Willis Building (London) 2004 and 2008 under the management of Mace and represented a significant addition to the City of London skyline, becoming its fourth-tallest building after Tower 42, 30 St Mary Axe and CityPoint. The core was topped out in July 2006 and the steelwork completed in September that year. Cladding began in July 2006 and the structure was externally completed by June 2007. It was internally fitted out and officially opened in April 2008. The building was the first in a wave of new tall towers for London's primary financial district. Others included The Pinnacle, the Leadenhall Building and the Heron
30 St Mary Axe of the building during the summer and warm the building in the winter using passive solar heating. The shafts also allow sunlight to pass through the building, making the work environment more pleasing, and keeping the lighting costs down. The primary methods for controlling wind-excited sways are to increase the stiffness, or increase damping with tuned/active mass dampers. To a design by Arup, its fully triangulated perimeter structure makes the building rigid enough without any extra reinforcements. Despite its overall curved glass shape, there is only one piece of curved glass on the building, the lens-shaped cap at the apex.