Structural fire protection in tunnels

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Tunnels , like bridges for roads or railway lines, are engineering structures. In order to meet the requirements that are placed on these structures, a special duty of care is required by the specialist engineer in the static, but also constructive processing. The additional technical contract conditions for civil engineering structures (ZTV-K, edition 1996) drawn up for this purpose therefore refer on the one hand to the relevant standards depending on the type of construction, such as DIN 1045 for reinforced concrete or DIN 4227 for prestressed concrete construction, and on the other hand to the additional technical Contract conditions and guidelines for the construction of road tunnels (ZTV tunnels). In the meantime the ZTV-K and the ZTV-Tunnels have been replaced by the ZTV-ING, so that the ZTV-ING, Part 5 “Tunneling”, with Sections 1 to 5, is now valid for the previous parts of the ZTV-Tunnels. Against this background, this article looks at the measures for road tunnels in Germany described in the RABT (guidelines for the equipment and operation of road tunnels) and the ZTV-ING, Part 5, also for structural fire protection . The guideline for railway tunnels (EBA) and the requirements according to the Rijkswaterstaat (RWS) are included. Finally, possible solutions for structural fire protection as part of an overall security concept required by the RABT are shown.

Requirements according to RABT or ZTV-ING, Part 5, Sections 1, 2, 3 and 4

RABT, edition 2006

In addition to information on the traffic area , operating equipment, operation or lighting, the RABT contains, for example, the following statements for ventilation: The dimensioning of fire ventilation must be designed for at least 30 MW. It is assumed that this rated fire output will be reached or exceeded after a few minutes. In the case of high truck mileage and thus the possibility of several vehicles being detected, the fire output must be increased to 100 MW. Since this can lead to no longer meaningful requirements for the ventilation system, cost-risk considerations must be carried out in individual cases and technically feasible and cost-justifiable solutions found. With regard to the regulations for the structural execution of road tunnels and their equipment, i.e. also for structural fire protection, the RABT refers to the ZTV-ING, Part 5. The overall safety concept for a road tunnel also required in the RABT must, in particular, make statements about damage prevention and damage reporting, but also for self-rescue and rescue of persons as well as for the assistance and fire-fighting of the rescue services. This information applies to all tunnels intended for motor vehicle traffic with a closed length of 80 m or more, but are not intended to replace the technical examination and planning in each individual case.

ZTV-ING, part 5 tunnel construction, section 1 (closed construction)

In order to ensure adequate structural fire protection, according to this section of the guideline, the tunnel inner shell must be designed in such a way that, in the event of fire,

  • no damage occurs that endangers the stability of the tunnel,
  • there are no permanent deformations of the construction that limit the serviceability of the tunnel and
  • the tightness is largely guaranteed.

For the construction, minimum structural measures or, in exceptional cases, computational verification procedures must ensure that the load-bearing reinforcement is not heated to more than 300 ° C in the event of a fire.

This should be ensured by a sufficient concrete cover of the load-bearing reinforcement of at least 6 cm, whereby the thickness of the inner shell must be at least 35 cm. Additional fire protection measures should then not be necessary. In the case of false ceilings, facing the traffic area, galvanized mesh reinforcement (N 94) must also be arranged to prevent concrete spalling, which must then have an overlap of 2 cm.

Ceiling and wall joints are to be provided as space joints with joint inserts made of building materials of class A (non-combustible) according to DIN 4102 .

In interior construction, the T 90 classification according to DIN 4102 with ETK / ISO curve is required for escape or connecting doors to separate fire compartments.

However, the fire curve used in Section 1 shows a fire resistance duration of 30 minutes at 1200 ° C for the structure , which is followed by a 110-minute cooling phase. In the meantime, the duration of the fire should generally be extended to 60 minutes if a fire assumed in a road tunnel "has a high probability of leading to a loss of the structure itself or an adjacent structure." This applies e.g. B. for tunnels with little overburden under water.

Depending on the duration of the fire, this curve progression is still shorter in direct comparison to the ETK / ISO curve, but significantly more aggressive in the rise, higher in the temperatures reached and more critical due to the long cooling phase and thus contradicts the T 90 requirement according to DIN 4102 for escape and connecting doors.

