With the vigorous development of the construction industry, the form of the building structure changes with each passing day. The steel structure has been widely valued by people because of its high strength, light weight, good earthquake resistance, fast construction, low construction cost, less occupied area of the structure and high degree of industrialization. Although the steel structure is non-combustible, it is easy to conduct heat. When its temperature reaches more than 540℃, its mechanical properties decline rapidly, and when the temperature reaches 600℃, the strength of the steel is almost zero. The fire resistance limit of bare steel is usually only 15-30min. According to the requirements of the code for fire prevention design of building, the requirements for the fire resistance limit of the columns, beams and roof bearing components are not ranging from 0.5-3.0h. May 5, 1998, Beijing Yuquanying Huandao furniture City fire, steel structure fire did not meet the standards, 13,000 m2 steel structure light body buildings collapsed, causing direct economic losses of 20.87 million yuan; Experts who were shocked at ****** after the September 11 attacks believe that the plane did not actually knock down the building, but because the plane damaged the fireproof coating on the steel structure of the building at the same time, and the fire exploded, making the steel structure exposed to the raging fire. After more than an hour, the structure softened, lost strength, and collapsed. Therefore, it is necessary to protect the steel structure against fire, so as to improve its fire resistance limit.
Fire protection measures for steel structures have varied in the past, including casting concrete shells, setting fire barriers such as firebricks, cladding with non-combustible materials such as gypsum board, or cooling with water inside hollow pipes. Now more is the use of spraying steel structure fireproof coating method to protect the steel structure, because of this measure of fire insulation performance is good, construction is not restricted by the structure geometry, generally do not need to increase auxiliary facilities, easy to implement, and some beautiful decorative effect has been widely used. However, there are some problems in the selection, durability and construction quality of fire retardant coatings for steel structure, which also bring some difficulties to the fire supervision work.
First, the selection of fire retardant coating for steel structure
(a) classification of steel structure fire retardant coatings
At present, according to GB14907-2002 "steel structure fire coatings general technical conditions" classification, steel structure fire coatings according to its coating thickness and performance characteristics can be divided into: ultra-thin, thin coating and thick three types. Ultra-thin steel structure fire retardant coating (CB), the coating thickness is not more than 3 mm, thin coating of steel structure fire retardant coating (B class) of coating thickness is 3 ~ 7 mm, these two kinds of coating during a fire can heat expansion foam, the formation of foamy coking heat insulation layer, so as to prevent heat transfer to the steel structure, slow temperature rise of the steel structure, fire protection effect. These two coatings are usually referred to as steel structure expansion type fire coatings. Thick steel structure fire retardant coating (H class) coating thickness of 7~45mm, they appear granular surface, small density, coating heat is not foaming, relying on its low thermal conductivity to delay the temperature rise of steel structure, fire protection, fire limit up to 0.5~3.0h, usually called this coating for steel structure fire insulation coating. The two coatings have different performance characteristics and are suitable for different occasions. All steel structure fire retardant coatings are divided into indoor and outdoor two types of use.
(2) there are problems in the selection of fire retardant coating for steel structure
The main problems in the selection of fireproof coating for steel structure: One is ultra-thin steel structure fire retardant coating has become a research and production of fire retardant coating of steel structure in our country developed by unit to hot spots, ultra-thin steel structure fire retardant coating for solvent system, commonly with thick and thin coated steel structure fire retardant coating, ultrathin steel structure fireproof coating decorative better, the coating is thinner and engineering the usage has been greatly reduced, Thus reduces the total cost of the project, is the current market to promote the variety. However, in the steel structure fire protection engineering appeared one-sided pursuit of coating thinner and thinner phenomenon, one-sided promotion of ultra-thin coating phenomenon, which is extremely unfavorable to provide reliable security for the building. Because the ultrathin steel structure fire retardant coating mainly depends on the physical and chemical reaction of organic components in the formula, any factors that affect the carbonization expansion of the coating will affect its ultimate fire performance, it is difficult to ensure that the coating can provide more than 2.0h fire protection for members after a long time.
