Structural Fire Protection

Introduction
Modern building codes specify minimum fire resistance requirements based on studies by fire protection engineers. The fire resistance rating is expressed as the number of hours a structural assembly is able to withstand when exposed to a standard time temperature test, ASTM E119, before the first critical point in its behavior is reached. Steel begins to lose its stiffness at temperatures in the neighborhood of 538oC (1000oF). ASTM E119 standard fire test requires that average temperature readings do not exceed 538oC (1000oF) for columns, and 593oC (1100oF) for beams. Individual readings must not exceed 538oC (1000oF) for columns and 649oC (1200oF) for beams. A fire exposure of a severity and duration sufficient to raise the temperature of steel above the fire criteria temperature will seriously impair the steel’s ability to sustain loads above unit stresses or plasticity load factors permitted by the AISC specifications. Under such an exposure, the members, upon which the stability of the structure depends, should be protected by fire resistant materials or systems. These must be capable of holding the average temperature of the steel below the limits specified for the fire test standard: Fire Protection Methods and Materials.

A wide range of products and systems are available to detect and protect structural steelwork from fire.

They are divided into three major categories;

• Passive Fire Protection
• Fixed Fire Protection
• Fire Detection and Alarm

To select the appropriate system(s) for fire protection and detection, the following factors should be considered:

• Appearance
• Mechanical Durability
• Compatibility with the Environment
• Compatibility with any Corrosion Protection System
• Space Requirements
• Construction Program
• Application or Fixing Rate
• Cost

This section is intended to educate the reader about the available fire protection systems. For further details, you need to contact fire protection specialists directly.

Passive Fire Protection Systems
In recent years passive fire protection systems have been extended to incorporate the fire protection of structural steel beams, columns, floor slabs and other building components such as firewalls, wall linings, partition walls and ducting systems.

All passive products comply with BS 476 part 22 or equivalent. The fire resistance varies from half an hour up to four hours depending upon the type of product and its application. The main passive fire protection systems are: boarded systems, intumescent products, spray applied systems and concrete encasements.

Boarded Systems 
These are pre-formed boards usually made from gypsum, mineral fibers or naturally occurring plate like materials such as vermiculite and mica using cement and/or silicate binders.

The thickness of such boards range from 6 to 80 mm depending on the material they are made of. Their fire resistance ranges from one hour to four hours.

Boards may be fixed to steelwork using mechanical methods, (screws, straps and/or galvanized angles) or they may be glued and pinned. A box configuration is generally used, except for very deep sections (above 800 mm) where it is advisable to have the board fire protection follow the section profile.

Boarded systems are particularly suitable for the protection of columns where smooth surfaces are required to achieve a neat appearance. They provide the most suitable finish for accommodating other trades.

Intumescent products   
Intumescent products are designed to seal bulkheads, small gaps or airline cracks where mechanical and electrical utilities pass from one area of a building to another. The products range from intumescent coating, applied to a substrate as a thin film, to mastic and pipe collars.

At ambient temperature the ingredients are perfectly stable and non-reactive. When the temperature increases (usually above 200oC) the intumescent ingredients undergo a chemical reaction and produce an expanded char layer sometimes fifty times thicker than the original film thickness. This char has low thermal conductivity and good insulating characteristics, which produce good thermal protection to the substrate.

 

Spray Applied Systems 
These are lightweight fire protection materials that are divided into two main classes:

• Based on vermiculite or perlite plus a binder, often cement.
• Based on mineral fibers such as rockwool.

This form of protection is generally applied directly to the steel surface and follows its profile. In some situations it is applied to an expanded steel lathing to form a hollow box protection. Many of the products designed for internal usage can be applied directly to unpainted steel.

However, only few materials are suitable for use in externally exposed situations where it is necessary to provide the steel with an adequate corrosion protection. Depending on their thickness, such products can achieve up to four hours of fire resistance. Some require mesh reinforcement to achieve longer periods of fire resistance. Spray applied systems provide the cheapest and fastest method of protection and are particularly suitable for protection of beams which are generally concealed by suspended ceilings.

Concrete Encasements 
One of the advantages of concrete is that it is a fire resistant material. When subjected to high temperatures it slowly releases its retained water. The temperature on the concrete face opposite to the fire does not exceed 100oC until all the water in the concrete has been released.

The selection of aggregate is critical to the fire resistance of concrete. Aggregate containing 60% or more of quartz, chert or granite is not as fire resistant as those containing limestone. Therefore, concrete with such aggregate must be increased in thickness to obtain a comparable fire resistance. The use of lightweight aggregates instead of stone greatly improves the concrete’s fire resistance.

Concrete with at least 60% limestone, shale or trap rock aggregates can provide up to 4 hours of fire resistance with only 50 mm thick encasement thickness. Due to its heavy weight concrete is mainly used to fire protect columns although it is widely used to protect steel floors and roof decks.

Fixed fire protection systems installed in buildings or structures are activated once any signs of fire appear. Their main purpose is to extinguish fire at its early stages. There are several fixed fire protection systems of which the three most popular are: sprinkler systems, gaseous extinguishing systems and foam systems.

 

Sprinkler Systems 
Sprinklers have long been acknowledged as the first line of attack in a fire situation. Some records show that, in fully sprinklered premises, 99% of all fires have been controlled by sprinkler systems.

The reason sprinklers are so effective is because they attack the seat of the fire before it has time to spread. There is no excessive use of water to extinguish the fire; 65% of fires in sprinklered locations have been controlled by five or fewer heads.

Gaseous Extinguishing System 
There will always be risk areas where there is a need to extinguish a fire at the earliest possible stage, to minimize damage to extremely valuable or sensitive equipment, and without risk to personnel. In such instances the use of a gaseous extinguishing system is a good choice.

One type of gaseous extinguishing systems is CO2 systems. Although carbon dioxide fire protection is not new, its unique fire protection benefits have enabled it not only to survive, but evolve over the years. Today’s CO2 systems utilize sophisticated detectors, hi-tech control panels and computer aided design of agent pipe network.

Gaseous systems are normally used in computer rooms, control rooms and switch-gear rooms, etc.

Foam Systems 
Low, medium or high expansion foam is now available for any industry that involves the use of highly flammable liquids that could be subject to a variety of intense fires.

When mixed with proper amounts of fresh or salt water and air in foam hardware, the system produces foam bubbles that are light enough to float on the surface of hydrocarbon fuels.

Fire fighting foam extinguishes and secures fuels in three stages. It creates a foam blanket, which deprives the fire of oxygen, it significantly reduces vaporization by up to 95% and the water content helps cool the fuel.

The latest foam systems have a long storage life. They normally are non-corrosive to most common construction materials and have minimal effect on the environment. Foam systems have a proven track record in protecting hazards in buildings such as process areas and aircraft hangers.

Fire Detection and Alarm Systems
The installation of fire detection and alarm systems is essential especially in buildings that contain a large number of people at one time such as offices, hospitals, shopping centers, factories, etc., to help in immediate and speedy evacuation of the building.

Generally, a fire detection and control system is temperature sensitive. It will respond to moderate temperature changes in the building. Such systems are normally designed so as not to respond to gradual increase in temperature, seasonal changes in ambient temperature, arc lights, welding torches, and infrared light or ultraviolet rays.

Furthermore, different alarm and communication systems are available and can be used depending on the type and function of the building. Such systems include voice alarm systems that provide manually or automatically, relayed instructions to aid evacuation; public address systems that provide general paging and announced facilities including music and speech reinforcement; and integrated systems used to complement and operate with fire detection systems.