WO2011106843A1

WO2011106843A1 - Improved performance based fire engineering building design method

Info

Publication number: WO2011106843A1
Application number: PCT/AU2011/000240
Authority: WO
Inventors: Bruce Mcleod Thomas
Original assignee: Modconnect Group Pty Ltd
Priority date: 2010-03-03
Filing date: 2011-03-03
Publication date: 2011-09-09
Also published as: WO2011106842A1; EP2542732A1; US20130055671A1; AU2011223504A1; NZ602482A; WO2011106843A9; NZ612750A; CN102869842A; AU2011223504B2

Abstract

The invention provides a building design method comprising the steps of determining a minimal fire risk/probability outcome which must be achieved by a building having a predetermined function and proportions designed to satisfy a relevant building code, selecting and customising the design of the fire safety sub-systems for the building to be designed which will result in the determined risk/probability outcome being achieved or bettered, and selecting a building design having a similar function and proportions which will provide the required risk probability outcomes such as structural stability in fire mode and other requirements in the building to satisfy a relevant building code

Description

IMPROVED PERFORMANCE BASED FIRE ENGINEERING BUILDING

DESIGN METHOD

This specification is derived from Australian Provisional Patent Application

No. 2010900876 the contents of which are incorporated herein by reference.

This invention relates to building design methods, and more particularly, to methods which rely on risk based fire engineering analysis. Under standard industry practice, the fire risk/probability outcome of a particular building design is the end result of an analysis of a particular building design which is obtained primarily from statistical or empirical data of reliabilities of various fire safety sub-systems based on design standards that exist at a point in time and specified in a particular building design.

Since many buildings are designed to include limited fire suppression and smoke management sub-systems, the infrastucture specifications for the building must be such as to satisfy the relevant building code in relation to fire safety. This usually results in the infrastructure design being based on traditional reinforced concrete or similar structures which are inherently expensive to construct. Examples of such building designs may be found in the patent literature, such as US 3,500,595 which prescribes perimeter steel beams and column systems held together by tensile rods, or US 3,514,910, which prescribes reinforced concrete structures. The present invention is based on the realisation that many present day building codes have the option of establishing compliance by the use of the equivalency concept and that if the design of a building is based on a reverse fire risk design approach, the fire safety sub-systems of the design are selected to meet or exceed a predetermined minimal risk or probability outcome to satisfy the provisions of a relevant building code, the structural design of the building may be less substantial in fire resistance whilst still meeting the requirements of the building code, thereby saving significant construction costs and still meeting the holistic performance criteria of the building code by establishing equivalence via the analytical probabilistic risk outcomes .

The invention provides a building design method comprising the steps of determining a minimal fire risk/probability outcome which must be achieved by a building having a predetermined function and proportions designed to satisfy a relevant building code, selecting and customising the design of the fire safety sub-systems for the building to be designed which will result in the determined risk/probability outcome being achieved or bettered, and selecting a building design having a similar function and proportions which will provide the required risk probability outcomes such as structural stability in fire mode and other requirements in the building to satisfy a relevant building code.

The fire safety sub-systems include structural fire resistance, occupant egress arrangements, smoke management systems, fire suppression systems such as sprinklers, and intervention by occupants and fire brigade.

In one form, the determination of the minimal fire risk/probability outcome includes performing a risk analysis for a hypothetical building design that meets the prescriptive provisions of the relevant building design code to derive a numerical probability outcome for occupant risk with respect to the whole or a portion of a building.

The selection of fire safety sub-systems includes the step of performing risk analysis of the selected fi e safety sub-systems to determine the reliabilities of the selected fire safety subsystems to ensure that they at least cumulatively meet or better the numerical probability outcome for the hypothetical design.

The reliabilities of the fire safety sub-systems may be determined by an iterative process using an event tree including each of the selected sub-systems to calculate the probability outcome for occupant risk.