Water crossings or tunnels below urban / residential areas require the risk assessments currently included in ZTV-ING, Part 5, with longer fire durations in order to meet the engineer's duty of care. In the 4th tube of the Elbe Tunnel, the fire duration was therefore extended to 90 minutes, plus a 110-minute cooling phase, even before the ZTV-ING, Part 5 was updated.

Appendix B describes and defines the use of PP fiber concrete.

Construction materials that do not meet the aforementioned requirements must be protected against the effects of fire by additional measures. The building materials used themselves must correspond to building material class A (non-flammable) according to DIN 4102 and must not release any substances that damage people or the building.

Fasteners for constructive tunnel installations and empty pipes must have the stainless steel material quality 1.4529 or 1.4547 and correspond to resistance class IV / strong according to EN ISO 3506 and EN 10088. The dowels used must be generally approved by the building authorities for non-static loads or be statically verified with correspondingly reduced loads. The load values ​​to be applied for pressure and suction are based on the clear cross-section of the tunnel for cladding. The requirements for the dowels also apply to the assembly of fire protection panels and cladding.

ZTV-ING, part 5 tunnel construction, section 2 (open construction)

The basic statements on structural fire protection and on the fasteners for open-cut tunnels are identical to those in Section 1. Here, too, reference is made to Appendix B in Section 1 with regard to PP fiber reinforced concrete.

For the construction, the nominal dimension of the concrete cover for the steel inserts inside and outside is specified as Cnom = 60 mm, and the minimum dimension is Cmin = 50 mm. For ceilings, the information on the concrete cover including the mesh reinforcement (N94) from Section 1 applies. In addition, there is the option for frames with or without a base, one and two cells, with component thicknesses of 0.80 m to 1.60 m and spans of up to 16 m a simplified mathematical proof for the fire load case. For this purpose, a temperature difference of 50 K in wall and ceiling with full rigidity of the concrete cross-section in condition I is to be assumed.

The verification itself is for the combination for extraordinary design situations according to DIN EN 1991-2 and for the required reinforcement cross-section and the reinforcement layout from the design for service loads according to DIN EN 1992-2.

In the case of a significantly higher degree of reinforcement than the state of use or deviating cross-sections and systems, however, the mathematical proof must then be carried out precisely.

Section 1 applies to fire protection measures in interior construction.

ZTV-ING, part 5 tunnel construction, section 3 (mechanical shield driving method)

For the construction method with tunnel boring machines , the requirements according to Section 1 also apply to the construction in general and with regard to the thermal effects.

In the case of single-shell segment constructions, with a nominal dimension of the concrete cover of 6 cm, the arranged impact walls also serve as fire protection. These are to be produced with a PP fiber concrete according to Section 1, Appendix B. The exposed ridge area is to be lined with fire protection systems. Pressure and suction effects from road traffic must be taken into account. As an alternative to lining with fire protection panels, e.g. B. Silicate fire protection boards, other proven protection systems, such as PP fibers added to the concrete, are also permitted.

In the case of 2-shell segment structures, no additional fire protection measures are required for the inner shell if the requirements of Section 1, No. 7 are complied with.

For interior work, please refer to section 1.

ZTV-ING, part 5 tunnel construction, section 4 (technical equipment)

In edition 12/07 of this section you will find information on the choice of materials for components and system parts in order to achieve a long service life. According to this, fastening elements and suspension structures for structural fixtures must be assigned to requirement class II, ie the use of steel with surface protection is excluded. The requirements for the material grades from Sections 1 and 2 therefore also apply here.

