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A Comparison Of Horizontal Projections and Spandrels as Protection Methods Against External Fire Spr
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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 [1]. 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 [2]. For example, in Sweden, a prescriptive spandrel configuration of at least 1.2 meters between windows in the facade is required [3]. 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 [2].

In the case with the Swedish building regulation (BBR) no horizontal projection alternative is given. The fact that no horizontal projection alternative is given in the regulation poses a problem when designing buildings with French and regular balconies. Because of these “design uncertainties” a study was initiated by Brandskyddslaget in cooperation with Lund University [2]. The study investigated how different configurations of horizontal projections influenced on the risk of external fire spread between vertical windows in the building facade, as well as to the level of protection given by the prescriptive design of the BBR. Figure 1 illustrates this comparison for a real fire scenario. The main results of the study, and some further elaborated work [4] on the subject presented at the 2016 Interflam conference in London are presented in this article.


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 [5] 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 [6] [7] [8]. The SP FIRE 105 [9] 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 [2]. 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 [4].



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 [4]. 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.

The results in the initial study [2] also show that horizontal projections with a depth less than 60 cm may offer the same protection as the 1.2 m spandrel height requirement in BBR for specific configurations. Also, combinations of a specific vertical safety distance and a less deep horizontal projection proves to offer the same protection as a specific height of 1.2 m. This means, by using the verification method presented in the studies [2] [4], that it is possible in many cases to adapt the design to suit a project's specific preferences as far as possible.

A future recommendation is to further investigate the possibilities of using the horizontal projections as a protection to improve the prescriptive requirements in BBR similar to the design of the New Zealand building regulations. The New Zealand building codes offers combinations of spandrels and horizontal projections in the prescriptive requirements as an alternative to only stating a vertical safety distance. A similar system is also seen in France. This would lead to flexible protection methods in BBR, still resulting in the same level of protection compared to today's spandrel height requirement of 1.2 m. Moreover, similar levels could then be expected in various buildings including the application of horizontal projections as compensation for the spandrel requirement. This is not the case today as the application of horizontal projections is done in a relatively arbitrary manner. The design of such advice may advantageously be developed by using the methodology presented in the studies [2] [4].

Markus Nilsson & Axel Mossberg are with Brandskyddslaget AB Consulting firm, Sweden


  1. J. H. Mammoser III and F. Battaglia, “A computational study on the use of balconies to reduce flame spread in high-rise apartment fires,” Fire Safety Journal, Vol. 39, pp. 277-296, 2004.
  2. M. Nilsson, “The impact of horizontal projections on external fire spread - A numerical comparative study,” Report nr. 5510, Lund University, Division of Fire Safety Engineering, Lund, 2016.
  3. Boverket, Boverkets byggregler (Swedish building regulations), BBR 23, Karlskrona: Boverket, 2016.
  4. M. Nilsson, A. Mossberg, B. Husted and J. Anderson, “Protection against external fire spread - Horizontal projections or spandrels?,” Paper presented at 14th International Fire Science & Engineering Conference, Vol. 2, pp. 1163-1174, Royal Holloway College, University of London, UK, 2016.
  5. F. Evegren, M. Rahm, M. Arvidsson and T. Hertzberg, “Fire testing of external combustible ship surfaces,” 11th International Symposium on Fire Safety Science, Christchurch, NZ: IAFSS, 2014.
  6. J. Anderson and R. Jansson, “Fire Dynamics in Facade Fire Tests: Measurement and Modeling,” Proceedings of Interflam 2013, pp. 93, Royal Holloway College, University of London UK, 2013.
  7. R. Jansson and J. Anderson, “Experimental and numerical investigation of fire dynamics in a facade test rig,” Proceedings of Fire Computer Modeling, p. 247, Santander, Spain, 18-19th October 2012.
  8. J. Anderson and R. Jansson, “Facade fire tests - measurements and modeling,” MATEC Web of Conferences 9, 02003, 2013.
  9. SP Fire Technology, SP Fire 105, “External wall assemblies and facade claddings: Reaction to fire,” Issue No: 5, SP Swedish National Testing and Research Institute: Fire Technology, Borås, 1994.
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