Fläkt Woods - The Case for Formal Guidelines to Ease Specification & Approval of Mechanical Smoke Extraction Systems
Flakt Woods - Ross Barritt-Mehta
The current lack of definitive guidance for auditing mechanical smoke extraction designs in high rise buildings means that their specification is often based on cost rather than objective technical performance standards. Ross Barritt-Mehta, Business Development Manager for Fire Safety at Fläkt Woods, part of Fläkt Group, explains why more specific criteria for assessment, together with a standardised provision of these systems, would make it easier for specifiers and building owners to choose the best option in line with the fire safety strategy.
In high-rise buildings, mechanical smoke shafts are a common solution to protect common escape routes and maintain tenable conditions for firefighting access during a fire. They take up less space compared to natural ventilation systems so are particularly suitable if space constraints or architectural restrictions prevent the use of simpler solutions, or if the owner wants to increase the building’s lettable area. Despite their simplicity and effectiveness, many specifiers, developers and construction procurement professionals are in the dark about how best to compare different solutions during early stages of a project.
This is primarily because mechanical smoke shafts do not yet appear in the Building Regulations, and are treated as a fire safety ‘engineered solution’ (in comparison, guidance for natural smoke shafts can be found in paragraph 2.26 of Approved Document B of the Building Regulations). This means that each design is treated as a new scenario, and judged using the appropriate parts of several related documents. The Smoke Control Association’s (SCA) ‘Guidance on Smoke Control to Common Escape Routes in Apartment Buildings’, contains some of the acceptance criteria for a mechanical ventilation and smoke shaft system. But this only applies to residential developments, with no regulatory equivalent for commercial or mixed-use premises.
Due to the lack of detail and exact minimum performance standards that mechanical smoke extraction and ventilation systems should meet, the level of detail provided by suppliers can vary depending on which stage the project is at. In early stage design, very little information tends to be available other than a general description of how the system would work. At tender stage, whilst a full bill of quantities clarifies what will be supplied, it is very difficult for specifiers and end clients to compare the different options and judge which would be best at implementing the fire strategy in the absence of detailed technical standards to refer to. The only clear criteria upon which they can make decisions on is simply price. This of course, is in no way the best method in ensuring that the option specified would meet goals outlined in the fire safety strategy.
Specifiers who don’t have an in-depth knowledge of fire safety systems only have the supplier’s information and promise of compliance with the relevant regulations, its past performance, and the strength of their brand to base procurement decisions on. It is only when a report needs to be submitted to the local authority for building approval prior to hand-over or at practical completion that the supplier would need to provide further detail on how the proposed system meets regulations – by then, a certain level of commitment would have been made to the supplier.
What’s more, because each development is treated as a bespoke design, project-specific Computational Fluid Dynamics (CFD) analysis is required in the report for Local Authority Building Control. So unless the whole mechanical smoke extraction system is supplied by Fläkt Woods, which is ratified by LABC Approval, more hoops are involved in this process.
In our opinion, key personnel involved in specifying such systems should be able to understand exactly what they are assessing and buying into from the outset, rather than simply relying on suppliers’ promises. After all, technical proof in the form of CFD analysis is not normally required until the construction stage in design and build projects.
If detailed guidance on the minimum performance standards for mechanical smoke extraction and ventilation systems was available, the process of comparing different designs during the early stages of a project, as well as the procedure for gaining local authority approval, would be easier.
After a decade of common use, a bank of data exists to assist in designing such systems, especially for residential buildings where one lobby is very similar to another. At Fläkt Woods, we have data from many projects, which details the changes to the system such as supply and extract points in relation to the stair door, building characteristics, travel distances and fire size. We can compare this to the required smoke extract rates from the fire floor to ensure smoke-free escape routes. By aggregating all of this information into a database, we have developed suggested extract rates for buildings over 11m high with up to 20 storeys.
If further information from previous designs supplied by other key players in the industry was available, along with local authority reports from approved projects throughout the years, a full body of evidence and knowledge could be produced to compile a set of technical standards and parameters. This could form the basis to extend Approved Document B, which would simplify the approval process, enable specifiers and developers to choose the most suitable - not just the cheapest – solution, and most of all, maximise fire safety in a tall building for the benefit of occupants and firefighting access.