Initial design
Both towers had always been planned with a rectangular plan, not a square one, with Castellana's front facing parallel to the avenue and Génova's facing south. The stairs were located in the back, that is, to the north in the tower that faced Genoa and to the west in the tower that faced Castellana.[18] The elevators were in the central core, in a group of three, one of them having a double cabin for use as a freight elevator.[18].
The project was conceived for residential use from the beginning. A multitude of designs and compositions of three, two and one home per floor were considered, including large overhanging outdoor terraces that were in high demand at the time.[18] Their destination was even considered as tourist apartments managed by a hotel chain, an innovative concept that had been successful in more touristy areas.[18] Finally, the most risky option was chosen: one home per floor with the best possible finishes and luxuries.[19] To facilitate its commercialization, a farm was built in Las Pueblas in real size raised above the ground a floor plan that faithfully recreated the interior layout and the materials used as a "show floor." The idea of residential use barely lasted since the stoppage of works prevented the correct marketing of the residential units. The pilot farm was used for many years by a family close to the Osinaldes until it was demolished.[19].
Final design
The building consists of a base or podium on which rise two twin orange-brown towers topped by two greenish crests joined by a plug-shaped finial from which the emergency stairs hang. The building has three elevators in each tower and two elevators on the parking floors.[20].
The base is reminiscent of the nearby building, at Génova 27, also built by Estudio Lamela, before its remodeling also by said studio between 2003 and 2005. The original project from 1974, in the midst of the oil crisis, sought "natural air conditioning and self-protection", which was achieved through inclined glass to receive the least possible insolation.[21] Above the podium rise the two twenty-story towers, one advanced in relation to the other. the other and not in parallel. The typical plan of the tower has a quadrangular shape, but it is not an exact square. With a floor area of 607.74 m², it consists of a reinforced concrete central core where four openings are located: three for elevators and a fourth for wiring and other installations. The façade has a double glass covering: the first is in the shape of saw teeth and the second, installed in the renovation that took place in the 1990s, is uniform with rounded corners. Each tower is topped by a quadrangular greenish crest joined by a semicircular socket-shaped structure.
suspended structure
If this building stands out for something, it is for its technical innovation, since for various reasons the floors had to be built in the opposite direction, that is, "from top to bottom." Choosing a traditional structure would have made it impossible for vehicles to circulate in the basement floors, since the necessary pillars would not have left enough space to place the ramps, and would have taken away useful space for parking spaces. Very necessary space due to the already reduced dimensions of the site (1700 m²).[22] In addition, the new municipal ordinance required a minimum of 150 parking spaces.[23] In this way, completely open floor plans were achieved, free of pillars and beams, allowing maximum flexibility in the distribution of the floor as well as greater use of the useful surface without hindering the horizontal conduits.[24] Furthermore, thanks to the transmission of loads through the core, one of the towers on Génova Street could be blown up to 9.5 meters, since there were no supports that interrupted the pedestrian passage.[25] However, the choice of this procedure meant an increase in construction costs between 10 and 15% compared to the traditional pillar structure.[26].
This model of suspended or hanging structure was novel because until then only seventeen towers had been built in the world with this system, such as the Commercial Union Building (today St. Helen's or Aviva Tower) or 20 Fenchurch Street (demolished) in London, the BP Building in Antwerp, the Standard Bank Center in Johannesburg or the BMW Tower in Munich, among others. [27] Subsequently, Spanish architecture adopted the hanging model, developing in other cases the experience originated in the Columbus Towers.[27].
The towers are designed away from the traditional scheme of a skyscraper or tower, since their main peculiarity is that the building's loads rise towards a head structure, located at the crown of the building and from there they are conducted, through a central concrete core towards the foundations. The loads are transmitted to the central core directly or through the head structure, ultimately being conducted to the ground through a footing with a surface area of 13x13 m and a depth of 7 m.[28].
Through the hanging structure, the horizontal supports are directed towards a structural system located at the top of each of the towers, made up of four beams crossed two by two with edges that decrease from 5 to 3 m, forming a tic-tac-toe board. These beams, also called radial beams, were made supported by a formwork on which, first, the lower half, then the upper half, and finally the 3-meter-deep perimeter tying beams were built, visually forming an irregular 3x3 grid.[28] All of them were made of post-tensioned reinforced concrete. In this inverted aerial foundation structure, enough space was left in the center to house the machine rooms for the elevators and facilities.[28].
A total of 18 post-tensioned concrete braces or pendulums hang from the perimeter beams, arranged at regular distances on the façade, which allow the fixation of the prefabricated anodized aluminum exterior enclosure that, in turn, houses the ducts for the air conditioning supply. The 21 floor slabs of the towers rest on this system through their outer edges. These slabs are made of lightweight reinforced concrete and are supported on the central core through brackets embedded in it.[28].