The dilemma of outdoor space in the event of fire

From the point of view of fire safety, every space in a building shall be considered interior until proven otherwise

A priori, if we ask anyone whether they are able to distinguish between interior space and outdoor space in a building, they will probably say yes.

The criterion behind the vast majority of these people would be to consider as outdoor everything that lies outside the building’s external envelope, basically from the perspective of climate conditioning.

The problem arises when we have to determine what is considered outdoor space, not from this perspective, but from the point of view of fire safety.

When a building is analysed in this regard, what we could call a “presumption of guilt” should be applied. In other words, every space in a building shall be considered interior until proven otherwise.

In fire-related cases, outdoor space is understood as that which, due to its smoke and heat dissipation conditions, will never reach certain critical fire conditions, so that evacuation through it, if it is passable, can be carried out under minimum safety conditions and, with regard to fire spread, this is not foreseeable if the requirements of CTE DB-SI 2 have been taken into account (or article 5.3 of Annex II of the Fire Safety Regulations for Industrial Establishments (RSCIEI), in the case of an industrial establishment).

In practice, this divergence in criteria can basically be concentrated in two types of spaces:

• Courtyards
• Porches

Courtyards

In the case of courtyards, the only problem is to determine at what point it is considered to have dimensions such that it represents an area for releasing smoke and fire, capable of extracting smoke and dissipating the heat of the flames that reach it, or, on the contrary, whether its dimensions are so small that it will act as a channel for fire transmission between floors.

In this case, the regulations do provide guidelines:

• In Barcelona, Protecció Civil, Prevenció, Extinció d’Incendis i Salvament service (SPCPEIS) regulates this through sheet 2.03 Ventilation Courtyards of the Technical Guide – Criteria for interpreting the Fire Protection Regulations, which refers to the application of art. 10 of the Ordinance Regulating Fire Protection Conditions (ORCPI 2008).
• Across Catalunya, it is document DT-6 Courtyards for the ventilation of protected and specially protected staircases, issued by the Taula per a la Interpretació de la Normativa de Seguretat Contra Incendis (TINSCI) of the Generalitat de Catalunya fire brigade, that establishes the conditions a courtyard must meet in order for a protected staircase to ventilate into it. Although it does not specifically mention this, it is understood that if a protected staircase can ventilate into it, considering it as exterior, the same consideration may be applied to other spaces with less restrictive safety requirements.
• In state regulations, the CTE DB-SI, in Annex SI A Terminology, in its validation of a courtyard onto which a staircase considered exterior may open, requires that a circle with a diameter of h/3 (h = height of the courtyard) can be inscribed in it. In the absence of other regulatory references, this could also be the criterion, following the same reasoning as in the previous point.

Porches

In the case of covered porch spaces, the absence of objective regulatory references is absolute; as stated at the beginning, in case of doubt, they should always be considered interior spaces. In any case, this doubt must always be resolved by the corresponding competent authority, applying its criteria and interpretation to each particular case.

The reason for this lack of definition basically lies in the behaviour of fire and smoke, in accordance with their tendency always to move upwards. This simple characteristic means that, as a general rule, when a fire or the smoke arising from it reaches a covered porch area, it tends to form a layer of smoke in the upper part which, to a greater or lesser extent, accumulates heat and unbreathable air, therefore affects structural elements and compromises evacuation through that space.

Obviously, when the configuration of the space is particularly favourable and straightforward, it may be possible to speculate, without entering into more detailed studies, as to whether the smoke layer in that space will not descend below reasonable levels to allow safe evacuation and whether the dissipation conditions will prevent excessively high temperatures. In these cases, when the authority (usually the fire brigade) shares the same view and accepts the proposed approach, this space may be considered outdoor.

Logically, the range of cases is practically infinite, and for this reason it is not possible to define constant, simple and objective regulatory criteria to determine whether a space can be considered outdoor. Almost whenever we are outside the most elementary assumptions, or these have dimensions beyond certain reasonable limits, we will be required to develop some type of study to justify the condition of outdoor space. In the most extreme case, it may even be necessary to justify it through performance-based design (PBD).

Although, as already stated, the competent authority will have the final say, the aim below is to set out elementary cases and the dimensions and proportions of these spaces that, based on the experience in project design that we have at ARCbcn in these cases, are considered likely to be accepted. The intention is therefore to provide a minimum reference that can help in the initial design process, attempting to avoid dramatic setbacks.

Taking into account that UNE 23585: Fire safety. Smoke and heat control systems. Requirements and calculation and design methods for designing a temperature control and smoke evacuation system (SCTEH) in the event of a steady-state fire, establishes 2,5 m as the minimum safe smoke-free height, any porch with a height close to this dimension should be ruled out as an outdoor space. We consider that they could reasonably begin to be validated from a height of 3 m.

In the case of tunnel porches, with ventilation at both ends, the improvement in ventilation conditions that this represents allows the dimensions to be adjusted further.

• Simple porch h = 3 m (1 façade): width (a) ≥ height (h) ≥ 1,5 x depth (f)
• Simple porch h > 4 m (1 façade): width (a) ≥ height (h) ≥ depth (f)
• Tunnel porch h = 3 m (2 façades): width (a) ≥ height (h) ≥ 0,5 x depth (f)
• Tunnel porch h > 4 m (2 façades): width (a) ≥ height (h) ≥ 0,3 x depth (f)

Obviously, this does not mean that spaces with more unfavourable conditions cannot be validated or that spaces with these conditions may not ultimately be rejected by the authorities, but at least these data can be taken as a reference starting point, so as not to approach this issue blindly.