Bond Strength of Square Steel Tube Concrete Members Experimental Study

Literature Overview

This experimental and analytical study by Huang Yijie, Xue Jianyang, Zhao Hongtie, and Wu Jianbin, published in the Journal of Architecture and Civil Engineering (2010, Vol. 27, No. 1, pp. 43-48), investigates the bond strength between steel tubes and concrete cores in square steel tube concrete (SSRC) members. Funded by the National Natural Science Foundation (Grant No. 50478044), the research is conducted at Tongji University and Xi'an University of Architecture and Technology. The authors conduct push-out tests on SSRC columns with varying aspect ratios and width-to-thickness ratios, then derive a semi-empirical semi-theoretical bond strength formula using elasticity and strength of materials theory.

Core Technical Content and Test Program

The push-out test is the standard method for measuring the bond strength between steel tubes and concrete cores in composite members. In this test, a concrete plug is pushed out of a steel tube sleeve, and the maximum force divided by the contact area gives the bond strength. The authors systematically vary the aspect ratio (height-to-width ratio) and width-to-thickness ratio (B/t) of square steel tubes to identify the key geometric parameters affecting bond strength.

The experimental results reveal that the bond strength of SSRC members is significantly influenced by the width-to-thickness ratio (B/t) but only slightly affected by the aspect ratio. Higher B/t ratios (thinner walls relative to width) result in lower bond strengths due to reduced lateral confinement effectiveness and increased steel tube flexibility.

Test Parameters and Bond Strength Results

Interpretation of Technical Points

The proposed semi-empirical semi-theoretical bond strength formula is based on the observation that failure occurs when the thin-layer concrete surrounding the core concrete is crushed. This "thin-layer concrete crushing" failure mode is distinct from the conventional pull-out failure observed in reinforced concrete members. The formula incorporates the elastic modulus of the composite section, the geometric parameters of the steel tube, and the compressive strength of the concrete.

The derivation uses elasticity theory to analyze the stress distribution in the thin concrete layer between the core concrete and the steel tube inner wall. Under axial loading, the differential Poisson's ratio between concrete and steel creates radial stresses that contribute to bond development. The steel tube's lateral restraint prevents the thin concrete layer from expanding laterally, creating a confined state that enhances the effective bond strength.

Bond Strength Formula Components

Integration with Engineering Practice

The bond strength between steel tubes and concrete cores is a critical design parameter that governs the load transfer mechanism in SSRC members. Inadequate bond strength can lead to premature debonding, loss of composite action, and reduced structural capacity. For steel tube fabrication, this research highlights the importance of controlling the inner surface quality of steel tubes, as surface roughness, scale, and oxide layers can significantly affect the bond development.

The width-to-thickness ratio (B/t) is a critical fabrication parameter that directly influences bond strength. Thinner-walled steel tubes provide less lateral confinement to the concrete core, resulting in lower bond strengths. However, thinner walls also reduce material cost and weight. The optimal B/t ratio must balance structural efficiency with bond strength requirements. For square steel tubes with B/t ratios exceeding 30, additional measures such as surface roughening, mechanical interlocks, or chemical bonding agents may be necessary to ensure adequate bond strength.

Fabrication Requirements for Bond Strength Optimization