Following the destruction of New York's World Trade Center (WTC) in the terror attacks of 11 September 2001, architects and structural engineers worldwide began re-evaluating the design of tall structures. The Shard's early conceptual designs were among the first in the UK to be amended following the publication of the US National Institute of Standards and Technology (NIST) report into the collapse of the WTC. The building is designed to maintain its stability under very onerous conditions, with its post-tensioned concrete and composite floors, load-bearing pillars and tapering shape giving it a sway tolerance of .