In one form, the probability for operational success of any sub-system is at least 90%. The probability of failure of unreliability of the selected fire safety sub-systems includes an assessment of each active component of each sub-system. This is set-up using a fault tree analysis with the unreliabilites of the various components addressed to derive the subsystem outcome

An embodiment of the invention will be described with reference to the accompanying drawings in which:

Figure 1 illustrates an event tree for a hypothetical building based on a deemed to satisfy design;

Figure 2 illustrates an event tree for a reverse risk performance design with two values known and sub-system reliability values unknown;

Figure 3 illustrates probability outcomes for evacuation success;

Figure 4 illustrates how the probability of successful evacuation can be determined; Figure 5 illustrates how the outcome of smoke detection and audible alerts can be assessed;

Figure 6 is a fault for the analysis of a water-based fire suppression sub-system having a single mains supply and water tank;

Figure 7 is a typical floor plan designed for review by the reverse risk method, and Figures 8A to 8C illustrates event trees for smoke spread for a sole occupancy unit to the corridor space, as illustrated in Figure 7, according to a deemed to satisfy design (Figure 8A) and according to reverse risk designs (Figures 8B and 8C).

Reverse risk engineering methodology is a concept for deriving the reliability outcomes of various fire safety sub-systems (which may be at variance to the prescriptive requirements of a building code) whether they are passive or active sub-systems that are to be provided within a building where the occupant risk profile (represented as a numerical value) and the likelihood of a fire scenario are known numerical (probabilistic) entities: It is inherent within the reverse risk methodology that the design aspect under consideration can be the whole of a building or a nominated part of the building that has a fundamental effect on the safety of occupants or the building construction. The use of reverse risk engineering is particularly beneficial in the design of performance based buildings that are required by legislation to achieve at least the design equivalence with respect to occupant safety that would otherwise exist where such a building were to meet the prescriptive (deemed to satisfy) provisions of the Building Code concerned.

In determining the probabilistic or occupant risk profile to be achieved it is necessary to assess the probabilistic Or risk outcome for a similar building that would (hypothetically) be designed to meet the prescriptive requirements of the relevant Building Regulations/Building Code. To demonstrate equivalency with the prescriptive designed building, a performance designed building must be able to achieve the same or better probabilistic or occupant risk profile(s). The performance designed building may have within its fire safety infrastructure fire safety sub-systems that are at variance to the prescriptive (deemed to satisfy) designed building.

The performance design approach using the reverse risk model is substantially different from the "standard" practice of risk engineering design for buildings. Under standard industry practice the risk/probabilistic outcome is the end result of such an analysis and is obtained inter alia, from known statistical/empirical data of reliabilities of required fire safety sub-systems. The sub-systems concerned are those that are specifically provided for under the prescriptive parameters of the relevant building regulations/building code that statutorily applies.

The reverse risk engineered situation must realise, to achieve equivalency the same probabilistic or occupant risk outcome but may have the fire safety sub-systems that are customised for the building at variance to the "standardised" prescriptive requirements of the relevant building regulations/building code.

To initiate the equivalency numerical benchmark it is necessary to have a known profile/outcome. This can be derived by undertaking an assessment of a hypothetical building of similar function and proportions that would meet the relevant prescriptive (deemed to satisfy) provisions of the building regulations/building code. The hypothetical building that meets the prescriptive deemed to satisfy provisions is required to have the probabilistic or occupant risk profiles(s) derived numerically for the purpose of setting the design benchmark or numerical target to be achieved for the performance designed building. The benchmark value becomes the design "target" value to be met in the development of the reliability criteria for the various fire safety subsystems for the performance building. Such sub-systems may be consistent with the various base design/construction standards that are required to be met under the BCA or designed from first principals where such sub-systems have reliability outcomes that are different to the base design or construction standards. The reliabilities of the relevant fire safety sub-systems are then determined in order to meet the "target" value concerned. Each fire safety sub-system is then able to be designed/"tailored" to suit the reliability which has been determined for it.

The reverse risk design takes a holistic approach with respect to the derivation of the probabilistic and reliability outcomes of the various fire and life safety sub-systems of the building that are both active and passive. The design integration of these sub-systems is utilised as a means to embrace the holistic assessment that is reflected in the "Event Trees" and "Fault Trees" contained in, Figs 1 and 2. Fire modelling undertaken consists of several different empirically based scenarios as outlined below.

Section 6.1 uses a simplified example to demonstrate the equivalence status of a performance design for a component of a building being the common corridor space (egress route) that provides access to a stairway for occupants seeking egress.■ The methodology used as illustrated by the example design can be extrapolated to include risk outcome for occupants by inclusion of the assessed consequence values within the risk definition.

PERFORMANCE REQUIREMENTS General

Risk is numerically defined as the product of the probability of an event and the consequences of the event for a given scenario. Where more than one scenario provides for a tangible probabilistic or risk outcome the scenarios concerned that give rise to such numerical outcomes are to be aggregated.

