Although China had outstanding achievements in iron structure in the early days, due to the shackles of the feudal system of more than 2,000 years, science was underdeveloped, so it stayed at the level of iron buildings for a long time. It was not until the end of the 19th century that China began to adopt modern steel structures. After the founding of New China, the application of steel structure has been greatly developed, far exceeding the past in terms of quantity or quality. In the design, manufacturing and installation technologies have reached a high level, mastered the design and construction technology of various complex buildings, many large-scale and complex steel structure workshops, large-span steel have been built throughout the country. Structural civil buildings and railway bridges, steel trusses of the Great Hall of the People in China, steel grids of gymnasiums in Beijing and Shanghai, and three-hinge steel arches of the Qin Shihuang Emperor's Horse and Horse Ware Exhibition Hall in Shaanxi.

 

1. Advantages of steel structure

 

1.1 The steel structure is light in weight. Although the steel has a large specific gravity, it can withstand large loads because of its good mechanical properties. Therefore, the steel structural parts have small cross-sectional dimensions. When the same span is used, the weight of the steel roof trusses is the most reinforced concrete. The 1/4 or 1/3 of the truss has a small weight and is easy to transport.

 

1.2. The material is uniform and conforms to the assumption of mechanical calculation. The internal structure of the steel is relatively uniform, close to the isotropic body, and almost completely elastic within a certain stress range. Therefore, the actual stress of the steel structure is in line with the results of the calculation of the mechanical mechanics.

 

1.3. The degree of mechanization of steel structure manufacturing and installation is high. The steel structure has a certain degree of technical intensive nature. The steel structure uses a single material, and is a finished product. It is easy to process, has a high degree of mechanization, is quick to build, and is easy to ensure quality. It is suitable for mass production in batches, from factory processing of components. To the site hoisting, the degree of industrialization is much higher than the cast-in-place reinforced concrete structure with labor-intensive nature.

 

1.4. Steel structure installation and construction is simple. The structure is produced by specialized metal components factory (beams, trusses, columns, etc.), which are connected by electric welding or bolts (or high-strength bolts) on the construction site. The construction and installation are simple and rapid, and the construction speed is increased, which has become the most cost-effective construction. The main factor, the structural cost problem has been reduced to a secondary position. Moreover, the bolted steel structure facilitates the transformation and demolition.

 

1.5. The flexibility of the steel structure in the layout of the floor is another important feature. In today's information age, the requirements for indoor planes, space and internal facilities are more stringent. The original layout and equipment may soon become less suitable or completely unsuitable, while steel structures offer greater possibilities for planar re-layouts, extending the life of the building.

 

Although steel structures have so many advantages, they have a fatal flaw: they are not fire resistant. Although steel is a non-combustible material, its mechanical properties such as yield strength and modulus of elasticity will decrease with increasing temperature under the action of high temperature of fire. At around 550 °C, the reduction is more obvious, generally around 15 minutes. It will lose its ability to bear the weight and collapse. The fire cases of steel structures at home and abroad have proved that the building was burned within 20 minutes after the fire, and finally formed a ruin. In May 1973, the fire in Tianjin Stadium, the fire in Shanghai Cultural Square in December 1969, and the fire in the fiberboard workshop of Beijing Erqi Locomotive and Rolling Stock Plant in August 1972 all exposed the Achilles heel of poor fire resistance of steel structure.

 

2. Fire protection method for steel structure

 

Due to the poor refractory energy of the steel structure, it will collapse and collapse quickly under the action of fire and high temperature. The fire resistance limit is only 15 minutes. If measures are taken, the steel structure is protected so that the temperature rises not exceed the critical temperature during the fire, and the steel structure is in the fire. In order to maintain stability, the protective measures taken on the steel structure can be divided into two main principles: the interception method and the grooming method.

 

2.1. Interception method

 

The principle of the interception method is to intercept or block the heat flow generated by the fire to the member, so that the temperature rise of the member within a specified time does not exceed its critical temperature. The method is to provide a layer of protective material on the surface of the component, and the high temperature generated by the fire is first transmitted to the protective material, and then the protective material is applied to the component. Since the selected material has a small thermal conductivity and a large heat capacity, the heat transfer to the member can be well blocked, thereby providing protection. The interception method is further divided into a spraying method, an encapsulation method, a shielding method, and a water spray method.

 

2.1.1. Spraying method

 

The spraying method is to spray the fireproof coating directly on the surface of the component with a spraying machine to form a protective layer. Steel structure fireproof coatings can be divided into organic fireproof coatings and inorganic fireproof coatings according to the different adhesives used. According to the coating thickness, they are divided into thin coating type and thick coating type. The coating thickness of thin coating type steel structure coating is generally 2 ~ 7mm, has a certain decorative effect, coating expansion and thickening at high temperature, with fire and heat insulation, fire resistance can reach 0.5 ~ 1.5 hours, this coating is also known as the structural expansion fire retardant coating. Thick coated steel structure fireproof coating thickness is generally 8 ~ 20mm, granular surface, low density, low thermal conductivity, fire resistance limit of 0.5 ~ 3.0 hours, this coating is also known as steel structure fire insulation coating.

