Fire protection measures for steel structures

 1. Fire resistance limit and fire resistance of steel structure 

The advantages of high strength and ductility determine that the steel structure has the characteristics of light deadweight, good seismic performance and large bearing capacity. Meanwhile, the steel structure can be processed in the field, the construction period is short, and the materials can be recycled.Therefore, no matter in the domestic or foreign steel structure buildings have been widely used.

But steel structures have an Achilles heel: poor fire resistance.In order to maintain the strength and stiffness of steel structure in fire for a long time and to guarantee the safety of people’s life and property, a variety of fire protection measures have been adopted in practical projects.According to different fire prevention principles, fire prevention measures are divided into heat resistance method and water cooling method.Heat resistance method can be divided into spraying method and encapsulation method (hollow encapsulation and solid encapsulation method).The water cooling method has the water pouring cooling method and the water flushing cooling method.In this paper, various fire prevention measures will be introduced in detail and their advantages and disadvantages will be compared.Resistance and fire resistance
The fire resistance limit of steel structure refers to the time during which the member loses its stability or integrity and its adiabatic resistance to fire during the standard fire resistance test.

Although the steel itself will not be on fire, but the steel material property is greatly affected by the temperature, but the impact toughness of the steel at 250℃ drops, over 300℃, yield point and ultimate strength significantly decreased.In actual fire, the load condition remains unchanged, and the critical temperature at which the steel structure loses its static equilibrium stability is about 500℃, while the general fire temperature reaches 800 ~ 1000℃.As a result, the steel structure will quickly appear plastic deformation under high fire temperature, resulting in local failure, and eventually resulting in the collapse of the whole steel structure failure.Fire prevention measures must be taken in steel structure building to make the building have enough fire resistance limit.Prevent the steel structure from heating up to the critical temperature rapidly in the fire, prevent the excessive deformation to the building collapse, so as to win precious time for fire fighting and personnel safety evacuation, avoid or reduce the loss caused by the fire.

2. Fire protection measures for steel structures

Steel structure fire protection measures according to the principle is divided into two categories: one is heat resistance method,the other is water cooling method.The purpose of these measures is consistent: to keep the temperature of the component from rising beyond its critical temperature at a specified time.The difference is that the heat resistance method prevents heat from being transferred to the components, while the water-cooling method allows heat to be transferred to the components and then transferred away for the purpose.

2.1 Resistance heat

Resistance heat method according to the resistance heat and heat resistance of coating material, fire retardant coating was divided into spraying method and the method of coating spraying method to build fire retardant coating by coating or spray coating method to protect and can be divided into the hollow coating method and solid coating method 

 2.1.1 spraying method

Generally USES fireproof paint coating or spray the surface of the steel, refractory insulating protection layer formation, improve the fire resistance of steel structure of this method is very light weight refractories for a long time, and should not be restricted steel component geometry has good economy and practicality, wide application.The variety of steel structure fire retardant coating is more, roughly divided into two categories: one is thin coating type fire retardant coating (B type), namely steel structure expansion fire retardant material;Another kind is a thick film coating (H) class B class fire-retardant coating, coating thickness is generally 2-7 mm makings for organic resin, have certain adornment effect, when high temperature expansion thickening of refractory limit of 0.5 ~ 1.5 H thin thin light weight coated steel structure fire retardant coating coating vibration resistance good indoor bare steel structure light roof steel structure, when its fireproof limit in 1.5 H and the following, appropriate chooses scumble H type steel structure fire retardant coating fireproof paint coating thickness for 8 ~ 50 mm commonly in granular surface main ingredients for the inorganic thermal insulation materials, smaller thermal conductivity of low density Refractory limit of 0.5 ~ 3.0 h thick coated steel structure fire retardant coating is generally not burning ageing resistance durability and reliable indoor concealed steel structure all steel structure and steel structure of multi-story factory buildings, when the rules above its fireproof limit in 1.5 h, should choose thick coated steel structure fire retardant coating

2.1.2 coating method

1) hollow coating method: generally use fire prevention board or brick, along the edge of the outside of the steel members, steel structure parcel up domestic petrochemical industry steel structure workshop is mostly adopt the method of laying brick wrapped steel members of steel structure to protect the method has the advantage of high strength Impact resistance, but the disadvantage is that takes up space larger construction more trouble with refractory light plate, such as fiber reinforced cement plasterboard monolayer slab for fire prevention covering method for box package of large steel components with low cost loss to decorate surface level off is smooth without environmental pollution aging resistance and other advantages, it has good prospects for promotion. 2) solid coating method: generally by pouring concrete, steel members wrapped up, completely closed steel structure pieces such as the world financial center Shanghai Pudong steel column is its advantage is that by the method of high strength, impact resistance, but the disadvantage is that concrete cover to take up the space is large Construction is troublesome, especially on steel beam and inclined bracing

2.2 Water cooling method

Water cooling method includes water pouring cooling method and water filling cooling method.

