Determining the degree of fire resistance of structures during construction.
Determining the degree of fire resistance of structures during construction
Construction materials used in the construction of buildings and structures are characterized by fire hazard, from which it follows that such a parameter as the degree of fire resistance of structureswill depend on the quality of the materials. Construction materials are characterized by flammability, ignitability, smoke-forming ability, toxicity and the degree of fire spread over the surface. Depending on the manifestation of one or another feature in a given specific case, in accordance with GOST, the following groups of materials are distinguished: non-flammable and flammable. The latter, in turn, are divided into slightly flammable, moderately flammable, normally flammable and highly flammable.
Fire resistance level of building structures
According to SNiP 11-2-80 — «Fire safety standards for the design of buildings and structures» and in accordance with CMEA standards 382-76 and 2437-80, building materials and structures are divided into three groups according to flammability: non-combustible, hardly combustible and combustible. The degree of fire resistance of structures is derived from these concepts. Non-combustible materials are those that do not ignite, smolder or char when exposed to fire or high temperatures, that is, they provide a high degree of fire resistance of structures. These are natural and artificial inorganic materials, gypsum and gypsum fiber boards with an organic matter content of less than 8%, mineral wool boards with a synthetic, bitumen or starch binder content of less than 6% by weight, as well as metals used in construction. Thus, fire doors are made of such materials and have a high degree of fire resistance.
Slow-burning materials are those that ignite, smoulder or char when exposed to fire or high temperatures and continue to burn or smoulder in the presence of an ignition source, and after its removal, the burning or smouldering stops. These are materials consisting of non-combustible and combustible substances — asphalt concrete, gypsum and concrete parts with organic fillers, clay-straw materials with a bulk density of at least 900 kg/m3, cement fiberboard, wood subjected to deep impregnation with fire retardants, felt soaked in clay solution, foam plastic, mineral wool slabs on a bitumen bond.
The fire resistance level of structures made of combustible materials is the lowest. Combustible materials ignite or smolder when exposed to fire or high temperatures and continue to burn or smolder after the ignition source has been removed. These are all organic materials that do not meet the requirements for non-combustible and hardly combustible materials. Defining the fire resistance of structures implies differentiating buildings and structures into five levels.
The level of fire resistance of building structures is determined by the fire resistance limits of the main building structures and the limits of fire spread along these structures.
Fire resistance limit of building structures
The definition of fire resistance of structures implies their ability to maintain their load-bearing and enclosing functions in fire conditions. Associated with this definition is such a concept asfire resistance limit of building structures— this is the duration of resistance of a building structure to the effects of high temperatures during a fire until its bearing and enclosing capacity is exhausted, it is customary to call it the fire resistance limit. Fire resistance limit of building structuresIt is customary to calculate in hours. The manifestation of any of the following signs indicates the extent to which the fire resistance limit of building structures is observed: the formation of through holes or through cracks in the enclosing structure (wall, partition, ceiling, roof) through which flame or combustion products can penetrate; an increase in temperature on the unheated surface of the enclosing structure by an average of more than 140°C or at any point on this surface by more than 180°C (compared to the initial temperature) or more than 220°C regardless of the initial temperature of the structure; loss of load-bearing capacity or stability by the structure, i.e., collapse.Fire resistance limit of building structures is not the only limiting characteristic, it also includes the flame propagation limit of building structures. This indicator is determined experimentally using the VNIIPO method (according to SNiP II-2-80). The fire propagation limit is characterized by the ability of building structures with any combination of layers of non-combustible, hardly combustible and combustible materials to burn independently. The flame propagation limit is measured in centimeters and represents the size of damage to the structure in the control zone for 15 minutes. Fire resistance limit of building structures is determined both experimentally and by calculation methods. They are usually called actual fire resistance limits Pf. The fire resistance limits established by standards for the main parts of buildings and structures are called required fire resistance limits. Requirements for fire-resistant wiring are also based on these general concepts.