Reinforced concrete (RC) structures possess inherent fire resistance due to relatively low thermal conductivity, high thermal capacity, and slower strength degradation of concrete with temperature. Nonetheless, fire exposure results in varying reduction in strength and stiffness RC members because of irreversible temperature induced degradation in mechanical properties concrete and rebar. Thus, uncertainty regarding extent of reduction in load bearing capacity RC structures (members) necessitates post-fire assessment of residual capacity to facilitate repair and (or) return to service conditions. Specifically, RC beams or slabs are particularly susceptible to fire damage arising from convective effects (hot gases) and impingement by flames near the ceiling, which do not affect vertical members as significantly. This PhD dissertation develops a comprehensive understanding on the residual response of fire damaged RC beams after exposure to combined effects of fire and structural loading through detailed experimental and numerical studies. A novel three stage approach to evaluate residual response of RC beams following fire exposure is conceptualized to overcome deficiencies in current assessment approaches. The three stages comprise of; Stage 1: evaluating the member response at room temperature during service (load) conditions as present prior to fire exposure; Stage 2: evaluating member response during heating and cooling phases as present in a fire incident, and during extended cool down phase of the member to simulate conditions as occurring after fire is extinguished or burnout conditions are attained; and Stage 3: evaluating residual response of the fire damaged member following complete cool down to room temperature. The proposed approach can account for the influence of critical factors such as, distinct temperature dependent material properties of concrete and rebar during heating, cooling, and residual phases, fire induced residual deformations, load level, and restraint conditions present during fire exposure in evaluating residual response of RC beams.Experimental and numerical studies were conducted to develop needed data for establishing applicability and validity of the proposed approach for tracing residual response. Material level tests were undertaken to establish temperature dependent bond strength relations for interfacial bond between rebar and concrete. Full scale fire resistance tests followed by residual capacity evaluation tests were conducted on six RC beams having different configurations. As part of numerical studies, a three-dimensional finite element based numerical model was developed to implement the proposed three-stage approach for evaluating residual response of fire exposed concrete beams, using general purpose software ABAQUS. The novelty of the developed model lies in explicit consideration of distinct thermomechanical properties of concrete and rebar during heating and cooling phase of fire exposure and residual (after cool down) phase, as well as in incorporation of plastic deflections occurring during fire exposure into post-fire residual response analysis of RC beams. Predictions from the developed model were validated against response parameters measured during tests published in literature and conducted as part of this study. The validated model was applied to conduct parametric studies to quantify the effect of critical parameters, namely, fire severity, load level, axial restraint, cross-sectional dimensions, and cover to reinforcement, on residual response of RC beams following fire exposure. Finally, findings from experimental and numerical studies, together with that reported in literature, were utilized to develop a five-step rational approach combining physical testing with simplified and advanced calculation methods, for practical post-fire assessment of residual capacity in concrete structures.
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