Bending Performance of Steel Box Module Composite Square Steel Tube Beams

Literature Overview

The paper by Chang Hongfei, Hu Lei, Song Xinyi, Qin Fuyang, An Aichen, and Zhang Shaohua from China University of Mining and Technology addresses an important practical problem in modular steel construction: the weak inter-module connection in stacked steel box modular structural systems. Published in the Journal of China University of Mining and Technology in 2023, this study proposes a novel connection method using square steel tube beams stacked in series, with shear connectors providing composite action between adjacent modules. The research combines experimental testing with finite element parameter analysis to evaluate the bending performance of the proposed composite beam system.

Design Concept and Connection Methodology

The proposed system involves stacking upper and lower square steel tube beams with shear connections at the interface to create a composite beam that acts as a single structural unit. The key innovation is the use of bolted shear connectors with connecting plates to transfer shear forces between the upper and lower beams, thereby preventing relative slip and enabling composite bending action.

The design concept addresses a well-known limitation of modular steel construction: individual modules often behave independently under load, leading to inefficient use of material and reduced overall structural performance. By introducing shear connectors at the module interface, the system achieves a degree of composite action that significantly improves the bending capacity and stiffness.

The parameter analysis examined several key design variables:

Experimental Results and Failure Modes

The experimental program revealed two distinct failure modes in the tested specimens:

1. Steel beam yielding: In this mode, the steel tube beams themselves reach their yield stress before the shear connectors fail. This is the more desirable failure mode from a ductility standpoint, as it allows for significant plastic deformation before collapse.
2. Shear connector failure: In this mode, the shear connectors (bolts and/or connecting plates) fail before the steel beams yield. This is a more brittle failure mode and is generally undesirable, as it leads to sudden loss of composite action and potential progressive collapse.

The experimental results demonstrated the significant improvement achieved by the shear connections:

The 70.65% improvement in initial bending stiffness is particularly noteworthy. In practical terms, this means that the connected composite beam exhibits significantly less deflection under service loads, which is often the governing design criterion for long-span structures. The stiffness improvement is achieved without a proportional increase in material weight, making the system economically attractive.

Composite Bending Mechanism

The study provides a clear explanation of the composite bending mechanism:

1. Shear connection prevents slip: The shear connectors at the interface between the upper and lower beams prevent relative longitudinal slip, forcing the two beams to deform compatibly.
2. Eccentric loading on upper beam: The shear connection exerts a compressive force on the upper beam at an eccentricity from its centroid, creating a bending moment in addition to the axial compression.
3. Eccentric loading on lower beam: Similarly, the shear connection exerts a tensile force on the lower beam at an eccentricity, creating a bending moment in addition to the axial tension.
4. Neutral axis migration: As the shear connection action increases, the neutral axes of both the upper and lower beams migrate toward the composite interface. The upper beam develops a compression-bending characteristic, while the lower beam develops a tension-bending characteristic.

This mechanism is fundamentally different from the behavior of an unconnected composite beam, where the upper and lower beams bend independently with their own neutral axes at their respective centroids. The migration of the neutral axes toward the interface effectively increases the moment of inertia of the composite section, explaining the substantial improvement in bending stiffness.

Welding and Fabrication Implications

The fabrication of square steel tube beams and the shear connectors involves several welding operations that must be carefully controlled:

• Square tube fabrication: Square steel tubes are typically fabricated from plate by roll forming and welding the longitudinal seam. The weld quality of the longitudinal seam is critical, as it is the primary load path for the tube under bending. The weld should be inspected by ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) to detect internal defects.
• Shear connector plate welding: The connecting plates are welded to the top flange of the lower beam and the bottom flange of the upper beam. These welds are subjected to combined shear and bearing stresses and must be designed and inspected accordingly. The weld type (fillet weld, groove weld, or a combination) depends on the connection design.
• Bolt hole preparation: The bolt holes in the connecting plates must be accurately drilled or punched to ensure proper fit-up of the bolts. Poorly fitted holes can lead to eccentric loading of the bolts and premature fatigue failure.
• Preheat and post-weld treatment: Depending on the steel grade and plate thickness, preheating may be required to prevent cold cracking in the heat-affected zone. Post-weld heat treatment may be necessary to relieve residual stresses, particularly in thick-section connections.

Parameter Analysis Results and Design Recommendations

The finite element parameter analysis provided quantitative guidance on the optimal design of the shear connections:

• Number of bolts: Increasing the number of bolts per shear span significantly improves the bending performance relative to the unconnected case. The improvement is most pronounced at lower bolt counts and diminishes at higher counts due to the non-linear distribution of shear forces among the bolts.
• Optimal bolt configuration: The study recommends 4 bolts of 20 mm diameter per shear span connection, with connecting plates of 240 mm length and 8 mm thickness. This configuration provides a good balance between performance and fabrication cost.
• Connection position: The position of the shear connections along the beam length affects the distribution of shear forces and should be optimized based on the loading pattern. For a simply supported beam under uniformly distributed load, connections should be distributed along the midspan region where the shear force is highest.

Engineering Practice Integration

The proposed composite beam system has potential applications in modular construction, where prefabricated modules are assembled on-site to form structural frames. The key advantage is the significant improvement in bending performance achieved through relatively simple shear connections. This makes the system suitable for applications where rapid construction is required, such as temporary structures, emergency buildings, and modular commercial facilities.

From a quality control perspective, the critical inspection points include:

1. Weld inspection of tube longitudinal seams: Full-length UT or PAUT inspection to detect internal defects.
2. Shear connector weld inspection: Visual testing and magnetic particle testing (MT) or dye penetrant testing (PT) for surface defects, and UT for volumetric defects in critical welds.
3. Bolt tension verification: Torque wrench testing or ultrasonic bolt elongation measurement to verify that the bolts are tightened to the specified preload.
4. Dimensional accuracy of the assembly: Verification of the alignment and fit-up of the upper and lower beams before bolt tightening.

Study Insights and Future Considerations

This study demonstrates that the introduction of shear connectors at the interface of stacked steel tube beams can dramatically improve the bending performance of modular steel structures. The 26.67% improvement in bending capacity and 70.65% improvement in initial stiffness represent a significant engineering advantage that can translate into material savings and reduced construction costs. The composite bending mechanism, in which the neutral axes migrate toward the interface, provides a clear physical explanation for the observed improvements. Future research should address the fatigue behavior of the shear connectors under cyclic loading, as this is critical for applications in wind and seismic environments. The welding engineer must also consider the effect of welding residual stresses on the fatigue life of the shear connector welds, as these stresses can significantly reduce the fatigue strength of the connection.