Post-fire bond behaviour in elliptical concrete filled steel tubes: Experiment and simulation

The post-fire bond behaviour between the steel tube and core concrete in elliptical concrete filled steel tubes (ECFST) is studied in this paper. A total of 15 post-fire push-out test specimens, including 9 elliptical, 3 circular and 3 square CFSTs, were designed and tested. The testing parameters included: (a) target temperatures (Ttar = 20 degrees C, 600 degrees C and 800 degrees C); (b) high temperature duration time (td = 30 min, 60 min, 180 min and 300 min); (c) length of long axis of elliptical section (2a = 200 mm and 300 mm); (d) section type (elliptical, circular and square). The test results indicated that the target temperature and high temperature duration time affected the post-fire bond strength of ECFSTs significantly, and the post-fire bond strength of ECFSTs could be increased with the increasing values of the two parameters. Based on the test data, a two-linear stage bond stress versus slip constitutive model considering the influence of both the temperature and fire-exposed surface area was proposed and verified to capture the post-fire bond behaviour in ECFSTs, which could be used in the refined bond behaviour numerical analysis of ECFSTs.