Numerical Investigation of Composite Floor Beam Resilience to Realistic Fire Scenarios

Abstract

This thesis provides a computational investigation of three separate composite steelbeam fire tests conducted at Lehigh University’s ATLSS Laboratory as well as parametricanalyses with various fire curves and levels of passive protection. The objective of thisstudy was to validate numerical models that conservatively capture structural failure ofcomposite floors subjected to fire, while striving for simplicity, to help further realizeperformance-based design and evaluation approaches for structural-fire resistance andresilience of secondary floor framing in steel buildings.The first pair of tests were identical structural systems with one having passive fireprotection and the other being unprotected subjected to the ASTM E119 fire curve.Thermal analysis of the steel was performed using a lumped mass approach, which can beimplemented via spreadsheet or a simple, explicit programmed solution. Thermal analysisof the slab was performed using a simple one-dimensional heat flow model. Two types offinite element analyses were performed: one composed of shell elements and anothercomposed of fiber-beam elements. The slab was unrestrained in all cases, so the effects ofslab continuity and membrane action were neglected. The structural models referencedboth lumped mass prediction temperatures as well as measured test temperatures as inputfor the temperature-dependent material properties of the specimens. The results of allmodels show conservative agreement with the experimentally observed behavior. Theplasticity of the section is analyzed over the duration of the tests using the concept ofwarping axial-moment failure envelopes which consider shifting of the effective centroiddue to the thermal gradient per three-sided heating. These models can be leveraged as part of a conservative performance-based approach to design composite floor assemblies forone-way flexural behavior under fire.The final validation case consisted of an unprotected composite beam subjected toa realistic fire curve with a decay phase. The objective of this study was to point towardsthe possibility of surviving a realistic fire scenario with a decay phase, as opposed to thecontinued growth of the ASTM E119 curve. The test fire curve closely matches the E119fire for 20 minutes prior to rapidly decaying. The test beam was shown to withstand theparametric fire curve computationally, resulting in relatively little damage, matching thetest observations reported. According to the ASTM E119 thermal criteria, the beam“failed” around 13 minutes. The same pair of SAFIR finite element models used in therunaway failure model validation were used to compare structural behavior of an assemblywhen it is permitted to cool in the case of fire suppression.The models of the realistic fire test were then parametrically extended to variouscombinations of active and passive protection, as well as different fire curve formulations.The fact that these models could capture failure per the first validation study allowed forthis extension to be confidently applied. The model comparisons highlight the current E119standard‘s lack of resiliency quantification. The standard may have potential to be used asa benchmarking tool in resiliency calculations, in turn making use of the plethora of datathat has already been obtained over the last several decades