The Windy City of Chicago in the USA is currently drawing the attention of the engineering world, not due to a new fantastic skyscraper proposal, but for a somewhat unusual extension to the existing Blue Cross - Blue Shield (BCBS) Tower.
Its not unusual for buildings around the world to have one or two stories added years after construction but in Chicago this has surprisingly never been done before despite the huge amount of high-rises it has. Usually in Chicago, developers simply demolish the existing building and go taller if they want to build up.
Thanks to architects Goettsch Partners and BCBS's foresight, the tower was designed to be able to cope with an extra 24 storeys being added years after its completion which means on completion it will be 57 storeys in height having practically doubled to 237 metres.
Having cleared out and relocating some 2000 plus employees, a load-baring structure has been spotted being erected along with several construction cranes on the roof of the building indicating that work is commencing.
Extending would normally pose problems such as the adding of extra elevator shafts. The consistency of the appearance of the facades and of course the most important factor of whether the foundations can take the additional weight but the original design planned ways around these connundrums by engineering in the extra weight giving BCBS the opportunity of expanding their existing offices when most firms would have to go through the inconvenience of moving.
A massive atrium which is open to the public through special arrangement runs up the north face of the building - as well as providing a nice relaxing space, two empty vertical shafts with an open staircase and several platforms in between exist to reserve space for extra elevators.
As for the facades, materials such as granite that would show minimal aging where chosen so when the time came for expansion there would be no unsightly mismatching going on.
Visually the scheme was originally split into two sections vertically. This would allow the rebuilding of the skyscraper to mimic what had been constructed before and retain the original feel and symmetry that the BCBS Tower featured.
Originally the company were looking for a much larger plot of land on which to site the building but as this at the time was not possible they opted for a smaller plot and adjusted the design to enable a vertical expansion to be possible in the future.
BCBS won't be taking all the space though as they expect to rent out approximately half of the new extension when it is completed in 2010 giving them additional space to take up in the future, next time without having to extend.