We have read with interest the article 'Coping with smoke in atria' (aj 21.1.99), and we would suggest that the rather general nature of many of the statements tend to oversimplify the difficulties associated with smoke control in atria.
For instance, there is no mention of the additional problems which arise when there is a sizeable fire load on the atrium floor and, indeed, when some quantities are developed for the two sketches showing smoke removal from an atrium, a very different picture arises.
If these diagrams are taken to represent an office building for which the design fire size would generally be taken to be 2MegW, and which has a floor-to-ceiling height of 3m, with a clear layer height below the smoke of 2.5m, probably imposed by the local authorities, then it is found (using the information given in bre 186) that the area of the common vertical duct will need to be 10m2 with a similar-sized opening for replacement air on each floor.
This is the area of a small office, and no client is going to look favourably on a suggestion that this area of lettable space should be lost on every floor. Add to this the fact that such a large extract opening into the smoke layer will cause 'plug-holing', so that at the top of the smoke layer this large extract area has to be divided into about ten smaller extract points distributed over the whole office area, and it becomes clear that the common-duct idea is not practicable.
Now consider the sketch which suggests putting the neutral plane at the head of the atrium. The position of the neutral plane is controlled by the area and the position of openings above and below this plane. Using again the 2MegW fire and a slab-to-slab dimension of 3.5m, it is found that in order to put the neutral plane in the position shown in the sketch, a roof vent area of 63m2 with a ground-floor fresh-air opening of 25m2 would be required.
This could well mean that the whole roof needs to be open.
We note that reference is made to the differential effects on natural ventilation in summer and winter, but no positive information is proved for the emergency condition.
No one should under-estimate the difficulties associated with smoke management in atrium buildings, and sketch solutions as shown in the article without the relevant requirement dimensions can be very misleading.
E Gordon Butcher & Alan Parnell
Fire Check Consultants, Tonbridge, Kent
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