Bespoke design v standardisation in smoke control for high rise buildings
By Ben Meek, Design Manager / Fire Engineer at SCS Group
Imagine designing a fire alarm system for an office or residential building from first principles:
1: Take a set of drawings for the building and start by assessing the fire risk and deciding what the likely size of fire would be.
2: Calculate the heat output and smoke temperature generated by the fire.
3: Consider where the fire would likely occur and perhaps model various locations to ensure the worst case scenario is covered.
4: Predict the likely sequence of events in a fire including how long the fire brigade would take to attend, the movement of people through the building and the amount of smoke entering escape routes.
5: Design smoke and heat detectors to suit the exact amount of smoke in the room and decide on the optimum location of these. Decide how many call points and sounders would be needed and where these would be positioned. Would manual controls be required by firefighters for this particular building (based on our interpretation of how they would tackle the fire)?
6: Once we have established what hardware is required, construct a cause and effect statement based on our assumptions. Finally, we might have this converted into a bespoke software programme to control the system for this building.
Think what the outcome of such an approach would be: Every new building, regardless of its similarity to other buildings of its type, having an individually designed fire alarm with varying numbers of devices and individual operating systems, maintenance requirements and cause and effect tables at the whim of the designer.
This approach would place a high level of responsibility on the designer to consider every possible outcome and the software engineer to design a system that would work in every case without any bugs being present that may cause the system to malfunction. This approach would undoubtedly be a high cost one due to design time and bespoke engineering and installation.
Finally, imagine that there are no qualifications or certifications required to design fire alarm systems, so anyone can call themselves a fire alarm specialist and design systems for any building regardless of size or number of occupants. Imagine there is no external certifying body, and each system is self-certified and never checked by a third party.
Does this seem illogical, nonsensical, or even dangerous? Well this is pretty much how the field of smoke control works in practice today. It is a completely unregulated part of the fire safety industry that is populated by specialist contractors of varying size and capability. There are no accreditations required to enter the industry or to call oneself a smoke control specialist.
Unlike in other areas of fire protection, for example sprinklers or fire alarms, there is simply no recognised qualification for designers of smoke control systems.
We find it incredibly worrying that the importance of smoke control system design is stressed repeatedly within the British Standards and also in guidance written by the Smoke Control Association (SCA) - the trade association for smoke control specialists - yet, nowhere is it stated what qualifications these designers should have. Fairly alarming when such a complex and important life-safety system is being created, wouldn’t you say?
You only have to read the SCA document “Guidance on Smoke Control to Common Escape Routes in Apartment Buildings” to see the importance placed on a smoke control system designer. The document details numerous tasks which need to be carried out by them. And remember, this designer is coming up with an incredibly important life-safety system which focuses on the safety of occupants of buildings and firefighting professionals.
Among the designer’s responsibilities listed in the SCA document are:
- Provide a detailed engineering analysis using the relevant prescriptive benchmarks and functional requirements for comparison.
- Agree performance criteria and accompanying fire scenarios with the authority having jurisdiction (AHJ) as part of the approval process, preferably in advance of detailed calculations or modelling.
- Where system performance is being assessed deterministically (and not compared to an ADB compliant one) set acceptance limits for one or more performance criteria based on tenability. These to be established on a case-by-case basis as part of the overall fire strategy.
- Choose the fire size and ventilation opening conditions and agree with the authorities having jurisdiction. The need to complete a sensitivity analysis in relation to the fire size and extent of the glazing failure may, in some cases, be required.
- Provide as a minimum the following information:
- A description of the residential area and the proposed ventilation system
- The design criteria and performance objectives of the analysis
- The scenarios investigated
- Details of the techniques used and related information
- The results of the analysis
- A statement as to whether the design criteria and objectives have been met
- Summary input/output data for the modelling used for time dependent analyses, graphical results should be presented wherever possible to quantitatively show conditions plotted against a time line. A sensitivity analysis should be carried out and presented such that it allows important outputs between different scenarios to be easily compared
- Complete an assessment of the conditions within spaces where single direction of travel exceeds 7.5m, to demonstrate that effective fire service intervention can take place.
