By Ron Burns
As mega stores get bigger, compartmentalising the development is a bigger challenge than ever.
Developments are getting larger and stores are occupying larger footprints than ever within these developments. In the early 1990s, I remember working on what was then the largest shopping centre in KwaZulu-Natal. The development boasted two mega stores exceeding 4 000m2. Today, developments are going up with stores double that size and also being multi-level. The building code refers to these buildings as ‘complex’. The challenge in these developments is to compartmentalise the ‘mega store’ — the compartmentalisation is generally ignored. Please overlook the sweeping generalisation, though; in the past 15 years, I have only been involved in the compartmentalisation of one mega store.
I distinctly remember getting a telephone call from the consulting engineer I had been working with. The client was distraught: how was it even possible for this unsightly grey smoke barrier to be installed in their store. Wikipedia describes this particular company as now having USD11.19-billion in assets and 50 000 employees. I was indeed surprised to note that they now had their first smoke barrier, but were disappointed. What a marketing opportunity lost: imagine telling the South African population that stores were being developed where lives would be safer, should a fire start. Perhaps my view is too simplistic.
“The cost of locking out an adjacent smoke zone is prohibitive, resulting in the sizing of the electrics to cater for both smoke zones being activated simultaneously.”
The consultant was less than animated on the phone, demanding to understand why the store in a development in an adjacent suburb (which was at least twice the size) did not have this smoke barrier installed. I was aware of this particular project. I had suggested the compartmentalisation of the void, which was rationalised out of the design. This rationalisation remains puzzling to me (best I do not rant). I responded to the barrage of questions to justify the barrier and asked, “Why are we justifying a compliant installation to the client? Their other store does not meet the building regulations. The barrier is installed because it is a legal requirement.” After a moment of silence, I was invited to the consultant’s office to discuss how to make the barrier more aesthetically pleasing.
The story does not have a happy ending. The barrier remained, but the size was drastically reduced. ‘Drastic’ meaning that the bottom of the barrier was lifted 200mm. Not too drastic? Overreacting? The original barrier depth was 800mm. This resulted in an extraction rate of 27.83m3/s per smoke zone, achieving a clear layer of 3.9m (AFFL). Reducing the depth of the barrier impacts on the extraction rate. The 200mm lift in the smoke barrier results in an increased extraction rate to 29.38m3/s per smoke zone, achieving a clear layer of 3.1m (AFFL). The decision to raise the smoke barrier now allows the generation of smoke to overwhelm the smoke extraction fan system, resulting in a flooded reservoir and spillage into the adjacent reservoir.
An air-conditioning engineer would by default adjust the blade angle on the fans and increase the extraction rate; however, this is not a toilet exhaust system. Smoke fans are not adjustable. The blades are pinned in the impeller — no opportunity for any adjustments. Smoke fans are selected at maximum pitch, based on the motor size. There are few complaints should a smoke exhaust fan expel slightly more smoke than the required extraction rate. This results in the clear smoke layer being higher than the design requirements. This is a good situation for hiding the occupants from the firefighters.
The fan sizing is not the main problem. The advanced construction state of this building makes it impossible to add the additional fans required to increase the extraction rate. With the fans already installed to exhaust 27.83m3/s of smoke, topping up the difference with a smaller fan will be difficult. The installation of an additional fan will result in a larger extraction rate than the additional 1.55m3/s required. Modifications to the existing control panel, the additional feed thereto, and the additional requirements for replacement air legitimise the ‘disastrous effect’ I claimed earlier.
Questions that need to be asked relating to the additional extraction rate, besides the three requirements above, are as follows:
Can the standby generator cater for the additional load? The cost of locking out an adjacent smoke zone is prohibitive, resulting in the sizing of the electrics to cater for both smoke zones being activated simultaneously. This is good engineering practice, as there may be smoke spillage into the adjacent smoke zone based on the location of the fire. The addition of two 5.5kW fans may overload the generator. This could result in the generator tripping on overload. No generator, no power. The effect on the system is zero emergency power supplied to the store. Zero emergency lighting, zero smoke extraction, zero life safety. A 200mm adjustment has multiple consequences.
“Correct application of smoke barriers also reduces extraction rates and results in a lower capital cost of the extraction components in the system.”
The replacement air system is legislated at a 5m/s velocity to prevent fanning of the fire. In the event of the replacement air fanning the fire, the fire spread can cause additional activation of the sprinkler system, which negatively impacts on the sprinklers being able to control the fire. The fire size has an exponential impact on smoke generation. Excessive smoke generation leads to a higher extraction rate, which has already been compromised by the lifting of the smoke barrier. The potential for losing the smoke zone continues to increase.
Once the smoke zone is flooded and the smoke migrates to an adjacent smoke reservoir, the smoke loses its buoyancy. The smoke descends to low level, reducing visibility and making escape from the building as well as firefighting difficult. The biggest concern, however, is the smoke being drawn back into the seat of the fire. This creates a situation of an incomplete process of combustion and the generation of more smoke, placing additional strain on a system, which the extraction system already cannot handle if the fire were to remain a clean-burning fire.
The modification of smoke barriers is not limited to trading stores. Barriers installed in mall zones are modified during the construction period based on multiple reasons. The smoke barrier is specifically chosen to assist the smoke extraction system, prevent the loss of life, and assist firefighting. Multistorey mall extraction systems are constantly compromised by poor decisions relating to smoke barriers. Directing low-level mall smoke to slab penetrations and upper level extraction location is seriously compromised when channelling screens and designed levels are modified to create a better ambience in the mall.
Buildings designed with atria require specific attention to channelling screens and compartmentalisation. This can only be achieved with smoke barriers. Smoke barriers are effectively the sheepdogs of smoke ventilation — rounding up the smoke and directing smoke to the extraction point in the simplest and quickest manner to exhaust smoke, preventing the unnecessary loss of life whilst protecting the building. Correct application of smoke barriers also reduces extraction rates and results in a lower capital cost of the extraction components in the system.
Next month we will delve into the portion of the EN 12101 code that governs smoke barriers. Part 1 allows the engineer confidence when specifying products to channel smoke and reduce extraction rates.
