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Smoke shafts

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By Ron Burns

I was invited to the FIREX 2018 International Exhibition and thought this was an opportunity not to be missed …

Ron Burns - Bio

FIREX is hosted in London, so since I thought that it would be interesting to see how they approach building protection from smoke and heat exhaust, I packed my bag and set off on the adventure. My agenda was to see what products were available for introduction into the South African market.

My opinion of insufficient certified products has always haunted me. Perhaps South Africa is a pace too far back for the products of interest. None of the ‘interesting’ items were on display. I was later to discover the reason for this. Products that I regarded as ‘interesting’ were treated as simple commodities. There were no high-temperature-rated fans, no smoke and fire dampers, no roof ventilators, and only one stand with electrical cables. The international focus seems to have shifted past those products. Instead, they were focusing on the application of the products.

There were stands housing certification and installation bodies. Fire-rated glazing was popular, as were fixings and sundry items. The focus was no longer on ensuring that the high-temperature equipment was tested to perform to the requirements — this is a given, but rather to ensure that the installation was carried out by competent people to ensure functionality. Discussions relating to installation, support, and routing were the focus.

“We need to expel the thought of a brick shaft as an acceptable method of transporting hot gases through a building. The brick shaft is not airtight.”

Though the topic was hardly discussed at FIREX, visiting the Grenfell Tower site was a priority for me. I was to discover that getting people to engage in a discussion would be difficult and emotionally charged.

The building stood tall, covered in a white cloth screening material, wrapped with no visible access. My eagerness to take photos passed as I read the first public notice: “No photos allowed — please respect the homes of people who have lost loved ones”, or words to that effect. Fire marshals were still on the site. It was a year and a week to the day that the disaster had struck. There were walls of tributes to the lives lost — 72 people perished in the fire. Two families were milling around the memorials, their grief apparent. It was a sobering moment; I texted family back in South Africa, describing the uneasiness of the environment.

Returning to the train station was difficult. The more I walked, seeking the station, the more memorials I saw: a church overrun with dedications and memorials, flats in adjacent streets posting sentiments of memories and reflecting on the lives lost. The overwhelming message was the call for justice — the call for imprisonment of the people who had disregarded the regulations when they installed the fire protection services. It made me reflect on the high-rise buildings back in South Africa.

How do we prevent a similar disaster from occurring in South Africa? Do we have the correct products? Are we applying the regulations with sufficient diligence?

I cannot recall being involved in any smoke ventilation systems in high-rise residential buildings — only high-rise office buildings. As I reflect on the application of the regulations, I wonder if all the decisions made will protect lives in a fire and smoke-engulfed building? Which products do we use? There are many floors that have no roofs; instead, they have a concrete floor above. The option of opening windows in the vertical façade is simply not a solution. In my opinion, a ducted extraction system is best suited to ensure smoke and heat exhaust from the fire floor. With this in mind, I would like to look at the equipment required to extract smoke from the fire floor.

A simple system incorporating the following equipment is required: a series of extraction fans, the installation of duct — brick shafts are simply no good — and smoke dampers allowing smoke and heat exhaust fumes into the duct. Cabling is critical to the performance of heat extraction systems, especially with cables passing between smoke and fire zones. Ensuring the protection of the panel requires care.

We need to expel the thought of a brick shaft as an acceptable method of transporting hot gases through a building. The brick shaft is not airtight. Brick shafts are constructed from porous material. Nothing new here; the industry has a solution: bag washing. But does a bag-washed brick shaft become airtight? Obviously not. Have you ever seen a swimming pool being constructed from bricks and mortar and then bag washed? Never — the structure will leak. Concrete shafts offer no more of a solution than bricks and mortar do.

Concrete roof slabs are waterproofed with membrane to stop water penetration, so why is the leaking factor ignored when extracting smoke? How are the friction losses calculated through a bag-washed brick shaft with an aspect ratio greater than 3:1 (say, a shaft 2 400 × 500 extracting 14m3/s?). Calculating the friction losses in that environment will be challenging at best.

The next solution is to plaster and paint the shaft. Let’s dig into that. The internal surfaces are plastered and painted, providing a smooth surface. The shaft is getting close in comparison to the smoothness coefficient of a sheet metal duct. A few questions spring to mind:

  1. The plaster and paint finish is internal to the duct; therefore, the suction pressure is constantly trying to pull the plaster off the wall. Should the plaster and paint finish not be external to the shaft, the suction pressure will then pull the plaster and paint onto the brick shaft — a fail-safe concept similar to the principle used when determining the door swing on a pressurised space? The recommendation remains to have the shaft plastered and painted on the internal surface.
  2. How is the brick shaft sealed where the top of the brickwork meets the underside of the slab? The structural engineer requires a gap for deflection between the brick walls and the slab over to prevent slab deflection loads being transferred to the brick walls. The pressure transfer onto the brick walls results in cracking of the walls, again leading to leaking shafts. Is this correctly fire-rated mastic applied to the manufacturer’s recommendations? Is the mastic designed to overcome the negative pressure created by the extraction system? Is the brick shaft going to remain airtight 10 years after construction?
  3. The additional financial requirements for creating an airtight brick shaft to act as a smoke exhaust duct may be prohibitive when the commissioner challenges if the commissioning of the extract system is diligently undertaken.

At the entry into the shaft, a smoke damper is required, while at the exit of the shaft, an extraction fan is required. These components are built into the shaft with porous material; the airtightness is not guaranteed. Imagine a 13-storey building with two smoke dampers per level measuring 2 400 × 500; that is, a perimeter of 5.8m with a 5mm gap equals a leakage area of 0.754m2, compared to the area of the two smoke dampers totalling 2.4m2. The leakage rate around the perimeter of incorrectly sealed fire dampers built into the brick shaft equals 31.4% of the fire floor damper inlet. One can only assume the balance of the leakage through the construction of the brick shaft.

The excessive leakage through the construction method is not detected at commissioning stage. In my experience, the commissioning of smoke systems is seldom performed to any recognised standard. In buildings of this nature, the performance of the fan is tested at the fan discharge — if tested. However, the discharge reading of the fans has zero indication of the extraction rate on the fire floor. The inlet through each smoke damper needs to be measured and verified against the design requirements. Generally, more care is taken to balance the toilet extraction system in a building than the smoke extraction system. I wonder how many handover documents for multi-level extraction systems have commissioning data recording the inlet readings of the smoke extraction shafts.

“Unless there is confirmed measurement through the dampers on each independent level that correlates to the design calculations, the designer has fallen short of ensuring that the required air volume is being extracted from the fire floor in a fire condition.”

The last point on the brick shafts comes from my formative years. While working on St Augustine’s Hospital, I elected to save the company money by using the plumbing riser shaft as the return-air duct. I was young and bright in those days, but the contracts engineer was not impressed. He pointed out that although I could not see the air, there was in reality no difference between air and water. He then went on to ask if we should scrap all the cold water piping and pressurise the plumbing duct with water and build the cold water taps into the brickwork. Naturally, this is a crazy suggestion; I could just imagine the leakage and flooding. No person, engineer or not, would consider this method of reticulating water through a building. It does, however, raise the point of a brick shaft under pressure, positive or negative, no airtight seal is possible. The result will be the drawing in of air from areas that the designer does not require.

In conclusion, unless there is confirmed measurement through the dampers on each independent level that correlates to the design calculations, the designer has fallen short of ensuring that the required air volume is being extracted from the fire floor in a fire condition. We can delve deeper into the duct requirements for a SHEVS system in the following article. I trust this has let your mind query the use of brick shafts for the transport of smoke fumes through a building.