Tenability criteria - Revision history

Emma: Add 3 watermarked SVG illustrations from the Advanced visual plan

2026-11-02T15:41:41Z

Add 3 watermarked SVG illustrations from the Advanced visual plan

← Older revision		Revision as of 16:41, 2 November 2026
Line 8:		Line 8:

	A physiological endpoint describes a particular effect, while a design criterion specifies how an assessment uses the available evidence. Pain, impairment of escape, incapacitation and death are distinct endpoints. A model predicting one of them should not be interpreted automatically as predicting another at the same exposure.<ref name="purser-framework"/>		A physiological endpoint describes a particular effect, while a design criterion specifies how an assessment uses the available evidence. Pain, impairment of escape, incapacitation and death are distinct endpoints. A model predicting one of them should not be interpreted automatically as predicting another at the same exposure.<ref name="purser-framework"/>
			[[File:fse-tenability-criteria-v1.svg\|thumb\|upright=1.5\|alt=Four distinct hazard checks feed a comparison of their limiting times under stated occupant and location assumptions.\|Tenability assessment retains separate hazard criteria before identifying the earliest limiting condition. Based on Purser (2002), pp. 2-85–2-91, and Ministry of Business, Innovation and Employment (2025), pp. 17–18.]]

	Purser describes a continuum of escape impairment. Smoke can reduce visibility, irritants can affect the eyes and breathing, and heat can make movement painful. These effects may slow or disrupt escape before the person reaches a nominal incapacitating dose. The analysis therefore needs to consider effective movement, rather than only the most severe outcome.<ref name="purser-framework"/>		Purser describes a continuum of escape impairment. Smoke can reduce visibility, irritants can affect the eyes and breathing, and heat can make movement painful. These effects may slow or disrupt escape before the person reaches a nominal incapacitating dose. The analysis therefore needs to consider effective movement, rather than only the most severe outcome.<ref name="purser-framework"/>
Line 50:		Line 51:

	The assessment position determines which exposure history is being tested. A point within a [[Hot gas layer\|hot gas layer]] can have different conditions from a lower position in the same room. The supplied New Zealand and ABCB methods specify evaluation at two metres above floor level for their stated occupant criteria.<ref name="nz-criteria"/><ref name="abcb-criteria"/>		The assessment position determines which exposure history is being tested. A point within a [[Hot gas layer\|hot gas layer]] can have different conditions from a lower position in the same room. The supplied New Zealand and ABCB methods specify evaluation at two metres above floor level for their stated occupant criteria.<ref name="nz-criteria"/><ref name="abcb-criteria"/>
			[[File:fse-tenability-criteria-v2.svg\|thumb\|upright=1.5\|alt=Fixed targets at the specified height differ from the exposure history accumulated by a moving person.\|The assessment location and movement assumptions determine which exposure history is being tested. Based on Australian Building Codes Board (2020), p. 110, Ministry of Business, Innovation and Employment (2025), p. 17, and Peacock et al. (2026), p. 37.]]

	That prescribed height is part of those methods. It should not be silently changed to obtain a favourable result or treated as a universal description of every occupant's breathing zone. A different population or task requires its own justified interpretation within the applicable assessment framework.<ref name="abcb-criteria"/><ref name="purser-variability"/>		That prescribed height is part of those methods. It should not be silently changed to obtain a favourable result or treated as a universal description of every occupant's breathing zone. A different population or task requires its own justified interpretation within the applicable assessment framework.<ref name="abcb-criteria"/><ref name="purser-variability"/>
Line 94:		Line 96:

