Fire safety of façades

Façade fires do not occur often, in comparison to other major structure fires only 1.3 – 3 % of the total number of structure fires [1], but relatively recently several high-profile façade fires have occurred around the world that have had major consequences such as the fires in the United Arab Emirates, Australia, China, France and the United Kingdom. ^[1-3]

These external fires spread rapidly from floor to floor and sometimes large parts of the whole building is engulfed in fire as became obvious in the very recent tragic fire in the Grenfell Tower in London in which 71 people died. This shows the importance to limit or delay fire spread to higher floors. Requirements built on large scale fire testing will decrease the risk of these types of fires together with quality systems to ensure that the buildings are constructed according to the tested construction. However, the recent fires and the continuous introduction of new materials and façade systems might call for an update of these tests and regulations.

Different countries have different regulations and tests for façades and there are several different tests used for verification and classification of façade systems, ranging from small scale tests to full scale tests ^[1]. A list of the different methods used in Europe is provided in Table 1. Since each country has their own building regulation there is a large spread in the requirements.

A test method needs to be able to assess all modes of vertical fire spread involving the façade such as ^[1]:

• Flames from a broken window causing a window of the floor above to break allowing for spreading the fire;
• Inadequate fire stops in the gap between floor slab and exterior wall allowing for flames and hot gases into the next compartment;
• Deflection or distortion of metallic façade materials, e.g. aluminum, leading to deterioration of the fire safety allowing for fire spread inside the façade;
• Inadequate fire stopping around service penetrations, windows etc.

The Swedish method and regulation

The experimental setup described in the SP Fire 105 method ^[6] developed in the 1980ies is intended for determining the fire behavior of external wall assemblies and façade claddings exposed to heat and flames from a pre-defined apartment fire. The SP Fire 105 method evaluates a large-scale façade fire where the test object is 4 x 6 m (width x height) and should resemble the real façade system as much as possible. The fire exposure lasts around 15 – 20 minutes. The fire source is 60 liters of heptane burning in trays with attached flame suppressors. The purpose of the tests is to determine if the façade system itself contributes too much to the fire, e.g. not allowing it to spread above the second floor. The performance criteria of the façade system are maximum temperatures of the combustion gases at the eave and maximum heat flux to the specimen in the middle of the first fictitious window. No flame-spread on the surface or inside the cross section above the second floor is allowed.

The accepted way to meet the provision of the Swedish building regulations for a building up to eight stories, is to test the façade system according to SP Fire 105 where the following conditions are to be met;

1. no major parts of the façade fall down, for example, large pieces of plaster, panels or glass panes, which could cause danger to people evacuating or to rescue personnel,
2. fire spread on the surface finish and inside the wall is limited to the bottom edge of the window two floors above the fire room, and
3. no exterior flames occur which could ignite the eaves located above the window two floors above the fire room. As an equivalent criterion, the gas temperature just below the eaves must not exceed 500 °C for a continuous period longer than 2 minutes or 450 °C for longer than 10 minutes.

For exterior walls in buildings with more than eight stories, in addition to criteria a–c in the test, the exterior wall must not increase the risk of fire spreading to another fire compartment in a floor above the fire room. As an equivalent criterion when testing according to SP FIRE 105, the total heat flow into the façade in the center of the window in the story above the fire room must not exceed 80 kW/m2.

Discussion regarding the BS 8414-1 and SP Fire 105 methods

Two series of tests were carried out outdoors in Zagreb, Croatia, one in March 2014 and one in May 2014, details are presented ^[4]. The tests were made in accordance with BS 8414-1:2002. The test specimen extends 6 m above the combustion chamber and is 2.6 m wide with a return wall (wing) of similar height and 1.5 m wide. In each test three façade rigs were used with different test specimens, i.e. three different façade systems. The three façades were prepared with different types of external thermal insulation composite systems (ETICS). The specimens were instrumented as defined in the standard where 8 thermocouples are placed at each of the two heights from the top of the combustion chamber, at 2.5 m and 5 m. In addition to these measurements, temperatures were measured at different heights from the combustion chamber with different types of thermocouples. Wall 1 consisted of noncombustible mineral wool insulation, Wall 2 of EPS insulation and fire stops at different heights of rock-wool insulation, and Wall 3 of EPS insulation.

The experimental results in this work showed that the fire exposure on the façade varies in both BS 8141-1 and in SP Fire 105. In these two methods, the amount of fuel to be used is specified, in BS 8141-1 a certain volume of wood and SP 105 Fire a certain volume of heptane, instead of, as in many other test methods (not necessarily façade testing), a certain fuel consumption rate or incident radiation. In addition, the geometry of the combustion chamber is specified. This means that it is not possible to control the fire exposure on the façade surface, and it may differ from test to test due to factors such as air movement around the combustion chamber and the geometry of the façade system. It was also found that the thickness of the test object affects the exposure on the façade surface because the convective heat transfer change and energy will be absorbed by the underside of the façade before the fire reaches the façade surface.

