For all materials, structural performance under fire conditions is a crucial design factor. When temperatures are high, steel loses strength, hence structural steelwork will typically need to be protected if the resistance of an unprotected steel member cannot be validated. In order to offer a practical answer, fire protective materials have been developed over the years. Parallel to this, extensive research into how structures operate when exposed to high temperatures resulted in the development of comprehensive design guidelines like ANSI/AISC 303-22.

Steel in Construction

One of the two main materials used during civil engineering constructions, steel is without a doubt the industry leader in structural materials. Important elements of a building are optimal resource usage, low cost, and well-organized structural systems. Creating tactics to better comprehend and handle the consequences of building materials is of great importance. Steel buildings are reusable and recyclable. One such building material that has this special quality is steel. Steel is easily recovered from other building materials and it can be recycled endlessly without losing quality.

In recent decades, steel constructions, like large-scale structures, skyscrapers, and bridges, have become prevalent in civil engineering. As technology and economies move toward sustainability, steel buildings are becoming increasingly common in residential buildings and the fabrication industry. Modeling and testing are the primary methods used to ascertain the behaviors of steel structures, including its seismic performance, ductility and bearing capacity.

Benefits of Steel in Construction

• Being Adaptable and Flexible

Long-span steel pieces can be utilized to build sizable, adaptable areas, giving them a very wide range of possible uses. Temporary or flexible partitioning solutions make it simple to split these rooms, and they can be adjusted later when the needs of the building change with little expense, interruption, or demolition. Depending on the stresses involved and the desired outcome, steel is also available in a variety of sizes and weights that make it appropriate for a wide range of uses in building.

• Construction Speed

The rapidity of construction, particularly where there is a potential for interruption to neighboring roads and structures, is one of the primary arguments for using structural steel. Erection, which is the process of connecting the structural pieces together on site, is a very simple procedure because steel is built to accurate measurements. Numerous cost savings can be gained by reducing the negative impact on the neighborhood and speeding up the development process.

• Sustainable in Nature

Steel offers many advantages for the environment, including being completely recyclable. The projected recovery rate of recycled steel from steel constructions in the US is 96%, and recycled steel makes about 50% of the world’s new steel output. Steel constructions that have been prefabricated and are extremely exact also enable simple, waste-free assembly on site, cutting down on both the time it takes to complete a project and the quantity of materials dumped in landfills.

Fire Safety in Construction

When a fuel and oxygen interact chemically, a succession of incredibly fast reactions take place that release heat and light, resulting in fire. The "fire triangle"—the presence of oxygen, heat, and a fuel source—is necessary for combustion to take place. Combustion manifests itself visually as flames.

Buildings need to be constructed in a way that minimizes heat and smoke concerns while providing an adequate level of fire protection. In addition to protecting contents and making sure that as much of a building can keep functioning after a fire and that it can be repaired, the main goal is to lower to within acceptable limits the possibility of death or injury to building occupants and others who could become involved, such as the rescuers and firefighters. Along with potential environmental damage, it’s also important to take into account the risk to neighboring houses.

Exposure of Structural Steel Members to Fire: During and After

During a fire occurrence, changes to the structural steel framing’s physical and material properties occur. Thermal elongation, strength decrease, and a drop in stiffness can occur. At 600°F, buckling and deformations are possible due to the property changes. Deformation and buckling can happen as low as 250°F if a beam or girder used in structural steel framing is entirely limited at its connections. However, due to the 50% drop in strength and stiffness, buckling of the member often happens at 1,300°F.

After exposure, the steel frame framing will start to constrict to its original shape as the fire event moves into the Decay Stage and the temperature drops. The steel members, however, will undergo “permanent set” and won’t revert to their original shape if inelastic deformations happened during the fire. Even though the fire has been put out, structural connections and steel members are still susceptible to failure. The combination of the permanent set, fire-related loads still present after the fire has been extinguished, along with the dead and live loads for which the member or connection was initially intended, might cause the members or connections to collapse under tension.

Effects of Fire on Unprotected Steel Structures

If the temperature does not rise above 1330°F, structural steel can almost completely regain its pre-heated qualities. The majority of structural steel is hot-rolled, low carbon steel with varied strengths. As the temperature rises, these steels’ characteristics alter. As the temperature of the steel rises, the following things can happen:

• Corrosion

Once the steel surfaces are exposed, surface corrosion will develop. The chemical reaction of oxidation, which is temperature-dependent, causes rust to form on steel. Steel can undergo thermally accelerated oxidation when exposed to high temperatures in a fire.