Tenability Criteria for Occupants

The spatial tenability limitations for occupant egress need to be specified so as to design the fire safety sub-systems to achieve them. The fire safety sub-systems that are part of the building infrastructure that give rise to tenable conditions (either in perpetuity or for a known time period) for successful occupant egress, when such sub-systems operate as designed, are limited with respect to their operational reliability. In addition the relevant performance based requirements of the Building Code are also to be included as an integral part of the design for the various fire safety sub-systems. Each of the nominated subsystems must be designed to meet the relevant performance requirements specified within the Building Code.

Performance Requirements (Legislative)

The performance requirements of the building code(s) concerned need to be met for compliance to be established. The deemed to satisfy requirements represent only one means of substantiating performance provided that they are not modified or varied. The relevant performance requirements within the building code being those for access and egress, smoke hazard management, spread of fire and structural stability for the time period necessary to achieve the nominated performance outcomes.

The performance requirements of building codes make mention of maintaining structural stability or preventing flame spread, taking into account any active systems installed in the building, the function of the building, the evacuation time and various other factors. It is therefore inherent that the building codes recognise that the structural adequacy of various elements of construction with respect to fire resistance should not be considered in isolation.

Fire Safety Sub-Systems & Reliability

The fire safety sub-systems infrastructure for a building is a fundamental aspect to the probabilistic or risk outcome. The hypothetical deemed to satisfy building will have the reliabilities of the relevant sub-systems predetermined from the building design standard based on compliant sub-system componentry. The fire safety sub-systems pertaining to the hypothetical prescriptive designed building (refer Fig. 1) are represented as: :

Sub-system W

Sub-system X

Sub-system Y

Sub-system Z

The reliabilities (as numerical values) generally obtained from the standards/codes upon which they are required to comply with. These reliabilities are known values and are represented on the "event tree" as P2, P3, P4 and P5 respectively (refer Figure 1).

The fire safety sub-systems that are applicable to the hypothetical prescriptive designed building are specifically orientated to the Building Code/Design Standard that applies. Their reliability is usually a known condition and the various probabilities of sub-system failure that lead to untenable conditions for the assessment of occupant risk can be derived.

In the case of the performance designed building the fire safety sub-systems are often at variance to the deemed to satisfy standards in that they are a customised design and their reliabilities are required to be determined for the particular building project concerned.

The event tree (as an initial starting point) therefore has two known probabilistic values viz: -

Probability of fire starts; and

Probability of aggregate fire scenarios and associated sub-system failures that lead to untenable conditions.

The fire safety sub-systems pertaining to the performance designed building using the reverse risk method (refer Fig. 2) are represented as: - Sub-system A <- Sub-system B

Sub-system C

Sub-system D The reliabilities (as numerical values) are not known and are to be derived by the use of iterative processes using the event tree. Once the iterative processes are complete each sub-system is developed with respect to componentry from first principles via fault trees! This form of derivation will require a comprehensive knowledge of the sub-system concerned as it is "built" on a component by component basis. This is reflected in the^'fault tree analysis (FTA) of Figs 5 and 6. In addition Fig. 2 depicts the ETA outline that provides the probability results that must be achieved for the nominated sub-systems.

DESIGN FIRES Fire Modeling

Design fire scenarios indicatively represent the type(s) of fires that can be reasonably expected given the circumstances that prevail within the space/room concerned. The mathematical modeling of design fires may be developed from empirical data obtained from fire tests undertaken by reputable testing authorities or may be developed from a theoretical basis from established engineering criteria. Unsteady design fires that have growth and decay stages are the most common use fires that provide varying outputs (heat release rates) at different points in time.

It is considered appropriate that more than one fire type ought to be used for modeling purposes as insitu conditions for a building may alter from time to time.

The design fires that are adopted will serve to enable the extent or portion of the population that are potentially exposed to untenable conditions in the event of a fire scenario. The portion of the population exposed to untenable conditions and subsequent inability to exit the conditions are considered to be the consequence value(s) of the risk outcome.

ASSESSMENT OF CONSEQUENCE General

The ability for occupants to egress from the space(s) that are subject to fire and the effects of fire is fundamental to occupant safety. Variables for the egress (occupant avoidance) sub-system provide for contingencies that may occur and give rise to different outcomes. Such variables include the different characteristics of occupants that may occupy the space(s) concerned and also whether such occupants are awake or asleep in the case of residential buildings.

