Torre Windsor's original structural design was carried out in the 1970s by Madrid-based tall building specialist Otep Internacional.
At least eight towers on Madrid's skyline are Otep creations, including the 250m Torre de Cristal.
Otep director general Pedro Jaun Blanco told NCE the original concept of the building was 'emblematic, with a very open floor plan that made maximum use of natural light'.
To meet these requirements Blanco went for a simple rectangular floor plan and a largely concrete structural frame, but with significant structural steel components. 'We chose small section steel columns for most of the perimeter to give maximum transparency, ' he added.
'And to reduce the number of columns internally we used steel beams to increase column centres along one axis.' An unusual feature by modern standards was a second external steel-framed escape stair in addition to the one inside the core.
Ground conditions meant pad foundations were sufficient.
There were four levels of car parking below ground, shared with other commercial buildings on this prime city centre site. To create a column-free perimeter at ground level, all live and dead loads from the perimeter columns had to be picked up by a massive concrete transfer structure at the third and fourth floors.
Blanco's key decision was to effectively stack two buildings on top of each other. At the 16th and 17th floors he pencilled in another heavily reinforced concrete transfer structure.
Above this point the building's structure was basically simple: 230mm deep waffle floors supported by concrete internal columns tapering from 1200m by 500mm to 500mm square and a concrete core - and by the 140mm by 120mm box section steel perimeter columns.
These perimeter columns carried up to 15% of live and dead floor loading via cast-in brackets. The concrete columns were on a 5m by 10m grid, with additional 360mm deep underfloor steel I-beams to support the floor over the 10m spans.
At the upper transfer structure all the perimeter column loads were fed into an enlarged core and concrete shear walls measuring 150mm by up to 2m.
Each row of shear walls extended out to the shorter facades.
The underfloor 10m span steel beams were still a feature.
Again, up to 15% of the floor loading was carried by the steel perimeter columns.
At every floor there were a number of slots boxed out of the slab around the core area for services. In the 1970s there was no requirement in Spanish building codes for such openings to be firestopped; indeed, there was no mandatory requirement for the steel structural elements to be protected at all.
The original cladding fitted between the floor slabs, so there were no gaps to be firestopped.
Floor soffits were plastered to create ceilings, concrete columns and shear walls were left exposed. Concrete was specified as Grade C25.