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Windy City Wonder

Clever engineering has meant that North America's tallest residential building will be solid as a rock despite its windy location. Jessica Rowson reports.

Nestled among the forest of skyscrapers on the Chicago skyline, the 92 storey Trump Tower is currently notching its way up to become the city's second tallest building. The 415m tower will be completed in January 2009.

The stepped concrete building, has been designed to reflect the height of nearby buildings by architect and engineer for the project Skidmore, Owings & Merrill (SOM).

The first step aligns with the 130m high Wrigley Building, the second the 179m high Marina City Towers, and the third the 212m high IBM Plaza, known as 330 North Wabash.

As important as these steps – also known as setbacks – are architecturally, they also have an important engineering role as they each contain an outrigger stability system. These 5.3m deep by 1.7m wide concrete monoliths transfer lateral loads between the perimeter columns and the central core.

SOM associate partner Robert Sinn explains that the lateral shear resistance of the core and overturning resistance of the perimeter structure are mobilised by linking them at discrete levels using outrigger trusses or beams.

He adds that this means just a few heavier vertical elements are needed on the perimeter to keep the building stable, freeing up the facade.

The outrigger beams take up a storey height and are heavily reinforced. In some areas conventional bars are even replaced by an equivalent area of steel plate to ease congestion. Contractor Bovis Lend Lease is using self compacting concrete to penetrate densely reinforced areas.


Surprisingly, the tall building does not require dampers to limit its movement. This is because of the stabilising effect of the heavy concrete core and columns and the setbacks.

The asymmetric setbacks change the cross section of the building, so changing the frequency of wind passing it. This means that vortices, which would cause the building to move more, cannot build up.

Any massive building needs massive foundations. The building sits on 30m long piles founded on bedrock. A permanent steel liner, which seals the excavation, cuts through 18m of stiff clay and 12m of boulders and fractured rock to form a socket in solid rock.

On completion the Trump Tower will hold the record for the world's highest residential building, but only for a year. After that it will be dwarfed by the 610m, 150 storey Chicago Spire.

The surrounding tall buildings dictated the heights of the Trump Tower steps

A. 415m high Trump Tower

B. 212m high 330 North Wabash

C. 179m high Marina Towers

D. 130M high Wrigley Building

FOUNDATIONS
A 3m deep piled raft was poured continuously over a period of 22 hours. The concrete was poured using conveyor belts so that minimal labour and very few vibrators were needed. the temperature had to be carefully 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.

Finite Element analysis
Engineers had to deal with the inherent problem of the uneven load distribution of a massive, asymmetrical building and its tendancy to move sideways under its own weight.

The solution was to carry out a time-based finite element analysis on the structure so that movements could be predicted and compensated for during construction. Bovis Lend Lease used these results to make millimetre adjustments at every storey to bring the building back to plumb.

Non-linear analysis predicted the short and long term displacement of Chicago's Trump Tower, which included the effects of creep and shrinkage. If no horizontal correction had been made during construction, the roof could have moved 300mm out of line, due to the combined effects of gravity, creep (time-dependent deformation) and shrinkage.

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