Recent tests at BRE

Several BS 8414-1 tests were performed at BRE according to the performance criteria defined in the BR135 document. The performance criteria for external and internal fire spread is that failure is deemed to have occurred if the temperature rises above the starting temperature of any of the external/internal thermocouples at level 2 exceeds 600 °C, for a period of at least 30 seconds, within 15 minutes of the start time. Tests were performed in three types of claddings: ACM with a polyethylene dominated core (this is the type as used on Grenfell tower); Fire retardant ACM cladding with a better performance with regards to its fire behavior and ACM cladding with a mineral core filling of limited combustibility. These claddings have been tested in combination with several insulants: a PIR insulation as used at Grenfell and a Mineral wool insulation. Test results are summarized in Table 2 where it is seen that one of Grenfell like compositions passed the test.

Discussion and summary

The experimental work is accompanied with simulations to further asses the different test methods to understand them better and discuss the arguments for and against the methods where the experimental results are used as validation of the numerical models. This work is necessary to assess the methods and what requirements are appropriate. The numerical models have then been used to assess small changes in the systems such as comparisons between thin and thick façade specimens, different soot yields of different fuels etc. It is found that, in most cases that the models can represent the experimental data rather well while considering the variation in experimental data and simulations, except very close to the fire source, see Anderson et al ^[4] for a more detailed discussion on this topic. One of the main issues that have been identified in comparing simulation results with experimental measurements are uncertainties stemming from natural variations in parameters used in the modelling or stemming from measurement uncertainties or effect of ambient conditions.

It has been recognized that the wind may have a significant effect on the test, influencing the fire source and mass loss rates as well as specific results on measurements due to movement of flames with respect to the measuring points that may affect the outcome of the test. The main conclusion from the literature study performed in this work is that a when large-scale test method is used to assess fire performance, a significant reduction of large-scale façade fires is found. However, it is important to perform the test within controlled ambient conditions to reduce the effects of wind and climate.

In the way forward, it is essential to ensure that the harmonized (within the EU) method for façades is robust, well repeatable and reproducible. It is still uncertain when a European method can be available. It is important to avoid or minimize any arbitrariness in the assessment of test results. To achieve this, the harmonized method to be developed must clearly specify measurements and clear requirements on e.g. fire source, falloff and combustion inside the façade system. There is now a large collaboration project ongoing including several fire laboratories to support the harmonization work. RISE is coordinating this project, funded by the European Commission, to propose a new classification system and test method. In the current proposal a combination of the British method BS 8414-1 and the German method DIN4102-20 is to be used. An obstacle in this work is that the testing methods and regulations currently used at national level in the member countries greatly differ from each other. The seemingly easiest way is to define a classification system where all countries specific regulations can be fulfilled with minimum change to the methods. This can e.g. be done by defining a classification system where size of the fire is specified, if windows are included or falling parts are recorded, work in this direction is under way.

For more information, go to www.ri.se

References

1. White, N. & Delichatsios, M., Fire Hazards of Exterior Wall Assemblies Containing Combustible Components, 1st edition, Springer Briefs in Fire, Springer, 2014.
2. G. Badrock, Post incident analysis report: Lacrosse Docklands, 25 November 2014, 2nd International Conference on Fire Safety of Façades, Lund 11-13 Maj 2016, MATEC Web of Conferences 46, 06002, 2016.
3. E. K. Asimakopoulou, D. I. Kolaitis, M. A. Founti, Geometrical characteristics of externally venting flames: Assessment of fire engineering design correlations using medium-scale compartment-façade fire tests, Journal of Loss Prevention in the Process Industries 44, 780-790, 2016.
4. Anderson, J., Boström, L. and Jansson McNamee, R. (2017) Fire safety of façades SP Report 2017:36.
5. BRE. Fire test report: DCLG BS 8414 test no.1. 7 August, test no.2. 3 August 2017, test no.3. 8 August 2017, test no.4. 11 August 2017, test no.5. 14 August 2017, test no.6. 25 August 2017, test no.7. 21 August 2017.
6. SP FIRE 105, – Method for fire testing of façade materials, Dnr 171-79-360 Department of Fire Technology, Swedish National Testing and Research Institute, 1994.

Dr. Johan Anderson

Dr. Lars Boström

Dr. Robert Jansson McNamee