Seismic Performance of Self-Compacting Self-Stress Rectangular Steel Concrete Columns

Literature Overview

The experimental study by Jia Hongyu, Li Awei, Li Fengge, and Han Chungang, published in Bulletin of the Chinese Ceramic Society in 2018, investigates the seismic performance of rectangular steel-concrete columns filled with self-compacting self-stress concrete. Funded by the Inner Mongolia University of Science and Technology (Project TMY201702), this research examines the influence of section aspect ratio on the hysteretic behavior, ductility, energy dissipation, and stiffness degradation of these composite columns.

Test Configuration and Material Characteristics

Three specimens with different length-to-width ratios were fabricated and subjected to cyclic loading tests. The use of self-compacting self-stress concrete represents an advanced material choice that combines the workability advantages of self-compacting concrete with the early-age strength development and reduced shrinkage of expansive self-stress concrete. This material combination is particularly beneficial for steel-concrete composite columns where the concrete-steel interface bond and long-term durability are critical.

Failure Mode Analysis and Weld Quality

The most critical finding of this study is that premature tearing of corner welds is identified as the root cause of the sudden load-carrying capacity drop in all specimens. This observation carries profound implications for the welding quality assurance of rectangular steel-concrete composite columns in seismic applications.

Corner welds in rectangular steel tubes are inherently more vulnerable than longitudinal or transverse welds because they experience complex multiaxial stress states under cyclic lateral loading. The corner region concentrates both bending stresses from the column and shear stresses from the concrete-steel interaction, creating a highly stressed zone where any weld defect can initiate catastrophic failure.

From a welding metallurgy perspective, the corner weld region is particularly susceptible to:

• Lack of fusion defects due to the geometric constraint of the corner geometry.
• Porosity caused by insufficient gas shielding in the tight corner region.
• Cracking in the heat-affected zone due to hydrogen-induced delayed cracking in susceptible microstructures.
• Residual stress concentration at the weld toe where cyclic fatigue cracks can initiate.

The self-stress concrete, while beneficial for early strength and reduced shrinkage, can exacerbate welding issues if not properly coordinated with the welding schedule. The expansive properties of self-stress concrete generate internal pressure that, if applied before weld residual stresses have fully relaxed, can create additional stress states in the weld metal and HAZ.

Hysteretic Behavior and Ductility Assessment

The paper documents that as the aspect ratio increases, the fullness of the hysteretic loops decreases, indicating greater susceptibility to shear failure or slip mechanisms. The skeleton curve shows a more pronounced descending branch for higher aspect ratio specimens, reflecting the progressive loss of composite action as corner welds fail.

The stiffness degradation pattern reveals that higher aspect ratio specimens experience more rapid stiffness loss after peak load, which is directly attributable to the premature weld failure. This has important implications for seismic design, as rapid stiffness degradation can lead to increased inter-story drifts and potential collapse mechanisms in multi-story buildings.

Engineering Practice Recommendations

Based on the experimental findings, the following engineering recommendations emerge for the design and fabrication of self-compacting self-stress rectangular steel-concrete columns:

• Strict limitation of section aspect ratio to prevent premature corner weld failure and maintain ductile seismic response.
• Mandatory full-penetration welding at all corner joints with qualified weld procedures specifically designed for corner geometries.
• Comprehensive NDT coverage of corner welds, including ultrasonic testing (UT) and magnetic particle testing (MT) for surface and near-surface defect detection.
• Coordination between welding schedule and concrete placement timing to ensure adequate weld stress relief before self-stress concrete expansion occurs.
• Post-weld inspection of the internal concrete-steel interface to verify that no weld spatter or debris impedes concrete placement and bond development.

Study Insights and Implications

This experimental study makes a compelling case that the seismic performance of steel-concrete composite columns is fundamentally governed by weld quality, particularly at geometric stress concentration points such as corners. The identification of corner weld tearing as the primary failure mechanism redirects attention from material property optimization alone to the critical role of fabrication quality in seismic resilience.

The use of self-compacting self-stress concrete offers genuine advantages in terms of early strength and reduced shrinkage, but these advantages can be negated if the welding quality is not correspondingly elevated. Engineers must adopt a systems-level approach that considers the interaction between material properties, weld quality, and seismic demand when designing composite columns for seismic zones.

The practical implication is clear: investment in welding quality assurance for composite columns is not merely a cost consideration but a fundamental requirement for seismic safety, and any reduction in weld quality standards for the sake of economy is counterproductive in the long term.