Fire protection

Steel sheet piles are a versatile and cost-effective solution for use as permanent retaining walls in underground car parks, tunnels, and other underground constructions. Their main functions include retaining soils and water during the temporary excavation phase and the permanent phase, as well as forming permanent exterior structural walls that can transmit high vertical loads from floors or from superstructures to the soil.

Advantages of using steel sheet piles

Reduced construction time: used as permanent structures, there is no need for additional temporary retaining walls, leading to shorter construction times.
Optimized space utilization: due to their small height, steel sheet pile walls maximize usable space, especially in urban areas where space is a premium.
Cost savings: reduce both construction costs and time due to a simplified construction sequence.
Environmental benefits: are recovered after their intended use, can be reused multiple times, and are 100% infinitely recyclable, which contributes to sustainability in construction projects.

Underground car parks | Fire resistance

This brochure provides assistance in the fire safety design of steel sheet piles used as permanent retaining walls in underground car parks or roadworks (tunnels, underpasses, ...).

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Fire Safety and Resistance

Steel sheet piles used in permanent underground structures are often overlooked due to misconceptions regarding their ability to withstand high temperatures in case of a fire. ArcelorMittal conducted extensive fire tests and numerical simulations to demonstrate that steel sheet pile walls can achieve significant fire resistance with the appropriate design and additional protection measures in case of need.

Fire Protection Strategies

Fire resistance for steel sheet piles can be achieved through a combination of active and passive protection measures

Active measures: include systems such as fire and smoke detectors with an automatic sprinkler system that quickly detect and limit the spread of fire.
Passive measures: aim to prevent excessive temperature buildup in load-bearing structures. This can be achieved using:
- Protective coatings: application of intumescent paints or sprays that expand in response to heat to provide an insulating layer.
- Insulating panels: attach panels to provide thermal insulation.
- Masonry: add brickwork or stone in front of the steel sheet pile wall to act as a thermal barrier.
- Concrete fill: fill the inner pans of the sheet piles with concrete to enhance heat transfer resistance.
- Complete concreting: cover completely the face of the sheet piles in concrete for higher fire protection.

Design considerations for fire loads

When designing underground structures with steel sheet piles, it is essential to consider fire load effects. The main factors include

Characteristics of combustible materials: The type and amount of combustible materials directly influence the fire's intensity.
Rate of Heat Release (RHR): The RHR of fire influences gas temperatures. The speed at which combustible materials burn can significantly affect structural safety. A slow, smoldering fire can lead to different temperature distributions compared to a rapidly developing fire.
Ventilation conditions: Ventilation affects how the fire develops, with well-ventilated areas experiencing faster fire spread and potentially higher temperatures.
Fire load density: The sum of all combustible material in a given area defines the fire load density (measured in MJ/m²). High fire load density directly impacts the fire's severity and the temperatures reached in the compartment.

Testing and simulation of fire behavior

The fire resistance characteristics of steel sheet piles have been extensively tested in a laboratory at the University of Liège, Belgium. These tests involved using different soil types and with different water contents to evaluate thermal conductivity and the impact of the soil on the behaviour of the steel sheet piles under high temperatures. Key outcomes of these studies include:

Thermal conductivity of soils

Fire resistance. Thermal conductivity Soils with different characteristics and soil moisture have a different thermal conductivity. Extensive FE-simulations (back-calculation of tests) allowed to calibrate the thermal properties of a two typical soils with different saturation levels.

Fire testing with soils

Fire resistance. Comparison of measured steel temperatures_Slider

Tests conducted with different types of soils (clay, sand) at various moisture levels showed that saturated soils reduce the heating rate of steel, enhancing fire resistance.

Fire testing setup

Tests were performed using a specially designed test furnace, with steel sheet piles retaining different soils. The temperatures of the sheet piles and in the soils were monitored at different locations both in the heating and cooling phases of the fire simulation.

Moment redistribution in a steel sheet pile structure

In steel sheet pile structures, the bending moment can be redistributed when reaching local plastification (applicable only to multi-strutted structures), which increases the resistance to the loads despite the reduction of the cross-sectional resistance due to the increase of the steel temperature.

This redistribution of the moments is useful during fire events, as rising temperatures decrease the steel's resistance. Nevertheless, the overall stability of the structure can be maintained, allowing it to safely reach an ultimate state under combined earth and water pressures.

Real fire development

In a natural fire, three different phases can be identified

Initial phase: combustibles begin to burn with temperature variations across the compartment, causing gradual fire spread.
Flashover phase: temperatures rise to 300-500°C, leading to sudden ignition ("flashover"), and the fully develops with a rapid temperature increase, with a peak value often exceeding 1000°C.
Decay phase: combustibles decrease, causing the gas temperature inside the compartment to fall.

A real fire in an underground car park assumes that the fire spreads from the first car over to a few adjacent vehicles (typically 4 or 6), with a delay of a few minutes. Such a behaviour is completely different from a typical ISO fire, for which the gas temperature increases indefinitely.

Computing structural behaviour during fire events

ArcelorMittal's engineers can provide assistance in verifying the fire resistance of any steel sheet pile structure under fire. They use advanced software such as SARI (developed by the University of Liège, Belgium) to simulate fire conditions and evaluate structural safety. The software allows detailed thermal and structural analysis, considering

Thermal properties of steel: steel’s strength decreases significantly at elevated temperatures. The mechanical properties can drop to about 10% of nominal values at 800°C. Structural analysis must therefore consider these changes to ensure adequate safety under fire conditions.
Plastic redistribution of the bending moments: steel’s ability to redistribute stresses helps to maintain stability even when localized plastification occurs, which is particularly useful during fire events.
Soil properties: impact on the calculations of the thermal conductivity of the soils and the water (acts as a sink, cooling effect).
Passive protection of steel: as an option, concrete encasing or other measures can be considered in the software.

Standardised fire curves

Fire resistance is typically assessed using standardized temperature/time curves like ISO-834 and ASTM-E119. The traditional definition of fire resistance is the time
expressed in minutes that a structural element is able to support the design loading when exposed to the standard fire before a specific condition of failure is reached (e.g. R30, R60 classifications).

However, these curves do not accurately reflect natural fire behavior. Recent advancements in fire engineering design focus on how structures perform in more realistic real fire scenarios, and this approach is gaining acceptance among regulatory authorities worldwide.

Soils characteristics and fire curve

The thermal properties of soil and its ability to absorb heat significantly reduce and delay the heating of steel sheet piles.

Soil type and water content play a crucial role.

The tests showed that for all the natural fire tests, the maximum temperature in the steel sheet pile is lower than in the case of an ISO fire.

Mercedes-Benz Car Park, Ghent, Belgium: Steel sheet piles were used to provide a permanent structural solution. Fire resistance was enhanced using passive protection methods.

Car Park in The Hague, Netherlands: This underground parking structure showcases the adaptability of steel sheet piles in providing fire resistance and bearing significant loads.

Technical Support

ArcelorMittal offers technical assistance in verifying the fire resistance of steel sheet pile structures, considering factors such as soil characteristics, load conditions, and fire scenarios (both standardized and natural).

With their ability to perform as both permanent retaining walls and structural bearing elements, steel sheet piles are a practical and cost-effective choice for underground construction and resilient against fire-related risks.

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