Behavior of concrete structures subjected to static and dynamic loading after fire exposure

Abstract

The resistance of concrete structures exposed to extreme loading conditions such as explosion, impact, industrial accidents, tsunami, earthquake or their combination represents one of the major topics in research today. Such loading conditions are characterized with high loading rates often acting in conjunction with fire exposure. Especially vulnerable are the structures located in the seismically active areas with high level of urbanization and proximity to HAZMAT landfills, which additionally exacerbate fire conflagrations. The behavior of concrete changes significantly when exposed to elevated temperatures resulting in the decrease of its mechanical properties. Reinforced concrete (RC), when exposed to high temperature culminates in a simultaneous thermal behavior of its two constituents, steel and concrete, that should be considered in the analysis. It is also known that the resistance, crack pattern and failure mode in concrete are strongly influenced by the loading rate. The dynamic response of RC structures previously exposed to fire changes significantly when compared to initially undamaged RC structures. The main objective of the present work is to further improve the existing rate sensitive thermo-mechanical model for concrete through the following: (i) the implementation of the experimentally obtained thermal dependence of concrete fracture energy in the thermo-mechanical model, (ii) the calculation of concrete thermally dependent mechanical properties by means of nonlocal (average) temperature and (iii) to perform parametric study on fastening elements and RC frames in order to investigate the interaction between the thermally induced damage and mechanical behavior of structures. The experimental investigations in the present work indicated that the concrete fracture energy has a declining tendency with the temperature increase, measured on small and mid-sized concrete beams. This is implemented in the thermo-mechanical model and it is indicated that the decrease of fracture energy has a relatively mild influence on reaction values in terms of loading rate. However, its effect on the fracture patterns and reaction-time histories can be considered as more significant. The influence of the nonlocal temperature is validated against the experimental results carried out on RC frames which had been thermally pre-damaged and subsequently loaded with impact. Currently there are almost no models that can realistically predict the structural behavior at this level of complexity. Furthermore, a parametric study is carried out to show the influence of preloading of single-headed stud anchor and anchor group with two and four studs, on the residual concrete edge failure capacity after fire exposure. The anchors are exposed to fire and loaded in shear, perpendicular to the free edge of the concrete member up to failure, in both hot and cold state (after cooling). The influence of different geometry configurations and initial conditions such as the edge distance, embedment depth, anchor diameter and duration of fire on the load-bearing behavior of anchors is investigated. It is demonstrated that the preloading has a strong negative influence on the residual load-bearing capacity of the concrete. Finally, the numerical parametric study is performed to investigate the influence of fire duration and the loading rate on the resistance of RC frames. The response of the RC structures strongly depends on whether it was loaded in hot or residual (cold) state, i.e. after being naturally cooled down to ambient temperature. Furthermore, an extensive numerical investigation on the influence of post-earthquake fire on the residual capacity of RC frames with and without ductile detailing is conducted. The numerical investigation encompassed the validation of the thermo-mechanical model in terms of temperature distributions, thermal deflections and load-bearing capacity against the test data and subsequent parametric analysis with different levels of fire exposure ranging from 15 to 120 min.