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The Chicago Spire

Chicago’s horizon is skyscraper heaven, but soon one building will stand head and shoulders above the rest. Jessica Rowson reports from the Windy City.

When built, the 150- storey Chicago Spire will be 610m tall. Compared to the proposed 541m tall Freedom Tower on Ground Zero in New York and the current tallest building in the United States – the 110-storey Sears tower in Chicago which stands 442m tall – this will be a real skyscraper
among tall buildings. It is designed by Spanish engineer architect Satiago Calatrava.

Not only will the height make it stand out from the crowd, but it also has a rather unusual shape as it twists into a spiral which soars skywards. The floor plate is based on a circle but the edge is pinched into cantilever points at even spaces around the outside, giving it the appearance
of a wide toothed cog. The cantilevers will be rotated to give the façade the appearance of a very elegant helter skelter.

“It looks complicated, but there’s high repetition which means less cost,” says David
McLean vice president of the Spire’s structural consultant Thornton Tomasetti.

“The [concrete] core remains in the same position, but the floor plates appear to slowly rotate in plan with each change in floor elevation. However the columns and the inner fl oor plates are repeated at each level and only the edges of the floor slab rotate.”

All great things must start somewhere and this project begins with some heavy duty ground work. Twenty, 3m diameter rock caisson piles will support the building’s central circular core and there are seven outer columns at ground level with a further pair of the 3m diameter rock caissons beneath each of these.

The rock caisson piles are large diameter concrete piles installed with a permanent casing typically used when very high loads need to be supported. These huge piles pass through the eight-storey basement and socket into the bedrock to an average depth of 3m.

Workers have already installed all of the 3m caisson piles as well as smaller intermediate bell or under-reamed piles. These under reamed piles support the columns that will in turn support
floor slabs in the eight storey basement.

There are two types of basement column, both are formed by driving 25m long, 910mm diameter tubular casings into the ground. The bottom of these casings form piles below lowest
basement slab. Most are then filled with reinforced concrete all the way up to ground level.

Where the loads are greatest the casings are only filled with reinforced concrete up to basement
slab level. Steel drop-in columns are then placed on top of them to form supports for the basement floors. The casings are later removed before basement excavation work begins.

“To construct the steel dropin columns we terminate the concrete caisson at the bottom basement level and leave an empty shaft,” says McLean.

“A steel column is hung into the shaft just above the installed caisson. The base plate area of the steel column is concreted in place and left to cure. Later the shaft is filled with sand or weak slurry to prevent the clay collapsing when the steel casing is removed.”

It is important to prevent the ground around the steel columns collapsing after the casings are removed, because there is a risk that underground voids could make the areas around the columns dangerous to work in.

The basement will be constructed top down from the ground floor. The finished ground floor slab will brace the slurry walls on all sides of the excavation.

On site, the visible construction work is the eight-storey circular coff erdam. This was created so that the core could be built before the top down construction starts as it will help support the basement slabs. The core forms part of the gravity and lateral stability system, along with four outrigger and transfer levels.

“Normally, for shorter towers, the concrete core takes the entire lateral load and the outside columns just support gravity loads,” explains McLean. “However with really tall buildings, one would require a very stiff core so outriggers are preferred at certain levels to transfer moments into the outer columns.”

In a typical rectangular tower, outriggers are usually large beam/truss elements that connect perimeter columns to the core and can be one or two storeys deep. The outriggers usually run through the core walls leaving space for lifts, stairs and services. But, the Spire’s circular shape and circular core meant that the designers had to find a different solution.

“Normally cores are rectangular and the main structural elements can be installed through the core,” says McLean. “With a circular core one cannot do that as the diagonal elements intersect
at the core centre making it very difficult to fit anything in the triangular spaces. “We decided on a ring system around the core walls connected at two floor levels, which would not interfere with the inner core layout. The ring elements are horizontal steel trusses which encircle the core wall.”

In addition, it was decided to include the outer column transfers within these outrigger systems. These outrigger/transfer levels were situated at levels 35 to 40, 72 to 74, 109 to 111 and
142 to 144 and designated as space for the plant rooms. The building floors are framed in composite steel.

“We needed concrete for a robust core, but an all concrete construction would have resulted in very large columns in the lower part of the building,” says McLean. “With steel we can use smaller columns.”

McLean says the one very important issue in composite construction is the differential movement between the concrete core and the steel columns. Concrete shrinks and creeps over time whereas steel shortens under loading, but at a different rate. This not such a problem with low rise buildings or buildings which are all steel or all concrete, as the columns and cores shrink at the same rate.

The core of the Spire is being constructed above its final elevation so that there is a slight fall in the level of the slab from the core outwards. As the concrete core shrinks, the floor level
should approach a flat surface.

The main basement work has yet to start but tenders for a general contractor have been invited. When one is chosen the Spire will really start to motor.

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