2.2.1 Water shower cooling method

The spray cooling method is to arrange an automatic or manual spray system on the upper part of the steel structure.In case of fire, the spraying system will be started to form a continuous water film on the surface of the steel structure. When the flame spreads to the surface of the steel structure, the water evaporation will take away the heat and delay the steel structure to reach its limit temperature.The water shower cooling method is used in the building of Civil Engineering College of Tongji University.

2.2.2 Water-filled cooling method

The water – filled cooling method is to fill water in hollow steel members.Through the circulation of water in the steel structure, the heat absorbed by the steel itself is absorbed.Thus, the steel structure can keep low temperature in the fire and will not lose its bearing capacity due to too high temperature rise.In order to prevent rust and freeze, water to add rust inhibitor and antifreeze.The steel columns of the 64-story U.S. Steel Company building in Pittsburgh are water-cooled.

3. Comparison of fire prevention measures

The heat resistance method can slow down the heat conduction velocity to the structural members through the heat resistance material.Generally speaking, the heat insulation method is economical and practical, and is widely used in practical projects.Water cooling method is an effective protective measure against fire, but it has not been well promoted in engineering field because of its special requirements on structural design and high cost.

Thermal resistance method is widely used in the fire protection of steel structure, so the following focuses on comparing the advantages and disadvantages of spray method and cladding method in thermal resistance measures.

3.1 fire resistance

In terms of fire resistance, cladding method is superior to spraying method.The fire resistance of concrete, firebrick and other envelopment materials is better than general fireproof coating.Additional, the fireproof performance of new fire prevention board also is superior to fire prevention coating.Its fire resistance limit is obviously higher than the same thickness of the steel structure fire insulation material, more than the expansion of fire coatings.

3.2 the durability

Because the durability of cladding material, such as concrete, is better, it is not easy to deteriorate over time.But the durability is always the steel structure fire retardant coating failed to solve the good problem.Whether used for outdoor or indoor, the organic component of the thin and ultra-thin fireproof coating may produce decomposition, degradation, aging and other problems, so that the coating peeling powder or loss of fire performance.

3.3 construction

The spraying method of steel structure fire prevention is simple and can be used without complicated tools.But spraying fireproof coating construction quality control is poor, the derusting of the base material, the coating thickness of fireproof coating and construction environment humidity is not easy to control;The construction of cladding method is complex, especially for inclined bracing and steel beam, but the construction is controllable and the quality is easy to be guaranteed.The fireproof limit can be controlled by changing the thickness of cladding material accurately.

3.4 environmental protection

Spraying method pollutes the environment during construction, especially under the action of high temperature, it can volatilize harmful gases.There is no toxic release in construction, normal use environment and high temperature of fire, which is beneficial to environmental protection and personnel safety in fire.

3.5 economy

Spraying method is simple, short construction period and low construction cost.But the price of fireproof coating is high, and because coating has the shortcoming such as ageing, its maintenance cost is higher.The construction cost of wrapping method is high, but the material price is cheap, and the maintenance cost is low.Generally speaking, the encapsulation method has good economic efficiency.

3.6 applicability

The spraying method is not limited by the geometry of the components, and is widely used for the protection of beams, columns, floors, roof and other components.It is especially suitable for the fire protection of light steel structure, grid structure and special-shaped steel structure.The cladding method is complex in construction, especially for steel beams and inclined bracing members. The cladding method is generally more used for columns and is not widely used for spraying.

3.7 Space Occupied

The volume of fire retardant coating used by spraying method is small, and envelopment method USES envelopment material such as concrete, fireproof brick, will take up space, reduce the use of space.And enveloping material quality is also larger.

 4. Summarize

The following conclusions can be drawn from the discussion:

1) The adoption 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)  By comparing the spraying method with the encapsulation method, the main advantages of the spraying method are simple in the construction process, and the appearance of components does not change greatly after spraying.The main advantages of the packing method are low cost, good fire performance and durability.

3) All kinds of fire prevention measures have their own advantages and disadvantages. In engineering application, they can learn from each other and make up for each other’s shortcomings.And can take different measures to set up multiple lines of fire defense.

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Tina

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