Calculation Method for Fire Resistance of Hanger Systems Under Vehicle Fires
To evaluate the fire resistance of hanger systems in suspension bridges,vehicle fires are classified into five levels.Levels 1 and 2 represent passenger vehicle fires,levels 3 and 4 correspond to truck fires,and level 5 represents tanker fires.These vehicle fires are characterized by distinct maximum heat release rates and burning durations.The proposed hierarchy was validated using existing vehicle fire experiments.Geometric features of flames are established for Levels 3,4,and 5 vehicle fires based on previous vehicle fire incidents.For passenger vehicle fires,a cylindrical flame radiation model was employed to compute spatial radiative heat flux,validated through three full-scale car fire tests.In the case of truck fires,a prismatic flame radiation model was used to calculate spatial radiative heat flux.For tanker fires with crosswinds,a computational fluid dynamics method validated by a liquefied natural gas trench fire test was employed to calculate the heat flux envelope on the hanger surface.An incremental temperature calculation formula for hanger cross-sections with radiative heat flux boundary conditions was derived,and validation was performed using finite element models.Using mechanical property tests of high-strength steel wires at high temperatures,a quantitative relationship between critical temperature and design safety factor of hangers is developed based on the ultimate load-carrying capacity at high temperatures.Finally,integrating the above outcomes,a five-step theoretical framework is proposed to evaluate the fire resistance of hanger systems under graded vehicle fires.This algorithm can serve as a reference for the assessment and fire-resistance design of hanger systems in suspension bridges.
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