Safety margin in fire safety design
Fire engineering includes known uncertainties in the method of analysing fire spread and evacuation due to the limited data and modelling tools available. For this reason all fire engineering designs have a safety margin.
If the alternative solution is a departure from C/VM2 or includes complex assessment (such as Available Safe Egress Time (ASET)-Required Safe Egress Time (RSET) analysis), a key point a fire engineer must address is the safety margin.
Factors of safety are needed in fire engineering to address issues such as:
- uncertainty in the input values resulting from limitations in the data collection
- known non-conservatisms in the method of analysis because of a simplified approach
- uncertainty in the method of analysis or its outputs, such as modelling or calculation uncertainty.
The use of factors of safety also gives a level of comfort, especially for critical design outcomes, to cover the possible variation of inputs in the design process.
Some acceptance criteria are quantified within the Building Code (ie tenability criterion related to visibility, Fractional Effective Dose (FED) (CO), and FED Thermal, and radiant flux levels to relevant boundaries).
The appropriate use of a safety margin can either be applied to design inputs as they enter an analysis, or to design or analysis outputs.
The current Verification Method C/VM2 is assessed against the acceptance criteria stated in the Building Code with a safety factor of one. This is because the necessary factors of safety and margins are considered to be built into the method itself (inherent in conservative input parameters, prescribed form of analysis, and method for measuring outputs).
It is critically important that if an engineer modifies an aspect of C/VM2 or undertakes a complex performance-based assessment as part of an alternative solution then the safety margin needs to be reviewed during the Fire Engineering Brief (FEB) stage and stated in the engineering assessment documents.
The engineer should discuss and agree the safety margin with the peer reviewer and regulatory stakeholders for the alternative solution during the FEB process.
Sensitivity analysis
It is appropriate is to carry out sensitivity analyses of key variables impacting the outcome of the engineering assessment. Undertaking a sensitivity analysis is an essential part of performance-based design. It tests what happens if any component of the system fails to operate, or if changes occur to particularly sensitive input values.