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controlled as the concrete cured due to the raft thickness. If the differential temperature across the depth was too large, stresses would set up and lead to micro cracking.
(New Civil Engineer International, October, 2007)
^TEXT4
"ROSSIA" WITH LOVE by Jessica Rowson
Europe's tallest skyscraper is being built in Moscow. If you're thinking of designing a tall building, make it at least 600 m or nobody will bat an eyelid. Moscow's latest addition to the 600 m plus club is the 612 m high, Rossia Tower, a cool 2 m higher than the Chicago Spire. Rossia's site is currently being cleared to make way for what will be Europe's tallest building. The skyscraper will incorporate retail and office space, a hotel and apartments on its 120 floors, three of them below ground level.
To the untrained eye, Rossia is an elongated pyramid, or rocket shaped structure, but on the inside, the structure tells a different story. At its base there are three colossal, high strength concrete abutments clamping the whole structure down. Each abutment forms the base of three wings of the building, from which columns radiate. The wings converge at a central spine, or concrete core, which runs the full height of the tower. Consultants Waterman International and Halvorson have designed the steel frame and composite floor structure. The plan and profile of the building take on the efficient geometry of a triangle to achieve maximum stability using the minimum amount of material.
Initially architect, Foster & Partners, designed the tower as three discrete blocks, arranged in a Y shape in plan. But this meant that each block was too slender, having a height to width ratio of 10:1. "Structural solutions were possible for this option of independent towers, but at these aspect ratios, the solutions would be inefficient," explains Waterman International project director Hugh Docherty. The decision was made to merge the blocks, so
they leaned into the central core. The sloping parallel columns could then brace the core laterally as well as carrying vertical loads. The result was a more efficient height to width ratio of 5:1. "So in terms of height to base, the building is squat," says Docherty. The design was starting to look like the familiar form of a cable stayed mast. However instead of tension cables, Rossia uses the sloping columns to act in compression — propping the central core and essentially acting like three dimensional arches.
The fan columns carry gravity load and wind overturning forces as direct axial loads. And as the weight of the building and its inhabitants exceeds the design wind load in the majority of the columns and core, there is little tension in the system. Piling contractor Soletanche is currently building a diaphragm wall on the site, but it will be at least six years before the 100 m tall mast crowns the building.
The tower's three wings comprise steel and concrete columns which fan out from the three massive abutments at the base. Visually, this gives the form of a tripod supporting the rest of the building — a structural form known for its efficiency. "Three legged stools are great. With four legs you start to bring in redundancy," says Waterman International project director Hugh Docherty. Having established the path for vertical and lateral loads, the remaining challenge was torsion. The facade of the wings is stiffened by a series of "reverse fan columns" which triangulate the facade. "The wings are designed as boxes with crossed bracing. These resist twisting," he explains. The rigid facade is further stiffened by steel chevron bracing up to the fourth floor on the outer edge of each wing. This provides sufficient torsional stiffness. But a structure with sloping columns causes other problems in the form of horizontal loads amassing at the base. "We used tension ties in the raft to stop the feet from spreading. We could have propped against diaphragm walls or relied on friction, but tension ties were the most controllable option," says Docherty. The construction sequence requires the fanning columns to be designed for erection loads. Later they will be encased in reinforced concrete to achieve the final strength for permanent loads.
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(New Civil Engineer International, February, 2008)
298 I Английский языкдля студентов строительных специальностей
^ТЕХГ 5
KARACHI CRACKER
Building a super tall tower in Karachi calls for international know-how and an understanding of local subcontractors' capabilities.
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-nigh 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
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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 $396 M 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 perimeter. Structurally, square cores are stiffer, Gregson notes. The cylinder was specified for architectural reasons and to achieve spatial efficiency within the circular footprint of the tower. But lack of stiffness has been more than made up for by increasing the diameter of the core to 31.5 m 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 11 m2, 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 dickers. 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 260 mm thick post-tensioned concrete, stiffened by a 400 mm deep edge beam. Columns will be tied into the circular ring by an 850 mm-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
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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 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 significant cost saving." Design has had to deal with the old problem of 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 75 mm, 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 75 mm 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 2 m wide by 3 m deep. Column sizes diminish as they rise up the building — first in width, then in depth.
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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 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. "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 3 m on opposing sides of the tower. As you go up the tower, the cantilever moves around a few degrees."
(New Civil Engineer International, May, 2008)
^ТЕХТб
WARSAW WONDER
Careful adaption of an existing two-storey basement in Poland's capital has meant that it can take the increased load from the new 54-storey Zlota 44 residential tower.
Warsaw, Poland's capital, is something of a surprise. The city was almost destroyed in the Second World War and fewer than a fifth of its buildings were left standing. Its redevelopment under the country's post-war communist government was, for the most part, relatively modest, with the notable exception of the impeccably reconstructed old town.
Elsewhere, stark, system-based construction produced a hard-to-love, Modernist architecture known as social realism. In recent years the city has embraced the obligatory glass-and-steel look of the modern city. But that too is set to change with an increasing number of iconic landmark buildings by high-profile international architects in the pipeline.
A new movement.
Zlota 44 is a 192 m-tall residential tower designed by US-based architect Daniel Libeskind, who was born in Poland to parents who had survived the Holocaust. The building is part of a new movement that is, according to Libeskind, redefining Warsaw "through culture, fashion and an unrivalled approach to living".
This may be so, but the project could also play an important role in redefining Poland's fledgling geotechnical community. Foundation design on the project is by Amp, whose Polish geotechnical group is led by Mariusz Leszczynski. Leszczynski, who cut his teeth working as a geotechnical designer for Buro Happold in the
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