9. Tunnel Response to Fire

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ROAD TUNNELS MANUAL

9. TUNNEL RESPONSE TO FIRE

All rights reserved. World Road Association (PIARC)


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PIARC ROAD TUNNELS MANUAL PIARC

9. Tunnel response to fire

The materials used in the structures and equipment of a tunnel should neither burn nor produce largequantities of toxic smoke if a fire occurs in the tunnel. In addition, in such an event, the structuresmust not collapse while users or emergency services personnel remain inside the tunnel and criticalsafety equipment must continue to function at least until evacuation and fire-fighting operations arecompleted.

These general objectives are dependent upon the reaction to fire of the materials and the resistance tofire of the structures and equipment:

The reaction to fire of a material characterises its ability to take part in a fire to which it isexposed, including by its own decomposition (e.g. combustion).This is discussed in Section 9.1.

The resistance to fire of a structure or a piece of equipment characterises its ability to keep onfulfilling its function despite the development of a fire. Structures are considered inSection 9.2,while equipment is considered inSection 9.3.

Contributors

This Chapter was written by Robin Hall and Working Group 4 of the C4 committee (2008-2011) inwhich:

Robin Hall (UK) coordinated the work and wrote the full chapterFathi Tarada (UK) and Ignacio del Rey (Spain) reviewed the full chapter.

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9.1. Reaction of Materials to fire

The materials used in tunnel construction have to possess adequate resistance to fire to ensure integrityduring evacuation and fire fighting.

Section VII.3 "Fire reaction of materials" of technical report 05.05.B "Fire and Smoke Control inTunnels" discusses the fire properties of tunnel materials, indicating that the specifications set formaterials should include requirements concerning their properties in the event of a fire. Desirableproperties include:

low flammability, which reduces rate of fire spread; low heat output, which reduces the fire size and hence the structural and life-safety impact; andminimisation or elimination of toxic products of fire.

Gases generated by a fire cannot be prevented, but the risks can be mitigated by the choice of thematerial and also the design of safety features, such as escape routes, to reduce exposure. Attention isalso drawn to the properties of wall-covering materials, including tiles and paints, waterproofing or

lighting equipment. The specifications set for such materials should also include requirementsconcerning their properties in the event of a fire.

The possibility that materials might produce chemically corrosive or toxic substances duringcombustion and that these might penetrate the surface of the concrete and cause subsequent corrosionshould also be considered. This also applies to any coatings that might be used. In case ofpolypropylene fibres being specified to reduce the risk of spalling, the issue of concrete durability afterany significant fire event should be considered. This is because there will be increased porosity withinthe concrete where fibres have melted, leading to increased vulnerability to carbonation or chlorideattack.

Road surfaces may be constructed from cement concrete or asphalt. TheRoute/Roads article "Effectsof Roadway Pavement on Fires in Road Tunnels" discusses the properties of these materials from a

fire safety point of view. Of these, cement concrete is the only one which is not combustible and doesnot raise any question as to its use in tunnels. However, studies and experiences from real fires haveshown that, in phases when safety of people is concerned, asphalt does not add significantly to the firesize (both heat release rate and total fire load) in the case of a road tunnel fire. Open asphalt is notadvisable in tunnels as a fuel spillage will be stored below the road surface.

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9.2. Resistance to fire of structures

The fire resistance of a structure can be characterised by the time which elapses between the start of afire and the time when the structure does not ensure its function any longer, due to unacceptabledeformation or collapse.

Chapter 7 "Design Criteria for Structure Resistance to Fire" of technical report 2007 05.16.B "Systemsand Equipment for Fire and Smoke Control in Road Tunnels" summarises the objectives of structuralfire resistance in tunnels as follows:

1. people inside the tunnel shall be able to self-evacuate (self-rescue) or be assisted to a safeplace (main objective)

2. rescue operations shall be possible under safe conditions3. protective measures shall be taken against collapse of tunnel structure and loss of property to

third parties

A supplementary objective is to limit the time during which traffic will be disrupted due to the repairsafter a fire.

An overview of the subject was published inChapter VII.4 "Fire resistance of structures" of technicalreport 1999 05.05.B "Fire and Smoke Control in Tunnels".

The fire resistance of structures is described in relation to different time-temperature curves.Figure 9.2-1 shows the ISO 834 curve, the Dutch RWS curve, German ZTV curve and aFrench'increased' Hydrocarbon curve, HCinc, in which the temperatures are multiplied by a factor of1300/1100 from the basic Hydrocarbon (HC) curve of Eurocode 1 Part 2-2.

Figure 9.2-1: Temperature versus time curves for ISO, HCinc, ZTV and RWS standards (Routes/RoadsNo. 324)

Design criteria for resistance to fire in tunnels have been agreed between the World Road Association(PIARC) and the International Tunnelling Association, as presented in the Routes/Roads article"PIARC Design Criteria for Resistance to Fire for Road Tunnel Structures" (2004), and published as aPIARC recommendation in Chapter 7 "Design Criteria for Structure Resistance to Fire" of technicalreport 2007 05.16.B. A summary of the proposals is presented in Table 9.2-2.

