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The fire resistance of the structure is very important to the safety of the building. The CPD, sponsored by Steel for Life, outlines the fire performance of steel, including key standards and fire resistance period 

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Intumescent paint coating on honeycomb beam

Many people know the steel structure in the fire. Rigorous testing, including full-scale building testing in the late 1990s, has led to the development of robust modeling and analysis techniques that require continuous improvement. For a typical multi-storey commercial building, fire protection accounts for about 10-15% of the cost of the steel frame.

This CPD introduces the fire protection of new steel structures. It will outline the performance of steel in terms of fire protection, legislation and standards related to fire resistance, fire resistance period and fire protection options.

In the late 1990s, at BRE's Cardington facility, a series of comprehensive fire tests were conducted on an eight-story steel frame building with a composite steel deck floor. Cardington fire test and observations of other large buildings

The fire showed that the performance of the composite steel deck floor plays a vital role in providing inherent fire resistance compared to the testing of isolated structural elements. Fire tests have shown that the board acts as a membrane supported by the perimeter beams and protected columns. 

Figure 1: This figure shows the effect of temperature on the strength of steel. Fire protection must enable the steel structure to maintain at least 60% of its strength at room temperature. The fire protection calculation of steel is usually based on a limit temperature of 550°C, when all four sides of the steel structure are exposed, and 620°C, when the fully loaded beam supports the concrete floor.

All hot-rolled structural steel profiles have a certain inherent fire resistance. This is a function of the size of the section, the degree of fire exposure, and the load carried by the section. The fire resistance is usually related to the structural part's ability to withstand the standard fire resistance test described in BS 476-20, BS ISO 834 and BS EN 1363-1. The test is performed in an approved furnace and follows a standard time-temperature curve that is the same for all materials.

Although the strength of steel decreases with increasing temperature, the decrease in strength has been quantified by standard fire tests, as shown in Figure 1 (above). For the partial protection of very large hot rolled profiles, light loads and upper flange concrete floor slabs, the inherent fire resistance can be as high as 50 minutes without additional protection. If the inherent fire resistance of the steel is lower than the fire resistance required to meet the structural stability regulations of the building, additional precautions must be taken. This usually takes the form of applying fire protection, insulating steel from elevated temperatures, and/or fire protection engineering solutions.

The building regulations stipulate that those who design and construct buildings have an obligation to ensure their safety and health. These are decentralized, so they are different in England, Wales, Northern Ireland and Scotland. Regulatory requirements are presented in a functional way—that is, they outline what the requirements are, not how to implement them.

Requirement B3(1) of the British Building Code stipulates: "The design and construction of buildings should ensure that in the event of a fire, their stability can be maintained for a reasonable period of time." 

Each decentralized UK government will issue guidance documents on how to comply with building codes. In the UK, the relevant document for fire protection is Approval Document B. It contains detailed information on structural fire resistance requirements to meet the designer's obligation to structural stability. For example, office buildings above 30m require 120 minutes for fire-resistant and sprinkler systems. An unsprinkled prefabricated building with a height of 18-30m requires 90 minutes to resist fire. 

In recent years, as a result of extensive research on the nature of fire, its propagation methods and the risk factors of building fires, BSI has issued BS 9999, "Code of Practice for Fire Safety in Building Design, Management, and Use." The purpose of this document is to provide a more transparent and flexible fire protection design method by using a structured risk approach. In many cases, using BS 9999 will result in a safer fire solution than using government publications. 

One of the most obvious changes in BS 9999 is the structural fire resistance requirements, as shown in Table 1 (below).

Open office building, two floors, <1000 square meters ground floor area

Open office building, 30-60m high 

Department store, 11-18m high

Warehouse building, medium risk, four floors

Leisure center, two floors 

The fire resistance requirements of buildings and their frames are defined in terms of fire resistance time and expressed in minutes (15, 30, 45, 60, 75, 90 or 120 minutes). 

It is important to note that fire resistance is not the length of time a structure may survive an actual fire, but a standard metric that compares the performance of different designs in a consistent manner. In a real fire, once the combustible material or fire load is consumed, the fire decays and/or moves. In a standard fire test, the temperature rises rapidly and indefinitely, so the situation is much more serious. 

The fire resistance test result is expressed as failure time according to one or more of the three standards of the product/component under consideration. these are:

In England, the minimum fire resistance time varies depending on the purpose and height of the building. The minimum fire resistance time for an open parking lot is 15 minutes; for offices with a height of more than 30m and sprinklers, the time is 120 minutes.

