The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 110

December 5, 2022 11:26 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 109, LWF looked at the difference between the previously discussed and prescriptive measures for fire safety and fire safety engineering design approaches. In part 110, we consider the tenability limits for design.

One of the most basic and essential considerations when designing fire safety measures is that of ensuring conditions are suitable for means of escape. Building occupants must be able to evacuate via an escape route without being subjected to any significant amount of smoke or heat from a fire.

Protection of an escape route can be through passive measures, such as fire-resisting construction or active measures, such as smoke control ventilation systems, or a combination of active and passive measures is usually required.

Smoke and heat can affect tenability conditions prior to occupants entering a protected route to escape, so this blog series will consider both elements in terms of design limits for short-term exposure. It should be noted that conditions in protected escape routes and any refuge areas should not approach these values.

Smoke and tenability of conditions

The invasion of smoke into an occupied area of a building is likely to affect visibility prior to becoming debilitating in other ways, such as in terms of breathing or toxicity. Fire engineering designs therefore usually concentrate on maintaining visibility, safe in the knowledge that if this is achieved, the other risks from smoke are not at a dangerous level.

While this is acceptable in many circumstances, the potential for the combustion of any highly toxic products should be considered, for example, in any building where toxic chemicals are stored or used in processing.

When building occupants must pass through a fire affected space before reaching the escape route or other place of safety, they should not be subjected to a loss of visibility due to smoke. Design limits are suggested, as follows:

  • Maintenance of an air layer relatively clear of smoke above eye level. Typically, this is 2-3.5 m above floor level, depending upon the building geometry and smoke-modelling technique.
  • The temperature of the smoke layer should not exceed 200 oC, to limit the downward radiant heat flux to less than 2.5 kW.m-2. Temperatures exceeding this can cause extreme skin pain.
  • Ensure visibility through smoke is sufficient for exits to be identified. People are reluctant to move through smoke when visibility is less than 10 m.

In part 111 of LWF’s series on fire engineering we will continue to look at the tenability limits for smoke, before discussing the tenability limits for heat from a fire. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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