Pseudo-Static Testing of Steel Tube Ultra-High Strength Concrete Columns

Literature Overview

Wei Jiangang, Zhou Jun, Yang Yan, and Chen Baochun from Fuzhou University and Fujian University of Technology published this study in Journal of Architecture and Civil Engineering in 2020, funded by the National Natural Science Foundation. The research presents pseudo-static (quasi-static) cyclic testing of 11 steel tube ultra-high strength concrete columns with varying axial compression ratios, steel ratios, and slenderness ratios.

Experimental Program and Parameters

The test matrix included three primary parameters:

• Axial compression ratio (multiple levels)
• Steel ratio (multiple levels)
• Slenderness ratio (multiple levels)

The use of ultra-high strength concrete (UHSC) introduces unique challenges and opportunities for composite column design. The high concrete strength alters the conventional confinement mechanics and may require different design approaches compared to conventional strength concrete.

Seismic Performance Indicators Analysis

The following table summarizes the parametric effects on seismic performance:

The consistent trend across all three parameters provides clear design guidance: lower axial compression ratios, higher steel ratios, and lower slenderness ratios all contribute to improved seismic performance.

Critical Finding on Code Applicability

A particularly important finding is that current code methods for calculating flexural stiffness and ultimate moment are not applicable to steel tube UHSC structures. The calculation results show significant deviation from experimental values, indicating that the material interaction mechanisms in UHSC-filled steel tubes differ fundamentally from those in conventional concrete-filled tubes.

This finding has direct implications for seismic design codes and highlights the need for code revisions to accommodate ultra-high strength concrete applications in composite structures.

Engineering Practice Integration

For steel pipe fabrication and welding, the use of UHSC introduces additional considerations. The extremely high compressive strength of UHSC means that the steel tube confinement becomes even more critical, as the concrete's own ductility is limited. The steel tube must provide adequate lateral confinement to prevent brittle concrete crushing.

Welding quality is paramount in these columns. Any weld defects in the steel tube could lead to premature local buckling, which would catastrophically reduce the column's confinement effectiveness. Non-destructive testing should be comprehensive, with particular attention to:

• Weld toe regions for crack initiation
• Weld root regions for incomplete fusion
• Heat-affected zone properties for potential softening

The axial compression ratio finding reinforces the importance of accurate load estimation in design. Overestimation of axial loads could lead to over-design of the column, but underestimation could result in inadequate confinement and brittle failure.

Key Reflections and Study Insights

The finding that existing code methods are inadequate for UHSC-filled steel tubes is a significant contribution to the field. It suggests that the confinement mechanics in these members operate differently from conventional concrete-filled tubes, possibly due to the reduced concrete ductility and altered stress-strain behavior at very high strength levels. From a metallurgical perspective, the steel tube material properties become even more critical when paired with UHSC, as the steel tube must sustain higher confinement stresses without local buckling. Engineers should consider using higher strength steel grades with adequate ductility for UHSC applications, and ensure that welding procedures maintain the required material properties in the heat-affected zone.