In addition, suspension structures are to be designed in such a way that “in the event of a fire, the clear space of the tunnel remains completely free and is in no way restricted by crashed or hanging parts of the system.” Consequently, Section 2.5.1 states: “The required load-bearing capacity and embedment depth of the Fastening means or anchoring in the concrete must also be verified for the conditions during and after exposure to fire (e.g. concrete spalling). "Author's note: The spalling during the fire in the Eurotunnel was between 26 and 40 cm on the unprotected concrete - and that over a length from 500 m. The anchors for technical installations would have to be designed accordingly on the bare concrete in order to meet the required due diligence of the specialist engineer. Breakthroughs with and without cables, cable ducts with or without cables and other comparable openings in ceilings and walls must be provided with "fire barriers" of at least 90 minutes fire resistance in accordance with DIN 4102, Part 12. According to DIN 4102, this corresponds to the functional integrity E 90 for cable ducts, but actually has nothing to do with classified partitioning or the temperature-time curves specified for tunnels. In any case, the components to which such "fire barriers" are connected must also have a fire resistance of 90 minutes or higher in accordance with DIN 4102. A logical conclusion that can and should therefore also be used for tunnel structures.

Requirements according to EBA guidelines

Due to the difficult accessibility of the fire scene, according to the EBA guideline, fire protection measures are also necessary for railway tunnels . They are intended to avert danger, limit damage, rescue people themselves, and provide assistance and assistance from rescue services. For structural fire protection, the temperature-time curve according to ZTV-ING, Part 5, applies in principle, but with a fire duration of 60 minutes at 1200 ° C.

According to the EBA guideline, it is even expected that the tunnel shell will flake off during this process, which should be limited in size by suitable structural measures. At the same time, of course, this serves the personal protection of victims and rescuers. However, there is also a discrepancy here: Function maintenance E 90 according to DIN 4102 for electrical cable systems compared to a fire duration of 60 minutes and a cooling phase according to the more aggressive tunnel fire curve for the structure. The information in this guideline applies to tunnels longer than 500 m. Requirements for the corrosion protection of fixtures and fasteners can be found in the 853 series of guidelines, specifically 853.0101 or 853.5001, issued by DB Netz AG.

Requirements according to BOStrab

For local public transport (ÖPNV), fire protection in systems is regulated by BOStrab . According to this as of November 2007, the operating systems must be designed in such a way that the development of a fire and the spread of fire and smoke are prevented. For structural fire protection, this includes

  • the arrangement of the spatial and structural separation
  • the fire resistance of structural separations and structures as well as
  • the fire behavior of the building materials used.

Accordingly, the classification F 90-A according to DIN 4102 is required for load-bearing components such as walls, columns and ceilings of tunnels and bus stops.

According to BOStrab: "The stability can be guaranteed if the load-bearing components are either made from appropriate building materials or are provided with additional fire protection devices (e.g. cladding, coating)."

RWS requirements

The RWS curve ( Rijkswaterstaat -Tunnelbrandkurve) applies in the Netherlands . Their requirements are even higher and with a maximum temperature of 1350 ° C they exceed the melting or sintering limit of some materials, including those that are supposed to provide structural protection as cladding.

After this tunnel fire curve, temperatures of 1200 ° C are reached within a very short time, which then continue to rise and peak in the 60th minute at 1350 ° C. By the 120th minute they drop back to 1200 ° C. During the examinations of fire protection systems according to these temperature loads

  • do not exceed 380 ° C on the concrete surface,
  • no more than 250 ° C occur on the reinforcement with a concrete cover of 25 mm,
  • the temperature at joint seals does not exceed 60 ° C,
  • the clothing and / or the lanyards do not fail and
  • as a result, no explosive flaking of the concrete occurs.

Effects of a fire on concrete

First of all, many laypeople probably wonder what should happen in the event of a fire on the concrete, rock or even the steel reinforcement lying in the concrete, because these building materials do not burn. Concrete and steel, however, show very high losses in strength, which are only around 50% at approx. 500 ° C. As a result of the associated loss of load-bearing capacity, contrary to the requirements of ZTV-ING, Part 5, permanent deformations of the structure are to be expected. The fire setting , in which has made the effect of temperature particularly in the area of very resistant rock advantage in mining damage of the rock is the technical application of what appears as a damaging effect in principle in a fire in the tunnel.

