Push-out tests were conducted on circular, rectangular and square CFT members (Shakir-Khalil, 1993a, 1993b). The main purpose of the experimental study was to investigate bond strength of the steel-concrete interface. In addition, the performance of shear connectors and load-slip behavior of the specimens were examined.
Experimental Study, Results and Discussions
Two types of test setup were prepared. In the first one, the specimens rested on the steel tube wall having a 1.97 in. gap at the bottom and an axial load was applied on to the concrete alone at the top. In the second setup, an axial force was applied to the concrete alone while steel brackets or plates were attached to the steel tube and placed on supports having no restraint for rotation. The specimens were also resting on the steel tube with 1.97 in. gap at the bottom. Ten series of tests, which were named as X, Y, A, B, C, D, E, F, G, and H were conducted. The specimens in test series Y were 9.84, 17.71, and 23.62 in. in length and had a gap of 1.97 in. at the bottom. All the remaining specimens had a length of 17.72 in. and an interface length of 15.75 in.. Mild steel Grade 43 was used to manufacture the steel tubes. Either Hilti nails or black bolts were used as shear connectors.
The test series X included sixteen rectangular specimens in groups of four. For each specimen, the first type of experiment test setup was used and the D/t ratio was equal to 24. The specimens in the first group were specified as control specimens, which had no shear connectors. The other specimens had black bolts as shear connectors. Two specimens in each group were loaded up to failure. Two cycles of repeated axial loading was applied to the remaining two specimens before loading them to failure. The cycles were made at load levels of 30%, 60% and 90% of the average failure load of the first two specimens. The control specimens had an average bond strength of 120 psi. The failure load of the specimens with shear connectors seemed to increase at the same rate with the number of bolts. The specimens with shear connectors exhibited almost a bilinear load- slip response in the elastic range. This showed that the shear connectors were observed to be effective after the steel-concrete bond resistance was lost. As the black bolts rotated, the steel tube walls also deformed and the bolts had a larger shear area combined with tensile resistance against slip. Both of these effects caused failure loads larger than expected. For the repeated axial loading, no significant detrimental effect on the load-slip response was observed.
In the test series Y, the first type of test setup was utilized and twenty-four specimens were tested in groups of four without any shear connectors. The D/t ratio was 30 for the square tubes and it was equal to 33.7 for the circular tubes. The length of the specimens in each group was selected among the values of 9.84, 17.72, and 25.59 in.. The interfaces of some of the tubes were oiled before casting of concrete. After the test, the friction marks inside the steel tubes showed that higher frictional forces were generated at the steel-concrete interface of the circular specimens compared to the square specimens. The average bond strength for the circular specimens was found to be 82% and 64% greater than the square specimens in the case of dry and oiled interfaces, respectively. Moreover, the bond strength of the dry specimens was approximately two times the bond strength of the equivalent oiled specimens. Square specimens of this test series had smaller bond strengths than rectangular specimens of the test series X. This was attributed to the smaller D/t ratio of the rectangular tubes making steel tube wall stiffer and less shrinkage of concrete in the rectangular tubes due to the smaller cross-section area. The strains recorded on the steel tubes showed a gradual increase from the top to the bottom. In the linear part of the load-slip curves, the oiled specimens had a higher slip rate. However, the load-slip curves of the oiled and dry specimens had the same characteristics after failure load.
Test series A and B each consisted of ten specimens and they were tested in the first type of test setup. Either Hilti nails or black bolts were used as shear connectors. In test series A, square specimens with a D/t ratio of 30 were tested. The specimens in test series B were circular and had a D/t ratio of 33.6. The control specimens of test series A, which did not have any shear connectors, exhibited an average bond strength of 29 psi. The shear connectors appeared to become effective following the loss of bond strength and the load-slip relationship of the specimens with shear connectors were bilinear in the elastic range. Some specimens were subjected to subsequent unloading and reloading during their post-peak load-slip response. Among those specimens, the ones having Hilti nails as shear connectors regained their failure load when they were reloaded. However, this was not possible for the bolted specimens as the bolts were sheared off at failure. The steel strains were found to increase gradually toward the bottom. The rate of strain increase for the bolted specimens was found to be greater at the location of bolts due to the load transfer from the concrete core to the steel tube through the bolts. In test series B, similar response to test series A was obtained and the bolted specimens could not reach their failure load after they were subjected subsequent unloading and reloading. Due to the rotation of the shear connectors, the steel tube walls distorted and this caused an increase in bond strength, which was not noticeable in test series A. The strain in the steel tube increased toward the bottom and the rate of strain increase was observed to change at the load level, at which the control specimens failed. In addition, the rate of strain increase was greater at the bolt locations due to load transfer from the concrete core to the steel tube through the bolts. As a result of the higher friction in the circular specimens of test series B, their failure load was about two times greater than the failure load of square specimens of test series A.