	Linear interpolation between the listed points gives a visibility threshold crossing at 165 s. The thermal dose reaches 0.3 at approximately 212 s, reported conservatively to three significant figures. Unrounded values are retained in the calculations. Under these two checks, visibility is therefore limiting. The arithmetic follows the first-criterion principle; the supplied criterion sources do not contain or validate this illustrative history.<ref name="nz-criteria"/><ref name="abcb-criteria"/>		Linear interpolation between the listed points gives a visibility threshold crossing at 165 s. The thermal dose reaches 0.3 at approximately 212 s, reported conservatively to three significant figures. Unrounded values are retained in the calculations. Under these two checks, visibility is therefore limiting. The arithmetic follows the first-criterion principle; the supplied criterion sources do not contain or validate this illustrative history.<ref name="nz-criteria"/><ref name="abcb-criteria"/>
			[[File:fse-tenability-criteria-v3.svg\|thumb\|upright=1.5\|alt=For invented histories, visibility crosses ten metres at 165 seconds and thermal dose crosses 0.3 at approximately 212 seconds.\|The illustrative visibility history reaches its criterion before the thermal dose history. Based on Australian Building Codes Board (2020), p. 110, and Ministry of Business, Innovation and Employment (2025), p. 17.]]

	<div class="fse-equation">t<sub>lim</sub> = min(t<sub>visibility</sub>, t<sub>thermal</sub>) = 165 s</div>		<div class="fse-equation">t<sub>lim</sub> = min(t<sub>visibility</sub>, t<sub>thermal</sub>) = 165 s</div>

Peter: New FSE wiki article: Tenability criteria (written by GPT-6)

2026-11-01T23:11:40Z

New FSE wiki article: Tenability criteria (written by GPT-6)

New page

'''Tenability criteria''' are the specified conditions used to judge whether a fire environment remains compatible with an intended human activity, especially escape. They relate environmental exposure to effects such as impaired vision, painful heating, toxic incapacitation or reduced ability to move. A criterion must identify the hazard, endpoint, assessment location and relevant exposure assumptions.<ref name="purser-framework">Purser (2002), pp. 2-85–2-91.</ref>

In evacuation analysis, these criteria help establish [[Available safe egress time|available safe egress time]] (ASET). The supplied New Zealand method defines ASET as the interval from ignition to the first applicable criterion being exceeded in a specified room. Several hazards may therefore be evaluated independently before the limiting time is identified.<ref name="nz-criteria">Ministry of Business, Innovation and Employment (2025), pp. 17–18, §2.1.6.</ref>

The criteria are not universal physiological constants. Different assessment methods specify different exposure measures and assumptions. Occupant characteristics, visual targets, movement and protective equipment also matter. A result is meaningful when the numerical threshold remains connected to its intended population, scenario and design framework.<ref name="abcb-criteria">Australian Building Codes Board (2020), pp. 110–113.</ref>

== Criteria, effects and engineering decisions ==

A physiological endpoint describes a particular effect, while a design criterion specifies how an assessment uses the available evidence. Pain, impairment of escape, incapacitation and death are distinct endpoints. A model predicting one of them should not be interpreted automatically as predicting another at the same exposure.<ref name="purser-framework"/>

Purser describes a continuum of escape impairment. Smoke can reduce visibility, irritants can affect the eyes and breathing, and heat can make movement painful. These effects may slow or disrupt escape before the person reaches a nominal incapacitating dose. The analysis therefore needs to consider effective movement, rather than only the most severe outcome.<ref name="purser-framework"/>

Criteria also form part of the broader [[Acceptance criteria|acceptance criteria]] for a design. Demonstrating that one environmental quantity remains below its limit does not establish the adequacy of the complete evacuation strategy. The selected scenarios, model suitability, exposure assumptions and comparison with evacuation time also contribute to the decision.<ref name="abcb-methods">Australian Building Codes Board (2020), pp. 107–110.</ref>

The distinction between nominal response and design margin is important. Human susceptibility varies, and response models cannot reproduce every individual. Purser identifies both population variability and uncertainty in the prediction of effects as reasons for conservative interpretation when models are used for design.<ref name="purser-variability">Purser (2002), pp. 2-89–2-90.</ref>

A criterion consequently needs more than a number and unit. Its record should state the response being avoided, the population represented, the relevant time dependence and the location at which it applies. These details determine whether the model output and the criterion describe the same physical question.<ref name="purser-framework"/><ref name="abcb-criteria"/>

