Fire following earthquake (FFE) events are cascading hazards that have caused great damage in the past and pose a continued threat to our communities. The increased risk for fire after a seismic event stems from potential damage to utilities such as gas or fuel pipelines, which can lead to ignition. Additionally, damage to water utilities can limit or prevent fire suppression efforts. Earthquake hazard studies performed to evaluate risks to Portland, OR and the California Bay Area found that seismic events in the areas could lead to damaging fires. The behavior of the lateral force resisting system and gravity system in a structure during a fire is critical to life safety. Damage to either of these systems could lead to loss of compartmentalization of a fire, which would allow further spread horizontally or vertically through a structure.
In order to model and observe FFE structural performance, the authors developed modeling methodologies in the open-source finite element (FE) program, OpenSees. The methodologies were developed through a series of validation studies, benchmarking the results of OpenSees against experimental testing and against another FE program. The validation studies spanned from simple to complex 2D thermal-mechanical modeling techniques. These validation studies were then made available to the public on the OpenSees documentation website. The benchmarked methodologies were applied to a 2D steel moment resisting frame (MRF) to model FFE hazards through two investigations.
The first FFE investigation examined the effects of three compartment fire locations and two fuel load levels (high and low). Results of the FFE analysis were compared to a fire-only hazard, investigating the influence of fire following earthquake would have on the steel moment frames behavior. The first study was also completed to identify vulnerabilities for further study in the second FFE investigation. Damage was greater in the first compartment fire location and was investigated further in the second FFE study.
To investigate the 2D steel moment frame further, a coupled Incremental Fire Analysis (IFA) and Incremental Dynamic Analysis (IDA) was performed. The IDA considered three ground-motions recorded in Oakland, CA during the Loma Prieta earthquake scaled to 1, 3, and 5 times Peak Ground Acceleration (PGA). Fire hazards for the IFA analysis were developed using 10 parametric time-temperature curves then scaled to temperatures, 600oC, 800oC, and 1000oC. Fire hazard Intensity Measure (IM) was maximum compartment gas temperature, and the Engineering Demand Parameter (EDP) was compartment beam displacement. Seismic hazard IM was PGA and the EDP selected was inter-story drift. Behavior of FFE and fire-only analysis was compared.