The causes of loss of strength in concrete are its internal physical or chemical changes. Physical changes here are volume increases or temperature- related inherent and constraining stresses as well as crack formation in and on the concrete cross-section. During the chemical changes, when the concrete is heated to a 1700-fold expansion, the capillary-bound water in the concrete is released, which then suddenly has to escape. In this way, explosively deep flaking occurs, the reinforcement is blasted free over a large area and flamed, the concrete itself becomes brittle or crumbly. The literature gives a value of> approx. 2 mass%. In such a scenario, however, the self-rescue and external rescue of people as required by the RABT as well as the assistance and fire-fighting of the rescue services should work.

Possible constructive protective measures and their implementation

There are three options for ensuring the structural fire protection of the tunnel structure:

  1. Clothing applied to the outside
  2. Sprayed fire protection
  3. New type of concrete formulations with plastic fiber components

Different types of clothing

  1. Concreted in as permanent formwork
  2. Subsequently dowelled directly
  3. Subsequently mounted on panel strips or mounting rails as a substructure

For all 3 types of clothing, the material of the clothing and the assembly equipment according to ZTV-ING, Part 5, Section 4, must be viewed as a unit and thus jointly proven for the conditions during and after fire exposure.

Different clothing systems

Fire protection panels made of concrete

According to the manufacturer, these are fire protection panels made of glass fiber lightweight concrete with cement as a binding agent, glass fiber approved by the building authorities for reinforcement and closed- cell glass foam granulate as an aggregate. For some time now, perlite has been used as an aggregate instead of foam glass granulate.

Despite the relatively low melting point of the glass fiber and glass foam granulate, this board meets the fire protection requirements of ZTV-ING up to 1200 ° C with a fire duration of 30 minutes plus a cooling phase. As a result of the use of perlite as a lightweight aggregate, higher temperature loads are now also met. The cladding can be retrofitted with or without backing strips or as permanent formwork. The plates intended for tunnels also meet all other requirements for the properties of a building material for tunnel structures.

Perforated sheets

These are corrosion-protected perforated sheets with an intumescent layer and a top coating of variable colors. The corrosion protection of these perforated sheets must be ensured if they are classified according to ZTV-ING, Part 5, Section 4, for requirement class II or resistance class IV /, if the named stainless steel grades are then used, which is expensive . Perforated sheets as cladding meet the fire protection requirements of the ZTV-ING for a fire duration of up to 30 minutes plus a cooling phase.

In direct comparison, they are the thinnest form of clothing. In connection with 30 mm spacer sleeves and compensation plates, however, the total structure is approx. 35 mm. The big advantage here, according to the manufacturer, is the transparency and the associated visual controllability of the load-bearing components.

Silicate fire protection boards

Silicate fire protection boards are manufactured on the basis of cement / concrete technology with high temperature resistant materials. The curing takes place partly without tension in the autoclave (steam hardener). In addition to the fire protection properties, all other statically and atmospherically relevant properties are also met for use in tunnels.

Due to the manufacturing process, silicate fire protection boards are proven to be suitable for very high temperatures of 1350 ° C to 1400 ° C and to achieve very high fire resistance times.

Silicate fire protection panels meet the requirements of ZTV-ING with 30, 60 (EBA curve), 90 and 115 minutes fire duration plus cooling phase. Furthermore, the 120-minute RWS curve, a RWS curve extended to 180 minutes and long-term examinations over 7 hours in compliance with the criteria are proven. All examinations carried out with silicate fire protection boards have been assessed by STUVA and STUVAtec in Cologne and the results have been described in two comprehensive reports.

Thereafter, both the reinforcement and the concrete surface were only moderately loaded in the above-mentioned periods of the different tunnel fire curves. The maximum temperatures reached were sometimes more than 50% below the permissible limit values ​​according to ZTV-ING, EBA or RWS.

Spray plaster systems

This form of fire protection is sprayed on as a porous dry mortar. Spray plaster systems are largely given a steel mesh reinforcement as a substructure in steel grade V5A in order to permanently prevent adhesion problems in the tunnel atmosphere. The reinforcement is anchored using suitable dowels. Spray plaster systems can, for example, contain ceramic fibers, perlite or vermiculite.

In terms of fire protection, the requirements of ZTV-ING, with a fire duration of 90 minutes plus a cooling phase and the RWS curve, are met. The finished surface of the approx. 30 mm to 75 mm thick fire protection coatings is very rough, additional measures are necessary in the area of ​​structural joints or connection points of tubbing.

