Axial Compression Performance of Grouted Connection Joints with Square Steel Tubes

Literature Overview

The paper by Li Tao, Xiao Bin, Li Yong, Li Yang, Wang Xiao-Ping, and Shao Yong-Bo (2023), published in the Journal of Xi'an University of Architecture and Technology, investigates the axial compression performance of grouted connection joints with square steel tubes used in modular prefabricated steel structures. The research is supported by multiple funding sources including the National Natural Science Foundation of China (Grant 51908437), Hainan Province Key R&D Program (ZDYF2021GXJS211), Hainan Provincial Science and Technology Special Project (ZDKJ2021024), and Central University Basic Research Business Fee Special Fund (2020IVB0090, 2020IVB009). The study proposes an improved grouted connection method incorporating shear keys, drawing inspiration from offshore wind turbine pile-foundation connections, and investigates the influence of shear key height (h) and spacing (s) on the ultimate load-bearing capacity of the joint.

Technical Background and Connection Design

Modular prefabricated steel structures offer significant advantages in construction speed, quality control, and adaptability, particularly for applications requiring rapid deployment such as offshore wind foundations, emergency shelters, and temporary structures. The connection between modular units is a critical design element that must transfer axial loads, shear forces, and moments while accommodating construction tolerances and thermal expansion.

The grouted connection method involves inserting an inner tube (or square tube) into an outer sleeve and filling the annular gap with high-strength grout material. This method provides a strong, monolithic connection that can effectively transfer axial loads through both the steel-to-steel interface and the grout-to-steel bond. However, conventional grouted connections may be susceptible to shear failure along the grout-steel interface, particularly under high axial loads combined with lateral forces.

The authors propose an improvement to the conventional grouted connection by incorporating shear keys (anti-shear keys) on the inner tube or outer sleeve surface. These shear keys act as mechanical interlocks that enhance the shear resistance of the connection and provide additional load transfer mechanisms beyond the frictional and adhesive bond between the grout and steel surfaces.

Test Specimen Design and Experimental Program

Seven square steel tube grouted connection specimens were designed and tested under axial compression loading. The key design variables investigated were:

The specimens were loaded to failure under axial compression, and the ultimate load-bearing capacity, failure mode, and deformation characteristics were recorded for each specimen. The test program was designed to systematically vary the h and s parameters to establish design guidelines for the shear key configuration.

Key Experimental Findings

The experimental results provide several important technical insights for the design of grouted connections with shear keys:

1. Material strength matching: The study reveals that the strength of the outer sleeve steel must be appropriately matched with the grout strength. If the sleeve steel is too weak relative to the grout, the connection will fail in the sleeve steel before the grout reaches its full capacity. Conversely, if the grout is too weak relative to the sleeve steel, the grout will fail in compression or shear before the steel reaches its yield strength. An optimal match ensures that both materials contribute fully to the connection's load-bearing capacity.
2. Effect of shear key quantity: Increasing the number of shear keys significantly improves the ultimate load-bearing capacity of the connection. This is because each additional shear key provides an additional mechanical interlock that resists shear sliding between the inner and outer tubes. The improvement is approximately linear with the number of shear keys, up to a point where other failure modes (such as sleeve steel yielding or grout crushing) become governing.
3. Diminishing returns with decreasing h/s ratio: While increasing the shear key height (h) and decreasing the spacing (s) generally improves the connection capacity, the efficiency of this improvement diminishes as the h/s ratio decreases. This is because very closely spaced or very tall shear keys create stress concentrations in the grout and can lead to premature grout failure between the keys.
4. Optimal spacing for a given number of keys: For a fixed number of shear keys, a moderate increase in the spacing (s) can slightly improve the ultimate load-bearing capacity. This counterintuitive result suggests that the grout between the keys requires sufficient space to develop full strength, and overly dense key arrangements can compromise the grout's compressive capacity.

Failure Mode Analysis

The failure modes observed in the tests provide insight into the load transfer mechanisms within the grouted connection. The primary failure modes include:

• Grout crushing: The grout between the shear keys fails in compression under high axial loads, typically occurring when the grout strength is insufficient relative to the applied stress.
• Sleeve steel yielding: The outer sleeve steel yields locally at the shear key locations due to stress concentration, particularly when the sleeve steel is relatively thin-walled.
• Shear key fracture: The shear keys themselves may fracture if they are too thin or if the grout provides insufficient lateral support.
• Interface debonding: In the absence of adequate shear keys, the grout-steel interface may debond under shear loading, leading to a sudden loss of connection capacity.

The shear keys effectively prevent interface debonding and redistribute the shear stresses over a larger area, resulting in a more ductile failure mode and higher ultimate capacity.

Engineering Practice and Design Recommendations

For engineers designing grouted connections in modular prefabricated steel structures, the following recommendations emerge from this study:

• Shear key configuration: The shear key height and spacing should be optimized to achieve the target load-bearing capacity while avoiding premature grout failure. A moderate h/s ratio is recommended, with specific values determined based on the grout strength and the applied load level.
• Material selection: The grout material should have sufficient compressive strength to match the sleeve steel strength. High-strength grout (f_g ≥ 60 MPa) is typically recommended for structural grouted connections. The sleeve steel grade should be selected to ensure that the sleeve does not yield before the grout reaches its full capacity.
• Quality control: The grouting process must be carefully controlled to ensure full void filling without air pockets or segregation. The shear key geometry must be manufactured to tight tolerances to ensure proper engagement with the grout. Pre-grouting inspection of the tube surfaces and shear key dimensions is essential.
• Design methodology: The study provides experimental data that can be used to develop design equations for the axial compression capacity of grouted connections with shear keys. Engineers should use these equations in combination with appropriate safety factors to ensure reliable connection design.

Study Insights and Implications

This research contributes meaningfully to the growing body of knowledge on grouted connections for modular prefabricated steel structures. The systematic investigation of shear key geometry parameters provides practical design guidance that can be directly applied to engineering projects. The finding that material strength matching is critical underscores the importance of a holistic design approach that considers the interaction between all connection components rather than treating each element in isolation.

The study also highlights an important trend in modern construction: the increasing use of modular and prefabricated steel structures, which requires innovative connection methods that can achieve monolithic structural performance while accommodating the practical constraints of modular construction. Grouted connections with shear keys represent a promising solution that combines the advantages of bolted connections (constructability, inspectability) with the advantages of welded connections (monolithic behavior, load transfer efficiency).

Future research should extend these findings to include cyclic loading (relevant for seismic and wind loading), the long-term durability of the grout-steel bond, and the effect of construction tolerances on connection performance. Engineers should also consider the cost implications of shear key fabrication and the additional inspection requirements for ensuring proper shear key geometry and grout filling quality.