Time-temperature curves

The impact of fires on building structures is simulated using time-temperature curves. These temperature curves form the basis for developing requirements for tunnel construction – such as the thickness of the fire-protection cladding and the design of the escape and rescue strategies in the event of fire. The aim of these specifications is to protect the tunnel structure from excessive temperatures in the event of fire. They include:

  • Limiting the maximum temperature on the concrete surface 
  • Limiting the maximum temperature on the reinforcing steel

The load capacity of the concrete structure can only be guaranteed if these temperature criteria, which are defined nationally or on a project-specific basis, are met.

The observance of these requirements and the relevant structural fire-protection measures, such as cladding with AESTUVER fire-protection boards, is documented in advance with extensive fire tests. The internationally recognized timetemperature curves are outlined below. They differ in terms of fire development, the time before the maximum temperature is reached and the duration of impact of the maximum temperature.

ISO curve
The ISO curve (ISO 834), also called the standard fire temperature curve, is the thermal stress generally applied in component analyses/tests in the building industry. This thermal stress represents the lowest level of stress and reflects temperature requirements resulting from interior fires in buildings. The timing of the ignition, smoldering and cooling phases is not taken into account.

ZTV-ING curve*/RABT curve
In accordance with ZTV-ING, the ZTV-ING curve applies to road tunnels in Germany regardless of their design and the type of traffic. It is vital to ensure that, under the temperature conditions of the ZTV-ING curve, the supporting reinforcement of the tunnel structure is not heated to a temperature exceeding 300°C and that only building materials of class A in accordance with DIN 4102 or equivalent are used. In addition, these building and general materials may not release any substances in the event of fire that are harmful to buildings or people.

EBA curve (EUREKA)
Railroad tunnels must be designed to be self-supporting according to the state of the art and their intended use. The guideline from the EBA stipulates a fire curve defined by the German Federal Railroad Authority (Eisenbahn-Bundesamt, EBA). This fire curve is based on a temperature profile that has to allow for temperatures in the tunnel reaching 1 200°C after just 5 minutes. The EBA fire curve simulates the time-based temperature profile of fire gases that can occur in railroad tunnel systems in the event of damage. The EBA fire curve was confirmed with fire tests on long-distance passenger train cars as part of an EU project (EUREKA EN 499).

Hydrocarbon curve (HC)
The hydrocarbon curve is an open fire curve from the 1970s and was developed for hydrocarbon fires in industrial and offshore plants. This curve has been modified to take into account the higher fire loads in tunnels. According to the curve, the fire development is similar to a gasoline or diesel pool fire, but can also be used qualitatively for a solids fire.

Modified hydrocarbon curve (HCM)
The modified hydrocarbon curve (HCM) was developed in France to take into account the increased safety requirements in tunnel structures, and is based on the hydrocarbon curve (HC). The HCM curve, derived from the hydrocarbon curve described previously, reaches a maximum temperature of 1,300°C (HC curve = maximum of 1,100°C). The HCM curve is being used increasingly in large international infrastructure projects.

Rijkswaterstaat curve (RWS)
The Rijkswaterstaat curve is an open fire curve based on a tanker fire with 45,000 liters of gasoline. In the modified variant, the temperature of 1,200°C, which is reached after the fire has been burning for 2 hours, is maintained for an additional hour.

N0, N1, N2, N3
In France, there are more far-reaching requirements for fire-protection in road tunnels, depending on the size and infrastructural importance, in accordance with the CETU** guideline. There are four categories: N0, N1, N2 and N3. The categories represent a combination of the different time-temperature curves and have the following meanings:

  • N0 = no requirement
  • N1 = ISO 120 minutes and HCM 60 minutes
  • N2 = HCM 120 minutes
  • N3 = HCM 120 minutes and ISO 240 minutes
* ZTV-ING: Zusätzliche Technische Vertragsbedingungen und Richtlinien für Ingenieurbauten (Additional Technical Terms of Contract and Guidelines for Civil Engineering Structures)
** Centre d’études des tunnels