Sentence:  The eastern tower and the smaller western satellite are linked by the lift core, which passes through the centre of the monorail turning circle and provides the structural <WSD>spine</WSD> of the building; the two towers are cantilevered from this <WSD>spine</WSD>. 
Meaning of the word marked with <WSD> tags:  Something resembling a backbone, such as a ridge, or a long, central structure from which other structures radiate.