Seismic tests and nonlinear model of beam-CFDST column joints with blind fasteners

Composite materials combine two or more materials to produce improved properties derived from the individual material properties. Over the past decades, a significant amount of research has been devoted to developing new composites and improving the properties of the existing ones. Recently, a novel concrete-filled double skin steel tubular (CFDST) composite has been developed by replacing a section of core concrete in concrete-filled steel tubular (CFST) with an inner steel tube. Thus, CFDST has improved bending stiffness, lower-self weight and improved fire resistance with broader application prospects than conventional CFST. To date, substantial research on CFDST structures has been carried out to deduce their mechanical properties. However, to increase the practical application of CFDST, there is a need for systematic research on both CFDST members and joints.

Generally, welded rigid joints or pinned connections commonly used in engineering practice fail to achieve the desired ductility under seismic actions. It also requires a large number of site welding that further possess construction complexities. Pinned connections fail to fully utilize beam capacity due to their inability to transfer the bending moment. To this end, blind fasteners have been adopted to solve these problems, and their applicability in CFDST has been experimentally investigated. These connections were found to improve mechanical performance under seismic actions. Nevertheless, static loading tests, which are commonly used, fail to reveal the dynamic performance of these connections under earthquakes. This can be addressed by conducting pseudo-dynamic tests and developing analytical models to estimate seismic performance, an area that is yet to be explored due to the lack of pseudo-dynamic tests on blind fastened CFDST column joints/connections.

To address these challenges, Professor Jingfeng Wang, Dr. Wanqian Wang and Dr. Lei Guo from Hefei University of Technology conducted pseudo-dynamic tests to investigate the dynamic response of beam-CFDST column joints with blind fasteners. They commenced their research work by prefabricating a series of joint specimens. The seismic test was performed to establish the effects of different types of end plates, beams and column hollow ratios. Analytical models were also developed to estimate the seismic behaviors of the specimens. The analysis of the shear strength-panel zone angle relationship based on the equivalent method involving modifying the shear strength via yield mechanism. The proposed strategy was finally validated by comparing the test and analytical model results. Their work is currently published in the Journal of Building Engineering.

The researchers reported that the blind bolted beam-CFDST column joints exhibited favorable seismic behaviors. An increase in the column hollow ratio resulted in a moderate increase in the ultimate strength, bearing capacity and initial stiffness of the specimen attributed to the presence of steel bar truss deck (SBTD). As a result, the specimen with extended end plated exhibited higher ultimate strength stiffness and bearing capacity than that with a flush endplate. The main failure modes of the bolted connections were identified: warpage of the end plate, local buckling in the beam flange, local crushing and crack in the SBTD slab. More importantly, the authors were the first to develop the analytical model for predicting the shear feature of CFDST joint panel zone, in which two potential yield mechanism of outer tube were accounted to derive the shear strength of confined compression strut. The component method was also employed to calculate the rotational performance of the joint. Finally, a nonlinear model was finally established, which could accurately predict the hysteresis curves of the connections under dynamic loads.

In summary, the researchers carried out pseudo-dynamic tests to establish the failure mechanism and dynamic response of a series of blind bolted connections to CFDST columns. The analytical models accurately predicted the seismic performance of the test specimens. The good agreement between the analytical and test results provided an effective method for assembling CFDSR column to beam connections whose joint performance under different loadings can be accurately predicted by the analytical model. In a statement to Advances in Engineering, the authors explained that the results would contribute to enhancing the practical applications of novel CFDST composites.