Programmed to reduce risk

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In a New Zealand first, a fire engineering computer program being developed by BRANZ will automate design fire generation and change current design practices with their inherent inconsistencies.

A major BRANZ fire research project will help ensure buildings in New Zealand are some of the safest in the world, enhancing our position as a leader in performance-based fire safety engineering. Due for completion in March 2013, the project output, a fire engineering software program, is being developed in collaboration with the University of Canterbury.

Removing design fire inconsistency

A criticism that could be levelled at fire safety engineering practitioners in New Zealand is the level of expert judgement, or subjectivity, required in some areas of fire design for buildings.

The most obvious example is when selecting the design fire that is the basis of any performance-based fire safety design. Current practice is for fire engineers to choose their own design fire based on information in the literature, guidance in the Building Code and standards and their own experience – which is where subjectivity, and therefore inconsistency, occurs. The new computer model, called B-RISK, addresses these issues by using an automated design fire generator.

The other significant breakthrough with the B-RISK model is that it provides a design tool that incorporates the risk and uncertainty that is a part of fire modelling, termed quantitative risk analysis. In reality, the designer simply cannot predict exactly what a real fire will do in an actual building, and hence there is risk and uncertainty inherent in the process.

B-RISK supports risk-informed design

B-RISK deals with risk and uncertainty on two fronts. Firstly, it is set up to use Monte Carlo simulation, a process by which the calculation process is repeated multiple times – literally thousands of times if the designer chooses to do so – but using input parameters that cover a range of possibilities. The design fire generator is at the heart of this process, producing a randomly varying design fire input for each repeat of the calculations.

Secondly, B-RISK also assumes that there is variability in the way that sprinklers respond, and this forms part of the calculation process where sprinklers are present.

B-RISK produces numerical results that are not just a single value but are a range of possible outcomes. Instead of existing models calculating, for example, that the radiation at head height of occupants will be 5 kW/m2 400 seconds after the fire breaks out, B-RISK will produce an output that says that there is an 80% probability that the radiation will not exceed 5 kW/m2 at 400 seconds.

This illustrates the difference between a deterministic or single output value model versus a probabilistic model where the output values quantify the uncertainty involved. It may appear to be a subtle difference, but from a risk analysis perspective, the innovative B-RISK model is now able to quantify the variability involved in building fires and, therefore, form the basis for making risk-informed design decisions. This will be a first for fire safety engineering in New Zealand and runs in tandem with changes to fire safety regulatory compliance.

Trial in October, launch March 2013

The finishing touches are currently being applied to the new B-RISK model. There are plans to hold a series of presentations in October 2012 to the fire engineering community as a field trial of the model before officially launching the final version next March. The launch will include a series of 2-day workshops that provide practitioners with detailed training in the use and application of B-RISK.

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