Occupants and firefighters illustrate the importance of population and task. The Australian Building Codes Board (ABCB) handbook discusses separate firefighter criteria based on protective clothing and breathing apparatus. It explicitly distinguishes those conditions from satisfactory design conditions for ordinary building occupants. Firefighter operating limits should therefore not be reused as occupant escape criteria.<ref name="abcb-criteria"/>

== Visibility and immediate impairment ==

[[Visibility in smoke|Visibility in smoke]] is frequently assessed because occupants need to recognise routes, obstacles and exit information. Optical attenuation reduces contrast, but the distance at which an object can be recognised also depends on illumination and the type of target. A light-emitting sign and a reflecting object need not have the same visibility.<ref name="mulholland-visibility">Mulholland (2002), pp. 2-263–2-267.</ref>

The visual task should therefore accompany the chosen criterion. A calculation for a backlit exit sign does not directly represent every object along the route. Mulholland also identifies visual acuity and adaptation to lighting as influences. Reporting a single visibility distance without these assumptions can imply greater precision than the underlying relationship supports.<ref name="mulholland-visibility"/>

Irritation adds another limitation. Some visibility correlations were derived from subjects looking through glass, which prevented direct exposure of their eyes to the smoke. An optical calculation using those correlations does not independently represent the reduction in visual performance caused by irritating gases or aerosols.<ref name="mulholland-visibility"/>

A [[Fractional effective concentration|fractional effective concentration]] approach addresses effects that depend primarily on the immediate concentration of an irritant. Purser describes a fractional irritant concentration for impairment of escape efficiency. It differs from a cumulative asphyxiant dose and can become limiting at a different time.<ref name="purser-framework"/>

These distinctions explain why a visibility criterion is an engineering choice for a specified application. It is not a universal distance below which every occupant immediately becomes incapable of escape. Familiarity, visual targets and the required task influence how an obscured environment affects movement.<ref name="purser-framework"/><ref name="mulholland-visibility"/>

Visibility can be the first limiting condition in many developing fires, but the ordering should not be assumed for every scenario. The ABCB handbook specifically calls for a supplementary carbon monoxide (CO) criterion for smouldering scenarios. Different combustion conditions and exposure histories can make another hazard controlling.<ref name="abcb-criteria"/>

== Toxic and thermal exposure ==

[[Fractional effective dose|Fractional effective dose]] (FED) accumulates an exposure relative to the dose associated with a stated effect. The calculation therefore depends on the complete relevant exposure history. A momentary concentration or temperature is not interchangeable with a cumulative dose, even when both are outputs from the same fire model.<ref name="purser-framework"/>

The contributing gases must be identified. The supplied New Zealand method states that its named CO dose assessment includes CO, carbon dioxide and oxygen. A different model may use another combination or another response expression. The label FED does not establish identical hazard coverage or identical numerical meaning.<ref name="nz-criteria"/>

The [[Effects of heat on people|effects of heat on people]] include whole-body overheating, skin pain and burns, and respiratory thermal injury. Temperature, heat flux, duration, humidity, clothing and activity affect these responses. A single air temperature cannot describe every thermal exposure mechanism or every relevant endpoint.<ref name="purser-thermal">Purser (2002), pp. 2-125–2-128.</ref>

Thermal dose models can account for both radiation and convection. The New Zealand method explicitly includes both in its thermal assessment. Their inclusion requires the relevant thermal inputs at the assessment location; a hot-gas temperature alone does not determine the incident radiation received from a nearby fire or opening.<ref name="nz-criteria"/><ref name="purser-thermal"/>

Dose and concentration criteria should remain separate unless the chosen method provides a justified combination. Adding a visibility fraction, irritant fraction and thermal dose into an unexplained total would mix different effects. Separate checks retain the ability to identify which physical mechanism limits escape first.<ref name="purser-framework"/><ref name="nz-criteria"/>

A nominal dose endpoint also differs from a guarantee of no injury. Some occupants are more susceptible, and effects can occur before the model's central endpoint. Delayed consequences can extend beyond the period of movement considered in an egress analysis. The selected criterion must therefore retain its stated purpose.<ref name="purser-variability"/><ref>Purser (2002), pp. 2-128–2-132.</ref>

