Experimental Study on Overall Stability Bearing Capacity of High-Strength Steel Tube Axial Compression Members

Literature Overview

This paper by Wang Hui, Li Xiaoyan, Sun Qing, and Xue Jianyang, published in Industrial Construction (2014, Vol. 44, No. 11), presents an experimental investigation of the overall stability bearing capacity of 18 Q690 welded steel tube axial compression members with varying slenderness ratios. The research is conducted jointly by Xi'an University of Architecture and Technology and Xi'an Jiaotong University. The study addresses the gap in design knowledge for high-strength steel (HSS) compression members, for which the conventional stability curves developed for lower-strength steels may not be directly applicable.

Core Technical Points

Material and Member Specifications

The study uses Q690 welded steel tubes, which have a minimum yield strength of 690 MPa. This is significantly higher than the conventional structural steel grades (Q235, Q345, Q390) for which most existing stability design curves were developed. The higher yield strength affects the stability behavior because the critical buckling stress may exceed the proportional limit, leading to inelastic buckling even at relatively low slenderness ratios.

Stability Behavior of High-Strength Steel Members

The stability behavior of compression members is governed by the interaction between the slenderness ratio and the material strength. For conventional steels, the Euler buckling stress is typically well above the yield stress, and inelastic buckling occurs over a wide range of slenderness ratios. For high-strength steels, the yield stress approaches the Euler buckling stress at lower slenderness ratios, resulting in a transition from flexural-torsional buckling to material yielding at lower slenderness ratios.

The study employed the inverse unit length method to determine the stability coefficient from the experimental results. This method back-calculates the effective unit length from the measured buckling load, allowing the determination of the stability coefficient as a function of the slenderness ratio. The resulting stability coefficient-slenderness ratio curves were compared with the GB 50017-2003 design curves, which were developed for conventional steel grades.

Comparison with Conventional Design Curves

The comparison between the experimentally derived stability curves for Q690 steel and the GB 50017-2003 recommended curves revealed that the conventional curves are not directly applicable to high-strength steel members. The high-strength steel members exhibit different buckling behavior due to the higher yield strength, which affects the inelastic buckling range and the post-buckling strength. The proposed stability coefficient calculation formula provides a more accurate prediction of the bearing capacity for Q690 steel tube compression members.

The numerical calculation results showed good agreement with the experimental data and provided a reasonable distribution of the stability coefficient-slenderness ratio curve. This confirms that the proposed formula can be used for the stability design of high-strength steel axial compression members, filling an important gap in the existing design codes.

Engineering Practice Implications

For engineers designing structures using Q690 or similar high-strength steel tubes, this study provides critical stability design data. The conventional design curves from GB 50017-2003 may be unconservative for high-strength steel members at certain slenderness ratios, potentially leading to unsafe designs. The proposed stability coefficient formula should be used for preliminary design, and the results should be verified by finite element analysis or, for critical members, by physical testing.

From a fabrication and welding perspective, Q690 welded steel tubes require careful attention to the welding procedure. The high strength of this steel grade means that the welding heat-affected zone (HAZ) is susceptible to cracking if the carbon equivalent (CE) is not controlled and if preheating is inadequate. The typical CE for Q690 steel is in the range of 0.45–0.55, which requires preheating temperatures of 100–200°C depending on the thickness and welding method. Post-weld heat treatment may be necessary for thick sections to relieve residual stresses and improve HAZ toughness.

The stability of welded steel tube compression members is also affected by the initial geometric imperfections, which include out-of-straightness, ovality, and residual stresses from the welding process. These imperfections reduce the actual buckling capacity compared to the theoretical Euler buckling load. The design formula should account for these imperfections through the use of appropriate reduction factors or equivalent imperfection amplitudes.

Key Questions and Reflections

One important question is the applicability of the proposed stability formula to other high-strength steel grades, such as Q890 or Q1100, which are increasingly being developed for specialized applications. The stability behavior may differ for these grades due to the even higher yield strength and potentially different strain-hardening characteristics. Additional experimental studies would be needed to extend the formula to these grades.

Another consideration is the effect of the loading eccentricity on the stability of high-strength steel tube members. In practice, perfectly axial loading is rarely achieved, and even small eccentricities can significantly reduce the buckling capacity of slender members. The study focuses on axial compression, but practical design must account for the combined effect of axial load and bending moment, which requires interaction equations that are also affected by the material strength.

Study Insights and Implications

This study provides essential experimental data and a validated stability design formula for Q690 welded steel tube axial compression members. The findings demonstrate that conventional design curves are not directly applicable to high-strength steel and that a modified approach is necessary. Engineers designing structures with high-strength steel tubes should use the proposed stability coefficient formula for accurate prediction of the buckling capacity. The study represents an important contribution to the rational design of high-strength steel structures and highlights the need for code updates to accommodate the increasing use of advanced high-strength steels in construction.