If you're thinking of designing a tall building, make it at least 600m or nobody will bat an eyelid. Moscow's latest addition to the 600m plus club is the 612m high, Rossia Tower, a cool 2m higher than the Chicago Spire (NCE 25 October).
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 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 cental 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 100m 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.