Damage Due to Wildfires
In recent years, wildfires have become a major concern. During a fire event, the mechanical properties of steel deteriorate under elevated temperatures. A reduction in yield strength, stiffness, and modulus of elasticity can occur. Deflections, local buckling, and twisting of steel members can also occur. Fire hazards in construction and T&D applications can result in significant safety in HCA, economic, and asset losses.
It should be noted that AISC, DOE, DOD and other similar organizations require metallurgical assessment of structures exposed to fire. It is also recommended to preform fitness to service and fire damage assessment for T&D structures after exposure to wildfires.
WILDFIRE EFFECT ON STEEL STRUCTURES
Both physical and mechanical properties can be significantly affected due to wildfire event. Based on the temperature and the duration of exposure, physical properties such as thermal conductivity, electrical conductivity, the coefficient of thermal expansion could be affected. Moreover, mechanical properties such as tensile strength, yield strength, ductility, hardness, and toughness could also be affected. In addition, permanent changes in the microstructure could take place.
DEFORMATION AND CREEP
Tensile strength of steel is always measured at room temperature, approximately +70 degrees Fahrenheit. However, as the temperature of steel is increased the tensile strength decreases. Therefore, at elevated temperatures a steel structure can deform under stress and its own weight or possibly even collapse. Deformation may cause cracking of the structure at bolt holes and welds.
Elevated temperatures associated with a fire can cause the steel to soften and permanently remain soft with lower tensile strength values. This may require field inspections to measure hardness and engineering calculations to re-rate the load carrying capacity of the tower. The degree of softening will depend upon the temperature reached, the time at temperature, and the cooling rate. Slow cooling rates generally result in softening
Elevated temperatures cause the steel to become harder with higher tensile strengths but lower impact strength. Harder, more brittle conditions can be achieved if the temperature exceeds the lower critical temperature of 1320 degrees Fahrenheit and is cooled rapidly by water or precipitation. The degree of hardening is dependent upon the carbon content and the alloying content of the steel. Temperatures above 1320 degrees Fahrenheit for a prolonged time can lead to grain growth which will negatively affect mechanical properties. Should lower portions of the structure be covered in vegetation at the time of the fire, and if the steel temperature exceeds the lower critical temperature of 1320 degrees Fahrenheit, carburization of the steel can occur. Carburization can cause a greater degree of hardening upon quenching and will increase the brittleness of the steel.
LIQUID METAL EMBRITTLEMENT
Galvanized steel is steel which has been coated with zinc. The melting point of zinc is 787 degrees Fahrenheit. Temperatures exceeding the melting point of zinc will cause a loss of the galvanized coating and can cause liquid metal embrittlement when liquid zinc penetrates the steel surface. Stainless steel is also affected by zinc liquid metal embrittlement
Organic coating can be charred or consumed entirely by fire
Here are some thoughts to consider for Condition Assessment and Wildfire Materials Assessment of T&D Structures.