Newsletter April 2021, Vol 002, Issue 02: Article



Probabilistic Risk Assessment of Occupant Evacuation in Office Buildings

by: Srinath Iyengar

A key question that needs to be answered is why undertake a probabilistic assessment of an office space when a deterministic approach would yield a design that complies with the BCA. The handbook for the fire safety verification methods (Australian Building Codes Board (ABCB), 2019b) highlights that risk assessments may be beneficial when comparing two systems. The example provided by the guide details that based on reliability and exposure, one system may be drastically worse than the other, though from a deterministic point of view they are equivalent however, there may be a different risk profile associated with them. With the introduction of probabilistic assessment methodology in the Fire Safety Verification handbook, Sabapathy et al. (2019) developed a broad framework that utilised computational fluid dynamics and evacuation models to determine an expected risk to life value. Sabapathy et al. (2019) applied the methodology to a six storey retail land office building complying with the Building Codes of Australia, then with the use of hazard analysis and a deterministic evacuation assessment, results were combined into an event tree to determine the probability of the system in relation to occupant life safety.

Based on the above study, a probabilistic approach can be used to calculate the risk to occupant life safety in a building during an evacuation. From the research undertaken on the subject building, Model 4 comprising a fire blocking one of the exits was determined to have the greatest impact to occupant evacuation.

This however relies solely on the assessment of the likelihood and consequence for an event sequence, similar to the above scenario occurring within the building. Nevertheless, it is safe to say that it is vital to establish a tolerable risk criteria for benchmark comparisons. This becomes imperative to assess the risk to occupant life in a particular scenario.

A conceptual framework that may be utilised to explore the evacuation of office buildings in a probabilistic manner is provided below.

  • Stage 1 - Evacuation: Undertake an assessment of the RSET in the subject building being analysed. This information would be utilised as part of the ASET analysis of the subject floor in order to determine the time at which conditions become untenable.

  • Stage 2 - Likelihood: This comprises an assessment of the failure of probabilities for the fire systems in the building being analysed. A block diagram would assist to represent the interaction between the various sub-systems in this regard. The block diagram for systems interaction can be converted into an event tree. From the event tree, the probabilities of the final events can be determined.

  • Stage 3 - Consequence: This comprises the evacuation assessment impact. The most critical scenario can then be evaluated, with the overall risk to occupant life being documented if necessary

The following limitations are noted as part of the study:

  • The analysis that was undertaken looked at a 2-storey office building with 2 exits available. It is noted that the open stair in the centre of the floor plate was not used in the egress strategy.

  • All occupants are considered to be able-bodied and not have any mobility impairments. The impact of disabled occupants within the floor plate was beyond the scope of the assessment and research task.

  • In line with general fire emergency procedures and BCA Clause E3.3, lifts, were not used as part of the evacuation strategy. Hence, although lifts are available in the building, occupants are considered to evacuate via the two fire stairs.

A probabilistic risk assessment approach can be utilised in conjunction with an evacuation analysis to determine the risk to occupant life safety within a building. The above limitations highlighted also portray key aspects that can be explored further, to provide alternative strategies that can reduce risk to occupant life.



SABAPATHY, P., DEPETRO, A. & MOINUDDIN, K. 2019. Probabilistic Risk Assessment of Life Safety for a Six-Storey Commercial Building with an Open Stair Interconnecting Four Storeys: A Case Study. Fire Technology, 55, 1405-1445.