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Table 9.2-2: PIARC and ITA recommendations

Notes

(1) 180 min may be required for very heavy traffic density of lorries carrying combustible goods.

(2) Safety is not a criterion and does not require any fire resistance (other than avoiding progressivecollapse).Taking into account other objectives may lead to the following requirements:

ISO 60 min in most cases;no protection at all if structural protection would be too expensive compared to cost and

inconvenience of repair works after a fire (e.g. light cover for noise protection).

(3) Safety is not a criterion and does not require any fire resistance (other than avoiding progressivecollapse).Taking into account other objectives may lead to the following requirements:

RWS/HCinc 120 min if strong protection is required because of property (e.g. tunnel under abuilding) or large influence on road network;

ISO 120 min in most cases, when this provides a reasonably inexpensive way to limit propertydamage;

no protection at all if structural protection would be too expensive compared to cost andinconvenience of repair works after a fire (e.g. light cover for noise protection).

(4) Other secondary structures: should be defined on a project-specific basis.

(5) In case of transverse ventilation.

(6) Shelters should be connected to the open air.

(7) A longer time may be considered if there is a very heavy volume of lorries carrying combustiblegoods and evacuation from the shelters is not possible within 120 min.

The consequences of failure will influence the requirements for fire resistance. This depends on the

type of tunnel. In an immersed tunnel, for example, a local collapse can cause the whole tunnel to beflooded whereas local collapse in a cut-and-cover tunnel may have very limited consequences. A basicrequirement is that progressive collapse must be prevented and vital longitudinal systems, such as anelectrical supply or communication cables, are not cut off.

The materials used in tunnel structures involve different precautions for fire protection. Section VII.3"Fire reaction of materials" of the report 1999 05.05.B "Fire and Smoke Control in Tunnels" discussesthe characteristics of rock tunnel linings versus reinforced concrete. The intensity of the heat generatedduring a major fire may cause reinforced concrete to lose its supporting function. The role ofinsulation using fire-resistant protection can be applied to prevent early damage to the structure. Thefire resistance of the total construction (type and depth of reinforcement/prestressing, additionalprotection, etc.) needs to be considered.

Spalling of concrete is caused by differences in temperature and expansion. It causes a danger for thereinforcement which is more easily exposed to high temperatures. It will generally not be a danger for

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evacuating people, but it may be dangerous for firemen. Various types of fire-resistant protection canbe used to reduce the risk and the effects of spalling, although it never can be completely preventeddue to the high temperatures that may occur.

Attention must be given to the fire resistance of the ventilation system so that its design performanceis not impaired by failure. Therefore it is necessary to examine the consequences of a local collapse of

a duct in case of fire.Escape routes are only used during the first phase of the fire for the escape of trapped people. It mustbe possible to use such routes for a period of at least 30 minutes. In cases where these routes are alsoused by the rescue and fire teams, the period may be longer.

To avoid fire spreading into an adjacent tube or escape route, emergency doors, emergency recessesand other equipment located between two traffic tubes, should remain intact during a specified periodof time. The whole emergency door and surrounding construction, including the door frame, shouldresist fire for at least a 30 minute fire exposure. For a door between two traffic tubes, a much longerresistance is required, for example 1 to 2 hours.


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9.3. Resistance of equipment to fire

In terms of resistance to high temperatures, tunnel equipment and cables can be broadly grouped aseither fire-rated or unprotected.

Protected equipment and cables with variable levels of resistance to fire include, for example: fire-resistant cables capable of withstanding 950C for 3 hours (CWZ specification);LS0H cables: 250C for 3 hours;ventilation fans: 250C for 1 or 2 hours

Unprotected items of equipment such as traffic signs, cameras and public address (PA) speakers haveworking temperatures typically up to 50C, and are likely to fail at relatively low temperatures.Materials include:

luminaires - laminated glass (fluorescent) or toughened glass (SON); aluminium alloy or steelhousings (working temperatures for SON luminaires typically about 120C)

traffic signs - polycarbonate screens, stainless steel housingscameras - lenses, aluminium housingspublic address (PA) horn speakers - glass-reinforced polyester (GRP).

Critical temperatures for materials used in such unprotected items include:

polymer-based materials such as polycarbonate will melt at temperatures in the region of 150Cand ignite at temperatures in the order of 300-400C;

silicone sealing - working temperatures typically go up to 200-250C;glass - working temperatures for toughened glass are typically up to 250-300C, cracks may

develop at temperatures greater than 600C;aluminium alloy - softens at 400C and melts at 660C.

All fittings used for the fixing of equipment to the structures should be considered in terms of theirbehaviour in fires.

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9. Tunnel Response to Fire