Probability Based High Temperature Engineering by Leo Razdolsky

Author:Leo Razdolsky
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.1.2 Temperature–Time Function

The analytical approach in the structural fire engineering field typically comprises thermal and subsequent structural analyses. When designing structures for thermal fire load, the first step is to calculate the temperature field and then the ultimate strength capacity, based on the temperatures assessed. This is possible by using the simplified (but conservative) design method or the more sophisticated global analysis and design in accordance with the structural code requirements. The simplification (where it is possible for the determination of thermal fire load only) is the key element of the methodology proposed here. The overall system of conservation of energy, mass, and momentum equations that are analyzed here is similar to the Fire Dynamics Simulator (FDS) model [2]. However, the limitations and simplifications are different because they are concentrated on a narrowly focused problem: temperature—time fire load. For example, the large eddy simulation technique, the mixture fraction combustion model, active fire protection systems, and so on are not needed in the case of assessing creep deformations and further down the probability-based structural fire resistance. The FDS model solves the conservation equations of mass, momentum, and energy using the finite-difference method and the solution is updated in time on a three-dimensional (3D) rectilinear grid. However, the thermal radiation is computed using a finite-volume technique. The method proposed in this book uses the spatial averaging of variables; therefore, it is similar to the two-zone method in this respect. Consequently, this method has an intermediate position between the FDS and two-zone methods.

Since the existence of heat within a substance is caused by molecular action, the greater the molecular activity, the more intense is the heat. Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. The energy in this case will be transferred from the higher speed particles to the slower ones with a net transfer of energy to the slower ones. Convection is heat transfer by mass motion of a fluid such as air when heated is caused to move away from the source of heat, carrying energy with it.

Radiated heat is one of the major sources of fire spread. This method of heat transmission is known as radiation of heat waves . Heat and light waves are similar in nature, but they differ in length per cycle. Heat waves are longer than light waves, and they are sometimes called infrared rays . Radiated heat will travel through space in all directions.

Compartment fire development can be described as being comprised of four stages: incipient, growth, fully developed, and decay. Flashover is not a stage of development, but simply a rapid transition between the growth and fully developed stages.

The overall structural fire resistance (SFR) design process can be separated into the activities illustrated in the following flowchart that emphasizes the fact that the ultimate strength (SFR) and overall stability of a structure very much depend on the assessed design fire scenario.

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