RESIDUAL RESPONSE OF REINFORCED CONCRETE BEAMS FOLLOWING FIRE EXPOSURE
Advisor – Venkatesh Kodur
Wednesday, December 4, 2019
12:00 pm to 2:00 pm
Room 2555D EB
Reinforced concrete (RC) structures exhibit high level of fire resistance owing to relatively low thermal conductivity, high thermal capacity, and slower strength degradation of concrete with temperature. Nonetheless, irreversible physiological changes in reinforcing steel and concrete resulting from high temperature exposure result in reduction in strength and stiffness properties of RC members. Thus, there is always uncertainty regarding extent of load bearing capacity retained in RC structures (members) after fire exposure owing to temperature induced degradation in mechanical properties, and extent of residual deflections leftover within a RC beam. From safety consideration, it is imperative to assess if sufficient residual capacity exists in RC members prior to re-occupancy of the structure after a fire incident. 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 directly.
This PhD dissertation develops comprehensive understanding on residual response of RC beams 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 the consideration of distinct material properties of reinforcing steel and concrete during heating and cooling phases of fire exposure and residual (after cool down) phase, as well as in incorporation of plastic deflections occurring during fire exposure of RC beams into post-fire residual response analysis. 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, results generated from experimental and numerical studies, together with that reported in literature, were utilized to develop a rational approach for assessing residual response of RC beams following fire exposure. This five step rational approach provides a general framework to apply visual assessment techniques and thumb rules, non-destructive testing, as well as simplified and advanced calculation methods, to assess the extent of fire-induced damage to a concrete structure. Overall, the research presented in this dissertation develops a comprehensive understanding on the residual response of fire damaged concrete beams after exposure to combined effects of fire and structural loading.