Xirong tells you: steel structure fire protection measures

Safety is the foundation of the construction industry, every project and every company is based on the principle of safety first, but every safety accident often occurs. Since July, there have been 18 major construction safety accidents in the construction industry across the country, resulting in a total of 75 deaths (of which 3 are missing), an increase of 169% compared with 28 deaths in the same period of 2015, and the number of 75 deaths is close to the total number of deaths and disappearances in the six months of the entire first half of 2016! The lesson of blood is shocking, and safety first must not be used as an empty slogan, but must be truly implemented in every link. Xirong steel structure construction, as a member of the steel structure industry, summarizes practical experience and methods in 16 years of steel structure construction experience for your reference.

As we all know, steel has the advantages of high strength and good ductility, which determines that the steel structure has the characteristics of light weight, good seismic performance and large bearing capacity, while the steel structure can be processed in the field, the construction period is short, and the material is recyclable. Since the 5.12 Wenchuan earthquake, steel structure buildings have risen in large numbers and become an industry that has rapidly popularized throughout the country.

But steel structures have an Achilles heel: poor fire resistance. Although the steel itself will not catch fire and burn, the properties of the steel are greatly affected by temperature, but the impact toughness of the steel at 250 °C decreases, exceeding 300 °C, and the yield point and ultimate strength decrease significantly. Under the actual fire, the load situation remains unchanged, and the critical temperature of the steel structure losing the static equilibrium stability is about 500 °C, while the general fire temperature reaches 800~1000 °C. Therefore, under the high temperature of the fire, the steel structure will soon appear plastic deformation, resulting in local damage, and eventually causing the overall collapse and failure of the steel structure.

In order to make the steel structure maintain its strength and rigidity in a fire for a more frequent time and protect people's lives and property, we have adopted a variety of fire protection measures in the actual project. According to the different fire prevention principles, the fire prevention measures are divided into heat resistance method and water cooling method. The heat barrier method can be divided into spraying method and encapsulation method (hollow encapsulation method and solid encapsulation method). The water cooling method includes the water shower cooling method and the flush cooling method. The purpose of these measures is the same: to increase the temperature of the component at a specified time by no more than its critical temperature. The difference is that the heat resistance method prevents the heat from being transferred to the component, while the water cooling method allows the heat to be transferred to the component and then conducts the heat away to achieve the purpose.

First, we will introduce various fire prevention measures in detail and compare their advantages and disadvantages.

1. Heat resistance method - spraying method

Generally, fireproof coatings are applied or sprayed on the surface of steel to form a refractory and thermal insulation protective layer and improve the fire resistance limit of steel structures. This method is simple to construct, light in weight, has a long fire resistance time, and is not limited by the geometry of the steel component. It has good economy and practicality, and is widely used. There are many varieties of steel structure fireproof coatings, which are roughly divided into two categories: one is thin-coated fireproof coatings (Class B), that is, steel structure expansion fireproof materials; The other category is thick coating (H).

Class B fireproof coating, the coating thickness is generally 2-7mm. The base material is organic resin, which has a certain decorative effect, and expands and thickens at high temperature. The fire resistance limit can reach 0.5~1.5h. Thin-coated steel structure fireproof coating has thin coating, light weight and good vibration resistance. Indoor exposed steel structure, roof steel structure, when the fire resistance limit is specified in 1.5h and below, it is advisable to use thinly coated steel structure fireproof coating.

The coating thickness of class H fireproof coatings is generally 8~50mm. It is granular in shape. The main component is inorganic thermal insulation material, which has low density and low thermal conductivity. The fire resistance limit can reach 0.5~3.0h. Thick-coated steel structure fireproof coatings are generally non-combustible, aging resistant and durable and reliable. Indoor concealed steel structure, high-rise all-steel structure and multi-storey workshop steel structure, when the fire resistance limit is specified above 1.5h, thick coated steel structure fireproof coating should be selected.

2. Heat resistance method - encapsulation method

1) Hollow encapsulation method: generally use fireproof board or refractory brick to wrap the steel component along the peripheral boundary of the steel component. Most of the steel structure workshops in the domestic petrochemical industry use the method of masonry refractory bricks to wrap steel components to protect the steel structure. The advantages of this method are high strength and impact resistance, but the disadvantage is that it occupies a large space and is more troublesome construction. Refractory lightweight boards such as fiber-reinforced cement board, gypsum board, vermiculite board, etc. are used as fireproof cladding. The method of box wrapping of large steel components has the advantages of smooth decoration surface, low cost, small loss, no environmental pollution, and aging resistance, and has good promotion prospects.

2) Solid encapsulation method: Generally, the steel components are wrapped by pouring concrete, and the steel components are completely closed. For example, the steel column of the Shanghai Pudong World Financial Building adopts this method. Its advantages are high strength and impact resistance, but the disadvantage is that the concrete protective layer occupies a large space and the construction is more troublesome, especially on steel beams and diagonal braces.

