How to Improve the Fire Resistance of Steel Structures

The high strength and ductility of steel give steel structures the advantages of light weight, excellent seismic performance, and high load-bearing capacity. Additionally, steel structures can be prefabricated off-site, have short construction cycles, and the steel itself is recyclable. As a result, steel structures are widely used in both domestic and international construction.

However, steel structures have one critical weakness: poor fire resistance. To ensure that steel structures maintain their strength and rigidity for as long as possible during a fire, thereby safeguarding human life and property, various fire protection measures are implemented in actual engineering projects. Based on different fire protection principles, these measures are categorized into thermal insulation methods and water cooling methods. Thermal insulation methods can be further divided into spray-applied methods and encapsulation methods (hollow and solid encapsulation). Water-cooling methods include water spray cooling and water jet cooling. This article will provide a detailed introduction to various fire protection measures and compare their advantages and disadvantages.

1. Fire Resistance Limit and Fire Performance of Steel Structures

The fire resistance limit of a steel structure refers to the duration during a standard fire test that a component resists the effects of fire, from the moment it is exposed to fire until it loses its stability, integrity, or thermal insulation.

Although steel itself does not ignite or burn, its mechanical properties are significantly affected by temperature. At 250°C, the impact toughness of steel decreases, and above 300°C, the yield point and ultimate strength drop significantly. In actual fire conditions, assuming the load remains constant, the critical temperature at which a steel structure loses its static equilibrium stability is approximately 500°C, whereas typical fire temperatures reach 800–1000°C. Consequently, under high fire temperatures, steel structures will rapidly undergo plastic deformation, leading to localized failure and ultimately resulting in the overall collapse and failure of the structure.

Fire protection measures must be implemented in steel structures to ensure the building has a sufficient fire resistance rating. These measures prevent the steel structure from rapidly heating up to the critical temperature during a fire, prevent excessive deformation that could lead to building collapse, and thereby buy valuable time for firefighting and the safe evacuation of occupants, avoiding or minimizing fire-related losses.

2. Fire Protection Measures for Steel Structures

Fire protection measures for steel structures are classified into two categories based on their principles: thermal insulation methods and water cooling methods. The objective of these measures is consistent: to ensure that the temperature of structural members does not exceed their critical temperature within a specified time frame. The difference lies in the fact that thermal insulation methods prevent heat from being transferred to the structural members, whereas water cooling methods allow heat to reach the members and then dissipate it to achieve the desired result.

2.1 Heat Barrier Method

The heat barrier method is divided into spray coating and encapsulation methods based on whether heat is blocked by fire-resistant coatings or encapsulation materials. The spray coating method protects components by applying or spraying fire-resistant coatings. The encapsulation method can be further divided into hollow encapsulation and solid encapsulation.

2.1.1 Spraying Method

Generally, fire-resistant coatings are applied or sprayed onto the surface of steel to form a fire-resistant and insulating protective layer, thereby improving the fire resistance rating of the steel structure. This method is simple to implement, lightweight, provides a relatively long fire resistance duration, and is not restricted by the geometric shape of the steel components. It offers good cost-effectiveness and practicality and is widely used. There are many types of fire-resistant coatings for steel structures, which are broadly divided into two categories: thin-film fire-resistant coatings (Class B), also known as intumescent fire-resistant coatings for steel structures; and thick-film coatings (Class H).

Class B fire-resistant coatings typically have a coating thickness of 2–7 mm. The binder is an organic resin, which provides a certain decorative effect and expands to increase thickness at high temperatures. The fire resistance rating can reach 0.5–1.5 hours. Thin-film fireproof coatings for steel structures feature a thin coating, light weight, and good vibration resistance. For exposed indoor steel structures and lightweight roof steel structures where the required fire resistance rating is 1.5 hours or less, thin-film fireproof coatings are recommended. Class H fireproof coatings generally have a coating thickness of 8–50 mm and a granular surface. Their main components are inorganic thermal insulation materials, which have low density and low thermal conductivity. The fire resistance rating can reach 0.5–3.0 hours. Thick-film steel structure fireproof coatings are generally non-combustible, resistant to aging, and offer reliable durability. For concealed indoor steel structures, high-rise all-steel structures, and multi-story industrial plant steel structures, where a fire resistance rating of 1.5 hours or higher is required, thick-film steel structure fireproof coatings should be selected.

 

2.1.2 Enclosure Method

1) Hollow Enclosure Method: This method typically involves using fire-resistant panels or firebricks to encase the steel members along their outer perimeter. In China’s petrochemical industry, steel-structured workshops are mostly protected by enclosing steel members with masonry firebricks. The advantages of this method include high strength and impact resistance; however, its disadvantages are that it occupies a large amount of space and is relatively complicated to construct. Using lightweight fire-resistant panels such as fiber-reinforced cement boards, gypsum boards, or vermiculite boards as the fireproof outer layer. The box-type wrapping method for large steel members offers advantages such as a smooth and flat finished surface, low cost, minimal material waste, no environmental pollution, and resistance to aging, making it a promising approach for widespread adoption.