Second, designers often simply indicate the use of steel structure fireproof coating protection measures, can not explain in detail which type of steel structure fireproof coating to choose, in the construction of simple consideration of price, beauty and other problems, resulting in the phenomenon of improper selection of fireproof coating. Three is often ignored when the selection of the building environment and the use of nature of the requirements. Open steel structure is exposed to the sun and rain, or the top of the high-rise building steel structure with transparent board, sunlight exposure, environmental conditions are harsh, should choose outdoor steel structure fireproof coating. Fourth, the technical performance only meets the indoor requirements of the paint for outdoor.
(3) the correct choice of steel structure fire retardant coating
1, outdoor should not use expansion type fireproof coating.
Intumescent fire coatings for wood structure, at first due to poor for outdoor weatherability, easy ageing, peeling, the reliability of the expansion in the humid environment, corrosion problems, and in the process of high temperature flame barbecue foaming, resin decomposition to produce smoke and toxic gases, etc., so without special measures, in tunnel, and people often stay outdoor buildings, Do not use inflatable fireproof paint. The projects in nuclear energy, electric power, petrochemical and chemical industries shall be mainly based on thick fireproof coatings.
2, carefully choose thin coating steel structure fire retardant coating, strictly control the selection of ultra-thin steel structure fire retardant coating.
It is necessary to strictly control the use of all steel structure buildings, especially steel columns, steel beams, steel floor slabs and steel evacuation stairs in large public and civil buildings. Steel column, steel floor, steel evacuation stairs and other main building components should give priority to the selection of steel wire mesh concrete and other fire protection materials, really difficult, can choose thick steel structure fire protection coating protection, carefully choose thin steel structure fire retardant coating, strict control of ultra-thin steel structure fire retardant coating selection. High-rise building steel structure and multi-layer steel structure workshop, should not use thin coating type and ultrathin steel structure fire retardant coating.
3, according to the building parts and fire resistance limit requirements to choose fire retardant coating
The hidden steel structure in the building, the appearance quality of the coating requirements are not high, should try to use thick steel structure fireproof coating. Bare STEEL NET frame, STEEL roof frame and roof bearing structure, when the specification of the fire limit requirements in 1.5h the following, can choose thin coating or ultrathin steel structure fire retardant coating, fire limit requirements more than 2.0h, should choose thick steel structure fire retardant coating.
Durability and use management of fireproof coating for steel structure
(a) durability of fire retardant coatings for steel structure
No matter which kind of fire retardant coating, the fire limit given in the test report is the test results before its use, and in the fire retardant coating coated for several years, fire retardant coating fire performance is still? At present, there is a lack of scientific evaluation on the service life of various types of fireproof coatings in the engineering environment. Fireproof coatings manufacturers cannot provide the life and replacement cycle that can be used in different use environments in the product description. The use of steel structure fireproof coating is holding the attitude of "once and for all". The durability research of fireproof coating of steel structure is already an imminent work, should formulate the evaluation standard and evaluation method of the durability of fireproof coating, so as to scientifically determine the cycle or fixed number of years for changing or maintaining fireproof coating. In addition, the maintenance of fire retardant coating is not a simple re-coating, re-coating on the old fire retardant coating base, whether it can meet the technical performance requirements? Refresh fire retardant coating is especially to be concealed project how to overcome its construction difficulty? These are topics that require further research.
(2) Use management of fireproof coating for steel structure
The DOMESTIC MANUFACTURER THAT PRODUCES STEEL STRUCTURE FIREPROOF COATING IS VERY MUCH, VARIETY ALSO IS MISCELLANEOUS, THERE IS NO UNIFIED TECHNICAL STANDARD, THE VARIETY OF ULTRATHIN STEEL STRUCTURE FIREPROOF COATING WITH EXCELLENT PERFORMANCE AND OUTDOOR STEEL STRUCTURE FIREPROOF COATING IS LESS, WHEN USING SHOULD BE VERY CAUTIOUS.
The coating type must be approved by the design unit, construction unit, construction unit and fire department. Fire retardant coating should be with the long-term use of buildings and also have long-term function, so after the acceptance of fire retardant coating construction, should also pay attention to maintenance management. One is to avoid other operations or accidental impact, wear and tear, rain and pollution damage.