Fig. 3 typically provides an outline of the means by which the probability of successful egress scenarios can be derived for a residential building consisting of multiple apartments accessed by a common corridor space at a typical floor. This can also be altered/.amended on an "ad-hoc" basis to accommodate other building occupations such as commercial, industrial, etc. As a result of the scenarios which are regarded as mutually exclusive being derived the overall probability of successful evacuation can be calculated (refer Fig. 4).

Reliability and uncertainty are contingencies upon which evacuation and safety of occupants is fundamentally addressed via the review of dependent variables. The circumstances which present the greatest hazard to occupants occurs when a fire is present and occupants are asleep. The time parameters for occupants to evacuate the fire floor therefore differ between when occupants are assumed to be asleep and awake respectively. The dominant or most conservative design consideration is therefore when occupants are asleep as this potentially results in significant pre-movement times prior to occupant movement taking place.

The Implication of Audible Alert & Occupant Response

Occupant response to audible alert will vary according to the initial condition if the occupant (eg: - awake, asleep, age, gender, etc.) and the type of cue received by the occupant concerned (eg: - local alarm, olfactory, etc). These variables are best represented on an event tree for analytical purposes. Fig. 3 provides a representation that delivers probabilistic outcomes for evacuation success (ie: - Pel and Pe2). Pel and Pe2 are considered to be mutually exclusive and therefore must be assessed as per Fig. 4.

For the purpose of assessing the consequence outcomes the event tree analyses (ETA) as per Fig. 3 has been adopted.

Audible alert for occupants both within and not within the space of fire origin occurs with the operation of the smoke and/or thermal detection systems. Failure of the. audible alert sub-system therefore occurs as a result of the failure of the smoke detection and/or thermal detection sub-systems. The signalling sub-system for the operation of the emergency warning sub-system is dependant of the smoke detection and/or thermal detection subsystems (refer Fig. 5). Some or all of these sub-systems may be at variance to the deemed to satisfy sub-systems that would be required as part of the hypothetical building design.

Consequence Outcome

The assessment of consequence has been addressed on the basis of the following parameters:-

Occupant characteristics

Type of audible cue received

Probability of appropriate response to audible cue

The proposed occupants are categorised with respect to their variable characteristics usually comprising age, gender, ambulant mobility, etc. The type of audible cue received is different for enclosures not being of the fire origin for the respective performance and DTS design models. The data has clearly indicated that the likelihood of an occupant doing nothing for a local audible alarm is significantly less for the performance design model than an occupant doing nothing for a corridor audible alarm. In addition, probabilities for actual cue recognition from being asleep for most occupant groups were significantly greater for the performance model. The consequence values (refer Fig. 4) for the performance model and the DTS model are therefore:-

Performance Model

Ci Pe_p x P

(b) DTS Model C₂ Pe_d x P Where:-

C_t = the population (consequence value) potentially subject to untenable conditions performance model

C₂ = the population (consequence value) potentially subject to untenable conditions DTS model CONTROL SUPPRESSION SUB-SYSTEMS

General Control or suppression fire safety sub-systems are often used as an integral part of the fire safety infrastructure of a building. These sub-systems are varied with respect to their actual design and operation. The selection of sub-systems for a particular building design is discretionary from a performance perspective. Such sub-systems may include but are not necessarily limited to the following: -

(a) Water based suppression/control sub-systems

(b) Gas suppression sub-systems

(c) Intervention sub-systems Each of these sub-systems can be assessed with respect to their operational reliabilities via the fault tree analysis (FTA) structure which encapsulates the relevant components of the sub-system concerned together with the reliabilities of each component concerned.

The design of a water based sub-system for the performance outcome (eg: - sprinkler sub- systems) is orientated to the customization of the sub-systems to achieve the reliability that is determined from the overall event tree analysis (ETA) that is appurtenant to the building as a whole. Fig. 6 provides an indicative example of a FTA componentry arrangement for a water based suppression sub-system. The fire safety sub-systems that are used for the control and/or suppression of a fire scenario are to be determined for their compliance with respect to the performance requirements of the relevant legislative provisions. In addition the sub-system reliability of operation is also to be derived. Such reliability derivation is undertaken by use of fault tree analysis (FTA) and event tree analysis (ETA). The aspect is important in deriving the outcome reliability of a sub-system with respect to what is required for the building as a whole. Other fire suppression water based sub-systems include hydrants, hosereels, mist spray, aqueous solutions, etc.

