The United States Air Force Memorial (USAFM), overlooking the Pentagon in Washington DC, comprises three stainless steel spires which evoke an image of aircraft in a ‘bomb-burst’ manoeuvre. The elegance and simplicity of their architectural form belies the complexity of their engineering design. Structurally they consist of a stiffened stainless steel shell with the lower two thirds of each filled with concrete. A second component, essential to the integrity of the structure, is also hidden by the steel skin; a series of large, steel-coated spheres, free to roll in oversized padded boxes, are located inside each spire. The purpose of these ‘impact dampers’ is to stabilise the motions of the spires in high winds. This article provides an overview of the spire structures and focuses on the challenges encountered during the design, development and test of the damping devices.