Many architects will have heard (I many times) that, according to the CTE, in the turns of stairs of restricted use (interior of houses) only entire plateaus can be left or split them at 45º, that is, the regulation only allows the inclusion of 4 steps in the 180º turn and 2 steps in the 90º turns. Interestingly, although many people blindly apply such an absurd restriction, very few have bothered to read the DB-SUA, as they would have quickly realized that it is a misinterpretation: Both the round-trip stairs (180º) and the “L” stairs (90º) are nothing more than curved staircases, since their axis is so and, therefore, the conditions of design that are specified for this type of stairs, regarding the minimum size of the tread (5cm inside the curve), the minimum dimension of the tread on the axis (22cm) and the maximum useful dimension (44cm in the outside of the curve). Applying these conditions, the plateaus divided at 45º would not comply and, for this reason, point 4.1.3 of DB SUA-1 states that: “Plateaus divided with steps at 45º may be arranged …”, as an exception to the rule. Thus, the exception has been taken as a rule, possibly due to the greater ease of application of the regulations in those terms, with the damage that this entails for the quality of the design and the safety of use and accessibility of the stairs (manifest intention of this DB), which are obviously undermined by the “imposition” of an unnatural and ergonomic model, whose inadequacy from a functional point of view is easily verifiable. Nor is it true that curved staircases have to constructively formalize the curve, since the “Criteria for the interpretation and application of the DB-SUA”, published by the Ministry of Housing, clearly stated that “The useful width of a ladder must be measured according to the perpendicular at each point to the line that defines the path of the route. In the plateaus in which said trajectory undergoes a turn, it is considered that the trajectory is defined by the arc of circumference whose center is located at the breaking point of the inside edge of the staircase. ” And in case there was any doubt, he continues saying: “… on plateaus with a 90º turn, the outer limit of the useful width would be a quarter of a circumference and on plateaus with a 180º turn, this limit would be a semicircle, and the design could be adjusted to these shapes. , although the most common are straight lines “. And, since the external measurement that is limited to 44cm is the useful dimension, we should not worry about the corner. When you try to design similar sections of stairs, you will realize that you have to play with the width of the stairs, with the diameter of the eye and with the number of steps to fit all the measures restricted by the CTE. That said, it is very true that DB-SUA, on the subject of restricted-use stairs, has some contradictions that (I know) will be polished in successive updates: If you try to fit three steps into a 90º turn you will see that it is impossible, since you will never be able to meet the maximum limit of 44cm outside. There is therefore the paradox that at 90º the turn can be designed with 2 or 4 steps, but never with 3, which is absurd if we consider that it is about setting limits on safety. Everything points to the fact that, in “restricted use”, this outer limit should be abolished. Meanwhile is what there is. If to the above is added the incongruity that a plateau with a 90º turn and 2 steps, with a small interior radius, does not comply, since it has to be considered a “curved path” (due to the existence of that interior radius) and therefore Exceeding 44 cm of footprint on the outside, while another plateau exactly the same, but without an inner radius is no longer a “curved path”, therefore it does not need to meet the 44 cm of exterior footprint and therefore complies, being clearly worse and more insecure than the previous one, it is concluded that you have to review that article, because it does not work. It’s not coherent. Some advocate eliminating the second option, that is, that in restricted use, plateaus (or perhaps not all, but stairs with a width of less than “X”) that are not justified as “curved path” cannot be split at 45º, given the insecurity of its null footprint on the inside, but only at 90º those that have a 180º turn. But that would have a harsh impact on interior staircases in close-quarters single-family homes. It is not that the CTE is a perfect document and free of errors, which it is not (especially in this DB), but as technicians we must know it and apply it rigorously, without believing interpretations made by third parties or “urban legends”, however widespread they are.
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