3. Water cooling method

1) Water shower cooling method

The water shower cooling method is to arrange an automatic or manual sprinkler system in the upper part of the steel structure. When a fire occurs, the sprinkler system is started, a continuous water film is formed on the surface of the steel structure, and when the flame spreads to the surface of the steel structure, the water evaporates to take away the heat and delays the steel structure building to reach its limit temperature. The water shower cooling method is used in the building of the School of Civil Engineering of Tongji University.

2) Filling water cooling method

The water-filled cooling method is to fill a hollow steel member with water. Through the circulation of water in the steel structure, the heat of the steel itself is absorbed. So that the steel structure can maintain a low temperature in the fire, and will not lose the bearing capacity due to excessive heating. In order to prevent rust and freezing, rust inhibitors and antifreeze should be added to the water. The steel columns of the 64-story U.S. Steel Company Building in Pittsburgh, USA, use the water-filled cooling method.

Second, the comparison of fire prevention measures

The heat barrier method slows down the speed of heat transfer to structural components through heat-resisting materials. In general, the heat insulation method has good economy and practicality, and is widely used in practical engineering. The water cooling method is an effective protective measure against fire, but because this method has special requirements for structural design and its high cost, it has not been well promoted in the field of engineering.

The heat resistance method is widely used in the fire protection of steel structures, so the following focuses on comparing the advantages and disadvantages of the spraying method and the encapsulation method in the heat resistance method.

1. Fire resistance

In terms of fire resistance, the encapsulation method is superior to the spraying method. Concrete, refractory bricks and other encapsulation materials have better fire resistance than general fireproof coatings. In addition, the fire resistance of the new fireproof board is also better than that of fireproof coating. Its fire resistance limit is significantly higher than that of steel structure fireproof insulation materials of the same thickness, and higher than that of intumescent fireproof coatings.

2. Durability

Due to the good durability of encapsulation materials such as concrete, it is not easy to deteriorate over time; Durability has always been a problem that steel structure fireproof coatings have failed to solve. Whether the organic components of thin and ultra-thin fireproof coatings are used outdoors or indoors, their organic components may cause decomposition, degradation, aging and other problems, so that the coating peels off and pulverizes or loses fire performance.

3. Constructability

The spraying method of steel structure fireproof is simple to apply and can be constructed without complex tools. However, the construction quality of the fireproof coating by spraying method is poorly controlled, and it is not easy to control the rust removal of the substrate, the coating thickness of the fireproof coating and the humidity of the construction environment. The construction of the encapsulation method is more complicated, especially for diagonal braces and steel beams, but the construction controllability is strong and the quality is easy to guarantee. The thickness of the encapsulation material can be changed more accurately to control the fire resistance limit.

4. Environmental protection

The spraying method pollutes the environment during construction, especially under the action of high temperature, it can volatilize harmful gases. The encapsulation method has no toxic emissions under construction, normal use environment and high fire temperature, which is conducive to environmental protection and personnel safety during fire.

5. Economy

The spraying method is simple in construction, short in construction period and low in construction cost. However, fireproof coatings have a high price, and their maintenance costs are high due to the shortcomings of coatings such as aging. The encapsulation method is expensive to construct, but the materials used are cheap and the maintenance costs are low. In general, the encapsulation method is more economical.

6. Applicability

The spraying method is not limited by the geometry of the components, and is mostly used for the protection of beams, columns, floors, roofs and other components. It is especially suitable for fire protection of steel structures in space structure systems such as steel structure, grid structure and special-shaped steel structure. The encapsulation method is complex in construction, especially for steel beams, diagonal braces and other components, and the encapsulation method is generally used for columns, and the scope of application is not as extensive as the spraying method.

7. Occupy space

The

fireproof coating used in the spraying method is small, while the encapsulation materials used in the encapsulation method, such as concrete and fireproof bricks, will occupy space and reduce the use of space. Moreover, the quality of the encapsulation material is also large.

III. Summary

Through the discussion, the following conclusions can be obtained: 1) The use of fire protection measures for steel structures should consider the influence of many factors, such as component type, construction difficulty, construction quality requirements, durability requirements and economic benefits; 2) Through the comparison of the spraying method and the encapsulation method, the main advantages of the spraying method are that the construction process is simple, and the shape of the components does not change greatly after spraying. The main advantages of the encapsulation method are low cost, good fire performance and durability; 3) Various fire prevention measures have their own advantages and disadvantages, in the application of the project, can learn from each other's strengths, a variety of fire prevention measures combined. And by taking different measures, multiple fire lines can be set up.

In the process of steel structure procurement and construction, there are

still some details to pay attention to: first of all, the procurement of materials, the current quality of fireproof materials on the market is uneven, the price is also a large gap, there are many manufacturers specializing in brand fireproof materials imitation, Xirong steel structure reminds the steel structure peers must polish their eyes and buy products from regular manufacturers. In fact, it is the installation or spraying of fireproof materials, and it is necessary to install and spray fireproof materials in accordance with national and industry standards, especially the key parts must be constructed in accordance with the standards, and do not lose the big because of the small.