What is worth pointing out is this very detailed document only relates to the common escape routes from apartment buildings – arguably the simplest buildings requiring smoke control provision.
There is usually a large degree of judgement used in compiling a fire-engineered smoke control solution for a building, and codes and standards are usually not specific enough to detail satisfactory acceptance tests. Therefore, in most cases it will never be known if the design was appropriate unless the system is called to perform in an emergency.
Despite this lack of prescriptive standards for smoke control provision and the potential consequences of poor system selection, required qualification and experience of the designer are not defined in any of the applicable documents.
In coming up with a system proposal, a fire engineer will develop a fire strategy for a building, which will specify a broad-brush approach to the type of smoke control provision required. This will then be developed by a specialist contractor, or often several specialist contractors in commercial competition, and the decision on who is appointed is largely a commercial one based on price. There are no clear ground rules for comparing alternative proposals.
If, as is commonly the case, the system proposed is not directly compliant with Building Regulations, it is deemed to be a fire-engineered solution that will require approval on a case-by-case basis by the Building Control Authority or authority having jurisdiction (AHJ).
The AHJ has to make a decision on the suitability of the submission even though they may not necessarily be well versed in such systems.
We believe these are major flaws in the current regime of smoke control design which could potentially compromise the safety of building occupants and firefighting personnel.
There are many competent specialists in the field, but without certifications and/or qualifications there is no clear way of distinguishing the good from the bad.
We believe there is a need for more guidance for the design of such systems to enable external auditing and verification by approving authorities and purchasers. We also think there needs to be a recognised training path and certification process for designers to instill faith in their output.
At the same time, there is a strong case to make for standardizing fire-engineered smoke control provision so equipment providers, installers and approving authorities can all work to the same basic requirements for groups of similar buildings, for example high-rise residential developments.
The most common type of system installed today is the mechanical smoke extract shaft (see guide at http://groupscs.co.uk/resources/) used to protect the escape routes in tall buildings. These systems, whilst they have been in use for many years, do not yet feature in the Building Regulations and are considered a fire-engineered solution. As such they require approval for use by the building control authority or authority having jurisdiction (AHJ) on a case-by-case basis.
The typical process is for a specialist to propose a system for a building backed by a design submission and often a computational fluid dynamics (CFD) model to prove that the proposal at least offers the same protection as the Building Regulations’ recommendations. The ethos is that every system is treated as a unique entity with the solution being designed exactly for the building.
In reality, although each building is modelled individually, the results are so similar that a set of industry standards have evolved, e.g. smoke shaft area 0.6m2, smoke lobby vent free area 0.6m2, volume extract 4-5m3/s. These are well known and very consistently applied within the industry – however, they are not contained within any published guidance. This methodology is already used for natural smoke shafts where the BRE carried out extensive research leading to a series of recommendations that are now in the Building Regulations on equipment sizes and specifications that are commonly in use today.
There are mechanical smoke shaft systems that have been approved by Local Authority Building Control (LABC), under its Type Approval scheme, and these essentially apply a standardised approach to protecting staircases and lobbies. Software used in such systems is tried and tested and configured on site in the same way that fire alarm systems are. In addition, all construction and maintenance information is available in advance which helps with construction programming.
Such systems will never cater for every possible case but will be effective in the vast majority of situations and offer the advantages of being based on a common set of parameters so will give consistent performance at a lower cost.
The alternative is to take a ‘first principles’ approach to every building - regardless of the level of complexity of the project - and rely on a designer to make an appropriate set of assumptions to give a safe outcome. As alluded to earlier, the major risk in this approach is that there is no reliable way of assessing the competence of the designer so it could be argued that this is an inherently riskier approach than using a standardised modular product.
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By Ben Meek, Design Manager / Fire Engineer at SCS Group Imagine designing a fire alarm system for an office or residential building from first principles: ...