Hsu and Lin 2006

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This study is focused on experimentally investigating the relationship between design details, such as embedment depth and stiffeners, and column base performance, such as strength and rigidity. Results from specimens tested under combined axial and lateral loads were used to define the effectiveness of the base connections. An empirical expression for base rigidity estimation is proposed for design references.

Experimental Study, Results, and Discussion

Seven square CFT members at various embedment depths were tested, including a specimen that was not embedded. All CFT members had side lengths of 350 mm and tube thickness of 7 mm (with a resulting D/t ratio of 50). The yield strength of the steel tube was 324 MPa. Each tube was fillet welded to a 32 mm thick end plate. Four ASTM A325 M27 anchor bolts were used to erect the CFT member in each foundation. Foundation concrete was cast in two pours: one for the anchor bolt mounting and a second to complete the foundation and fill CFT members. First and second pours had strengths of 24.3 and 35.4 MPa respectively. Three specimens were stiffened with different stiffener lengths embedded into the foundations. Stiffeners were made from 75 mm wide by 16 mm thick ASTM A36 steel plates and were attached to the CFT members by fillet welding. This was conducted to investigate whether surface failure could be moved from the highly stressed CFT embedded area to the surface of the foundation so that foundation integrity could be sustained.

Specimens were first subjected to only lateral loads within the elastic range. This was done to obtain the flexural rigidity of the base connection for future comparisons and establish the relationship between the base rigidity and corresponding fabrication details. After the elastic load tests, the specimens were unloaded and then tested under combined axial and cyclic lateral loads.

The cyclic lateral loading revealed that inelastic elongation occurred in the anchor bolts of the non-embedded specimen. This specimen also exhibited poor hysteretic performance. Anchor bolt yielding was significantly alleviated when the CFT members were embedded in the foundations. Stiffening was found to have a large impact on critical embedment depth. The critical unstiffened embedment depth was 350 mm while the critical stiffened embedment depth was only 175 mm. Stiffeners also resulted in better energy dissipation and higher stiffness in specimens. An empirical expression for base rigidity estimation is proposed for design references.

References

  • Hsu, H. L., and Lin, H. W. (2006). “Improving seismic performance of concrete-filled tube to base connections.” Journal of Constructional Steel Research, 62(12), 1333–1340. doi:10.1016/j.jcsr.2006.02.002