# Kenarangi and Bruneau 2020

## Experimental Study, Results & Discussion

Seven circular concrete filled steel tubes (CFST’s) and reinforced concrete filled steel tubes (RCFST’s) were cyclically loaded in order to investigate the shear strength under double-curvature setup until failure. The specimens were constructed with electric resistance welded steel pipes. Of the seven specimens, four were reinforced, and had the same outer diameter, wall thickness, height, and steel properties, and were reinforced with different size and type of reinforcing. Specimens were tested with a pantograph device in which an actuator applies force to a loading beam in order to pass the force at mid-height of the shear specimens. The specimens reach first yield at the end of the 4th cycle, and testing is continued after the first yield by subjecting the specimens to displacement amplitudes equal to multiples of the equivalent yield displacement until failure of the specimens occurs. A finite element model was developed such that individual loading protocols for each specimen was designed to match the yield points. Slippage occurred between the loading beam and top of the specimen, and was measured using relative displacement between LEDs on the beam and on top of the specimen. The slippage was in the magnitude of 2.5 mm for the concrete-filled specimens, and occurred after the first yield. For all but one specimen, excessive diagonal deformation on the steel surface was observed after the specimens reached maximum strength. This occurred due to failure of the diagonal compressive concrete strut that developed due to increased lateral displacement. After reaching maximum strength, the specimens exhibited decreased strength until cracking developed, and a sudden loss of strength occurred. Failure occurred due to fracture of the steel tube on the tensile sides. In many specimens, the infill concrete within the shear span was pulverized into fine particles, and sheared aggregates were observed in the crushed infill concrete. The composite shear strength of the specimens was calculated using two equations. The first equation, according to AASHTO, in which the shear strength of concrete and steel are added. The second equation, from WSDOT, in which three separate shear strength parameters are summed. The experimental values were then compared to the calculated, and it was found that there was little difference between the WSDOT calculated values, whereas the AASHTO equation produced shear strength values that were roughly half of the experimental values. The longitudinal reinforcement had little effect on the specimen shear strength, and similarly, the presence of spiral reinforcement did not have a significant impact on the shear strength.

## Analytical Study

A finite element model of half of the test setup was made as the test setup is symmetric. The concrete was modeled using a Winfrith material model with constant stress solid elements, and the steel was modeled using a bilinear elastoplastic material with 1% strain hardening. A pretension force of 70% of the yield strength was applied to the bolts, and thermal expansion was also taken into account by using the thickness of the bolted plates in the connection and the diameter of the bolts. Similarly to the experimental study, the RCFST shafts were similar to the CFST specimens, thus the finite-element analysis results only display CFST shear specimens, and the hysteresis curves are compared with the experimental results. Failure was not exhibited in the analysis as there was no failure criteria defined for the concrete and steel. It was observed that the strength of the concrete continued increasing at larger drifts, and this increase was consistent with the development of a diagonal compression strut in the concrete. A Von-Mises stress contour at the point where the steel tube yields displays that the steel yielding began at the center of the cross-section, close to midspan and propagated toward the unstiffened span ends.

In conclusion, the longitudinal reinforcement did not have a significant effect on the strength of the RCFST specimens, and the shear strength only increased by .25% and 4% for 1% and 2.2% longitudinal reinforcement, respectively. Similarly, the transverse reinforcement also did not have a significant effect on strength and ductility of RCFST specimens. The AASHTO equation, which included summing the individual shear strengths of a hollow steel tube and concrete section underestimated the strength of the shafts (by a factor of over 2.0), whereas the WSDOT equation provided closer results to the experimental results.

## References

Kenarangi, H., and Bruneau, M. (2020). “Investigation of Cyclic-Shear Behavior of Circular-Reinforced Concrete-Filled Steel Tubes” Journal of Structural Engineering, 146(5) doi:10.1061/(ASCE)ST.1943- 541X.0002598.