Steel does not burn in case of fire. Structural steel melts at 1,500 deg C, often there is a misconception that structural steel would fail when it reaches its melting point. It is very rare that fires would be able to reach those temperatures especially in cellulosic fires (unless steel was under direct flame exposure in very specific cases). However, heating causes structural steel to loose it’s load bearing capacity and will cause steel to expand. These effects play a significant role in the failure of structural steel buildings when exposed to fire.
Expansion of structural steel would lead to additional stresses on the steel element it self and the connection joints. Additional stresses such as shear on bolts would cause failure. Failure in connection bolts would lead to increased load on other structural elements or failure in floors that would increase the load on the subsequent floor when the upper floor or portion of the floor collapses [1].
Additional to the thermal expansion that increases the stresses on structural steel. Heating steel would lead to the reduction of it’s load baring capacity. This would increase the probability of failure of an element. When the steel is heated to 538 deg C (1000 deg F) it would lose approximately 50% of it’s load baring capacity. This point is the failure point in fire resistance rating tests such as ASTM E119 [2]. Figure below shows the relation between the temperature of steel and the yield strength [1].
As structural steel is heated the load bearing capacity decreases, and the load is translated into other elements. These effects would lead steel to be uncapable of taking the torsional, compression or tension loads and fail. In combination with thermal expansion effects this will lead a building or a portion of the structural steel to collapse.
The above effects are directly related to the heating of the structural steel. To protect (or delay) the failure of structural steel it is insulated to delay the steel of reaching the critical temperatures that would cause them to fail. ASTM E119, is one of the standardized tests for structural steel it assumes failure at 1000 deg F at which fully loaded columns are typically found to fail [2]. In BS 5950 : part 8, the structural steel limiting temperature (failure temperature) is 550 deg C for 4 sided columns, 520 deg C for hollow sections and 620 deg C for 3-sided beam with concrete floor on top flange [3].
The BS 476 part 21 and ASTM E119 standard tests provide a standard method of determining the time required for structural steel to reach the limiting temperature under ISO 834 fire curve. Both tests have very similar testing performance, however, have slightly different temperature curves due differences in testing apparatus for example type of thermocouples. Both tests would provide a set of limitations for the section sizes tested, type of fire protection material and it’s thickness and manufacturer (brand and model). This is very important and will be detailed further in the following paragraphs.
Not all structural steel would heat similarly. From high school physics, one of the material properties is heat capacitance which is the amount of energy required in joules to elevated the temperature of 1 kilogram (kg) of material by 1 deg C (or 1 K). This will mean that the more mass we have the longer it would take for the same fire to increase it’s temperature. Another aspect is related to the science of fire and heat transfer. Fire increases the temperature of air and transfers heat to structures and materials by either convection, radiation and material to material conduction. The heat transfer is usually represented in a heat flux (kW/m2) or in other terms (energy per time per surface area of material). This will mean that the larger the surface area of the steel the faster it would heat up.
In BS 476, this is represented in the surface area over volume ratio (A/V) or Heated perimeter of the section over the cross section area (Hp/A), while in ASTM E119, this is represented by Weight in pounds over Heated perimeter in inches (W/D) factor for I sections and Cross section area over heated perimeter (A/P) for hollow sections. The higher the AV, Hp/A, factor the thicker insulation it should receive to protect it from fire exposure, while it is the opposite for ASTM the lower the W/D or A/P factors the thicker insulation it should receive to protect it from fire exposure. These tests serve as a standardized test to allow testing and certification of different material types from different manufacturers to achieve the same level of protection. This also allows specifiers to specify a certain level of protection to achieve either prescriptive of performance based codes. The actual fire can either be less or more sever than the testing conditions and the hourly rating is specific to the conditions in the standardized test and cannot be compared to a real fire.
Methods for protecting structural steel:
Often the word “fireproofing” is mentioned as a commercial term for providing structural steel with an hourly rating of fire resistance. This term is scientifically not accurate, as there is nothing fireproof. Products under fireproofing category will provide steel with a level of fire resistance to failure to allow safe evacuation, and fire suppression activities. There are three main methods for protecting structural steel, which are as follows:
Intumescent Paint:
Intumescent paint expands under the influence of heat. Typically, fifty times more than it’s original thickness insulating the steel with a char layer. Intumescent paint would require intense heat to start the reaction when temperature of the paint is at 200 deg C or above. The intumescent reaction would typically need steel plates thicker than 4mm. Depending on the Hp/A, W/D,..etc factors manufacturers test their intumescent paint to a range of steel factors. Not all intumescent paints are capable of protecting your project. Stakeholders, consultants, engineers, should ensure that the product specified is capable of protecting the full range of your structural steel factors at the required fire resistance rating to the standard limiting temperature as discussed in this article earlier. Different types of intumescent paint with different bases such as water based, solvent based, epoxy intumescent. Each type would have advantages and disadvantages and different service capabilities in terms of environmental exposure. However, the topic of choosing the right intumescent paint will require a full article thus, it will be discussed in a separate article in the near future.
Cementitious Spray or Spray Applied Fire Resistive Material (SFRM):
Cementitious fireproofing insulates the steel by water retention and insulating capabilities of materials in the mix such as vermiculite. Cementitious is typically a dry mix bag and is mixed and pumped on site with a concrete plaster spray pump. The thicknesses are determined similarly to intumescent paint by utilizing third party lab testing that would provide the steel factor vs. the dry film thickness and the critical failure temperature. Same precautions should be taken into consideration when specifying or using an SFRM product. Cementitious fireproofing products can be Protland cement or Gypsum based. They come in different densities ranging form low, medium to high density. The correct method of choosing the product would be discussed further in a separate article.
Fire Boarding and Wraps:
Fire boarding and fire wraps are one of the ways of protecting structural steel. However, careful attention should be taken into account when using them. Some examples are calcium silicate boards, magnesium oxide boards, and endothermic matts. The Hp would be different as the perimeter exposed to fire will no longer be the steel cross section rather the boarding perimeter.
Author: Moath Quraini, Technical and Commercial Manager at Passifire.
Author Background: Bachelor of Science in Mechanical Engineering, Masters of Engineering in Mechanical Engineering, Graduate Certificate in Fire Protection Engineering, Master of Engineering in Fire Protection Engineering.
Contact: moath@passifire-sa.com
References:
[1] http://911research.wtc7.net/~nin11evi/911research/mirrors/guardian2/fire/SLamont.htm
[2] Hurley, M. J. (2016). SFPE handbook of fire protection engineering (5th ed.), NY: Springer.
[3] ISO 834-10:2014 Fire resistance tests — Elements of building construction — Part 10: Specific requirements to determine the contribution of applied fire protection materials to structural steel elements.
[4] BS 5950: part 8: Code of Practice for Fire Resistant Design
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