Layered Steel Pipe Buckling-Restrained Brace with Long Slits - Hysteretic Performance and Seismic Energy Dissipation Analysis

Literature Overview

This paper by Zhang Dongbin, Pan Peng, Wang Mengzi, Deng Kailai, and Chen Yabin from Tsinghua University (published in China Civil Engineering Journal, Vol. 49, Issue 12, 2016, pp. 9–15) presents the design and experimental investigation of a novel layered steel pipe buckling-restrained brace (BRB) with long slit openings. The research is funded by the National Natural Science Foundation of China (Grants 51422809, 51578314) and the Beijing Science and Technology Program. The study addresses the need for improved seismic energy dissipation devices with enhanced ductility and stable hysteretic behavior.

Design Configuration and Material Specification

The proposed BRB utilizes three different sizes of Q235 steel pipe sections arranged in a layered configuration. The core steel pipe, which bears the axial force, is weakened by cutting long slits (elongated openings) to promote controlled plastic deformation during seismic events. This approach differs from conventional BRBs that rely on solid steel core bars or solid pipes without geometric weakening.

Test Matrix Summary

Material Properties

Experimental Results and Technical Analysis

The five specimens were subjected to axial low-cycle reciprocating loading to evaluate their bearing capacity, deformation capacity, hysteretic energy dissipation, skeleton curves, and failure modes. The key findings include:

Hysteretic Performance

The layered steel pipe BRB with long slits demonstrated excellent deformation capacity and low-cycle fatigue performance when the opening rate was within a reasonable range. The hysteretic loops were full and stable, indicating good energy dissipation characteristics. The devices provided an additional equivalent damping ratio of 30%–42%, which is competitive with and in many cases superior to conventional BRB systems.

Failure Mode Analysis

The long slit openings serve as stress concentrators that initiate and localize plastic deformation. This controlled yielding mechanism prevents global buckling of the core pipe while ensuring that energy dissipation occurs through ductile plastic deformation of the steel at the slit locations. The layered configuration provides confinement to the core pipe, preventing local buckling at the slit openings.

Skeleton Curve Characteristics

The skeleton curves showed a clear bilinear behavior with an initial elastic slope followed by a stable post-yield plateau. The post-yield strength ratio was maintained across multiple cycles, indicating minimal strength degradation. This stability is critical for seismic protection applications where the device must function reliably throughout a major earthquake event.

Engineering Practice Considerations

From a steel pipe fabrication and welding perspective, this BRB design raises several important manufacturing considerations:

1. Slit Cutting Quality: The long slit openings in the core pipe require precise cutting (plasma, laser, or oxy-fuel) with smooth edges to avoid stress concentration at the slit tips. Any burrs, notches, or surface defects at the slit edges can initiate premature cracking under cyclic loading.
2. Welding of Layered Configuration: The assembly of three different pipe sizes requires careful welding design. The fillet welds connecting the layers must be designed to avoid brittle fracture during cyclic deformation. Welding procedure specification (WPS) should include preheat and interpass temperature control for Q235 steel to minimize residual stress effects.
3. Opening Rate Optimization: The research indicates that the opening rate must be within a "reasonable range" for optimal performance. Too low an opening rate results in insufficient ductility, while too high an opening rate compromises the bearing capacity. This optimization requires careful engineering judgment and potentially parametric studies for each specific application.
4. Dimensional Tolerances: The layered configuration requires tight dimensional tolerances on the pipe diameters to ensure proper fit and uniform confinement pressure. Deviations in outer diameter of the inner pipes or inner diameter of the outer pipes can lead to uneven stress distribution.

Study Insights and Implications

This research demonstrates that geometric weakening through long slit openings is an effective strategy for enhancing the seismic performance of steel pipe BRBs. The 30%–42% equivalent damping ratio is a substantial contribution to structural energy dissipation, potentially allowing for reduced lateral force demands on the overall structural system. The layered pipe configuration is particularly attractive because it leverages standard steel pipe products, reducing fabrication complexity compared to custom-shaped core elements.

A notable reflection is that the use of Q235 steel, while providing excellent ductility, may require careful consideration in corrosive environments or high-cycle fatigue applications. For offshore or coastal applications, higher-grade steel (Q345 or Q390) with appropriate corrosion protection may be necessary, and the slit opening dimensions would need to be re-optimized for the different material properties.

The research provides a valuable foundation for developing standardized design procedures for slit-weakened layered pipe BRBs, which could be incorporated into future seismic design codes for steel structures.