Incremental collapse threshold for pushout resistance of circular concrete filled steel tubular columns

This paper presents the results of a series of pushout tests using static loading (SL) and variable repeated loading (VRL) on concrete filled steel tubular (CFT) circular stub columns. The main parameters examined in this paper were the strength and age of concrete and the loading protocol. Under SL tests, the interface bond strength in CFT columns filled with normal strength concrete was found to be higher than that with high strength concrete. The SL test results showed that the interface bond strength varied from 0.41 to 0.85 MPa but from 0.33 to 0.66 MPa under VRL tests. A lower bound for the incremental collapse threshold of the pushout resistance of 70% of the static collapse load was empirically derived. Also an expression of the average growth of slip per loading cycle was empirically derived and recommended for design purposes. A comparison between the bond strength of the columns obtained from the present and previous test results, and available design codes is presented. Two newly derived bond strength limits were experimentally obtained and proposed for the design of structures subjected to either predominantly static or predominantly cyclic loading.