The Kio towers
Madrid's Puerta de Europa, or "Europe Gate", is made up of two symmetrical towers leaning together over the Paseo de la Castellana at Plaza de Castilla. They are also known as the Kio Towers.
They were designed on the old idea of giving Madrid a north-facing city gate. The original design is by American architect John Burgee. Geometrically speaking, the towers are parallelepipeds with square, horizontal bases measuring 35 metres on each side. They are 115 metres tall, and two sides of each tower are inclined at a 14.3° angle. This means the crown of each building is 30 metres off-base, so the base width and the roof width overlap by barely 5 metres, which is the minimum area it can be and still leave room for the lifts.
The towers have a total of 25 storeys (one of which is a mezzanine for technical facilities) and three basement levels. On top of each tower's roof is a heliport, designed for helicopters weighing up to 4,000 kilograms and for use in case of emergency evacuation.
Due to the towers' inclination, the foundations were designed with a 15,000-ton counterweight to keep the buildings from toppling over. Anchored to the counterweight are the post-tensioning cables that are bonded to the structure.
Given the inclination, many components of the towers required original solutions. For instance, because even the windows on the inclined faces need to be reached for cleaning, a special gondola had to be designed that slides along a metal rail attached by rollers to tracks that cover the entire façade, making the gondola cling to the inclination of the plane.