1.1 Introduction to Fire and Fire Safety Concept

1.1.1 Process of Fire Development

Fire is a chemical phenomenon occurring as a result of thermal processes. Fires are either hydrocarbon or cellulosic, based on their fuel sources. Cellulosic fires are caused by the combustion of materials such as floor coverings, furnishings and wall coverings. On the other hand, hydrocarbon fires are sourced by the combustion of oil and petrochemicals, which burn at high temperatures. Concerning the steel members exposed to a fire threat, the level of temperature increment on the face-steel profile depends on thermal inertia, exposed surface area (section factor) and the protective coating. As the rate and amount of heat flow from the fire environment to steel members increase, the temperature, and thus the risk of failure for the structural member, will also increase. Since the steel has a very high thermal conductivity, exposed member surface easily transmits the conveyed heat flow from the fire source to the whole structure in a short period of time. It also is a well known fact that heat is transmitted in between elements having different temperatures. Actually, heat is a form of thermal energy transmitted over the material surface, from high temperature to low temperature mediums, by way of conduction, radiation or convection modes

1.1.2 Fire Severity and Combustible Materials

The severity of the fire case that may occur in a building depends on some related factors. Namely, the fire load (as a factor showing the amount of combustible materials in the medium, confined indoor fire area, probability of occurrence and protective measures), amount of released heat, dimensions of the building, air ventilation conditions and building openings. The combustible material types that exist in the building area is an important factor in determining the fire growth level as well. That is why it is expected for a shopping centre or a library to have a fast growing fire rate in comparison to a house or office building. The risk of structural failure is quite low, unless a fire converts from a compartment fire to a fully developed fire, known as flashover.

1.1.3 Objectives of Fire Safety

Even if materials are referred to as non-combustible, like structural steel, this does not guarantee the fire safety of a structure. Steel structural properties and its yield strength considerably decrease as it absorbs heat upon exposure to a high temperature level. The main purpose of building fire safety is to reduce the risk of both life and property losses by using effective design and construction measures. Although the probability of fire damage cannot be completely eliminated, building fire safety can be attained at an acceptable minimised risk level by using some proper design features.

Designing a building to ensure a determined level of fire safety is a comprehensive issue requiring the consideration of some related factors, such as the building type, the area and height of the building, located fire exits, the number of occupants, and the quantity and type of combustible materials (potential fire load) expected to be present in the building. It is also very important to consider the fire spread between attached buildings or building parts which can be used for different purposes. As mentioned previously, a steel building's structural members are expected to have fire resistance to prevent any structural failure for a determined period of time, as stated by the related fire codes, to give the building occupants more time to escape and allow the fire service to control it. The required fire resistance periods for the different steel building types can be easily found in country-specific building codes.

On the other hand it must be kept in mind that, steel building members may easily collapse during a fire if the temperature is allowed to reach a critical value. The fire resistance of a steel member is related to some important factors including the section size, the perimeter of the section exposed to fire, place of the member in the protective structural assembly and the steel material thickness. Depending upon structural assembly and the fire occurrence, even the exposed steel may resist fire up to 30 minutes. However the structural steel needs to be protected against fire using the proper insulating materials and methods to control the situation and resist for longer periods. In essence the structure's fire safety is measured by the time of resistance regarding the supplied evacuation time and the level of failure. It also is important to assure that the fire resistance time is sufficient as such that the structure is able to carry the building loads during the duration of the fire.

1.1.4 Controlling the Fire Spread

Fire intensity and the duration of a fire, which is measured by the temperature change of the disseminated gases in the fire compartment, are closely related to the amount of combustible materials, ventilation conditions and the thermal properties of the structural elements. In order to reduce structural damage, a local fire must be prevented from becoming a fully developed fire which is not easy to control. That is why it is very important to have good ventilation in the fire area in order to evacuate the heat and the gases. Sound fire engineering design also is important to prevent a fire spread beyond the fire compartment, providing for both the fire- resistant partitions for that purpose and maintaining the steel member protection against the direct fire effect. It also will be very useful to have a fire detection system in order to gain time for effective fire fighting. In addition, appropriate extinguishers and sufficient water supply lines must be maintained to be used during a fire fight.

1.1.5 Building Material Fire Resistance and Fire Resistance Ratings

While designing the whole structural assembly for fire resistance, it also is a requirement to consider the fire rating values of each assembling structural member separately. That is why it is important to have certified, tested values for the materials' fire ratings before using any type of fire protection material in the steel structure design. Some times it may be difficult to supply the required fire insulation material within the local supply chain that meets the limit for the fire resistance period. Under such circumstances, the designer can consider using additional insulation material layers in order to reach the required design value.

1.1.6 Behaviour of Steel Structures in Fire

As previously mentioned, steel loses strength and stiffness at high temperature levels just like any other building materials. Although the effective yield stress for design purposes, generally is taken as zero at 1200 ⁰C, in actuality the yield value does not fall to zero unless the steel reaches its melting point, 1550 ⁰C. This melting point hardly will be reached in building fires. Although the steel is a non-combustible material itself, it has a high heat conducting value, which adversely affects the structural performance during a fire exposure. That is why it is important to create a fire design for the steel buildings.

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