Page 68 - Build 161

Fire safety
Quantifying how much is safe
The goal of the project is to come up with some simple universal guidance on what is safe. The easiest way to do this is with a percentage area exemption, where it is allowable for a certain proportion of the total wall and ceiling lining to be exposed.
As is often the case, though, simple end- user guidance is not quite as easy to quantify in a research context. For example, there are many di erent products of di erent thick- nesses and densities in a range of di erent con gurations.
Ease of ignition a ects safety
The key characteristic a ecting safety is the ease of ignition of the material. At the extreme end of the spectrum is paper, which is essentially very thin wood, and this very quickly ignites and is burnt. As the wood gets thicker, it gets progressively more difficult to ignite up to the point beyond which the thickness no longer a ects the ignition. There is a similar trend with density, with denser wood being more di cult to ignite.
Fires generally start from a small point of ignition and then grow in size. Once ignited, the increasing contribution of the timber lining to a growing re is caused by the ames spreading in all directions over the surface of the material. This surface ame spread is basically progressive ignition of yet-to-ignite timber immediately ahead of the ame front.
Tricky exercise to choose safe number
The more di cult the timber is to ignite therefore, due to thickness and density, the slower the timber contributes to re
growth and the safer (relatively) the mate- rial is. However, the reverse trend is that the thicker and denser the material, the longer the duration of the re, which can reduce the level of safety. The location of the fuel in the building (for example, on the walls or ceiling) also has a bearing.
This is a very simpli ed description, but it indicates how choosing a universally safe number is a tricky exercise.
Modest scale experiments pose chal- lenges
The other practical challenge with this project is that BRANZ can only do interme- diate-scale experiments, testing products ina3.6mlongby2.4mwideby2.4mhigh compartment.
From the experiments conducted, a safe area of timber lining can be confidently derived for a room in a building of less than 10 m2 oor area and standard ceiling height. Real buildings, however, have open spaces well in excess of this size but for which the same safe criterion also needs to be speci ed.
Taking from tests to real scale
The technical challenge for the research team is to decide how the smaller-scale data can be extended to the real scale.
Modelling surface flame spread with computer simulations is one option being employed, but this has some limitations. BRANZ re modelling software B-RISK can predict the contribution of timber linings to re growth, and the research project aims to extend this modelling capability further.
A related aspect of the research is asking the question whether it is possible to do
fire engineering calculations from first principles when timber linings are used. If the timber thickness, density, quantity and location in the compartment are known, can the re engineer accurately predict the resulting level of re safety for occupants? This is proving to be an interesting avenue of enquiry for the research team.
The other approach being explored is to determine the contribution of the timber lining to the heat release rate of the re in the building. This is the rate at which heat energy is generated by the re. It is a measure of how quickly a re grows, to what maximum level and how long it will last and is the usual starting point for re engineering modelling. The combustible contents of the building will contribute a certain amount to the heat release rate, and BRANZ will try to quantify the additional contribution from the timber linings.
Upper wall zone most influential in results
To date, a series of experiments has been carried out in the ≈10 m2 floor area compartment and the heat release rate measured for different configurations of timber lining. BRANZ has been able to demonstrate that not just the quantity but also the location has an impact on the heat release rate.
The timber lining was placed in three separate zones – the lower half of the walls, the upper half of the walls and the ceiling – in different combinations. This has provided several data points to work out which of the three zones is the biggest contributor.
66 — August/September 2017 — Build 161
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