inclinations and opacities which, combined with the different shades of aluminium, alter the colour balance of the tower as a function of time of day and season of the year. The construction used 25,000 m3 of concrete and 125 tonnes of steel. Form and structure The tower is made up of two concentric concrete oval cylinders which do not come in contact with each other. The outer cylinder, which completely encases the inner cylinder is covered by a dome of glass and steel which gives the tower its characteristic shape of a bullet. This outer cylinder with a thickness of 45 cm at the base and 25 cm at the top contains approximately 4,500 windows, while the inner cylinder measuring 50 cm at the base and 30 cm at the highest point, is where the lifts, stairs and facilities are located. Distribution The building contains 38 floors of which 34 are above ground and four floors are underground. Two of the underground floors host an auditorium for 316 people in addition to other services areas while the other two are intended for parking. Of the 34 floors located above the ground, 28 are for offices, three are technical plants hosting building facilities, one is dedicated to multipurpose rooms, another houses the cafeteria and the last is intended to be a viewing platform just below the top of the tower. Lighting One of the most characteristic elements of the building is its nocturnal illumination. The tower has more than 4,500 luminous devices that can operate independently using LED technology and enables the generation of images on the outside of the tower. The system is capable of creating 16 million colours, thanks to a sophisticated system of hardware and software. It has the ability to quickly transition between colours which can create a shocking effect. During Christmas, New Year and other events the tower changes color in animated computer controlled sequences. The unique lighting system of the building, dubbed by its creator Yann Kersalé as diffraction, who defined it as "a vaporous cloud of colour that seeks moiré", is often used in the celebration of various events such as the naming of Barcelona as the capital of the Union for the Mediterranean or the 50th anniversary of Treaty of Rome. A symbol of Barcelona Once it was finished, the Agbar Tower quickly became an architectural icon of the city of Barcelona and