The ability to determine the reliability of a sprinkler system is best done using fault tree analysis (FTA). FTA also reflects componentry of the system and the operational configuration. The reliability of each component for system operation is to be an integral part of the data base needed. The various components are configured so that they are expressed as a series and/or redundancy within the operational parameters of the subsystem.

Design Reliability of System Design

The design reliability of a water based suppression sub-system is reflected by example in the particular "fault tree" analysis given in Fig. 6. The sub-system reliability using the "fault tree" analysis approach enables the system to be tailored/customised to suit the particular building probability profile required. Components for effective redundancy can be added or subtracted from the sub-system infrastructure.

REVERSE RISK DESIGN

Design Methodology

The concept of reverse risk analysis can be applied to any building where a numerical risk outcome that is consistent with a hypothetical building that is designed in accordance with the prescriptive requirements of the relevant building code is also achieved or bettered for the performance designed building. The benchmark for the performance design is to achieve equivalence for the building or the component of the building being addressed for consideration. A simplified reverse risk design example has been undertaken to illustrate the methodology as a practical application. The probability of fire starts in this instance is given as 3.1 1 x 10^"2/year and the fire is assumed to be capable of growth subject to one or . more fire safety sub-systems being able to prevent such growth and detrimentally affect the corridor space. The design example in this instance addresses the potential for the common corridor space as indicated in Fig. 7 to become untenable thereby preventing occupants from reaching the stairway.

The rationale behind reverse risk pertains to the capacity to establish that a building design may be at variance with the prescriptive (deemed to satisfy) provisions of a particular design code and that such design variance does not undermine the inherent fire safety outcome of the building notwithstanding that the reliabilities of the fire safety sub-systems may significantly depart from the prescriptive provisions of the building code or standardconcerned. In effect this process allows the building designers the ability to design the fire safety sub-systems from first principles and if need be introduce an entirely different subsystem that otherwise would not be within the scope of the prescriptive design requirements (eg: - lift evacuation under fire mode).

Under such design process the requirement that the said building is able to be demonstrated as at least equivalent to or superior to a similar hypothetical building that meets the relevant prescriptive provisions of the design code. Essentially it is necessary to carry out two risk analyses. One for a hypothetical building that meets the prescriptive provisions of the relevant design code and another for the subject building which is at design variance (ie: - the performance design) to the prescriptive provisions. The hypothetical design and the performance design are able to be compared on a numerical probabilistic or risk basis. The hypothetical building meeting the prescriptive requirements is assessed to derive a numerical outcome for occupant risk. The numerical probabilistic or risk outcome pursuant to the hypothetical design therefore becomes the "benchmark" or "target" to be met with respect to the performance design which is at variance with respect to its fire safety sub-systems to the prescriptive (Dts) ·. provisions.

In the design example the requirement pertaining to design equivalency relates to the corridor component. The design is to assess the likelihood of the common corridor space of a typical residential floor of the building (refer Fig. 7) becoming untenable. In the case of the Building Code of Australia (BCA) the prescriptive (deemed to satisfy) requirements for a multi-storey residential apartment building would essentially realise a building floor plate with two (2) stairs that are connected by a common corridor (egress route). The residential apartments (SOU's) are entered from the common corridor space. The hypothetical building would also be compartmented such that the bounding envelope of the apartments/ SOU's are required to have walls separating adjacent spaces with a fire resistance level of 60 minutes (ie: - FRL -/60/60) for non loadbearing elements. This includes the apartments entry door (ie: - FRL -/60-30) which is to be self closing. Ancillary fire safety sub-systems such as smoke detection and audible alert (ie: - emergency warning) are also fundamental in warning occupants of a fire scenario and to commence evacuation. The sub-systems for the prescriptive design for the probability of a corridor untenability are therefore the passive wall system (including door) separating the SOU from the corridor space and the operation of the sprinkler system. The reliability values of the prescriptive designed fire safety sub-systems are known values as indicated in Fig. 8A.

Fig. 8B represents the interim position of the performance design prior to the iterative procedure being undertaken to determine the reliabilities of the fire safety sub-systems A, B, C and D. The probability of smoke spread from the SOU of the fire origin into the corridor space is derived in event tree Fig. 8A. In addition the consequence value may also be determined from the likelihood of the occupants responding to the audible warning system. The consequence assessment has not been included for simplification purposes. The sub-systems appurtenant to the example have been designated W, X, Y and Z and functionally identified in Fig. 8A.

