According to the National Fire Protection Association1, there were over 1,600,000 fires reported in the United States in 2005. This number indicates that, on average, one structure catches fire every 62 seconds. Fires in 2005 caused 3763 deaths and $10.7 billion in property damage. Many of these fires are initiated by explosions, which are often due to ignition of a dust cloud or gas leak, unintended/uncontrolled chemical reaction in an industrial plant, or a malicious act. The air-pressure wave resulting from an explosion, and the intense heat generated by a subsequent fire, produce extreme loads that often badly damage buildings and other structures.
Investigation of structures that have been subjected to explosive pressure and/or intense heat requires knowledge of both structural mechanics and material science. Different construction materials respond differently to high temperature. Structural steel begins to soften at about 200°C and begins to weaken at about 400°C, but usually regains virtually all of its strength on cooling. Concrete, on the other hand, begins to lose strength at about 300°C, and by 600°C, has little residual strength. Although timber already begins to lose some strength at 100°C and typically ignites by 300°C, char protects the interior material, providing time for occupants to exit the structure prior to collapse. In addition to thermal damage to the material itself, the expansion associated with heating a structure can cause significant distortion and damage, even in areas distant from the worst heat.
Pressure loads on structures and components due to rapidly expanding gas associated with explosions result in damage mechanisms that can be much different from those due to wind, snow, and earthquake (for which structures are typically designed). Explosion damage mechanisms depend on the magnitude of the pressure spike, the shape and duration of the spike, the potential for venting of confined spaces, possible complex reflections of the blast wave, and the dynamic properties of the components and materials. Understanding these mechanisms is important for investigating the true extent of the damage. In addition, estimates of the pressure pulse required to generate the observed damage can be useful for identifying potential ignition location, blast characteristics, and fuel source.
Exponent has investigated and analyzed thousands of fires and explosions, ranging from high-profile disasters at major industrial facilities to insurance claims for fires at single-family homes. Our structural engineers often work side-by-side with our fire protection engineers and thermal scientists to gain a complete understanding of the source and nature of the fire or explosion, the damage mechanisms and extent of damage to the remaining facility, and the most appropriate repairs. Knowledge gained from these investigations has also been useful to clients interested in mitigating the risk of future explosions or fire.
Our services include:
- Fast response to determine what areas are safe for investigations and what measures are required to stabilize the site
- Evaluation of the nature and extent of the damage
- Repair of structural fire and explosion damage
- In cooperation with our Thermal Sciences practice, we provide fire cause, origin, and propagation analysis; fire protection engineering and explosion and detonation investigations; and smoke and plume propagation modeling