Hysteresis Model Research of Steel Tube-Recycled Concrete Frames Under Cyclic Loading

Literature Overview

This paper by Zhang Xianggang, Chen Zongping, Xue Jianyang, and Su Yisheng from Henan Polytechnic University and Guangxi University was published in World Information on Earthquake Engineering in 2016, Volume 32, Issue 1, pages 277–283. The research investigates the hysteresis model of steel tube-confined recycled concrete (CFST) frames under low-cycle reversed loading. Two full-scale frame specimens were tested: one with circular steel tube columns and one with square steel tube columns, both combined with reinforced recycled concrete beams.

Core Technical Viewpoints

The study establishes a three-segment bilinear hysteresis model characterized by relative yield point, relative peak point, and relative failure point. The model incorporates hysteresis curves and stiffness degradation to capture the cyclic behavior of the CFST recycled concrete frames. The test specimens exhibited beam-end shear-flexural or flexural failure, with beams yielding before columns—a desirable seismic design philosophy that ensures plastic hinges form in the beams rather than the columns.

The hysteresis curves were found to be basically symmetric and exhibited a relatively full spindle shape, indicating good energy dissipation capacity. The special treatment of sudden load capacity drops at certain displacement amplitudes was a notable methodological contribution, as such drops can significantly affect the accuracy of the hysteresis model if not properly addressed.

Interpretation of Technical Points

From a steel pipe and welding perspective, the recycled concrete used in the CFST columns introduces additional complexity to the manufacturing process. Recycled concrete contains coarse aggregate derived from crushed concrete waste, which has different mechanical properties compared to virgin aggregate. The recycled aggregate typically has higher water absorption, lower density, and reduced strength, which can affect the compaction quality inside the steel tube during concrete pouring.

The steel tube itself must be manufactured to ensure proper concrete confinement. For circular tubes, the manufacturing process typically involves hot-dip galvanizing or corrosion-resistant coating to protect against the alkaline environment of concrete. The welding of tube segments must be performed with full-penetration butt welds to ensure structural integrity, and the weld quality must be verified through ultrasonic testing (UT) or radiographic testing (RT) in accordance with standards such as GB/T 3323 or ISO 17636.

The hysteresis model development is significant for structural engineers who need to perform nonlinear seismic analysis of CFST recycled concrete frames. The three-segment bilinear model simplifies the complex cyclic behavior into a computationally efficient representation while maintaining accuracy. The special treatment of sudden load drops addresses a practical issue observed in testing, where localized damage or cracking can cause abrupt reductions in load capacity.

Process and Standards Analysis

The welding of steel tube columns to the beam connections is critical for the overall frame performance. The column-to-beam connection typically involves welding of connection plates or end plates to the steel tube. The weld design must accommodate the relative rotation between the beam and column during seismic loading. If the weld is too rigid, it may concentrate stresses and lead to premature fracture. If too flexible, it may not provide adequate load transfer.

For square steel tubes, the corner regions are particularly vulnerable to stress concentration under cyclic loading. The manufacturing process for square tubes often involves cold-bending of flat steel plates, which introduces residual stresses at the corners. These residual stresses can interact with the cyclic stresses during seismic loading, potentially accelerating fatigue damage. The welding of stiffener plates at the corners of square tubes can help mitigate this issue, but the weld quality must be carefully controlled.

Integration with Engineering Practice

The use of recycled concrete in CFST columns is an important sustainability initiative that reduces construction waste and conserves natural resources. However, from a manufacturing and quality control perspective, recycled concrete poses challenges. The recycled aggregate has variable properties depending on the source material and recycling process, which can lead to inconsistencies in the concrete mix. The compaction of recycled concrete inside steel tubes requires careful attention to ensure full filling without voids, particularly at the tube corners for square sections.

The hysteresis model developed in this study can be directly applied in structural analysis software for seismic performance evaluation of CFST recycled concrete frames. Engineers should note that the model parameters (yield displacement, peak displacement, failure displacement) are derived from experimental data and may need adjustment for different tube sizes, concrete grades, and loading conditions. The model's accuracy should be validated against additional experimental data before being applied to critical structures.

The beam-first-yielding mechanism observed in the tests is a key design objective in seismic-resistant structures. From a welding standpoint, this means that the beam-to-column connections should be designed to allow plastic hinge formation in the beam while maintaining the integrity of the column. The weld design at the beam end should accommodate the expected rotation without cracking, which typically requires a ductile weld metal and a well-controlled welding procedure.

Key Questions and Reflections

A significant question is how the recycled concrete properties affect the long-term durability of the CFST columns. Recycled aggregate can have higher permeability and lower resistance to chloride ingress, which may accelerate corrosion of the steel tube from the inside. This is particularly concerning for structures in aggressive environments. The study does not address long-term durability, which should be considered in future research.

Another reflection is on the welding procedure specification (WPS) for steel tubes used with recycled concrete. The recycled concrete may have different thermal properties compared to virgin concrete, which can affect the cooling rate of the weld during fabrication. A faster cooling rate can lead to harder, more brittle microstructures in the HAZ, reducing ductility. The welding procedure should include appropriate preheating and post-weld heat treatment (PWHT) if necessary to maintain weld ductility.

Study Insights and Implications

This study makes a valuable contribution to the seismic design of CFST recycled concrete frames by providing a practical hysteresis model validated through experimental testing. For steel pipe manufacturers and welding engineers, the key implications are the need for high-quality welds at tube-to-beam connections, careful control of tube manufacturing tolerances to ensure proper concrete confinement, and attention to the long-term durability of steel tubes exposed to recycled concrete. The three-segment bilinear model offers a useful tool for seismic analysis, but its application should be accompanied by appropriate safety factors to account for uncertainties in recycled concrete properties and welding quality. Future research should extend the hysteresis model to include the effects of tube size, wall thickness, and concrete grade variations to provide a more comprehensive design tool for engineers.