2) Solid Encapsulation Method: This method typically involves pouring concrete to completely enclose the steel members. For example, the steel columns of the Shanghai World Financial Center in Pudong were constructed using this method. Its advantages include high strength and impact resistance, but its disadvantages are that the concrete protective layer occupies a large amount of space and construction is relatively complicated, particularly on steel beams and bracing.

2.2 Water Cooling Methods

Water cooling methods include water spray cooling and water-filled cooling.

2.2.1 Water Spray Cooling Method

The water spray cooling method involves installing an automatic or manual sprinkler system on the upper part of the steel structure. In the event of a fire, the sprinkler system is activated to form a continuous water film on the surface of the steel structure. When flames spread to the surface, the evaporation of water absorbs heat, delaying the steel structure from reaching its critical temperature. This method was implemented in the Civil Engineering Building at Tongji University.

2.2.2 Water-Filling Cooling Method

The water-filling cooling method involves filling hollow steel members with water. As the water circulates within the steel structure, it absorbs the heat generated by the steel itself. This allows the steel structure to maintain a lower temperature during a fire, preventing it from losing its load-bearing capacity due to excessive heating. To prevent corrosion and freezing, rust inhibitors and antifreeze agents must be added to the water. The steel columns of the 64-story U.S. Steel Building in Pittsburgh, USA, employed the water-filled cooling method.

3. Comparison of Fire Protection Measures

The thermal barrier method uses heat-resistant materials to slow the rate at which heat is conducted to structural members. Overall, the thermal insulation method offers better cost-effectiveness and practicality and is widely used in actual engineering projects. The water-cooling method is an effective protective measure against fire; however, because this method imposes specific requirements on structural design and involves higher costs, it has not yet been widely adopted in the engineering field.
Since the thermal barrier method is widely used in fire protection for steel structures, the following section focuses on comparing the advantages and disadvantages of the spray-applied and encapsulation methods within this category.

3.1 Fire Resistance

In terms of fire resistance, the encapsulation method outperforms the spray-application method. Encapsulation materials such as concrete and firebricks offer better fire resistance than conventional fire-retardant coatings. Additionally, new types of fire-resistant panels also exhibit superior fire resistance compared to fire-retardant coatings. Their fire resistance rating is significantly higher than that of steel structure fireproof and thermal insulation materials of the same thickness, and even higher than that of intumescent fire-retardant coatings.

3.2 Durability

Since encapsulation materials such as concrete have good durability, their performance does not degrade easily over time; however, durability has long been an unresolved issue for fire-resistant coatings for steel structures. Whether used indoors or outdoors, thin-film and ultra-thin fire-resistant coatings—which are primarily composed of organic components—may experience decomposition, degradation, and aging of their organic components, causing the coating to peel, chalk, or lose its fire-resistant properties.

3.3 Workability

The spray application method for steel structure fire protection is simple and requires no complex tools. However, the quality of fireproof coatings applied by spraying is difficult to control, as factors such as substrate rust removal, coating thickness, and ambient humidity are hard to manage. The encapsulation method is more complex, particularly for bracing and steel beams, but it offers greater control over the application process and ensures consistent quality. The fire resistance rating can be precisely adjusted by varying the thickness of the encapsulating material.

3.4 Environmental Impact

The spray application method causes environmental pollution during construction, particularly as it can emit harmful gases under high temperatures. The wrapping method produces no toxic emissions during construction, normal use, or under high fire temperatures, which benefits environmental protection and ensures personnel safety during a fire.

3.5 Cost-Effectiveness

The spray-applied method is simple to implement, has a short construction period, and involves low construction costs. However, fire-resistant coatings are expensive, and due to drawbacks such as aging, maintenance costs are relatively high. The encapsulation method has higher construction costs, but the materials used are inexpensive, and maintenance costs are low. Overall, the encapsulation method offers better cost-effectiveness.

3.6 Applicability

The spray-applied method is not limited by the geometry of structural members and is commonly used to protect various components such as beams, columns, floor slabs, and roof structures. It is particularly suitable for fire protection of spatial structural systems, including light-gauge steel structures, space frame structures, and irregular steel structures. The encapsulation method involves complex construction, especially for components like steel beams and bracing; it is generally used more frequently for columns and has a narrower scope of application than the spray-applied method.

3.7 Space Occupancy

The fire-resistant coatings used in the spraying method occupy a smaller volume, whereas the encapsulation materials used in the encapsulation method—such as concrete and fire-resistant bricks—take up space and reduce usable area. Furthermore, encapsulation materials are also heavier.

4. Summary

Based on the discussion, the following conclusions can be drawn: 1) The selection of fire protection measures for steel structures must consider the influence of various factors, such as component type, construction difficulty, quality requirements, durability requirements, and economic benefits; 2) A comparison of the spray-applied method and the encapsulation method reveals that the primary advantage of the spray-applied method lies in its simple construction process and the fact that the external shape of the components remains largely unchanged after application. The main advantages of the encapsulation method are its lower cost, as well as its superior fire resistance and durability; 3) Various fire protection measures have their own strengths and weaknesses. In engineering applications, one can leverage their respective strengths and compensate for their weaknesses by combining multiple fire protection measures. Furthermore, by implementing different measures, multiple lines of fire defense can be established.