Test series C had six square specimens with a D/t ratio of 30 and test series D included two circular specimens with a D/t ratio of 33.6. In both test series, each specimen had two 7.87 in. high steel brackets. The brackets were attached at the middle of two opposite sides for both of the circular and for two of the square specimens. For the remaining four square specimens, the steel brackets were located at two opposite corners. Shear connectors were used only in two square specimens, which had brackets at their corners and Hilti nails were selected as shear connectors. In test series C, the longitudinal strains were much lower than the ones measured in the previous test series. In addition, tensile strains were recorded on the steel tube below the brackets. This resulted circumferential contraction and improved the resistance to slip. The control specimens with steel brackets at the corners had 17% more failure load than the ones with steel brackets at the sides. The average bond strength for control specimens was 140 psi. The failure load of the control specimens and the specimens with shear connectors were 5 times and 2 times that of the equivalent specimens in test series A, respectively. The control specimens experienced sudden drop in strength after failure with a slip of about 0.118 in.. However, for the specimens with shear connectors, no deterioration of strength in post failure region was observed. The specimens in test series D had an average bond strength of more than 580 psi. Their failure was due to excessive deformation and tearing of the steel brackets. The amount of slip was insignificant at the end of the tests. The failure loads of the specimens in test series D were much greater than the failure loads in test series B.
Test series E and F consisted of specimens with steel brackets at the sides. Test series E had five pairs of square specimens having D/t ratio 31.7 and test series F had five pairs of circular specimens having a D/t ratio of 34.8. Only Hilti nails were used as shear connectors. In test series E, the control specimens exhibited a bond strength of 58 psi. The specimens having nails at the same steel walls with the brackets had better load-slip response than the ones having nails at different steel walls. Most of the specimens did not have a definite failure load. After the test, this was attributed to large deformation of the Hilti nails without releasing their grip in the concrete core. In addition, bracket distortion and separation of steel and concrete along the bracket height also occurred for some of the specimens at the end of the test. In test series F, two control specimens had failures at the brackets. Thus these specimens were retested by seating them on their base. This method of testing was applied for the remaining specimens. They exhibited no definite failure and the tests were stopped when the applied loads reached four times the estimated failure loads.
In test series G and H, four specimens were tested. The specimens in test series G were square and had a D/t ratio of 30. The ones in test series H were circular and their D/t ratio was 33.7. The control specimens were tested by seating them on their base and the other specimens were tested by supporting them on the side plates attached to steel tubes. The side plates were placed either at the top of the steel tube or at a distance of 3.94 or 7.87 in. below the top of the steel tube. No shear connectors were provided for the specimens. In test series G, the control specimen and the specimen with side plate at the top of the steel tube experienced sudden slip followed by a drop in the applied load. They had 61 and 67 psi bond strengths, respectively. The other specimens had a bilinear load-slip response with no definite failure load. The change of slope of their load slip curve occurred approximately at the failure load of the first two specimens. For these specimens, it was possible that tensile strains occurred in the steel tubes and this increased the resistance to slip by contracting the steel tubes laterally. Thus these tests were terminated when excessive slip took place. The specimens of test series H showed similar responses with the specimens in test series G. However, the bond strengths and failure loads in test series H was more than two times the ones obtained in test series G. In addition, the specimens of test series H exhibited larger slip under increasing load compared to equivalent specimens in test series D.
Shakir-Khalil, H. (1993a) “Pushout Strengths of Concrete-Filled Steel Hollow Sections,” The Structural Engineer, Vol. 71, No. 13, July 6, pp. 230-233.
Shakir-Khalil, H. (1993b). “Resistance of Concrete-Filled Steel Tubes to Pushout Forces,” The Structural Engineer, Vol. 71, No. 13, July 6, pp. 234-243.