they are elongated triangles with rounded corners, measuring 2m wide by 3m deep. Column sizes diminish as they rise up the building – first in width, then in depth.

Mega columns are required over the height of the tower's hotel atrium, slicing through the topmost 25 storeys. "Five columns are left free-standing by the atrium," Gregson says. Perimeter beams every five floors provide lateral restraint – columns follow the cigar-like profile of the tower's facade, so are subject to considerable outward force. Otherwise, the columns are structurally independent.

Gregson says that lower down the tower, axial shortening could have been reduced by specifying very high-strength concrete. "But we want to keep the concrete mix within the realms of what is feasible in Pakistan." Achieving C100 would require the use of exotic additives and very precise mix control. C65 concrete will be easier to batch and more forgiving in construction.

Concessions to the local construction market have also been made in the arrangement of columns and in the tower's foundations. Karachi Port Tower's facade is dominated by a spiral that twists through 180o over its height. "We initially looked at following the spiral with the columns, so they would have been raking," recalls Gregson. However, "to make them work it would have required very heavy reinforcement and precise steel fixing. Because there's no precedent for a building of this height in Pakistan, we felt it sensible not to add avoidable complexity."

Though columns are oriented to the curvature of the facade, an alternative way of expressing the spiral was found, says Gregson. "The spiral is achieved by cantilevering the floorplate by just over 3m on opposing sides of the tower. As you go up the tower, the cantilever moves around a few degrees."

the bigger picture
Karachi is 200km from the nearest seismic fault line and is generally regarded as at low to medium risk from earthquakes Đ UBC seismic zone 2B. But, because nothing even a fifth as tall as Karachi Port Tower has been attempted in Pakistan before, Mott MacDonald decided to reassess the seismic risk.

"Our specialist geotechnical engineers have just completed the analysis and concluded that for the purpose of this project Karachi should be treated as seismic zone 3," says Mott MacDonald director Steve Gregson. "Complying with structural requirements for zone 2 is easier.
Taking it up to zone 3 introduces some major structural issues."

And "sloshing" tuned mass dampers will be installed on the tower's roof. These will consist of water tanks connected by large-diameter pipes with tuned baffles.

Quake engineering

 Karachi Port Tower forms the focal point in a scheme involving construction of a conference centre and five residential towers.

 Underlying the complex will be a double-storey, 2,000-space subterranean car park.

 The only vehicles seen above ground on site will be the Port Trust's fleet of electrically powered service trucks.

 Power cuts are commonplace in Karachi. To prevent the tower being plunged into darkness, it is being equipped with twin 132kV power supplies - in Karachi the high voltage supplies are less affected by cuts than low voltage lines. In case of blackout, the complex also has its own generating plant.

 Karachi Port Tower will have its own water and waste water treatment plants.

Focal point: Karachi Port tower is part of a larger scheme

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