Mechanical Properties of CFST Column with Steel Beam Connection Using Outer Sleeve Tube

Literature Overview

The study by Liu Jian, Gao Kui, Zhou Guangen, and Chen Yuan (Guangzhou University and Zhejiang Dongnan Steel Structure Co., Ltd., 2015) presents a novel end-plate bolted connection between a concrete-filled steel tube (CFST) column with an outer sleeve tube and a steel beam. The research addresses the well-known challenge of connecting steel beams to CFST columns, which is complicated by the presence of concrete inside the steel tube. Funded by the Ministry of Housing and Urban-Rural Development Science and Technology Plan Project (2012-k2-8), Guangdong Provincial Education Department Science and Technology Innovation Project (2012-KJCX-0084), and a horizontal project from Zhejiang Dongnan Steel Structure Co., Ltd. (SM-0716-01), this work combines experimental data with finite element analysis to characterize the connection behavior.

Core Technical Findings

The proposed connection uses an outer sleeve tube welded to the CFST column, with the steel beam connected to this sleeve via a single-side bolted end-plate arrangement. The study establishes an M-θr (moment-rotation) practical model for this connection type and identifies the primary factors influencing initial stiffness and bearing capacity.

Parameter	Effect on Connection Performance
Outer sleeve tube thickness	Significant positive effect on initial stiffness and capacity
End-plate thickness	Moderate positive effect on bearing capacity
Bolt grade and number	Direct influence on ultimate moment capacity
Concrete strength grade	Minor effect (concrete does not directly participate in connection)
Initial geometric imperfection	Reduces ultimate capacity by approximately 5–10%
Beam-to-column width ratio	Affects stress concentration at the sleeve-column junction

Technical Interpretation of the Outer Sleeve Concept

Rationale for the Outer Sleeve Design

Traditional connections between steel beams and CFST columns face several challenges:

Concrete interference: Direct welding of connection plates to the CFST tube wall is complicated by the presence of concrete, which can cause excessive deformation of the tube wall under cyclic loading.
Limited deformation capacity: The concrete inside the tube restricts the plastic deformation of the steel tube wall, reducing the ductility of the connection.
Complex fabrication: Connections that penetrate the tube wall require careful consideration of concrete removal, welding access, and potential damage to the concrete.

The outer sleeve tube approach solves these problems by providing a dedicated steel element that absorbs the connection stresses without directly involving the concrete-filled tube. The sleeve acts as a transition element between the rigid CFST column and the flexible beam connection.

Finite Element Modeling Approach

The authors developed finite element models considering:

Initial geometric imperfections: Modeled using the first buckling mode shape scaled to the measured imperfection amplitude (typically L/1000 per EN 10219).
Concrete plastic damage constitutive model: Based on Mazars damage model or a simplified Drucker-Prager model with damage parameters calibrated against experimental data.
Contact definition: Penalty contact with friction coefficient of 0.3 between the sleeve inner surface and the CFST tube outer surface.
Bolt preload: Modeled using pre-tensioned bolt elements with yield strength of 8.8 or 10.9 grade per GB/T 3098.1.

Welding and Fabrication Considerations

From a steel pipe fabrication and welding perspective, the outer sleeve connection introduces specific technical requirements:

Sleeve-to-Column Welding

Weld Parameter	Specification
Weld type	Full-penetration fillet weld or T-butt weld
Welding process	SMAW or FCAW for field conditions; GTAW for workshop
Weld procedure qualification	ISO 15614-1 or AWS D1.1
Preheat temperature	80–120°C for Q345 steel with wall thickness > 20 mm
Interpass temperature	Maximum 250°C
Post-weld heat treatment	Recommended for stress relief if wall thickness > 25 mm
NDT requirements	100% MT or PT for surface; 100% UT for volumetric

Sleeve Tube Selection

The outer sleeve tube should be selected based on:

Material matching: Same grade as the CFST column (typically Q345B or Q390B per GB/T 1591-2018).
Dimensional tolerances: Ovality ≤ 1% of diameter, straightness ≤ 0.5% of length (per EN 10219-2006).
Wall thickness: Typically 10–20 mm, selected to achieve the desired connection stiffness while maintaining ductility.
Surface finish: Clean, free of scale and oxide, to ensure proper weld quality.

M-θr Practical Model

The authors propose a practical calculation method for the M-θr curve that accounts for:

Initial elastic stiffness: Determined by the sleeve tube section properties and bolt group stiffness.
Yield moment: Governed by the plastic moment of the sleeve section, reduced by the stress concentration factor at the sleeve-column junction.
Post-yield hardening: Limited by the concrete confinement effect on the sleeve tube deformation.
Ultimate moment: Reached when the sleeve tube undergoes local buckling or the bolts reach their ultimate strength.

The practical model shows good agreement with finite element results, with deviations typically within 10–15% across the full range of rotation angles.

Engineering Practice Integration

Application Scenarios

The outer sleeve connection is particularly suitable for:

High-rise buildings with CFST columns where seismic performance is critical.
Industrial structures where column connections need to be field-installable without extensive welding.
Retrofit projects where existing CFST columns need to be connected to new steel beams.

Quality Control Checklist

Verify sleeve tube material certificate (mill test report per EN 10204 3.1).
Inspect sleeve tube dimensional accuracy before welding.
Qualify welding procedure per ISO 15614-1 with impact testing at -20°C minimum.
Perform 100% magnetic particle inspection (MT) on all sleeve-to-column welds.
Verify bolt torque to 70% of proof load per GB/T 1231.
Conduct connection load test if required by the design code.

Key Questions and Reflections

The study does not extensively address the cyclic (seismic) behavior of the outer sleeve connection. In seismic design, the connection must undergo repeated loading-unloading cycles without significant degradation. The outer sleeve tube, being a thin-walled section, is susceptible to local buckling under cyclic loading. Further research on the fatigue and cyclic performance of this connection type would be valuable.

Additionally, the effect of welding residual stress on the connection performance warrants investigation. The sleeve-to-column weld introduces significant residual stresses that can affect the initiation and propagation of cracks under cyclic loading. A residual stress measurement study using the hole-drilling method (per ASTM E837) would provide valuable data for improving the design approach.

Study Insights and Implications

This research demonstrates that the outer sleeve tube concept is a viable solution for connecting steel beams to CFST columns, offering improved initial stiffness and bearing capacity compared to traditional connections. For steel pipe fabricators, the key implication is that the sleeve tube must be manufactured to high dimensional accuracy and surface quality standards, as any deviation can compromise the connection performance. Welding engineers must ensure that the sleeve-to-column weld is fully qualified and inspected, as this weld is critical to the overall connection integrity. The proposed M-θr practical model provides a design tool that can be readily incorporated into structural analysis software, facilitating the widespread adoption of this connection type in practice.