Combined bolted–welded joints use both bolting and welding methods to connect several members, resulting in a versatile and robust solution for structural connections. However, very limited studies have focused on the residual stress distribution and fatigue behavior of these joints. In this paper, a total of eight specimens of double lap joints using bolts and fillet welds were fabricated and tested to measure the residual stress distribution. A finite element model was also developed for predicting the residual stress and residual deformation, and then it was validated against the test results. The effects of different welding parameters on the residual stress and residual deformation were evaluated, including the welding sequence (four different welding sequences) and welding process (welding speeds of 4 mm/s, 6 mm/s and 10 mm/s; welding powers of 5000 W, 6000 W and 7000 W; and post-weld heat treatments of no insulation, insulation at 200 °C and insulation at 300 °C). The fatigue behaviors of combined bolted–welded joints with and without residual stresses were compared in terms of the fatigue life of crack propagation. It was shown that the maximum residual stress was approximately 450 MPa, far exceeding the yield strength of steel plate of 335 MPa, while welding sequence 1 produced the smallest residual stresses. Due to the presence of welding residual stresses, the fatigue life of combined bolted–welded joints was reduced by nearly 40%, which indicated that the fatigue life of the joint would be overestimated without considering the residual stresses.

Curved composite twin-girder bridges are suitable for mountainous areas, due to their advantages of light self-weight, excellent mechanical performance, and fewer construction requirements. It has been found that many composite twin-girder bridges collapsed due to fatigue failure. However, the literature review showed no relevant studies on the fatigue performance of curved composite twin-girder bridges. Because of this, the specimen of 1:2 scale curved composite twin-girder bridge in accordance with the design scheme of Xizhen Bridge in China was designed and tested. Three possible fatigue details were selected: cruciform connections, transverse attachments, and transverse splices named Class I, Class II, and Class III. For the test data of nominal stress (NS), equations were proposed to convert the strain value into the internal force of the fatigue detail position. The stress caused by torsion accounts for 2.8% of the total stress, which is almost negligible. The fatigue evaluation process based on the hot spot stress (HSS) S-N curve method is presented. The HSS method is more conservative than the NS S-N curve method in predicting the fatigue life of complex structures with high-stress concentrations.

In order to promote the development of bridge assembly technology and accelerate the application of rectangular steel-tube–concrete composite truss bridges, this study focuses on the Yellow River Diversion Jiqing Main Canal Bridge as the engineering example and conducts a numerical analysis of a rectangular steel-tube–concrete composite truss bridge. Based on the results of the analysis, structural optimization is achieved in three dimensions—structural design, construction methods, and force analysis—leading to the establishment of key design parameters for through-type ultra-high-performance rectangular steel-tube–concrete composite truss bridges. The results show that filling the hollow sections with ultra-high-strength concrete can significantly enhance the load-bearing capacity. Additionally, employing prestressed concrete components addresses the bending and tensile load capacity challenges of composite structures, thus maximizing the material strength advantages. The proposed preliminary design scheme incorporates prestressed PBL-reinforced tie rods filled with ultra-high-performance concrete with optimal design parameters, such as high span ratios, wide span ratios, and ideal segment lengths, are suggested to ensure that the strength, stiffness, and stability comply with relevant standards. While ensuring that the structure meets safety, applicability, and durability criteria, the preliminary design scheme reduces steel usage by 23.5%, concrete usage by 11.6%, and overall costs by 17.29% compared to the original design. The proposed design demonstrates distinct advantages over the original in terms of mechanical performance, construction efficiency, economic viability, and durability, highlighting its promising application potential.