== Where and when conditions are assessed ==

The assessment position determines which exposure history is being tested. A point within a [[Hot gas layer|hot gas layer]] can have different conditions from a lower position in the same room. The supplied New Zealand and ABCB methods specify evaluation at two metres above floor level for their stated occupant criteria.<ref name="nz-criteria"/><ref name="abcb-criteria"/>

That prescribed height is part of those methods. It should not be silently changed to obtain a favourable result or treated as a universal description of every occupant's breathing zone. A different population or task requires its own justified interpretation within the applicable assessment framework.<ref name="abcb-criteria"/><ref name="purser-variability"/>

The distinction between a stationary target and a moving occupant is also important. CFAST describes tenability calculations at user-specified target positions. Such an output represents conditions at the selected target. A person moving through several spaces has an exposure history that also depends on movement and time spent in each environment.<ref name="cfast-targets">Peacock et al. (2026), p. 37.</ref> Any averaging used by the model should remain explicit when assigning a local exposure.<ref name="abcb-methods"/>

An accumulated dose must be associated with the exposure period used by its model. It cannot be interpreted correctly if the record omits when exposure begins or which concentrations were encountered. Instantaneous visibility and cumulative thermal or toxic dose consequently need different treatment of the time history.<ref name="purser-framework"/><ref name="cfast-targets"/>

Fire location introduces a further spatial choice. The New Zealand method notes that several fire locations may produce the lowest ASET for a given escape route and requires consideration of multiple rooms. A result for one favourable location does not establish that the same route remains tenable for all relevant fires.<ref name="nz-criteria"/>

All time comparisons also need a common origin. The documented New Zealand definition starts ASET at ignition and compares it with the required safe egress time (RSET). An alarm-relative movement time cannot be subtracted directly from an ignition-relative ASET without accounting for the preceding detection and response period.<ref>Ministry of Business, Innovation and Employment (2025), pp. 14, 17.</ref><ref name="abcb-evacuation">Australian Building Codes Board (2020), p. 109.</ref>

== Examples of a specified criterion set ==

The supplied 2020 ABCB handbook provides a concrete example of framework-specific criteria. It identifies untenability when thermal FED exceeds 0.3 or visibility falls below the applicable distance, evaluated at the prescribed two-metre height. The visibility criterion normally uses ten metres within that framework.<ref name="abcb-criteria"/>

The same source permits five metres for rooms smaller than 100 m2 or where the distance to an exit is no more than five metres. It assumes backlit exit signs unless the design brief establishes another target. These qualifications are part of the criterion and should accompany the numerical values.<ref name="abcb-criteria"/>

The handbook also requires supplementary consideration of CO for smouldering scenarios, with the criterion established through the project brief. This prevents the thermal and visibility checks from being mistaken for a complete treatment of every possible fire atmosphere. The applicable scenario determines which additional exposures need assessment.<ref name="abcb-criteria"/>

These are descriptions of the supplied edition's method, rather than universal limits or a statement of current requirements in every jurisdiction. Their engineering value is that the hazard, height, visual target and exceptions are defined together. Importing only the most favourable number would discard that context.<ref name="abcb-criteria"/>

The ABCB handbook also discusses deriving temperature-based substitutes for visibility in some comparative assessments. It notes that species production depends on the materials and combustion regime. Such a substitution therefore requires its own supported relationship; it does not mean that temperature universally predicts visibility or toxicity.<ref>Australian Building Codes Board (2020), p. 111.</ref>

The surrounding model must remain suitable for its application. The same handbook requires review of verification, validation, input relevance and the method's field of application. A well-defined criterion cannot compensate for a fire or exposure model that is unsuitable for the geometry and scenario being assessed.<ref name="abcb-methods"/>

== Worked identification of the limiting time ==

Consider an illustrative room of 150 m2 with a distance to the exit greater than five metres. For this exercise, use the ABCB ten-metre visibility criterion and thermal FED criterion of 0.3 at two metres height. The following histories are invented calculation inputs, not measurements or fire-model predictions.<ref name="abcb-criteria"/>

