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CROSS Safety Report

The application of rule-based guidance and potential alternative approaches

Report ID: 1142 Published: 17 February 2023 Region: CROSS-UK


Overview

A reporter raised with CROSS a series of questions and opinions about the treatment of fire risk in buildings and rule based guidance requirements. 

Key Learning Outcomes

Designers, Architects, Fire Engineers, Building Services Engineers:

  • Buildings that rely on active fire precautions to meet safety requirements should be tested for the impact of failures of such systems and/or occupancy by persons not taken into account by standards, such as persons with reduced mobility
  • Designers need to be aware that following guidance blindly may not meet all needs of the occupants 

  • Cognisance of where rule-based guidance has come from and what has informed it is also key for the proper application of the rules
     

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A reporter raised with CROSS a series of questions and opinions about the treatment of risk in buildings and rule based guidance requirements. 

For the purposes of this report, the reporter considers that any building where fire safety is dependent on systems other than passive fire protection can be considered a complex building. This can also usually be the case for Higher Risk Residential Buildings (HRRBs), or buildings where the occupancy characteristics may necessitate the employment of more sophisticated analyses and solutions. This can sometimes be sparked by changes in the design of safety solutions due to natural effects and specific triggers (for example the stack effect in ventilation systems, or different approaches in the design of sprinkler systems beyond a specific number of floors and heights).

It has become generally common knowledge in the fire safety industry that, as it is phrased in Approved Document B, 'those with responsibility for meeting the requirements of the regulations will need to consider for themselves whether following the guidance in the approved documents is likely to meet those requirements in the particular circumstances of their case'.

Built-in assumptions and latest research

The reporter is of the mind that in order to most successfully meet the requirements of the regulations in cases of complex buildings, designers ought to be aware of the historical developmental processes of technical guidance to cultivate an understanding of the built-in assumptions. Additionally, designers should keep themselves aware of the latest research findings in the field, because the results may sometimes expand and add to existing guidance provisions. These might be utlised before the lengthy process of formally incorporating them in their practice. Reciprocally, there may be research findings that contradict existing guidance provisions, and practitioners should be wary of them until they are addressed by the appropriate guidance bodies.

Consideration of persons of reduced mobility

An example of such a case may be the ongoing consideration of persons of reduced mobility (PRMs) as occupants in a building. A fire safety solution that follows available guidance, and may be perfectly acceptable without such considerations, can potentially, under an updated perspective, be found wanting if provisions for PRMs are needed. According to the reporter, these distinctions become harder to navigate and address as the levels of complexity increase in the safety arrangements of a residential building.

This becomes even more complicated when one considers the deterministic philosophy that is employed in rule-based guidance. While levels of redundancy exist, these are not explicitly considered. The reporter cites that the underlying assumptions in some historical guidance were that some safety systems will always activate and operate successfully. The reporter thinks that this might not be realistic and will not always hold. While for the majority of the cases the guidance is very successful in implicitly providing acceptable safety levels, there may be a high-consequence safety environment where, according to them, an explicit risk-based approach may be more appropriate.

Assumptions around smoke control systems

The reporter is of the mind that only through a complete probabilistic risk analysis can single points of failure, along with the interdependencies amongst safety systems, be explicitly and thoroughly considered. An example of that can be the assumption that a fire is always suppressed (either by fixed active suppression systems or fire brigade intervention) in order to design a smoke control system. This may not always be true as such systems have a failure rate which may be big or small. A designer may assume that some layers of safety can be stripped (e.g. the removal of passive smoke seals in doors because they are not necessary in the scenario that suppression is successful and the smoke control system can cope with the smoke production). However in the low probability/high consequence event that the initial safety system fails and the fire is not suppressed, then the smoke control system is overwhelmed.  This would lead to untenable conditions in protected spaces where smoke may also migrate to other compartments due to the non-existence of smoke seals. Similarly, the use of firefighting lifts and the resulting piston effect can impact the pressurisation system’s performance, which may not be explicitly considered in some aspects of rule-based guidance due to its scope and the complexity of the chosen solution.

Another example, when it comes to interdependencies, can be the existence of an Evacuation Alert System, either being a partial or full alarm; this choice can affect the capabilities of a smoke control system. It will define how many doors will be open when the system is in operation and whether it can cope with the demand. This can be of importance because the tenability levels of a protected space are dependent on these capabilities. Designers ought to be aware of such considerations. 

The relative ‘simplification’ that occurs in the codification of knowledge through technical guidance and standards does not necessarily mean that designers should follow it without due consideration of all potential scenarios. It must be kept in mind that there is potentially more analysis to be done in order to account for the impact of technical solutions upon other safety measures.

It can be hard to decide when the analysis should shift to the more sophisticated risk-based approach, like the one outlined in BS 7974, given how there remains some uncertainty around what is (or is not) deemed a 'common building situation' and when rule-based guidance is applicable. It is recommended, in the opinion of the reporter, that detailed robustness and performance checks are conducted to validate the appropriateness of solutions, and not simply to assume that adequacy is achieved if prescriptive rules are followed.

detailed robustness and performance checks are conducted to validate the appropriateness of solutions, and not assume adequacy is achieved if prescriptive rules are followed

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-AUS Expert Panel page.

The panel recognises that there is a range of opinions on what a complex building is, but support the key message in this report. Regarding the definition of a complex building, a long-standing, and used, expectation of sprinklers in ADB does not necessarily result in a complex building, or an uncommon building situation. The reverse could also be true, in the case of a complex premises relying on passive measures. Each and every case needs to be considered on its own (holistic) merits.

Passive fire safety measures can also have a failure rate due to bad commissioning, maintenance, or misuse, and this uncertainty should ideally be accounted for in designs. Designers ought to test various failure mechanisms of their fire safety measures and check the potential consequences so as to spot weak points. This might not be appropriate or necessary for simpler projects, but the best approach is always going to be a multi-layered compilation of measures so that single points of failure are excluded from the design.

also acknowledged that designers should not be following guidance blindly

It is also acknowledged that designers should not be following guidance blindly, assuming that all needs of the occupants will be met by placing all occupants into a simple category that then informs the safety measures required. Placing all potential occupants of a premises, over its lifecycle, into one category (Purpose Group under ADB) is implicitly making assumptions about a level of mobility, and the assistance required in event of fire that the designers may need to explicitly account for in complex situations.

Cognisance of where rule-based guidance has come from and what has informed it is also key for the proper application of these rules.

Building Safety Regulator

This report is also of relevance regarding the new regime under the Building Safety Act 2022 as overseen by the Building Safety Regulator, specifically the Safety Case regime. With the limited information available so far, it could be expected that support for a Safety Case that purely references to following the recommendations of guidance may not satisfy the Safety Case requirements, and designers will need to explicitly demonstrate safety with appropriate evidence.

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