 

There are three fire protection principles for steel structure fireproof coatings: First, the coating shields the steel substrate so that the steel structure is not directly exposed to high flame temperatures. Second, the water vapor or other non-combustible gas liberated by some substances after the absorption of the coating absorbs heat, reduces the flame temperature and burning speed, and dilutes the oxygen. Third, the coating itself is porous and lightweight, and forms a carbonized foam layer after being heated, which prevents heat from being rapidly transferred to the steel substrate, delays the reduction of the strength of the steel substrate, thereby improving the fire endurance of the steel structure.

 

The main varieties of thin-coated steel structure fireproof coatings are: LB, SG-1, SB-2, SS-1 steel structure expansion fireproof coating. The main varieties of thick coated steel structure fireproof coatings are: LG steel structure fireproof thermal insulation coating, STI-A, JG276, ST-86, SB-1, SG-2 steel structure fireproof coating.

 

When spraying steel structure fireproof coating, the thickness of the spray must reach the design value, and the joint part should be properly thickened. When one of the following conditions is encountered, a steel mesh connected to the steel structure should be set in the coating to ensure the coating is firm.

 

- withstand the vibration of the beam

——The design layer thickness is more than 40mm

——Coating paint with strength less than 0.05MPa

——beams with web height greater than 1.5m

 

Spray coating is the most widely used and can be used for fire protection of any steel component.

 

2.1.2. Encapsulation method

 

The encapsulation method is to make a refractory protective layer on the surface of the steel structure and enclose the components. The specific practices are as follows:

 

2.1.2.1. Use cast-in-place concrete as a refractory protective layer. The materials used are concrete, lightweight concrete and aerated concrete. These materials are both non-combustible and have a large heat capacity. The use of a refractory protective layer can slow down the heating of the components. Since the surface layer of the concrete is easily peeled off at high temperatures in the fire, a steel mesh can be applied to the surface of the steel to further improve its fire resistance.

 

2.1.2.2. Use mortar or gray clay as a refractory protective layer. The materials used are generally mortar, light magma, perlite mortar or gray clay, vermiculite mortar or lime cement. The above materials have good fire resistance, and the construction method is often applied to the metal mesh.

 

2.1.2.3. Use mineral fibres. Its materials are asbestos, rock wool and slag wool. The specific construction method is to mix the mineral fiber with the cement, and then spray it with the special spray gun and the water spray at the same time to form a sponge-like covering layer, and then smooth or let it be concave and convex. The above method can be directly sprayed on the steel member, or can be sprayed onto the metal mesh thereon, and the latter effect is better.

 

2.1.2.4. Use lightweight prefabricated panels as a refractory protective layer. The materials used are lightweight concrete slabs, foam concrete slabs, calcium silicate molded sheets, asbestos-formed sheets, etc., which are coated with the above-mentioned prefabricated panels, and the inter-board joints can be stapled and bonded. This construction method is simple in construction and short in construction period, and is advantageous for industrialization. At the same time, the functions of bearing (steel structure) and fireproof (prefabricated board) are clearly defined, and the repair after fire is simple and does not affect the function of the main structure, so it has good recovery.

 

2.1.3. Shielding method

 

The shielding method is to store the steel structure in the wall or ceiling of the refractory material, and to make the refractory ceiling under the steel beam and the steel roof truss. In the fire, the heating of the steel beam and the steel roof truss can be greatly delayed, and the fire resistance of the steel structure is greatly improved. Ability, and this method can also increase the aesthetics of the room, but pay attention to the seams and holes in the ceiling should be tight to prevent bonfire.

 

2.1.4. Water spray method

 

The water spray method is to set up a spray water supply pipe network at the top of the structure. When a fire occurs, the water starts to be automatically started (or manually), and a continuous flowing water film is formed on the surface of the component to protect the water.

 

From the above, it is known that the common feature of these methods is to try to reduce the heat flux to the components, which is called the interception method.

 

2.2. Grooming method

 

Unlike the interception method, the grooming method allows heat to be transferred to the component and then tries to conduct or consume the heat, which also protects the temperature of the component from rising to a critical temperature.

 

The grooming method is currently mainly a method of water-filled cooling protection. The method is filled with water in a hollow closed section (mainly a column), and the component transfers heat absorbed from the fire field to the water during a fire, and relies on evaporation of water to consume heat or to transfer heat away by circulation, and the component temperature can be maintained at 100. °C or so. In theory, this is the most effective way to protect steel structures. When the system is working, the component is equivalent to a container filled with water and is heated like a boiling water pot. As long as the water source is replenished and sufficient water level is maintained, and the specific heat and heat of vaporization of the water are large, the heat absorbed by the components will be continuously consumed or led away.

 

The cooling water can be replenished by a high water tank or a water supply network or a fire truck. The vapor is discharged from the exhaust port. When the column height is too large, it can be divided into several circulation systems to prevent the water pressure at the bottom of the column from being too large. To prevent corrosion or water ice, the water should be mixed with a rust inhibitor and an antifreeze.

 

The water cooling method can be combined with a single column self-forming system or multiple columns. The former relies only on the evaporation of water, which can evaporate heat, and can also form a cycle by the temperature difference of the water, and direct the heat to the column with a lower temperature in the non-fire zone.