In the case of the performance design the nominated sub-systems A, B, C and D are different to the deemed to satisfy/prescriptive requirements of the BCA. The reliabilities of the sub-systems have been established via iterative processes via the event tree (Fig. 8C) to achieve the probability for the untenable conditions within the common corridor space. The respective probability values of the prescriptive design being 2.211 x lO^/year and the performance design 6.22 x 10^'5/year are able to be compared to establish the equivalency or better outcome and subsequent compliance with the BCA for the performance design. Given that the numerical outcome is a known position for the prescriptive design there is a need to be able to undertake a reverse risk analysis to ensure that the performance design building is at least equivalent to or better than the occupant risk that is apparent for the hypothetical building. The holistic approach undertaken for the design is multi-faceted and ought to be considered as an overall building system design. It is not solely dependent on one aspect of the design, but all design components and fire safety sub-systems are embraced as a means of achieving the desired outcomes. The risk analysis relates to the occupants that may be located at the fire floor and/or other floors within the building. The means by which the design is appraised as "reverse risk" analysis whereby the devised risk target is a known value and the unreliabilities fire safety sub-systems are determined to achieve the nominated targeted outcome. Sub-System Reliability

In order to determine an appropriate way to treat sub-system reliability, the probability of failure is to be determined. This may be done by the use of several different methods where the adoption of any given method depends on the complexity of the particular sub- system concerned. For relatively simple systems such as smoke detectors the use of an exponential one random variable probability distribution basis with respect to time to failure.

System reliability essentially relates to the reliability of the components that make up the system as a whole. Components which formulate the operational logic of the system may be in series, parallel (where some levels of redundancy exist) or a combination of series and parallel depending on the format, complexity and redundancy within the overall subsystem.

There are several ways by which to determine the reliability of nominated fire safety sub- systems. Prescriptive based sub-systems have been designed in accordance with a building code or building standard will have established reliabilities based on historical empirical data. Passive sub-systems such as barrier resistance (eg: - walls, doors and structural elements) is generally found in established literature. Active sub-systems such as mechanical smoke management, audible alert/warning smoke detection and suppression sub-systems are considered to be appropriately assessed with respect to reliability by the use of fault tree analysis (FTA). Each active sub-system appurtenant to the building is broken down into its components and the probability of failure or unreliability of the respective components is integrated within the FTA. The FTA also reflects the logic or rationale of how each component functions with respect to other components that comprise the particular sub-system under consideration.

Fault tree analysis is particularly useful for complex sub-systems that combine components which have both series and parallel configurations. Fault tree analyses are indicated in Fig. 5.

The set-up FTA configurations and reliability values for each of the fire safety sub systems for both the hypothetical deemed to satisfy (DTS) designed building and the performance based designed building is of fundamental importance. The configurations for the fire safety sub-systems pertaining to the performance based building are expected to be at variance (in varying degrees) or may be entirely different to the fire safety sub-systems that would otherwise be in accordance with the prescriptive (DTS) based designs.

The ability for a building designer to "piece" together an entire sub-system from an absolute first principles on a component by component basis, derive it's functional reliability and establish the sub-systems capability to comply with the relevant building ode (in particular the Building Code of Australia - BCA), is the inherent "cornerstone" of the reverse risk engineering design methodology

Probability combinations and their subsequent derivations are addressed via fault tree analysis and reflect the circumstances as to whether such combinations are in series and/or in parallel.

Combinations inherently provide for system operational redundancy where such systems are represented where more than one component comprising a sub-system needs to fail for the sub-system concerned to fail. This is represented by the "AND GATE" within the FTA. The "OR GATE" represents the events likelihood of failure where any one of the components will result in the failure of the sub-system concerned (refer Fig 6).

