The tower's unusual structural geometry uses less steel than a conventional frame and thus represents less embodied energy.
This from Foster's website - http://www.fosterandpartners.com/Pro...4/Default.aspx
"The new building is also distinctive in environmental terms. It is constructed using 80 percent recycled steel and designed to consume 25 percent less energy than its conventional neighbours. As a result, it is expected to be the first new office building in the city to be given a gold rating under the US Green Buildings Councils Leadership in Energy and Environmental Design (LEED) programme..."