In-Plane Mechanical Performance of CFST Truss Arch with Initial Stress

Literature Overview

The paper by Xiao Zerong (2020), published in Highway Transportation Research (Vol. 37, No. 1, pp. 66-73), presents experimental research on the in-plane mechanical performance of CFST truss arch structures subjected to initial stress. This study addresses a practical and often neglected issue in large-span CFST truss arch bridge engineering: the effect of residual or initial stresses, arising from fabrication, welding, and erection processes, on the structural performance and failure behavior. The experimental investigation focuses on the influence of initial stress on the in-plane load-bearing capacity, deformation characteristics, and failure mode of CFST truss arches.

Core Experimental Findings and Deformation Characteristics

The experimental results reveal distinctive deformation patterns and failure behaviors that are critical for the design and construction of CFST truss arch structures:

Performance Parameter	Without Initial Stress	With Initial Stress	Change
Ultimate load-bearing capacity	Baseline	Reduced	-7.3%
Maximum vertical displacement at ultimate load	Baseline	Reduced	-12.1%
Onset of elastic-plastic transition	Higher load level	Lower load level	Earlier transition
Failure mode	Chord weld fracture, diagonal member fracture	Same mode, more severe	Accelerated damage
Deformation symmetry	Symmetric	Symmetric (M-shaped)	Unchanged pattern

Deformation Pattern Analysis

Under mid-span concentrated load, the CFST truss arch with initial stress exhibits a symmetric "M"-shaped deformation profile:

The L/3 to 2L/3 segment shows significant downward deformation, with maximum displacement at mid-span.
The left arch foot to L/3 segment and the 2L/3 to right arch foot segment experience upward deformation.
The vertical web members at the arch crown are the first to buckle, indicating that the crown region is the critical zone for stability.
Before reaching ultimate load, no joint failure occurs, demonstrating good overall structural integrity.
At ultimate load, deformation accelerates rapidly, followed by diagonal member fracture and chord weld fracture.

Welding and Fabrication Quality Implications

The study directly highlights the impact of fabrication and welding quality on the structural performance of CFST truss arches. The initial stresses examined in this research are representative of the residual stresses that inevitably arise during the manufacturing and erection of steel structures:

Sources of Initial Stress in CFST Truss Arch Fabrication

Longitudinal weld residual stress: The HFW or LSAW longitudinal welds in the steel tubes generate residual stresses that can reach 60-80% of the yield strength in the weld heat-affected zone (HAZ).
Cross-weld residual stress: The circumferential welds at chord member splices and diagonal member connections introduce additional residual stresses.
Thermal distortion: The welding heat input causes angular and longitudinal distortion that, if not corrected, results in geometric imperfections that act as initial imperfections in the structural analysis.
Erection stress: The assembly and erection process, including temporary bracing removal and load transfer, introduces stresses that may remain in the structure.

Impact on Weld Quality and NDT Requirements

The 7.3% reduction in ultimate capacity and 12.1% reduction in deformation capacity due to initial stress have direct implications for welding quality control:

Weld acceptance criteria: The presence of initial stress means that weld defects, even if within acceptable limits per standard criteria, may have a more detrimental effect on structural performance than anticipated in stress-free conditions.
Post-weld treatment: Stress relief heat treatment (PWHT) or mechanical stress relief should be considered for critical welds in CFST truss arch components to reduce the initial stress level.
Weld sequence optimization: The welding sequence should be designed to minimize cumulative residual stress, using symmetric welding patterns and appropriate fixturing.
NDT sensitivity: Given the reduced tolerance for defects under initial stress conditions, NDT acceptance criteria should be more stringent for welds in CFST truss arch structures.

Engineering Practice Integration

For the design and construction of CFST truss arch bridges, the following practices are recommended based on the study findings:

Design Considerations

Account for the initial stress effect in structural analysis by applying an appropriate reduction factor to the predicted capacity, or by incorporating initial imperfections and residual stresses in the FE model.
Design the arch crown region with additional redundancy, as this is the critical zone where buckling initiates.
Ensure adequate weld design at chord member splices, as these are the final failure points in the structure.

Fabrication and Construction Controls

Implement a comprehensive weld sequence plan that minimizes residual stress accumulation.
Perform stress relief treatment on critical components before assembly.
Conduct dimensional inspection after welding to verify that distortion is within acceptable limits.
Use high-quality welding consumables and qualified welding procedures to ensure weld metal properties match or exceed base metal properties.

Inspection and Monitoring

Perform 100% UT or PAUT inspection on all chord splice welds and diagonal member connection welds.
Implement strain monitoring at the arch crown during construction to detect stress concentrations.
Conduct post-erection stress measurement using strain gauges or magnetic methods to quantify the actual initial stress level.

Study Insights and Reflections

This experimental study provides critical evidence that the initial stress state, arising from fabrication and welding processes, significantly affects the structural performance of CFST truss arches. The 7.3% capacity reduction and 12.1% deformation reduction are not trivial—they represent a meaningful degradation that can compromise the safety margin of the structure. The finding that initial stress does not change the failure mode but accelerates and intensifies the damage progression is particularly concerning, as it suggests that structures with higher initial stress levels may exhibit more sudden and catastrophic failure behavior.

From a welding engineering perspective, this study underscores the importance of residual stress management in the fabrication of structural steel components. The welding community should recognize that residual stress is not merely a theoretical concern—it has measurable and significant effects on structural performance. The adoption of welding procedures that minimize residual stress, such as low-heat-input processes, multi-pass welding with interpass temperature control, and symmetric welding sequences, should be standard practice for CFST truss arch fabrication. Additionally, the study highlights the need for more sophisticated structural analysis methods that incorporate the actual residual stress state of fabricated components, rather than assuming ideal stress-free conditions. The experimental methodology presented—combining controlled initial stress application with full-scale testing—provides a valuable framework for future research on the effects of fabrication-induced imperfections on structural performance.