The exterior area of ​​the Leadenhall building uses epoxy intumescent paint to improve durability

Unprotected steel structures are generally considered to have an inherent fire resistance of 15 minutes. For higher fire resistance periods, fire protection is usually required. Passive fire protection materials protect steel structures from high temperatures (this is in contrast to active fire protection systems (such as sprinkler systems)). Passive fire protection materials can be divided into two categories: reactive, in which intumescent film coatings are the most common example, and non-reactive, the most common being panels and spray coatings. 

Intumescent film coatings usually have three components: primer, primer and sealer. The primer is the part that reacts in a fire and usually contains the following ingredients:

They are mainly used in buildings with fire resistance requirements of 30, 60 and 90 minutes, although some products can provide 120 minutes. Coating can be carried out on-site or off-site. They can be water-based or solvent-based, depending on the intended use of the structure. Both can be used to obtain attractive surface finishes. The film expansion material can easily cover complex shapes, and the later protection service installation is relatively simple.

Panels are widely used in the UK for structural fire protection, regardless of whether the protection system is full view or hidden. They can be applied to unpainted steel structures and provide a clean boxed appearance for the specifier. The application is a dry transaction, so it is unlikely to interfere with other transactions on site, and since the board is manufactured in the factory, the thickness can be guaranteed. 

Circuit board protection is roughly divided into two categories: lightweight and heavyweight. Lightweight panels are usually 150-250kg/m³, which is usually not suitable for decorative finishes. They are cheaper than their heavyweight counterparts and are usually used where aesthetics are not important. Heavyweight wood boards are usually 700-950kg/m³, and decorative finishes are usually accepted. Both types of circuit boards can be used under limited external conditions, but the manufacturer’s recommendations should be sought. 

Spray protection is widely used in the United States, but less common in the United Kingdom. It can be used to cover complex shapes and details. The cost will not increase significantly with the increase in thickness, because most of the application costs are in labor and equipment. Some materials can also be used for exterior and hydrocarbon fire applications. The disadvantage is that spray is not suitable for aesthetic purposes, and since the application is wet trading, this may affect other field operations. To prevent overspraying, it may be necessary to provide cost subsidies. 

The flexible carpet system was developed to meet the demand for easy-to-apply fireproof materials that can be used for complex shapes and details, but the application is a dry transaction. The number of manufacturers of these products is limited. 

Until the late 1970s, concrete was by far the most common form of fire protection for steel structures. However, the introduction of lightweight proprietary systems, such as sheet, spray and thin-film intumescent coatings, has drastically reduced its usage. Occasionally, traditional methods are used, such as block packaging. Concrete is often used where resistance to impact damage, abrasion, and weather exposure is important-such as in warehouses, underground parking lots or external structures. The main disadvantages are higher cost, higher space utilization compared with light-weight systems (because the large protective thickness will take up valuable space around the column) and weight.

The Professional Fire Protection Association provides detailed guidelines on the installation of paint, panel and spray protection systems. BRE publishes information on the thickness of the concrete shell during a specific fire resistance period. It can also be found in BS EN 1994-1-2.

Fire protection engineering solutions, including wood planks and unprotected secondary steel structures

Fire safety engineering can be viewed as a set of measures designed to obtain the greatest benefit from the methods available to prevent, control or limit the consequences of fire. 

The simplest form of structural fire protection engineering is to use codes to design individual building elements. It has also developed structural fire protection engineering technology that uses simplified methods to assemble building elements, and engineers hardly need professional knowledge. These can be used to design simplified sub-component models and use the understanding of the behavior of composite structures in fire from the Cardington fire test.

Using these models, combining the residual strength of steel composite beams with the strength of the floor slab in a fire, allows designers to leave a large number of secondary beams unprotected in buildings that require 30-120 minutes of fire resistance, thereby saving significant costs , Although some compensation functions may be required, such as adding reinforcement in the slab.

UBM's CPD distance learning program is open to anyone seeking to develop knowledge and skills. Each module also provides an opportunity for members of professional institutions to earn 30 to 90 minutes of credits to meet their annual CPD requirements.

This article has been recognized by the CPD certification service agency. To earn CPD credits, read the article, then click the link below to complete your details and answer the questions. You will receive the results immediately, and if all questions are answered correctly, you will be able to download your CPD certificate immediately.

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