The question always arose: why exactly 9 floors in the houses, and for example, not 10? But somehow it was not a leisure to find a response to him, and here it seemed to have a conversation with the builders.
So, one builder, working in the 60-90s of the last century on construction, said that one of the main reasons for such a floor is the height of the standard mechanized staircase of the fire truck - 28 meters.
It is this permissible height from the fire fare to the upper floor window is spelled out in regulatory documents. Considering that the standard height of the floor is 2.8-3.0 meters, add the height of the foundation or base, almost in all cases, with a rare exception, it turns out that the fire staircase is removed only before the ninth floor. In buildings above 28 meters, an unlucky staircase H1 is required (construction norms and rules of unnecessary stairs are divided into the following categories:
H1 is the stairs, the entrance to which is carried out through the open space outside the building; H2 - staircases with the device of an additional airfield; H3 - staircases, the entrance to which is carried out through specially created zones with the side of the air) with a passage through an open balcony. And this is too much more appreciated the final cost of the square meter and such a staircase occupies a much more place in the building.This solution is justified in the buildings of 14-18 floors. Do not forget that and the savings were primarily and on Ghosts on a 9-storey building requires 1 elevator, on 10 or more floors already 2.
In addition to the second (freight) elevator in 9-storey buildings, the smoke removal systems, air support and special evacuation paths were required. This significantly influenced the cost. The specific value of the square meter in the 12th candle was significantly higher than in the 9th floor. Well, something like this, one more white spot in my world erased, I hope the information was someone interesting.