Articol ActaTechnica Civil.eng Darmon

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Acta Technica Napocensis, Series Civil Engineering and Architecture, Vol . 2010.

THE FIRE SPREAD OUTSIDE OF A BUILDING

RUXANDRA DRMON1

Articolul este o trecere n revist a studiilor i cercetrilor desprerspndirea flcrilor n exteriorul cldirilor, forma flcrilor ce ies prindeschiderile de fereast, mecanismele de rspndire a incendiilor pe faade sau lacldiri nvecinate, precum i cteva dintre prevederile normativelor de proiectarepentru prevenirea extinderii incendiilor. n final, sunt date cteva direcii pentrucercetrile viitoare n acest domeniu

.The article is a review of the research about the fire spread outside of a

building, the shape of the venting plume which expand out of a window, themechanisms of fire spread upward on a building and to an adjacent building and some of the existing fire design regulations to prevent the fire spread. Asconclusions, are given some directions for future research in this domain.

Key words: exterior flame, flame spread, window openings

1. Introduction

There are existing various studies and experimental data about thecompartment fires. Were carried out extensive analysis about the stages of a fireinside an enclosure, the fire spread mechanisms inside a building, the fire hazardevaluation methods, the ways of escape for users, assessment of damages cause bya compartment fire etc. The research is quite limited about the fire spread outsideof the burning room.

The first studies about exterior venting fire plume, were performed byYokoy [1] in 1960 to assess the risks associated with the fire spread from windowopenings in buildings. He described for the first time the venting plume trajectorydepending on the temperatures and flame velocity distribution along the plumeaxis. By setting the critical temperature when standard glass fails at approximate500 C, Yokoi found a way to estimate the necessary length of the spandrel

between window openings, to prevent the ignition of an upper level from theflames emerging from lower levels.

Webster [2] determined the high of the flames emerging from a windowopening, performing fire tests on natural scale considering cubic rooms with


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openings on one side. The results, correlated with a non-dimensional analysis,considering the flame temperature around 1000 F (538 C), were in accord withthe research results obtained by Yokoi.

To determine the flame size during fires inside the unprotected steelstructures, Seigel [3] considerated the plume as a horizontal jet emerging fromwindow openings The correlations stated by Seigel for flame length can be usedwhen fuel load and burning rates of materials are known. He also assumed a flametip temperature of 538 C, because flame temperatures below this value do not

pose a significant risk to exposed steel structures.Thomas and Law [4] determined the maximum flame high over the

window openings considering the effect of supplying air to the room, which

enhances the burning rate and the effect of the window geometry on the flametrajectory, measuring directly the radiative heat transfer. Law drafted a DesignGuide [5] which details the effect of the venting plume on the steel stucturesexterior. The issue of the guide was succeeded by the elaboration of a Handbookwritten out by Law and OBrian [6], which explained and simplified theinformation previously presented by Law.

Oleskiewicz [7] performed researced about the radiative heat flow andtotal heat flow emerging from window openings, modifying Laws [5] original

plume shape and changing the conservative assumption of the flame with aconstant thickness with a conical flame.

Natural scale tests conducted aimed to develop mechanisms of fire spread

outside of buildings, the building fire response, the fire behaviour materials andalso, the fire effect on facade cladding systems.

2. Mechanisms of fire spread outside of the building

A fire which reached the fully developed stage inside a room, can spreadoutside of the building to the upper floors of to the ajacent buildings throughseveral fire spread mechanisms.

A first fire spread mechanism is leap-frogging, the progressive upwardflame spread, when the plume emerging from the window openings of a lowerlever, alights the combustible materials within a level above, through direct

contact, if the window failed or it is open, or through radiative heat transfer.The fire can spread outside of the building or to adjacent buildings whenthe combustible cladding systems used for exterior walls are ignited .

The fire spread to adjacent buildings occurs indirectly through radiativeheat transfer, when the distances between buildings are too short, or by the directignition of combustible materials used for roofs or facades due to the burning

particles carried by air currents.


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The Fire Spread Outside of a Building

To prevent the fire spread to the neighbourhood, the romanian norm forfire safety, P118-99, recommends to be provided some minimum safety distances

between the buildings, in accord with their fire resistance degree.

Table 1.Safety distances between the buildings.

Fire resistancedegree

Minimum safety distances [m] to buildings having fireresistance degree

I - II III IV - V

I - II 6 8 10

III 8 10 12

IV - V 10 12 15

3. Factors which influence the flame spread outside of the building

The factors which influence directly the flame shape and the fire severityoutside of a building are related with the conditions from the room where the firestarted, but also with the environmental conditions like wind vellocity anddirection. Inside an enclosure, several factors can lead to the flashover occurenceand this may result the flame spread outside the openings, or, conversely the fireextinguishment, if the thermal load is insufficient.