{| class="wikitable"
! Time after ignition, s !! Illustrative visibility, m !! Illustrative thermal FED
|-
| 0 || 30 || 0.00
|-
| 60 || 24 || 0.02
|-
| 120 || 16 || 0.05
|-
| 180 || 8 || 0.17
|-
| 240 || 4 || 0.41
|}

Linear interpolation between the listed points gives a visibility threshold crossing at 165 s. The thermal dose reaches 0.3 at approximately 212 s, reported conservatively to three significant figures. Unrounded values are retained in the calculations. Under these two checks, visibility is therefore limiting. The arithmetic follows the first-criterion principle; the supplied criterion sources do not contain or validate this illustrative history.<ref name="nz-criteria"/><ref name="abcb-criteria"/>

<div class="fse-equation">tlim = min(tvisibility, tthermal) = 165 s</div>

{| class="wikitable"
! Symbol !! Meaning in the example !! Unit
|-
| tvisibility || Interpolated crossing time for the ten-metre criterion || s
|-
| tthermal || Interpolated crossing time for thermal FED 0.3 || s
|-
| tlim || Earlier crossing among the two evaluated criteria || s
|}

If an illustrative required egress time is 150 s from the same origin, the arithmetic difference is 15 s. That difference alone does not establish an acceptable design margin. The other applicable hazards, scenario completeness, evacuation assumptions and uncertainties have not been evaluated in this numerical exercise.<ref name="purser-framework"/><ref name="abcb-methods"/>

The interpolation is a stated numerical assumption. Actual conditions may not change linearly between stored outputs. A coarse output interval can therefore conceal a materially different crossing time. The reported precision should reflect the temporal resolution and the behaviour of the modelled quantities.<ref name="cfast-targets"/><ref name="abcb-methods"/>

The example also demonstrates why only checking the final time is insufficient. The criterion first becomes limiting before the last listed result. Identifying that first crossing preserves the relationship between evolving conditions and evacuation, rather than reducing a time-dependent assessment to one final snapshot.<ref name="nz-criteria"/>

== Uncertainty and reporting ==

Uncertainty enters through the design fire, product yields, transport, optical relationships, physiological response and evacuation behaviour. The ABCB handbook distinguishes model suitability from the selection of inputs, while Purser discusses uncertainty in human response. A numerical threshold does not remove either source of uncertainty.<ref name="abcb-methods"/><ref name="purser-variability"/>

A comparative assessment may share assumptions between a proposed design and a reference building. Shared assumptions can reduce some sensitivity, but the comparison still needs a suitable method and relevant inputs. Where a changed assumption could alter the outcome, its effect needs to remain visible in the assessment.<ref name="abcb-methods"/>

A useful result identifies the first limiting criterion, its location and time, the scenario producing it and the evacuation time used for comparison. It also states the visual target, exposure model and occupant assumptions. This allows the conclusion to be checked without reconstructing hidden choices from a final ASET number.<ref name="nz-criteria"/><ref name="abcb-criteria"/>

The resulting statement is consequently specific: the selected criteria are or are not satisfied for the assessed conditions and assumptions. It should preserve the distinction between that engineering finding and a prediction of every individual's response. That distinction keeps the tenability assessment connected to its evidence and intended use.<ref name="purser-variability"/><ref name="abcb-methods"/>

== See also ==
* [[Fire toxicity]]
* [[Optical density]]

== Notes ==
<references/>

== References ==
* Australian Building Codes Board. (2020). ''Fire safety verification method: Handbook'' (Version 1.1).
* Ministry of Business, Innovation and Employment. (2025). ''C protection from fire: Verification Method C/VM2'' (2nd ed.).
* Mulholland, G. W. (2002). Smoke production and properties. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-258–2-268). National Fire Protection Association.
* Peacock, R. D., McGrattan, K. B., Forney, G. P., & Reneke, P. A. (2026). ''CFAST: Consolidated model of fire growth and smoke transport, version 7. Volume 1: Technical reference guide'' (NIST Technical Note 1889v1, revision 7.7.7). National Institute of Standards and Technology.
* Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-83–2-171). National Fire Protection Association.

[[Category:Smoke, Toxicity & Tenability]]