Occupant Risk

General

Different fire scenarios create different hazard conditions and therefore potentially infinite circumstances for analysis. It is not feasible (nor is it necessary) to assess all scenarios and therefore selected situations have been considered to demonstrate the comparison between the prescriptive and performance (risk) based designs. It should be appreciated that it is not possible to remove risk with respect to fire; however, it is possible to assess the likely scenarios and recommend ways in which risk can be reduced. In effect a balance needs to be achieved between the capital efficiency of fire safety and functional operation of the building itself for the purpose of meeting legislative compliance. Risk 'R' associated with an event occurrence may be defined as the product of the following:-

(a) The probability 'Ρ_Γ' of the fire event occurrence within a given time period, and

(b) The resulting consequence 'C_r' of that occurrence

For the purpose of assessing consequence numerically the probability of the number of people who are likely to be exposed to untenable conditions given the sub-system failure are to be analysed. Consequence may be then numerically represented by:-

C_r = Prf no

Where: - P_rf = the probability of failure of appropriate occupant response to cue alert within a nominated time

no = the number of people within the fire affected zone/space. The consequence values for the performance design and the hypothetical deemed to satisfy design are therefore assessable given the different audible alarm and sprinkler conditions that apply.

The means by which the number of occupants that are potentially exposed to untenable conditions is assessed is derived from deterministic modelling using established modelling software. In addition untenable conditions must also be defined.

The risk is therefore represented by the following relationship: - R ∑ [Prs . C_r]

Where Prs is the cumulative probability of a fire occurrence capable of causing untenable conditions. The assessment of the respective reliabilities of the performance based fire safety subsystems is undertaken on an iterative basis on_. the ETA (this is addressed on an active spreadsheet Excel or similar). Once the respective reliabilities of the nominated subsystems have been established to meet the targeted or benchmark outcome a further iterative process is undertaken on the fault tree analysis (FTA) spreadsheet to determine which components and/or which configuration achieves the reliability outcomes of the nominated sub-systems concerned, Probability of Fire Initiation

General Ignition frequency /(A) within the various building categories depends inter alia, on the floor area of the building. Analysis of statistics of floor area has shown that the flexible function form for modelling dependence of /(A) on A (floor area) that may be adopted is: -

/(A) = Ci A^R + C₂ A^s

Where: - A the floor area under consideration

C, constant dependant on building function

c₂ constant dependant on building function

R power law index dependant on building function

S power law index dependant on building function

Other statistic methods may also be used to reasonably establish the frequency of fire starts applicable to a building. Probability Distribution

The probability distribution of the number of small fires "v" that are initiated in a building during a nominated time period for the building is "Poisson" and the probability distribution is:-

P_v [ exp (-wt_D) x (wt_D)v ] / v!

Where: - rate of fire starts (m^'2.y^"')

time period under consideration

the number of fire starts within a time period Determination of Outcomes

Hypothetical Deemed to Satisfy Assessment The egressing population is required to evacuate via the stairwell and/or the lift system from the fire floor. In the case of the floor above and the floor below, occupants evacuate via the stairwell as do the residual floors of the building upon receipt of subsequent audible alert. Generally the inherent assumption within the analysis which pertains to the logic that a corridor at the fire floor which is tenable will give rise to the stair shaft and lift shaft also being tenable. Tenable conditions for the common corridor are required to apply for successful evacuation from the fire floor and tenable conditions must apply to the lift shaft and the stair shaft for occupants to evacuate from the floor immediately above and immediately below the fire floor. The probability values given in Tables 6.1 and 6.2 are for illustrative purposes only by way of an example for consideration. The numerical values are subject to change depending on building area and sub-systems configuration. The design methodology however, remains constant in the derivation of the performance and prescriptive (Dts) outcomes. The risk outcome for the hypothetical building therefore establishes the target to be achieved for the performance designed building. Fig. 1 represents the means to establish the standard arrangements representing occupant risk for occupants exposed to untenable conditions. For the nominated fire safety sub-systems W, X, Y and Z the probability values of these sub-systems (ie: - P2, P3, P4 and P5) have known numerical values. The probability values P6A and P7A can then be derived as indicated in Fig. 1. Table 6.1

Likelihood of Occupant Exposure at Fire Floor - Prescriptive Design The likelihood of the egress route being untenable within a given time frame may be derived using an Event Tree (Refer Fig. 1). The probability. value P6A is the likelihood of egress route untenability. The scenarios 6, 8, 14 and 16 are the scenarios that potentially give rise to smoke occurrence in an exit route resulting in untenability. The numerical values of such probabilities are aggregated to derive the value of P6A.