Plastic fiber concrete

More recently, even with high-strength concretes, there has been a transition to the concrete by adding plastic fibers , e.g. B. polypropylene (PP) with approx. 3 kg / m³ concrete, to give a pore system to minimize spalling. When exposed to temperature, the plastic fibers melt and give space to the water vapor that is created . In this way, steam tensions in the interior and thus possible concrete flaking are to be reduced. In the meantime, investigations according to ZTV-ING, with different fire durations plus cooling phase, as well as the RWS tunnel fire curve have been carried out with such systems. The concrete spalling has been proven to be reduced to a maximum of 10 mm below the original surface of the examined component. In contrast to externally applied fire protection cladding, concrete spalling cannot be completely prevented by adding PP.

Used fire protection materials attached to the outside of the concrete as a “sacrificial layer” can then be replaced relatively easily and thus inexpensively. In the case of an unclad concrete interspersed with plastic fibers, however, this also takes on the function of the “sacrificial layer”. This means that the PP fibers used for fire protection and the loss of strength will, beyond the reduced amount of spalling, extend into deep component levels and make it necessary to renovate the concrete at much higher costs.

Current major fire investigations

In September 2003 , the first of four “full- or large-scale tunnel fire tests” was carried out in the Runehamar tunnel near Åndalsnes , Norway , as part of a research project under real conditions. This is part of the UPTUN program (Upgrading methods for fire safety in existing TUNnels), under the leadership of the testing institutes SP (Sweden), SINTEF (Norway) and TNO (Netherlands).

Compared to the Eureka project EU 499 Firetun (1990 to 1992) in the Repparfjord Tunnel, Norway, which was around 19 years ago, the fire tests carried out this time were even larger.

This program was financially and technically supported by the companies Promat International (silicate fire protection panels), Gerco (assembly of the silicate fire protection lining), BIG / Tempest (mobile jet fans) and the European Union. To protect the disused rock tunnel, a self-supporting tubular steel construction was installed in the area of ​​the fire investigations. The lining with silicate fire protection boards mounted on it was 25 mm, single-layer for the side walls, and 25 + 20 mm, two-layer for the ceiling area. It had to withstand repeated fire exposure from four major fires and did so.

The first fire test on September 18, 2003 resulted in a 203 MW fire which, following the course of the RWS curve, reached 1365 ° C within 35 minutes. This was followed by a cooling phase. This high fire intensity and temperature corresponded to a single simulated truck with a load of around 10 t made up of approx. 80% wooden pallets and approx. 20% plastic pallets. Three more large fire tests (158 MW, 125 MW for cupboards and sofas and 70 MW) with the same protective lining of the tunnel followed.

Conclusion

The UPTUN studies show that a single burning truck with a simulated everyday load (i.e. no tank truck) can reach high fire intensities of 203 MW with temperatures of up to 1365 ° C. These findings do not reflect the guidelines for tunnel structures applicable in Germany. The maximum temperatures of 1365 ° C achieved in the UPTUN program with the fire of a single vehicle are not covered by the rating curves of the ZTV-ING, Part 5, and the EBA guideline in Germany. Only the RWS curve shows comparable values ​​with a maximum temperature of 1350 ° C.

In the past major tunnel fires, several vehicles were always involved due to the fire spreading, and hours passed before the fire loads were consumed and thus cooled down. In contrast, the full fire phase of the current rating curves in Germany is quite short, while the assumed 110-minute cooling phase here covers a sufficiently large spectrum. As in building construction, the dimensioning curves described for fires are intended to test fire protection cladding in real fire investigations and to prove the specified requirements, since purely mathematical evidence can determine temperature developments on the component to be protected, but ignore many eventualities in the course of a fire.

Fire investigations are therefore to be carried out as large as possible and close to the later installation situation. Compared to the installation of technical protective measures or the renovation of the structure, which is only protected with increased concrete cover and / or additional reinforcement, after exposure to fire, fire protection cladding is comparatively cheap at around 3 ‰ of the total construction costs. However, they are missing in the overall safety concept for tunnels required by the RABT.

bibliography

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