Citigroup Center

Citigroup Center is a 59-storey skyscraper in Midtown Manhattan, New York City, at 601 Lexington Avenue. It is about 279 metres tall and opened in 1977 for Citibank. Its most striking features are a roof sliced at a 45-degree angle and a base that stands on four tall stilts placed in the middle of each side, not at the corners.

The stilts have a reason: a church. St. Peter's Lutheran Church owned a corner of the block, and the builders agreed to put a new church there. The tower's corner floats above it, and the church is built as a separate structure. Engineers also had to solve a hidden wind problem soon after the tower opened, which makes it one of New York's best engineering stories.

Why is it on stilts?

The developers quietly bought most of the land on the block from 1968 to 1975. St. Peter's Church sold its old building and air rights, the space above it, in exchange for money and a new church built at the base of the tower. To make room, the skyscraper was lifted on four stilts, and its northwest corner reaches out over the church.

The wind problem

In 1978 it was found that the tower could be at risk in strong winds blowing at its corners, partly because some steel joints had been bolted instead of welded. Workers added steel plates at night in 1978, and the building was made safe. The story became public only in 1995. The tower also has a tuned mass damper, a heavy weight that helps reduce swaying.

On the globe

The model shows a square tower of aluminium and glass bands lifted on four stilts, topped by a roof that slopes at 45 degrees. Look for the slanted top.