• Deformation of Steel Members

Steel expands as a result of thermal expansion as temperatures rise. These alterations are transient unless additional causes are present. When steel is constrained during a fire, internal stresses may be created that could lead to buckles or other long-lasting deformations in the steel components.

• Strength Reduction

The strength of the steel reduces as the temperature rises above room temperature:

1. Carbon steel retains about 90% of its yield strength at 400°F.

2. It keeps around 60% of its yield strength at a temperature of 800 °F.

3. About half of the yield strength of carbon steel is lost at 1050 °F.

   High temperatures frequently only temporarily reduce strength.

Fire Protection Systems For Steel Structures

Systems for preventing fire from spreading through steel buildings are intended to do so for a predetermined period of time. There are numerous fire protection methods that can be deployed. Designers specify fire protection measures.

There are two fire protection systems namely; Passive and Active systems. The following list includes various passive fire protection systems commonly utilized with steel constructions:

Board and Blanket System

This kind of fire prevention system is popular due to a number of factors, including the fact that it is hygienic, economical, water-resistant, and adaptable to steel parts that are not painted. Furthermore, in addition to being sturdy and long-lasting, it also works with decorative treatments.

Steel columns and beams with uniform forms can use a boarded fire prevention system. Depending on the material used to produce the board and the required fire rating, the thickness will vary. Boards may be made from a variety of materials, including calcium silicate, gypsum plaster, mineral fiberboard with resin or gypsum, and maybe light fillers like vermiculite.

Additionally, there are two basic categories of boarded fire prevention systems: heavy weight boards and lightweight boards. The former is appropriate in situations where aesthetic view is a top priority because it blends well with aesthetically pleasing finishes.

Regarding blanket fire protection systems, they are utilized for steel elements, such as truss members, whose irregular geometries prevent them from being safeguarded by board fire protection systems.

Intumescent Coating Steel Structure Fire Protection System

Intumescent coating is a type of paint that is inactive at room temperature but becomes active as the temperature rises to between 200 and 250C. Painting that is intumescent goes through intricate chemical processes and expands significantly when it activates.

The intumescent coating’s swelling could cause it to thicken up to fifty times more than it did initially. This added thickness would offer fire protection and up to two hours of fire resistance. The steel's characteristics would not change at this temperature, therefore the steel structure is satisfactorily protected by the fire protection system.

The benefits of intumescent fire protection systems are their ease of use, durability, ability to cover intricate shapes and design details, rapid application to the surface of steel elements, and ability to be repaired and maintained.

However, drawbacks include high expense, the requirement for extensive care and effort to maintain good quality coating, and the risk of damage if it is placed off-site.

Both building sites and non-construction sites can use this kind of protective system. The former is utilized to get aesthetic views, whereas the latter is frequently taken into account if aesthetic appearance is not a top priority.

Sprayed Cementitious or Gypsum Based Coatings

Gypsum or cement-based materials are used to create sprayed coatings. They are also combined with lightweight aggregate. This fire protection system has a very low price and offers complete fire protection for up to 4 hours. Additionally, it is suited for intricate steel parts and structural elements like bolts and connections and can be applied fast in addition to being dependable and effective. It cannot, however, sustain mechanical harm due to a lack of strength. The project’s timing and expense may be impacted by the messy and wet spray application method.

Active fire protection strategies involve taking immediate, physical action to slow the spread of fire or smoke. Most frequently, these systems are smoke control and fire sprinkler systems that respond to both manual and automatic signals to carry out their intended duty.

Active fire defense also includes fire alarm systems. They are used to activate extinguishing systems or to alert building inhabitants and the fire service when they detect smoke or heat, which is how they typically detect the presence of fire.

To put out or control the fire, extinguishing systems like fire sprinklers are used. Typically, smoke control systems are meant to prevent the spread of smoke, keeping exit routes usable for a predetermined amount of time. The building's occupants are alerted by the fire alarm, which also alerts the emergency services that are responding to the alarm. This is done through the activation of the fire sprinkler system as well as manual or automatic detection devices.

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