Fatigue Assessment of Concrete-Filled Steel Tube Truss Arch Bridge Joints Based on Hot-Spot Stress Method

Literature Overview

The paper by Jiang Lei, Liu Yongjian, Liu Bin, Long Xin, and Pu Beichen (2022), published in Bridge Construction (Vol. 52, No. 3, pp. 69-76), presents a fatigue assessment methodology for CFST truss arch bridge joints using the hot-spot stress approach. Funded by the National Natural Science Foundation of China (52008026), the China Postdoctoral Science Foundation (2021M692746), and the Shaanxi Provincial Natural Science Foundation (2021JQ-272), the research is conducted jointly by Chang'an University and two design institutes. The study develops a full-bridge multi-scale 3D FE model to compute joint hot-spot stress amplitudes and establishes S-N curves for CFST joints through regression analysis.

Core Technical Content

Hot-Spot Stress Methodology

The hot-spot stress method is a well-established approach in fatigue analysis of welded structures, where the stress at a critical geometric discontinuity is extracted from a FE model using stress extrapolation or volume averaging techniques. For CFST truss arch bridge joints, the method accounts for the complex stress state at the intersection of the main tube and brace tubes, where stress concentrations are amplified by the geometric discontinuity and the composite action of steel and concrete.

FE Model and S-N Curve Development

The authors establish a full-bridge multi-scale 3D FE model to compute the hot-spot stress amplitude at critical joint locations. Regression analysis of the computed stress amplitudes yields S-N curves specific to CFST joints, which differ from conventional steel tube joints due to the presence of the concrete infill that modifies the stress distribution and crack initiation behavior.

Fatigue Assessment Results

The assessment of an existing CFST truss arch bridge identified the most fatigue-vulnerable location as the upper-chord joint near the 1/4 span point of the arch rib. The initial crack location is at the crown point on the surface of the main tube at the joint. Using the fatigue load model II from the Code for Design of Steel Bridges for Highway (JTG D64—2015), the predicted fatigue life is 20,210,049 cycles. Using the fatigue load spectrum method, the predicted life is 27,311,265 cycles, with a multi-vehicle longitudinal correction factor of 14.9%. The study concludes that the JTG D64—2015 predictions are conservative, resulting in over-designed fatigue details.

Technical Parameters and Standards Comparison

Welding Quality and Fatigue Implications

From my expertise in welding metallurgy and quality control, the fatigue performance of CFST truss arch bridge joints is intimately linked to the welding quality at the tube intersections. The crown point identified as the crack initiation location is precisely where the brace tube weld meets the main tube surface, and this is also the location of maximum stress concentration. The weld quality at this location—including penetration depth, weld toe geometry, absence of undercut, and residual stress state—directly governs the fatigue life.

The HAZ of the weld connecting the brace tube to the main tube undergoes significant microstructural transformation, including grain growth in the coarse-grained HAZ (CGHAZ) and potential formation of martensite in high-carbon equivalent steels. These microstructural changes can reduce local toughness and create preferential crack initiation sites. Post-weld treatment, such as TIG dressing of the weld toe or shot peening, can significantly improve fatigue performance by reducing the stress concentration factor and introducing beneficial compressive residual stresses.

The concrete infill inside the main tube provides a degree of constraint on the steel tube deformation, which can either improve or reduce fatigue performance depending on the loading mode. Under cyclic axial loading, the concrete provides additional support that reduces the strain range in the steel tube. However, under cyclic bending, the bond between the concrete and steel tube can experience fatigue degradation, potentially leading to bond failure that alters the stress distribution and accelerates fatigue crack growth.

Engineering Practice Integration

The finding that JTG D64—2015 predictions are conservative is important for bridge engineers, as it suggests that fatigue detail designs based on this code may be unnecessarily conservative for CFST joints. However, this does not diminish the importance of welding quality control. Even if the nominal fatigue life is adequate, poor welding practices can introduce local defects that dramatically reduce the actual fatigue life below the predicted value. Therefore, rigorous welding inspection protocols—including visual inspection (VT), ultrasonic testing (UT) for internal defects, and magnetic particle testing (MT) for surface cracks—remain essential for ensuring the fatigue reliability of CFST truss arch bridge joints.

Study Insights and Implications

This research provides a rigorous methodology for fatigue assessment of CFST truss arch bridge joints that accounts for the unique composite behavior of the structure. The development of S-N curves specific to CFST joints, derived from multi-scale FE analysis, represents a significant advancement over applying conventional steel joint S-N curves to composite members. For welding engineers, the study reinforces the critical role of weld quality in determining fatigue life, particularly at the crown point of tube intersections. The identification of the 1/4 span upper-chord joint as the most vulnerable location provides a clear target for inspection and maintenance programs during bridge operation.