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Fire - How it Affects Structural Steel Framing

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As forensic engineers, it is often our responsibility to assess the structural integrity of a building after a natural disaster or an event from unfortunate circumstances. It is also our responsibility to determine the necessary repairs that need to be made to the structural framing of the building in order to return it to a safe and functional state. This blog focuses on one of the unfortunate events that we tend to see in our professional lives; fire and how it affects structural steel framing. Within this blog, general fire science will be discussed, along with how the physical properties of steel change due to high temperatures, what happens to structural steel during a fire and after, and evidence that we as forensic engineers need to look for when assessing steel framing after a fire event. This blog is based on the American Institute of Steel Construction (AISC) publication Facts for Steel Buildings – Fire.

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Fire - How it Affects Structural Steel Framing

  1. 1. THE VERTEX COMPANIES, INC. | CONFIDENTIAL 1 Blake Mayberry May 2017
  2. 2. Facts for Steel Buildings General Fire Science THE VERTEX COMPANIES, INC. | CONFIDENTIAL 2 • Incipient/Ignition – Initial or beginning stage of the fire where an ignition source ignites a combustible fuel. • Growth – Includes the initial flame as the heat source. Additional fuel ignites and both the size and heat of the fire increases. • Burning – Temperature and heat flux are so great that all exposed surfaces are burning. • Decay – When the fuel within the compartment is exhausted and the fire dies out.
  3. 3. Facts for Steel Buildings General Fire Science THE VERTEX COMPANIES, INC. | CONFIDENTIAL 3 • Flashover – The abrupt transition from relatively light burning of a small number of combustibles in the room to full-room involvement. In essence, it is a state in which off the combustible contents of the room experience a nearly simultaneous ignition. • Time to Reach Flashover – Depends on the amount and properties of the combustible materials, the volume of the compartment, and ventilation conditions.
  4. 4. Facts for Steel Buildings General Fire Science THE VERTEX COMPANIES, INC. | CONFIDENTIAL 4 At What Temperature and How Long Does a Normal Fire Burn? • Variables for temperature and duration • Amount and configuration of combustibles • Ventilation conditions • Properties of the enclosure/compartment • Weather conditions • Temperature • Office, residential, or retail occupancy • Fully developed = 1,500°F • Could peak at 2,300°F under more severe conditions • Duration • At peak temperature = 10 to 20 minutes
  5. 5. Facts for Steel Buildings General Fire Science 5THE VERTEX COMPANIES, INC. | CONFIDENTIAL How Does Heat Transfer From the Fire to the Surrounding Structure? • Conduction – Heat transfer through solid materials • Density • Specific Heat • Thermal Conductivity • Convection – Heat transfer through fluids • Can be gases or liquids • Radiation – Heat transfer by electromagnetic waves • Based on the gas temperature
  6. 6. Facts for Steel Buildings High Temperature Effects on Steel THE VERTEX COMPANIES, INC. | CONFIDENTIAL 6 • The mechanical properties of steel deteriorate under elevated temperature • Reduction in yield strength, stiffness, and modulus of elasticity • Deflections of many inches, local buckling, and twisting can occur • Based on the graphs • At 1,100°F steel retains approximately 50% of its strength and stiffness • At 1,300°F steel retains approximately 20% of its strength and stiffness • At 2,200°F a near-total depletion of strength occurs • These temperatures are those in the steel specimen and may not be identical to the actual fire temperature.
  7. 7. Facts for Steel Buildings High Temperature Effects on Steel THE VERTEX COMPANIES, INC. | CONFIDENTIAL 7 • Reductions in strength and stiffness are temporary • For temperatures that do not exceed 1,300°F for 20 minutes • More than likely deformed, the steel will regain its pre-fire properties • However, longer exposures will likely result in annealing and reduction of yield strength • How does fire affect steel connections? • Typically more material at the connection – angles, bolts, plates • Typically less exposure to heat from the fire than structural members • Connections will see additional forces during the fire due to the thermal expansion and possible deformations of the steel members • Below temperatures of 1,100°F • The strength of welds are not affected • A325 high strength bolts are not affected • A490 high strength bolts are not affected
  8. 8. Facts for Steel Buildings Strength and Reparability of Steel After a Fire – What Happens to Structural Steel During a Fire? THE VERTEX COMPANIES, INC. | CONFIDENTIAL 8 • Two basic fire exposures • Within the compartment • Exterior to the compartment on outside curtain wall • Temperature of the steel will differ from the fire temperature • Heat transfer mechanism • If steel is protected • Proximity to the fire • Ventilation of the compartment • Steel properties should return upon cooling • 1,300°F for more than 20 minutes
  9. 9. Facts for Steel Buildings Strength and Reparability of Steel After a Fire – What Happens to Structural Steel During a Fire? THE VERTEX COMPANIES, INC. | CONFIDENTIAL 9 • Physical and material property changes • Thermal elongation can occur • Reduction in strength • Reduction in stiffness • Deformations and buckling can occur at 600°F • If completely constrained connections deformation and buckling can occur in the member at 250°F • Typically at 1,300°F buckling of the member will occur • Due to the 50% reduction in strength and stiffness • Increasing end rotations and vertical deflections due to dead and live loads
  10. 10. THE VERTEX COMPANIES, INC. | CONFIDENTIAL 10 • Visible deformations are not by themselves indicators that the steel had a temperature >1,300°F for more that 20 minutes • Change in external appearance can also occur • If < 1,300°F • Tightly adherent mill scale will remain and the color will look normal • If > 1,300°F for more than 20 minutes • Steel surface will become oxidized • Possible pitting • Loss of cross-sectional area thickness • The appearance of significantly “burned” steel is typically light gray or white • Can also take the color of the fuel contents in the room from combustible residue • Paint applied to the steel will typically burn off at around 300°F Facts for Steel Buildings Strength and Reparability of Steel After a Fire – What Happens to Structural Steel During a Fire?
  11. 11. THE VERTEX COMPANIES, INC. | CONFIDENTIAL 11 • The steel will contract as its temperature decreases • If inelastic deformations occurred during the fire • The steel members will experience “permanent set” and will not return to its original shape • Failure can still occur in structural members and connections • Due to permanent set, residual loads from the fire, dead/live loads • Typically fail in tension • Not exposed to temperatures >1,300°F for more than 20 minutes • Acceptable/possibly better metallurgical properties upon cooling Facts for Steel Buildings Strength and Reparability of Steel After a Fire – What Happens to Structural Steel During a Fire?
  12. 12. THE VERTEX COMPANIES, INC. | CONFIDENTIAL 12 Facts for Steel Buildings Strength and Reparability of Steel After a Fire – What Happens to Structural Steel During a Fire? • AISC recommends the structure to be assessed into three categories: Category 1 Straight members that appear to be unaffected by the fire, including those that have slight distortions that are not easily visually observable Category 2 Members that are noticeably deformed but that could be heat straightened, if economically justified Category 3 Members that are so severely deformed that repair would be economically unfeasible when compared to the cost of replacement
  13. 13. For More Information THE VERTEX COMPANIES, INC. | CONFIDENTIAL 13 Blake Mayberry 919.238.4356 (ph) bmayberry@vertexeng.com

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