The potential risk of occupant exposure to untenable conditions within an egress route (hence preventing egress from the fire affected floor) is given as P7A. The numerical value of P7A is derived from the product of the sum of the probabilities that give rise to the untenable event and the value of the consequence C2 viz: -

P7A = P6A x C₂

Performance Assessment In undertaking the performance design using a reverse risk application it is important to define which components/aspects of the building are being addressed and the occupants that relate thereto (eg: - risk to occupants located at the initial fire floor; risk to occupants not located at the fire floor; risk to occupants using the stairway, etc.). The fire safety sub-systems pertaining to the performance design (ie: - sub-systems A, B, C and D) may or may not resemble any of the fire safety sub-systems pertaining to the prescriptive hypothetical designed building (eg: - sub-system 'W for the hypothetical designed building may be a water based suppression sub-system and Sub-system 'A' for the reverse risk performance based building may be a gas based suppression sub-system).

The performance based sub-systems A, B, C and D are unknown (refer Fig. 2) with respect to their respective reliability probabilities P2, P3, P4 and P5. These must have their numerical values determined so as to achieve the end risk outcome of P7A or better (ie: - P7A is the risk target to be achieved for equivalency or bettered by having a lower numerical value).

It may be noted that to achieve equivalency or better the value of P7A will need to be consistent for both the hypothetical and performance designed buildings. All other values (except for PI) need not be the same (eg: - P2, pertaining to sub-systems W is not the same as P2 pertaining to sub-system A, etc.). Similarly the values of the consequences C| and C₂ are unlikely to be the same.

The reliabilities and unreliabilities of each of the fire safety sub-systems are developed using fault tree analysis (FTA). The means by which the untenable conditions are assessed with respect to probability is undertaken by the use of event tree analysis (ETA). Each of the scenarios of the ETA that gives rise to untenable conditions is then aggregated (summed) to arrive at a probability that represents the likelihood of untenable conditions occurring within the space concerned. The risk to occupant exposure is the product of the derived probability for untenable conditions and the number of occupants that are likely to be exposed to each scenario that gives rise to untenable conditions. Table 6.2

Likelihood of Occupant Exposure at Fire Floor - Performance Design The likelihood of a fire occurrence within the nominated portion of the building together with the relevant fire safety sub-system failure(s) and hence the corridor/egress route being untenable may be derived from the Event Tree in Fig. 2.

The potential risk of occupant exposure to untenable conditions within an egress/corridor route and inhibiting or preventing effective egress is given as P7A. The numerical value of P7A is derived from the product of the sum of the probabilities that give rise to the untenable event and the value of the consequence Q viz: -

P7A = P6A.C1

Assessment of Equivalency

In the assessment of equivalency in probabilistic terms for a building or part of a building the numerical value of P6A for the performance designed building must be less than or equal to the P6A numerical value for the hypothetical prescriptively designed building. It will be appreciated from the above description, and from the accompanying drawings that the reverse risk design approach may be used to achieve a predetermined building outcome involving significant constructional savings while not compromising the fire risk or probability outcome required to satisfy the relevant building code.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. The invention provides a building design method comprising the steps of determining a minimal fire risk/probability outcome which must be achieved by a building having a predetermined function and proportions designed to satisfy a relevant building code, selecting and customising the design of the fire safety subsystems for the building to be designed which -will result in the determined risk/probability outcome being achieved or bettered, and selecting a building design having a similar function and proportions which will provide the required risk probability outcomes such as structural stability in fire mode and other requirements in the building to satisfy a relevant building code.

2. - The method of claim 1, wherein the fire safety sub-systems include occupant egress arrangements, smoke management systems, fire suppression systems such as sprinklers, and intervention by occupants and fire brigade.

3. The method of claim 1 or 2, wherein the determination of a minimal probability outcome includes performing a risk analysis for a hypothetical building design that meets the prescriptive provisions of the relevant building design code to derive a numerical probability outcome for occupant risk with respect to the whole or a portion of a building.

4. The method of any one of claimsl to 3, wherein the selection of fire safety subsystems includes the step of determining the reliabilities of the selected fire safety sub-systems to ensure that they at least cumulatively meet or better the numerical probability outcome for the hypothetical design.

5. the method of any preceding claim, wherein the reliability's of the fire safety subsystems are determined by an iterative process using an event tree including each of the selected sub-systems to calculate the probability outcome for occupant risk.

6. The method of claim 4 or 5, wherein the probability for operational success of any sub-system is at least 90%.

The method of claim 4, 5 or 6, wherein the probability of failure or unreliability of

each sub-system includes an assessment of each active component of each subsystem.