Fire engineering is a crucial aspect of structural design. In straightforward cases, it could simply be a matter of dimensional check for fire compliance – making sure things are thick enough, including overall section thickness, concrete cover, fire retardant layer, etc. For more complex situations, and especially for infrastructure projects, a more rigorous approach to fire engineering is required. This blog post outlines the overall fire engineering approach and touches on some basic principles.
Functional types of fire protection
Depending on the functionality of the structural element, three types of fire protection function could apply to the structural element under consideration, called ‘REI’:
R = structural (mechanical) resistance, meaning the structure must stand up without collapsing in fire. This is typically the most primary requirement for fire protection for a load-bearing structural member.
E = Separation, meaning the structure must sufficiently enclose the fire without letting it to spread. This usually means sealing off the gaps through which fire can spread.
I = thermal insulation, meaning the structural must not pass the high temperature to the other side too quickly. This can apply where there are people or temperature sensitive equipment on the other side.
Spalling prevention for structural concrete
Concrete is generally an extremely good material in fire as it does not catch fire or release any toxic fume, and is a very good thermal insulation. Concrete starts to lose strength at 300 degrees, but as long as the surface cover does not spall off, high temperature does not easily penetrate into the core of the concrete, especially for a thick section, due to its excellent thermal insulation. When spalling occurs, an inner layer of concrete will be revealed and temperature rises quickly. Reinforcement is also exposed and quickly loses its structural strength. Therefore minimising the risk of spalling is a key design driver for concrete structures.
A commonly adopted approach for protecting a concrete structure against explosive spalling subject to vehicle fires (RABT ZTV, see later) is to use polypropylene fibres in the concrete mix. These fibres melt at relatively low temperature creating channels to release vapour, which is believed to be the main hazard that leads to explosive spalling.
Careful selection of aggregate is also an important consideration. Some aggregates such as Limestone or Granite have the closest expansion rate to the cement, meaning less risk of differential expansion which leads to cracking and spalling.
Fire model
The expected/estimated ‘behaviour’ of a real fire is often simulated by a ‘Time-Temperature Curve’ model, which describes how temperature is changed over the course of development of a fire. The Time-temperature Curve could take a number of forms depending on the assumptions behind:
The ‘Cellulosic’ fire can be understood as the ‘normal’ or ‘default’ fire. It simulates the fire behaviour in a building for normal residential and commercial use. The temperature of the fire rises relatively gradually and stays at high temperature.
The ‘Hydrocarbon’ fire represents the fire behaviour of something that are highly flammable, such as petroleum and chemicals. The characteristics of such fires is that the temperature quickly shoots to the high end and stays there, and therefore is more onerous than the Cellulosic fire.
RABT ZTV fire is a short duration but intensive fire situation, representative of transportation vehicle fires including cars and trains. The fire shoots to the highest temperature within 5 minutes and die down after 30 minutes for cars and 60 mins for trains. The biggest threat of RABT ZTV fire to a concrete structure is the explosive spalling of concrete, due to the sudden shock of high temperature resulting in a sharp temperature gradient in the section. This fire curve is usually specified for road and rail tunnels.
Approaches for fire design:
Tabulated Data – a deemed-to-satisfy method, to be used in simplistic situations where the both the structure and the fire type is commonplace and standard. It allows the designer to come up with fire compliant design without actually having to do all calculations.
Simplified Calculation Methods, such as the 500 0C Isotherm method. In this method, concrete above 500 degrees Celsius has its strength fully neglected (assumed to have collapsed), whereas concrete below 500 degrees Celsius has full strength retained.
Advanced Calculation Models. Most of the time, a specialist is needed for this. Note that the two simpler approaches above are both done specific to certain structural members or part of the structure, in isolation to the whole picture. But if the structure is complex, the advanced calculation models can help the designer understand the complex interactions between different parts of the whole structure.
General steps for advanced fire engineering:
An advanced fire analysis is usually conducted with finite element software packages and generally speaking comprises 3 steps:
1. Definition of fire behaviour. Fire is seen as a potential threat to the structure and a hypothetic model, usually in the form of a ‘temperature-time curve’ is assumed to simulate the heat release behaviour of the fire itself. This depends on what fire hazard the structure is exposed to (e.g. what is the structure used for) and the layout of the structure (e.g. the behaviour of the fire in a structure with open space and good ventilation is definitely different from that of the fire in a well confined space).
2. Analysis of heat transfer. Once the behaviour of the origin of the threat is understood as the previous step, the next step is to see how the heat released from it transfers and propagates through the structure, i.e. this step is more about the material. This depends on a large number of factors such as thermal conductivity, specific heat, density, moisture of the material used in the structure.
3. Quantification of structural impact. With the analysis of step 2, we will be able to get a clearer picture of the temperature of each part of the structure at a given time of the fire. This converts into reduction in structural capacity of the material and thermal expansion, with which the failure modes of the structure can be understood.
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