BS 5588 Part 1 and Building Regulations Approved Document B, both advocate that internal common corridors of flats require opening ventilators, which should open automatically where escape is only available in a single direction. Where the corridor is a long one the requirement is for cross ventilation, each ventilator providing an area of 1.5 square metres, even though the corridor is required to be sub-divided by self-closing fire doors.
These measures have been around for many years. An explanation as to the reason for this provision was given In British Standard CP3: Chapter IV(1962), Part 1:-
" Ventilation. (i) The provision of cross-ventilation (i.e. ventilation at two points to produce a through draught) to a main corridor can contribute to safety by diluting and dispersing smoke, but the principle has some practical objections and weaknesses. The amount of smoke generated by a fire can be considerable. If the entrance door of a flat which opens on to a corridor should fail to restrain the smoke (and this possibility is assumed throughout) only the rapid movement of a large quantity of fresh air would make the escape route reasonably safe. But, in the first instance, it is not possible to ensure this condition at all times, since the horizontal movement of air within a building will depend upon the direction and speed of the wind at the time of the fire. And, secondly, as the provision must be a permanent one if credit is to be given to it at all, there will be many occasions when the draught will make the corridor unbearable for normal use. Any attempt to remedy the second objection by reducing the size of the permanent openings will increase the first objection that the movement of air at the time of a fire may be inadequate for the purpose. For these reasons the principle of using cross-ventilation as a basis for the design of means of escape has not been adopted in this Code, but this is not to deny that facilities for the removal of smoke from corridors will prove beneficial. If permanent ventilation can be afforded by some means without producing too severe an effect upon living conditions in the building, some gain in safety will result."
The approach currently adopted has come about as a consequence of the development of new equipment that makes its comparatively easy to provide automatic opening ventilators activated by smoke detectors. There have been other developments since then that have not been taken into account.
It can be seen that the measure originally advocated was to dilute and disperse the smoke. The two criteria are not compatible. For the smoke to be diluted there is a need for it to be mixed with a flow of fresh air The effect is to increase the quantity but reduce the density of the smoke. To disperse the smoke it is better to keep the smoke plume together and undiluted as far as practicable until it is able to pass out of the building.
Approved Document B (2000 Edition) describes the purpose of ventilation in common corridors to be to disperse smoke and afford protection to the common stairs. Whilst dropping any reference to dilution, the implication has to be that the most critical function of the ventilation is to provide an alternative path for smoke to leave a corridor other than by means of the stairway. AD B (2000) has also reduced the area of ventilation necessary to 1 square metre, a recognition perhaps that the area previously advocated was excessive.
Airflow due to cross ventilation varies as a function of wind speed and height. A fixed area of ventilation provides differing levels of ventilation depending on its location. The amount of ventilation varies considerably as a consequence of the wind speed.
If we are to disperse smoke from a corridor and relieve any pressure that might drive smoke into stairways we now have sufficient knowledge to provide a more credible technical solution. There is no justification for these massive areas of opening except perhaps to assist with the original concept of smoke dilution.
Despite the inclusion of 1.5 square metre ventilation shafts in the 2006 Approved Document B, FiSEC have developed a technique for ventilating smoke from fully internal corridors using significantly smaller Passive Stack Smoke Ventilation shafts. These not only save on floor area but also reduce the external impact making them more acceptable to planning officers.
Using the National Institute of Standards and Technology's Fire Dynamics Simulator, a Computational Fluid Dynamic Modelling programme, the performance of a PSSV was compared to the performance of a 1.5 metre high by 1 metre wide fully open window.
The scenario modelled comprised a fire in the living room of a flat with both the door to the internal corridor of the flat wide open and the front door into the common hallway wide open. The common hallway in turn led to a common escape stairway with a closed but leaky door fitted between them and a 1 metre by 1 metre open rooflight located over the top of the stairway.
Where the common corridor was ventilated by the open window smoke passing into the common hallway spread into the common stairway and continued to flow for the duration of the evaluation. Where ventilation was provided by a PSSV shaft in the common corridor, although there was some initial smoke flow into the common stairway this quickly ceased and all the smoke flowing out of the flat passed up the PSSV shaft exhausting itself harmlessly outside the building.
The potential benefits of this technique are significant, as plans for flats can be developed without the need to position common corridors and stairways adjacent to potentially valuable external walls.
Further details including a design for the PSSV terminal at the top of the shaft are available from our Lincolnshire Office.