Bearing Capacity Calculation Method for Square Hollow Sandwich Steel Tube Concrete Compression-Torsion Members

Literature Overview

This paper by Guo Lixiang, Li Ting, Yang Jian, and Huang Hong from Jiangxi Hangxiao Steel Structure Co., Ltd., East China Jiaotong University, and Qingdao University of Technology (Linyi) was published in "Railway Construction" (2014, Vol. 54, No. 7, pp. 4-7). The study develops a simplified bearing capacity calculation method for square hollow sandwich steel tube concrete (SHS-SC) members under combined compression and torsion, validated through finite element analysis and comparison with existing experimental data.

Core Technical Content

Structural System Description

The square hollow sandwich steel tube concrete member consists of:

• Outer steel tube: Square hollow section providing primary structural resistance
• Inner steel tube: Concentric square hollow section creating a sandwich configuration
• Concrete fill: Concrete placed in the annular space between outer and inner tubes
• Hollow core: The inner tube creates a hollow center, reducing self-weight while maintaining structural performance

Parameter Study Results

The finite element analysis investigated the influence of various parameters on the T/Tu-N/Nu interaction curve (normalized torsion-normal force interaction):

The dominant influence of slenderness ratio on the interaction curve is a critical finding, as it indicates that the failure mode transitions from material-based (strength) to geometry-based (buckling) as slenderness increases.

Simplified Calculation Method

Based on the parameter study results, the authors proposed a simplified bearing capacity equation that:

• Incorporates slenderness ratio as the primary variable
• Uses reduced interaction between axial and torsional capacities
• Accounts for the sandwich structural configuration
• Provides design values that match finite element and experimental results

Technical Analysis

Finite Element Model Validation

The FEA model was validated against existing experimental results, demonstrating:

• Good agreement in ultimate load predictions
• Consistent failure mode identification
• Reasonable prediction of deformation patterns

Slenderness Ratio Effect

The significant influence of slenderness ratio can be attributed to:

1. Buckling sensitivity: Higher slenderness increases susceptibility to local and overall buckling under combined loading
2. Stress gradient: Torsional stress distribution interacts with compressive stress more significantly in slender sections
3. Post-buckling behavior: Slender members exhibit more pronounced post-buckling strength degradation
4. Imperfection sensitivity: Real members have geometric imperfections that are more critical in slender configurations

Sandwich Configuration Benefits

Engineering Practice Integration

Design Application

The simplified calculation method enables:

1. Preliminary design: Quick evaluation of member adequacy under combined compression-torsion
2. Parametric optimization: Systematic investigation of section properties and material combinations
3. Code compliance checking: Verification against design code requirements
4. Cost optimization: Balance between steel usage and structural performance

Typical Application Scenarios

Square hollow sandwich steel tube concrete members are particularly suitable for:

• Railway bridges: Where torsional loads from track irregularities and train dynamics are significant
• High-rise buildings: Where weight reduction is critical and torsional resistance is needed
• Special structures: Where combined compression-torsion loading is dominant
• Long-span structures: Where self-weight reduction provides significant economic benefit

Design Considerations

Key Reflections and Insights

This research addresses a structurally important but under-studied configuration. The sandwich steel tube concrete concept combines the benefits of composite construction (high strength, good ductility, fire resistance) with the weight reduction advantages of hollow sections. The finding that slenderness ratio dominates the interaction behavior while material properties have relatively minor influence is both surprising and practically significant.

The dominance of slenderness ratio suggests that for sandwich steel tube concrete members, geometric design (section dimensions, wall thickness ratios) is more critical than material selection. This has important implications for design optimization: rather than specifying higher-strength materials (which are more expensive), designers should focus on optimizing the geometric proportions to control slenderness.

The simplified calculation method proposed in this paper provides a practical tool for engineers, but several considerations should be noted:

1. The method is based on a specific section geometry (square hollow sandwich) and may not directly apply to circular or other cross-section shapes
2. The parameter study found minimal influence of material properties, but this conclusion is specific to the range of parameters investigated
3. The method assumes elastic-perfectly plastic material behavior, which may not accurately represent real material response under combined loading
4. The interaction curve is approximate and should be used with appropriate safety factors in design

For practical engineering applications, the method provides a valuable starting point for member sizing. However, for critical structures or unusual loading conditions, detailed finite element analysis or experimental validation is recommended. The research also highlights the need for standardization of sandwich steel tube concrete design provisions in structural codes, as current codes primarily address solid-filled or single-tube concrete members.

The work represents a meaningful contribution to the understanding of composite structural behavior and provides engineers with a practical design tool for an innovative structural system that combines structural efficiency with constructability advantages.