|A Comparison Of Horizontal Projections and Spandrels as Protection Methods Against External Fire Spr|
A Comparison Of Horizontal Projections and Spandrels as Protection Methods Against External Fire Spread
By Markus Nilsson & Axel Mossberg
External fire spread between fire compartments is a risk that has been observed in both experimental and numerical research . A common way that different countries have dealt with this problem is by setting prescriptive spandrel (i.e. a vertical safety distance) and/or horizontal projection configurations in their respective building regulations . For example, in Sweden, a prescriptive spandrel configuration of at least 1.2 meters between windows in the facade is required . However, the spandrel and/or horizontal projection configurations differ significantly between countries. A conclusion drawn from a review of different building codes is that the level of protection differs and that more research on the subject of external fire spread between fire compartments is needed .
Can external fires be modelled in a credible way?
In order to obtain valid results, it was first necessary to evaluate FDS as a calculation tool for modelling external fire spread. This was done by performing a validation study of FDS 6.2.0 against a large-scale fire test with a geometry as closely linked as possible to the problem area. The validation study was performed using experimental data from a large-scale fire test  on a SP FIRE 105 test rig in Borås, Sweden. On this setup numerical work has been performed by SP using FDS version 5.5.3   . The SP FIRE 105  test method specifies a procedure to determine reaction to fire properties of different assemblies of materials, insulation, and claddings when exposed to fire from a simulated apartment fire where flames emerge through a large window opening. For a detailed description of the setup and for all results the reader is referred to the validation study in the initial work . Altogether FDS 6.2.0 was deemed well suited as a calculation tool for modelling external fire spread, the conclusions drawn from the validation study were then taken into consideration when performing the simulations in the comparative analysis.
Performing a comparative analysis
In the comparative analysis a smaller apartment was built up in FDS with two opening configurations in the building facade: a door or a window. By studying the adiabatic surface temperature at different heights along the facade, the consequence of the external flames was compared between scenarios built up by spandrel configurations and scenarios with horizontal projections between openings as the only difference. The setups used for the different scenarios in FDS are described in Figure 2.
By comparing the output data from these scenarios, the impact of horizontal projections on external fire spread was shown at different heights above the underlying opening by observing the difference in the output data in different graphs. One example of the diagrams presented is shown in Figure 3 .
The results in the diagram refer to the window configuration in the building facade and to various horizontal projections as per the illustration in Figure 2 and the spandrel configuration respectively. Balcony-scenarios resulting in consistently lower values than the spandrel-case at each height mean the existence of these balconies results in lesser consequences at the facade on all heights compared with the Spandrel-case. Furthermore, if these values are below the grey horizontal line (BBR Limit), these balconies are considered to result in lesser consequences on the facade at all heights compared with the accepted level in the prescriptive part of the Swedish building regulations (BBR). The latter since the BBR Limit highlights the consequence at 1.2 m above the opening in the spandrel case.
The wider the window, the greater the effects
In general, there is a distinct difference in the consequences at the facade depending on the opening configuration. A fire plume ejecting through a narrower door type is causing higher gas velocities through the opening, resulting in a fire plume ejecting further away from the facade. The outcome of a wider window type on the other hand is lower velocities and hence a fire plume ejecting closer to the building exterior . This is visualized below by comparing the external flames in FDS.
As seen for the door configuration in Figure 4, the horizontal projections mainly protect the facade by shielding the radiative and convective heat from the fire. However, for the window configuration, the horizontal projections are seen to project the fire plume further away from the facade, since the plume is closer to the facade in its original state. Because of this, the impact of the horizontal projection on the external flames is greater, which is also seen in the results in the diagrams by the difference in temperatures. This further suggests that a horizontal projection offers higher protection compared to spandrels when the underlying openings are wider.
Conclusions and recommendations to the future
The results shows that the use of a 60 cm deep horizontal projection results in less severe consequences above the projection compared with scenarios built up by different spandrel heights. The horizontal projections used were 20 cm thick rectangular non-combustible balconies with open sides and no separation walls positioned at two different heights and of two different widths. The results also suggests that the use of these balconies in most cases result in lesser consequences at the facade compared with the accepted level in the prescriptive part of the Swedish building regulations (BBR). This means that in many cases, a spandrel height of at least 1.2 m as stated by the BBR can be replaced by a 60 cm deep horizontal projection positioned at any height above the underlying opening. However, this is only a valid conclusion for scenarios with a wider window type in the building facade. Individual scenarios with a narrower opening type in combination with a low positioned horizontal projection require a wider projection in order to obtain lower consequences than the accepted level in BBR.