Seismic Performance of Bottom-Strengthened I-Shaped Steel-Concrete Column

Literature Overview

This paper, published in the Journal of Earthquake Engineering and Engineering Vibration (2013, Vol. 33, No. 1), presents research by Hui Cun, Cao Wanlin, Dong Hongying, and Xu Fangfang from Beijing University of Technology on the seismic performance of I-shaped steel-concrete columns with bottom strengthening. The study is supported by the National Natural Science Foundation of China (50978005) and the Beijing Municipal High-Level Talent Program (PHR20100502). This work addresses a practical challenge in seismic-resistant structural design where steel-concrete composite columns require enhanced ductility and energy dissipation capacity at plastic hinge regions.

Structural Configuration and Strengthening Strategy

The research introduces three column configurations for comparison:

• Model 1: Conventional I-shaped steel-concrete column without strengthening
• Model 2: I-shaped steel-concrete column with steel plates welded to the outer faces of both flanges at the bottom region
• Model 3: I-shaped steel-concrete column with steel plates welded to the entire perimeter of both flanges at the bottom region

The strengthening is applied only to the bottom region of the columns, which corresponds to the expected plastic hinge location under seismic loading. This targeted strengthening strategy is economically efficient and addresses the most critical region for seismic performance.

Strengthening Configuration Details

Test Program and Results

Low-cycle repeated loading tests were conducted on three scaled models representing the three configurations. The tests evaluated:

• Ultimate bearing capacity
• Stiffness and stiffness degradation
• Ductility (displacement ductility ratio)
• Hysteresis energy dissipation capacity
• Damage progression and failure modes

Key Performance Comparisons

Bearing Capacity Calculation Formulas

The authors develop calculation formulas for both normal section and shear section bearing capacity of I-shaped steel-concrete columns. The formulas account for:

• The contribution of the steel I-section (flanges and web)
• The contribution of the infilled concrete
• The interaction effects between steel and concrete
• The strengthening effect of the welded steel plates at the bottom region

The calculated results show good agreement with experimental measurements, validating the proposed calculation methods.

Welding and Fabrication Considerations

From a steel pipe and structural fabrication perspective, this research raises several important technical points:

• Welding sequence: The welding of strengthening plates to the I-shaped flanges must follow a controlled sequence to minimize residual stresses and distortions. Sequential welding from the center outward, with intermittent welds, is recommended to control warping.
• Weld quality: The strengthening plates are welded to the flange surfaces, creating fillet or full-penetration welds depending on design requirements. Weld quality directly affects the effectiveness of the strengthening. Pre-weld inspection, in-process monitoring, and post-weld NDT (MT or UT) are essential.
• Material compatibility: The strengthening steel plates should have mechanical properties compatible with the I-section steel to avoid brittle failure at the weld interface. Matching yield strength and elongation properties between base material and weld metal is critical.
• Heat-affected zone (HAZ) control: The welding heat input must be controlled to minimize HAZ softening or embrittlement in the base steel, particularly in regions where plastic hinge formation is expected. Low heat input welding processes (GTAW or low-current SMAW) are preferable.

Engineering Practice Implications

• Seismic design strategy: The bottom strengthening approach provides a practical solution for enhancing the seismic performance of existing or new I-shaped steel-concrete columns without requiring full column replacement. This is particularly valuable for retrofitting existing structures.
• Design code compliance: The proposed bearing capacity formulas should be validated against existing code provisions (GB 50011, GB 50017) to determine whether current codes adequately capture the strengthening effects.
• Construction sequencing: In seismic zones, the strengthening plates at the bottom of columns should be installed and welded before concrete placement, ensuring full bond between the strengthened steel section and the infilled concrete.

Critical Reflection

The research demonstrates clear benefits of bottom strengthening for I-shaped steel-concrete columns in terms of bearing capacity, ductility, and energy dissipation. However, the study is limited to three scaled models, and the scaling effects on seismic performance—particularly on failure modes and ductility—should be considered when extrapolating results to full-scale structures. The paper does not address the long-term performance of the welded joints under cyclic loading, which is a critical concern for seismic applications where fatigue cracking at weld toes can initiate progressive failure. Additionally, the interaction between the strengthening plates and the confining effect on the infilled concrete deserves further investigation, as the local confinement provided by the plates may influence concrete crushing behavior differently from the global confinement provided by the I-section.

Summary

This paper presents a systematic investigation into the seismic performance of bottom-strengthened I-shaped steel-concrete columns, demonstrating that targeted strengthening at plastic hinge regions significantly improves bearing capacity, ductility, and energy dissipation. The proposed bearing capacity calculation formulas provide practical tools for design application. For structural engineers and steel fabricators, the key takeaways are that bottom strengthening is an effective retrofit strategy, welding quality is critical to strengthening effectiveness, and HAZ control during welding is essential to maintain material properties in regions subjected to cyclic plastic deformation.