Current fire protection strategy for a building often incorporates a combination of active and passive fire protection measures. Active measures, such as fire alarm and detection systems or sprinklers, require either human intervention or automatic activation and help control fire spread and its effect as needed at the time of the fire. Passive fire protection measures are built into the structural system by: Choice of building materials Dimensions of building components Compartmentation, and Fire protection materials
These control fire spread and its effect by providing sufficient fire resistance to prevent loss of structural stability within a prescribed time period, which is based on the building’s occupancy and fire safety objectives. Materials and construction assemblies that provide fire resistance, measured in terms of fire endurance time, are commonly referred to as fire resistance-rated-construction or fire-resistive materials and construction in the current two model building codes: International Building Code 2006 (ICC 2006) and the Building Construction and Safety Code, NFPA 5000 (NFPA 2006).
Fire Effect on structures
Fire can affect a building’s structural capacity in two ways: 1. Prolonged exposure of structural components or subsystems to elevated temperatures degrades their engineering properties, thus resulting in the reduction of overall structural capacity. 2. Exposure to elevated temperature may induce internal forces (due to restraint of thermal expansion) or axial deformations in structural members due to plastic and creep strains or buckling, which may adversely affect the global stability of the building. For steel structures, the effect of geometric nonlinearity is likely to be significant because of large deformation that may occur. For concrete structures, lateral displacement of columns at slab–column joints due to thermal expansion of the slabs might pose additional risk to the global stability of the structure. Further, because steel has relatively small thermal mass and high thermal conductivity, temperature is more likely to be nearly uniform across most steel sections while concrete components can have steep thermal gradients near their surface, which may cause surface spalling. Consideration of the evolution of the building’s structural capacity and global stability requires a performance based fire engineering approach that explicitly considers structural fire loads in the design process to achieve a rational fire safety design.
Building regulations require certain elements of a structure to have fire resistance. Whether or not an element requires fire resistance depends on the size of the building, what it will be used for and what the function of the element is.
Fire safe insulation, steel construction, When exposed to fire, all commonly used structural materials lose some of their mechanical strength. Heavily loaded steel will lose its designed safety margin at temperatures around 550°C – regardless of the grade of steel. To protect the structural steel in your building, use PAROC fire protection slabs. Depending on the application, you can use one of three methods for fire protection: profile, box and solid.
The bigger volume of steel in the exposed area, the better fire resistance it has. How quickly the steel structure heats up in a fire can simply be described as the relation between the surface exposed to the fire and the steel volume of the profile. This relation is called the section factor A/V. A high section factor gives a quick temperature raise of the steel. In practice, this means that thin steel structures demand thicker protection.
Analysis; STRUCTURAL STEEL EXPOSURE TO FIRE
All the materials reduce their strength when subjected to fire but steel can recover strength for incombustible nature. When steel exposes to fire it absorbs thermal energy, after a certain time of cooling it returns either stable or unstable condition. During this heating and cooling operation the members may be Scrapped due to large deformation, Perfect for its straightness behavior after fire exposure, Reusable by straightening.
Fire Loads And Fire Rating
The combustible elements which are generated the ultimate heat in a structure is termed as fire load. Nowadays in the building design the rate of fire combustion material is concerned. Any structural elements mainly design to resist this fire, if it crosses the limit it will fail and collapse by losing strength. Fire rating is determined according to the fire safety of the building. Fire loading and fire rating are correlated in terms of the design issue. Fire rating is proportional to fire load and it depends on the combustible material and ventilation factor.
Thermal Expansion
Thermal expansion is the property to indicate the time of heating of the structure. Variation of the thermal expansion is the cause of failure and this effect is more essential in case of fire.
It is the main cause of additional pressure on connection and it creates sagging of the structure, when cooling down it tries to return its original stage and creates buckling
Modulus of Elasticity
It is deformed by the application of force, Modulus of elasticity of steel usually 230×103 MPa. This value decreases with increasing temperature.
Factors Influencing The Behavior of Structural Steel In The Fire
Loading: Loading is one of the major factors which affect the behavior of steel structure. Decreasing the applied load on the structure the fire endurance and temperature increases. If the applied load crosses the limit or reach the ultimate strength it creates failure. So, it always is lower to increase the fire resistance of structure
Connection: The connections of column to beam in modern steel buildings are generally designed with shear connections. Bracing element resists the forces. If the structure deforms because of fire loading moments are transferred to connection due to reduce the moment at mid span. Due to this, it increases the load ratio and fire resistance of the beam
End restraint: Deflection and fire resistance of steel structure depends on its end restraint. For the same fire loading, the simply supported beam survives more than the rotation end restraint. Application of extra axial load on the beam increases the deflection and further heating it decreases the deflection
Interaction effect: It is the effect of the different elements of the whole element. Interaction creates a great bonding between the metals and the damage of a material is not a big deal in that case as the subsequent matter can sustain the load which is applied through the load path.
Localization: Compartmentation is the way of localization of fire in buildings. Thermal expansion in the heated parts can extend the instability which can divert the load and weakling the members.
Tensile action: It occurs in the composite section of the steel framed building. When steel section resists the load, crack initiates in the concrete portion. It improves the resistance of a structure to provide an alternative load path and also minimize the need for fire protection.
The distribution of temperature: It mainly varies along the cross section or the length of the member. Variation of temperature in section shows better resistivity than the uniform distribution of temperature.
Conclusion
Building constructions, including steel are used widely because of the opportunities that they give to various objectives. Steel is often used in construction due to a wonderful combination of technological and operational properties, exemplified by many unique architectural constructions in the world.
Info and image: paroc.com, nvlpubs.nist.gov/nistpubs, worldscientificnews.com, theseus.fi/bitstream, zigurat, entremelodias.co, casfire.cn