In terms of load transfer, the design of the joints in concrete-filled steel tubular (CFST) arch bridges is more critical than that in buildings due to the higher likelihood of steel–concrete-interface debonding. To improve the contact at the steel–concrete interface, a novel arch rib was manufactured by longitudinally welding perfobond-rib-shear connectors to the inner surface of a steel tube and then filling the tube with concrete. In this study, extensive numerical and analytical investigations on the mechanism of introducing loads into CFST arch ribs through perfobond-rib-shear connectors were carried out. A deck CFST arch bridge, namely, the Shuangbao Bridge in China, was selected as a typical application location. The design parameters, including the geometric dimensions of the perfobond-rib-shear connector and the arrangement of the perfobond rib along the cross-section and longitudinal section of the arch rib, were evaluated. The design flow for the joint with perfobond-rib-shear connectors between the vertical columns and the CFST arch ribs was proposed. To improve the load-transfer efficiency, the design scheme of the joint in the Shuangbao Bridge was optimized by replacing the weld studs with perfobond ribs. Significant increases of 1.84–4.02 in the shear resistance were found for the perfobond ribs compared to the welded studs. Additionally, the fabrication of the perfobond ribs was more convenient compared to that of the welded studs.

In recent years, square hollow section (SHS) joints with concrete-filled square hollow section (CFSHS) chords have increasingly been used in truss bridges where the fatigue life reliability is a critical issue. In this paper, a finite element analysis was performed to investigate the SCFs in SHS-CFSHS T-joints under in-plane bending in the brace, axial force in the chord and in-plane bending in the chord. The finite element models were developed and validated with experimental results. Then a parametric study was conducted with a reasonable range of three key non-dimensional parameters, i.e., β (width ratio between brace and chord), 2γ (width–to–wall thickness ratio of the chord) and τ (wall thickness ratio between brace and chord). Consequently, the stress concentration factor formulae for the fatigue design of SHS-CFSHS T-joints were proposed through multiple regression analysis. For in-plane bending in the brace, the maximum stress concentration factors were found at lines B and C for thick-walled chords (2γ = 25.0), while the stress concentration factors at all the lines need to be checked for thin-walled chords (2γ < 16.0). Under axial force in the chord and in-plane bending in the chord, only stress concentration factors at lines C and D needed to be considered. A comparison of stress concentration factors between SHS-SHS and SHS-CFSHS joints showed reductions of 10~26% and 14~31% in stress concentration factors in SHS-CFSHS joints under axial force in the brace and in-plane bending in the brace, respectively. In addition, it showed a general increase in stress concentration factors in SHS-CFSHS joints under axial force and in-plane bending in the chord. This reduction is attributed to the reduction in chord face deformation benefiting from the in-filled concrete. Meanwhile, the stress concentration factors caused by loads in the chord are much lower than those caused by loads in the brace. This work complements earlier studies on SHS-CFSHS T-joints under axial force.

During the operational phase of a bridge, the crossbeam, acting as a supporting member, plays an important role in keeping the cross-sectional shape constant in addition to resisting against various lateral and longitudinal loads and distributing the dead and the live loads to the adjacent main girders. The complex functional requirements lead to a complex internal force composition of the crossbeam. When subjected to torque, the two main beams of the twin I-girder bridge will have deformation in opposite longitudinal directions (known as warping deformation) to counteract the torque. The existing research has not considered the impact of main beam warping deformation on the internal force of the crossbeam. Based on the existing research, this article further considers the impact of main beam warping deformation on the internal force of the crossbeam, making the calculation of the internal force of the crossbeam more accurate. The results show that the torsional characteristics of the continuous twin I-girder bridge can be calculated using Vlasov’s theory of thin-walled structures combined with the displacement method. As for the effect of the crossbeam on the torsional stiffness of the structure, it can be managed by making the crossbeam stiffness continuous; however, in general, the equivalent stiffness is small compared to the stiffness of the main beam and it can be ignored. The crossbeam can be simplified to a bar with two solid ends for the internal force calculation whose formula is proposed in this paper, based on the existing frame model, and it can further consider the influence of warping deformation of the main beam on the internal force of the beam, and the calculation accuracy is high.