In case that flashover occurs, the venting plume will be affected by factorslike system of smoke control or automatic venting system which may influencethe fire evolution in this stage.

After emerging out of an opening the fire plume spreads on the buildingfacade. The cladding system, the kind of materials used for lining the frontage,

plays an important role in fire spread. In this stage, another determinant factor canbe the window geometry. Experimental tests showed that the heat transfer byconvection is higher for narrower windows and lower when the window openingis squared or it has the width longer that the high. Larger glass sheets have a

higher breaking load limit than smaller sheets. Kerski-Rahkonen [8] demonstratedin his studies that the glass breaking is caused by the induced thermal efforts, dueto the differences of temperature within the glass sheet, from center towards theshaded border. The moment when the glass fails corresponds with the momentwhen is reached the breaking effort in the shaded part of the glass. Also, thewindow joinery can accelerate the glass failure, if it is made by combustiblematerials, like PVC, which melts before the glass.

The geometry of facade can influence the flame spread. Oleszkiewicz [9]


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has shown in his experimental results how the horizontal or vertical projections ona facade can affect the exterior fire plume trajectory. For example, horizontal

projections, such as balconies or aprons deflect the fire plume and diminishdamage caused by fire on a wall above the window opening where the flames areemerging outwards, while vertical projections, channel the fire plume upward,thus increasing the intensity of fire exposure to the wall above.

Fig. 1. The influence of horizontal and vertical projections towards venting flames emerging thewindow openings [9] .

To prevent the upward flame spread on the building faade, the fire designnorms from different countries recommend certain minimum size for spandrels

between floors and for horizontal projections, respective the minimum necessarywidth for a balcony to be effective in fire spread prevention. In figure 2 it isreproduced a drawing with the minimum accepted dimensions from The BuildingCode of Australia. In New Zealand the spandrel between two successive window

openings has to be at least 150 cm high.The romanian fire safety norm P 118-99 prescribes that the boundary walls

in storied high and very high buildings to be designed and built up so that torestrict the fire spread from one floor to another, being provided with separatioshaving minimum 120 cm, fire-proof minimum 30 minutes between glazing. Inaddition, the casement has to be from non-combustible materials.


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The Fire Spread Outside of a Building

Fig. 2. Deemed-to-satisfy solution from The Building Code of Australia .

4. Conclusions

The lack of research about the mechanisms of fire spread outside of aburning room, upward building facade or to the adjacent buildings is a problem inRomnia, in actual conditions when a lot of buildings were insulated withexpanded polystyrene. The danger to flame out and to spread a burning fire to theneighbourhood threatens dozens of storied buildings.

The legislation and fire design norms are outdated and are in a process ofharmonization with european regulations. There are needed fire safety specialistsand designers to provide a new system of technical requirements performance

based and in accord with the actual state in Romnia and with european norms.

REFERENCES

[1] S. Yokoy, Study on the Prevention of Fire-Spread Caused by a Hot Upward Current, Reportof the Building Research Institute, Japan, Report no. 34 , 1960

[2] C.T. Webster, M.M. Raftery, The Burning of Fires in Rooms: Part II Test with Cribs and HighVentilation on Various Scales Joint Fire Research Organisation, Borehamwood, 1959

[3]L.G.Seigel, The Projection of Flames from Burning Buildings, Fire Technology, Vol.5, No.1,pp.43-51, 1969

[4] P.H.Thomas, M. Law, The Projection of Flames from Buildings on Fire, Fire PreventionScience and Technology, No. 10, pp.19-26, 1972

[5] M.Law, Fire Safety of External Building Elements The Design Approach, American Iron

and Steel Engineering Journal, Second Quarter, pp. 59-74, 1978[6] M.Law, T.OBrian, Fire Safety of Bare External Structural Steel, Construction SteelResearch and Development Organisation, London, 1989

[7] I. Oleszkiewicz, Heat Transfer from a Window Plume to a Building Faade, HTD- Vol.123,Colledted papers in Heat Transfer, Book No. H00526, pp. 75-86, 1989.

[8] O. Kerski-Rahkonen, Breaking of Window Glass Close to Fire, II: Circular Panes, Fire andMaterials, Vol.15, pp. 11-16, 1991

[9]I.Oleszkiewicz, Fire exposure to Exterior Walls and Flame Spread on Combustible Cladding,Fire Technology, pp 357-375, 1990

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Articol ActaTechnica Civil.eng Darmon