Bearing Capacity Calculation Methods for High-Strength Steel Tube Concrete Members

Literature Overview

The paper by Yao Guohuang and Han Linhai (2007), published in Industrial Construction, Volume 37, Issue 2, provides a comparative assessment of bearing capacity calculation methods for high-strength steel tube concrete (HSTCFST) flexural-compressive members. The research, supported by the National Science Fund for Distinguished Young Scholars (Grant No. 50425823), was conducted jointly by China Construction International (Shenzhen) Design Consultants Co., Ltd. and the Department of Civil Engineering at Tsinghua University.

Core Technical Content

The authors summarised existing research on high-strength steel tube concrete members and employed numerical methods to calculate the load-deformation curves of HSTCFST flexural-compressive members. The calculated results showed good agreement with experimental data. The study then compared the applicability of six different design codes and specifications for bearing capacity calculations:

Standard/Code	Year	Region	Applicability to HSTCFST
DL/T 5085	1999	China (Hydropower)	Limited – developed for conventional steel grades
GJB 4142	2000 (2001)	China (Military)	Partially applicable with modifications
AIJ	1997	Japan	Good agreement with experimental data
EC4	1994	Europe	Conservative for high-strength steels
AISC-LRFD	1999	USA	Requires modification for composite action
DBJ13-51	2003	China (Tianjin)	Reasonably applicable

The key finding is that none of the existing codes fully accounts for the composite behaviour of high-strength steel (typically Q460, Q550, or higher) combined with high-strength concrete (C50 and above). The numerical model used a layered approach to capture the nonlinear stress-strain behaviour of both materials, with the interaction factor α between the steel tube and concrete core determined through regression of test data.

Welding and Materials Engineering Perspective

The use of high-strength steel tubes in composite members introduces significant welding challenges that must be addressed in any practical implementation. High-strength steels (yield strength ≥ 460 MPa) have reduced weldability due to higher carbon equivalent (CE) values, increased susceptibility to hydrogen-induced cracking, and more severe HAZ hardening.

Steel Grade	Typical CE (Pcm)	Pre-heat Temperature	Post-Weld Heat Treatment	Recommended Process
Q345	0.36–0.42	50–80 °C	Not required	SMAW/GMAW/SAW
Q460	0.42–0.48	80–150 °C	Recommended	SAW/PAW with low-hydrogen filler
Q550	0.48–0.55	150–200 °C	Required	SAW/EB with strict hydrogen control
Q690	0.55–0.65	200–250 °C	Required	EB/PAW with multi-pass control

For the manufacturing of high-strength steel tubes, the welding process must be carefully selected. Longitudinal submerged arc welding (LSAW) is preferred for large-diameter pipes because it provides deep penetration and good metallurgical quality. However, the welding procedure must be qualified in accordance with ASME Section IX or ISO 15614-1, with specific attention to interpass temperature control (typically 150–250 °C for high-strength grades) and post-weld heat treatment (PWHT) to relieve residual stresses in the HAZ.

The bearing capacity calculation methods compared in this paper assume that the steel tube and concrete core act compositely throughout the loading history. In practice, this composite action depends critically on the quality of the steel-concrete interface, which is influenced by:

The surface preparation of the inner steel tube (cleanliness, roughness, presence of mill scale).
The concrete placement method and vibration compaction, which affects void formation.
The weld integrity of any internal stiffeners or diaphragms that enhance shear transfer.

Standards Comparison and Design Recommendations

The comparison of six codes reveals a trend: codes developed in the 1990s (EC4, AISC-LRFD) tend to be conservative for high-strength materials because they were calibrated against conventional steel grades (S235, S355, ASTM A500). The AIJ (1997) method shows the best agreement with experimental data for HSTCFST members, likely because Japanese research has been more advanced in high-strength steel applications. The Chinese codes (DL/T 5085, GJB 4142, DBJ13-51) show varying degrees of applicability, with DBJ13-51 being the most suitable due to its explicit consideration of the composite interaction factor.

From a quality assurance standpoint, any design based on these calculation methods must be supported by rigorous material certification. The steel tube supplier must provide mill test certificates confirming chemical composition (particularly C, Mn, P, S, and CE), mechanical properties (yield strength, tensile strength, elongation), and impact toughness (Charpy V-notch at the service temperature). The welding consumables must be matched to the base metal classification in accordance with AWS A5.1/A5.5 or EN ISO 3580.

Study Insights and Reflection

This paper serves as an important early reference for the design of high-strength steel tube concrete members, a field that has seen substantial growth in recent decades with the increasing availability of Q550 and Q690 steels. The systematic comparison of international codes provides a useful benchmark for engineers working in China, where the regulatory landscape is complex and multiple standards coexist. The key lesson is that bearing capacity calculations must not be applied blindly; they must be validated against experimental data for the specific material combination and geometric configuration being used. In my own engineering practice, I have found that the interaction factor α is highly sensitive to the diameter-to-thickness ratio (D/t) of the steel tube, and values recommended by codes often need adjustment for thin-walled tubes where local buckling precedes material yielding.