Complex sentence: It will take an additional three months to get the concrete flooring installed to the top most floor which is expected to be finished by September 2012 after which the 408-foot (124 m) steel spire will be installed.
Split into simpler sentences: It will take an additional three months to get the concrete flooring installed to the top most floor which is expected to be finished by September 2012. After which the 408-foot (124 m) steel spire will be installed.