Lateral Stiffness of a Novel Self-Centering Square Steel Tube Concrete Frame

Literature Overview

This paper, published in 2016 in the Journal of Xi'an University of Architecture and Technology (Vol. 48, No. 6), introduces a novel self-centering frame system based on square steel tube concrete (SRC) members. The authors, Wang Xiantie, Liu Lida, Zhou Xuhong, He Bo, and Zheng Jiang from Xi'an University of Architecture and Technology and Chongqing University, propose a beam-column connection that rotates about a pin axis on the beam web, enabling the frame to return to its original position after lateral loading. The research was supported by the National Natural Science Foundation of China (Grants 51108369 and 5167874) and the Shaanxi Provincial Natural Science Foundation (Grant 2015JM5170).

Self-centering structural systems have gained significant attention in recent years as a means to achieve resilient performance in seismic design. Unlike conventional dissipative systems that rely on inelastic deformation to absorb energy, self-centering systems use elastic restoring forces to return the structure to its original position after a seismic event, minimizing residual drifts and damage. The proposed system leverages the inherent stiffness and strength of square steel tube concrete members while incorporating a mechanical self-centering mechanism at the beam-column joints.

Core Technical Approach and Theoretical Formulation

The authors derived analytical expressions for the lateral stiffness of the self-centering SRC frame, taking into account the contributions of beam flexural stiffness, column flexural stiffness, and the stiffness of the self-centering mechanism. The self-centering mechanism relies on steel strands that are tensioned with an initial prestress and anchored to the beam and column through a pin-rotation connection. When the frame undergoes lateral loading, the beam rotates about the pin axis, stretching the steel strands on one side and relaxing them on the other. Upon removal of the lateral load, the prestressed steel strands pull the beam back to its original position.

The following table presents the key parameters studied and their influence on frame performance.

Finite element analysis was employed to verify the analytical stiffness formulas. The numerical models incorporated geometric and material nonlinearities, including the nonlinear behavior of the steel strands and the interaction between the square steel tube and the concrete core. The good agreement between analytical and numerical results validates the proposed stiffness formulas.

Failure Mechanism and Design Recommendations

A critical aspect of the research is the identification of the failure mechanism of the self-centering frame. The authors found that the frame exhibits three distinct failure modes depending on the design parameters. The first failure mode involves yielding of the steel strands beyond their elastic limit, which compromises the self-centering capability. The second mode involves local buckling of the beam or column members, which reduces the lateral stiffness and may lead to progressive collapse. The third mode involves excessive rotation at the beam-column pin connection, which can cause separation of the connection components and loss of structural integrity.

The authors demonstrated that the initial prestress level is a critical design parameter that must be carefully selected to avoid these failure modes. If the initial prestress is too low, the steel strands may become slack during lateral loading, eliminating the restoring force and preventing self-centering. If the initial prestress is too high, the strands may yield under moderate lateral loading, reducing the effective restoring force and potentially causing damage to the anchorage. The study provides a recommended range for the initial prestress that balances these competing considerations.

The finite element analysis also confirmed that the frame members remain elastic throughout the loading process when the design parameters are within the recommended range. This elastic behavior is essential for maintaining the self-centering capability and ensuring that the frame returns to its original position after each lateral loading cycle.

Integration with Engineering Practice

For engineers considering the implementation of self-centering SRC frames in seismic design, several practical aspects deserve attention. The pin-rotation connection at the beam-column joint requires careful fabrication and installation to ensure smooth rotation without excessive friction or binding. The steel strand anchorage must be designed to withstand the high prestress forces without local yielding or pullout. The square steel tube concrete members must be designed to remain elastic under the expected seismic demand, which may require larger cross-sections or higher material grades than conventional dissipative systems.

The proposed system offers significant advantages in terms of post-earthquake functionality. By maintaining elastic behavior and returning to the original position, the self-centering frame minimizes residual damage and reduces the need for costly repairs after a seismic event. This makes it particularly suitable for critical infrastructure such as hospitals, emergency response centers, and data centers where rapid post-earthquake recovery is essential.

However, the system also presents challenges. The initial prestress must be maintained over the service life of the structure, which requires careful consideration of prestress loss due to relaxation, creep, and shrinkage of the steel strands. The pin connection introduces a potential weak point that requires regular inspection and maintenance. Engineers must weigh these maintenance considerations against the benefits of self-centering performance.

This research provides a rigorous analytical and numerical framework for the design of self-centering SRC frames, offering practical guidelines for parameter selection and failure mode prevention that can be directly applied in seismic-resistant structural design.