Mechanical Behavior of Circumferentially Prestressed Three-Tube Buckling-Restrained Brace

Literature Overview

This paper, authored by Gao Qingshui, Tian He, Yang Yi, Zhang Chu, Xiao Shaowen, and Hu Shuqing from Guangdong Electric Power Research Institute, Guangdong Electric Power Institute Energy Technology Co., Ltd., and Zhongtong Steel Structure Co., Ltd., was published in the journal Progress in Steel Building Structures, Volume 21, Issue 1, 2019, pages 51–60. The research is supported by the Guangdong Electric Power Company Science and Technology Project (No. GDKJQQ20153008). The study proposes a novel three-tube buckling-restrained brace (TTBRB) with circumferential prestress and investigates its hysteretic performance through validated finite element analysis.

Core Technical Content and Structural Configuration

The proposed TTBRB consists of three concentric steel tubes: an inner core tube that provides axial stiffness, load-bearing capacity, and seismic energy dissipation; an outer jacket tube that restrains overall buckling of the core tube; and an inner jacket tube that restrains local buckling of the core tube. Between the core tube and the jacket tubes, high-molecular-weight polyethylene (PE) friction-reducing layers are installed to minimize friction during axial deformation.

The key innovation of this design is the application of circumferential prestress to the core tube through interference fit assembly of the jacket tubes. This prestress mechanism allows engineers to modify the tensile or compressive yield strength of the core tube, thereby controlling the brace's yielding behavior under seismic loading.

Technical Analysis of Prestress Mechanism and Hysteretic Performance

The circumferential prestress mechanism operates through interference fit assembly. When the jacket tubes are assembled with a small clearance relative to the core tube, the radial compression of the jacket tubes generates circumferential tensile stress in the core tube. This prestress modifies the von Mises stress state in the core tube, effectively raising or lowering the axial yield stress depending on the prestress level and the direction of axial loading.

The finite element analysis, validated against experimental data, investigated the effects of two key parameters on the TTBRB hysteretic performance: the gap between the jacket tubes and the core tube, and the magnitude of circumferential prestress applied to the core tube. The analysis revealed that when the gap is small, the circumferential deformation of the core tube under axial loading is constrained by the jacket tubes, generating additional circumferential stress. When combined with the applied circumferential prestress, this interaction significantly alters the axial tensile and compressive yield strengths of the TTBRB.

A particularly important finding is that when only the outer jacket tube and the core tube have a gap (while the inner jacket tube maintains contact), the TTBRB can yield prematurely in tension while maintaining its compressive yield strength. This asymmetric yielding behavior is identified as the preferred configuration for three-tube buckling-restrained braces.

Engineering Practice Implications

The TTBRB design offers several advantages over conventional buckling-restrained braces (BRBs). Traditional BRBs typically use a solid-section core embedded in a concrete or mortar jacket. The hollow tube core of the TTBRB provides a larger radius of gyration for the same cross-sectional area, improving buckling resistance. The elimination of concrete fill material significantly reduces the self-weight of the brace and avoids the degradation of energy dissipation capacity that can occur due to concrete cracking and damage during cyclic loading.

For welding engineers, the fabrication of TTBRB components requires attention to several critical aspects. The core tube and jacket tubes are typically manufactured as seamless or welded steel tubes, and the quality of these tubes directly affects the brace performance. The PE friction-reducing layers must be installed with care to ensure uniform thickness and proper bonding. The interference fit assembly requires precise dimensional control, with tolerances on the order of 0.1–0.5 mm for the gaps between tubes.

The asymmetric yielding behavior, where the brace yields in tension before compression, is particularly useful for seismic energy dissipation. During an earthquake, the brace undergoes cyclic tension and compression. The asymmetric yielding ensures that energy dissipation is concentrated in the tensile yielding phase, where the material behavior is more stable and predictable than in the compressive yielding phase, where local buckling can complicate the response.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the long-term durability of the PE friction-reducing layers under cyclic loading and environmental exposure is not addressed. PE materials can degrade over time due to UV exposure, temperature cycling, and mechanical wear, which could affect the friction characteristics and ultimately the brace performance. Second, the study does not examine the effect of manufacturing imperfections such as ovality, out-of-straightness, and weld residual stress on the interference fit and prestress distribution.

Additionally, the study focuses on finite element analysis with experimental validation but does not provide detailed information on the experimental setup, loading protocol, or test results. A comprehensive experimental program would be valuable for validating the numerical models under various loading conditions and for establishing design guidelines.

Study Insights and Implications

This research presents an innovative structural component design that addresses several limitations of conventional buckling-restrained braces. The circumferential prestress mechanism provides a novel approach to controlling the yielding behavior of the core tube, offering design flexibility that is not available in traditional BRBs. The elimination of concrete fill material and the use of hollow tube components result in a lighter, more durable, and more predictable seismic isolation device. For steel pipe and welding engineers, the TTBRB design highlights the importance of precision manufacturing, material quality control, and the integration of structural mechanics with material science in the development of advanced seismic protection systems. The study exemplifies how fundamental understanding of material behavior and structural mechanics can lead to innovative engineering solutions that improve seismic safety and structural performance.