Seismic Retrofit Design of Prefabricated CFST Column-Steel Beam End Plate Bolted Connections

Literature Overview

This study by Li Liyi, Ma Jiangping, Liu Qingying, and Yang Yan from the School of Architecture and Art Design at Xi'an Peihua University, published in the journal Adhesives in 2022, investigates the seismic performance of prefabricated concrete-filled steel tube (CFST) column to steel beam end plate bolted connections. Five test specimens were subjected to pseudo-static loading to evaluate the influence of end plate thickness, bolt diameter, and concrete strength on the connection's load-carrying capacity, stiffness, and ductility.

Experimental Design and Key Parameters

The experimental program was designed to systematically vary three critical design parameters while maintaining other geometric and material properties constant. The five specimens covered a range of end plate thicknesses and bolt diameters commonly encountered in practice. The pseudo-static test protocol involved cyclic displacement loading to simulate seismic demand, with displacement amplitudes increasing progressively to capture the full hysteretic behavior from elastic to post-peak stages.

Core Findings and Technical Analysis

The study identifies these connections as semi-rigid nodes, which is a critical classification for seismic design. Semi-rigid connections exhibit intermediate behavior between pinned and fully rigid connections, with moment-rotation response that depends on the specific geometry and material properties. The key findings are:

1. The seismic strength of the connection increases monotonically with both end plate thickness and bolt diameter. This is physically intuitive: thicker end plates resist bending deformation more effectively, and larger bolts provide greater tensile and shear resistance.
2. When the end plate and steel beam govern the load-carrying capacity (rather than the CFST column itself), the influence on the overall frame's load capacity, stiffness, and ductility is relatively small. This finding is significant because it suggests that connection design can be optimized without significantly compromising global frame behavior.
3. The connections can effectively improve the ductility of the frame, which is the primary objective of seismic design. The hysteretic loops exhibited stable energy dissipation characteristics, indicating good seismic performance.

Connection with Welding and Fabrication Practice

From a fabrication standpoint, end plate bolted connections are favored in prefabricated construction because they eliminate field welding requirements, reducing quality risks and construction time. However, the end plate itself is typically attached to the CFST column through welding, and the quality of this weld is paramount. The weld between the end plate and the steel tube must be designed and executed to meet the requirements of semi-rigid connection behavior.

In practice, the weld type used for attaching the end plate to the CFST column is typically a full-penetration groove weld or a combination of fillet and groove welds. The weld quality directly affects the connection's moment capacity and ductility. Common weld defects such as incomplete fusion, porosity, and lack of penetration can significantly reduce the effective cross-section of the weld and create stress concentration points that initiate fracture under cyclic loading.

The bolted connection introduces another quality consideration: the bolt hole fabrication tolerance and bolt fit-up. In prefabricated applications, the bolt holes must be accurately drilled or punched in both the end plate and the beam end plate to ensure proper alignment and avoid forced assembly that could induce residual stresses.

Standards and Design Considerations

The design of these connections should comply with relevant standards including GB 50011 (Code for Seismic Design of Buildings), GB 50017 (Standard for Design of Steel Structures), and potentially ASCE 41 for seismic evaluation and retrofit. The semi-rigid classification requires careful consideration in structural analysis models, as assuming either fully rigid or fully pinned behavior can lead to non-conservative or overly conservative designs.

Critical Reflections

The study's finding that connection design has limited influence on global frame behavior when the end plate and beam govern capacity is somewhat counterintuitive and warrants careful interpretation. It suggests that there is design flexibility in connection sizing without compromising seismic performance, which is valuable for cost optimization. However, engineers must be cautious not to over-interpret this finding, as it applies specifically to the tested parameter range and loading conditions.

The classification of these connections as semi-rigid is important for structural analysis. In practice, the moment-rotation curve of a semi-rigid connection must be accurately characterized for use in frame analysis software. The pinning effect at the connection reduces the effective stiffness of the frame, which can lead to increased story drifts under seismic loading. The pseudo-static test data provides the experimental basis for developing these moment-rotation relationships.

Study Insights and Practical Recommendations

This research contributes valuable experimental data for the seismic design of prefabricated CFST structures. The key practical implication is that end plate bolted connections can provide adequate seismic performance when properly designed, offering a viable alternative to welded connections in prefabricated construction. For engineers involved in steel pipe and fitting supply, the study reinforces the importance of providing accurate material certifications and weld quality documentation for CFST columns used in seismic zones.

The parametric study approach is methodologically sound, but the limited number of specimens (five) constrains the generalizability of the findings. Future research should expand the parameter space to include variations in connection geometry, loading angle, and cyclic load history to provide a more comprehensive design basis.