Study on the mechanical behaviours of box-girder bridges reinforced with concrete-filled steel tube trusses

To address the uneven transverse load distribution in multi-box girder bridges lacking intermediate diaphragms, this study proposed a novel strengthening strategy of using concrete-filled steel tube trusses (CFSTTs) to replace traditional diaphragms. Experimental and numerical results demonstrated that the CFSTTs significantly enhanced the structural performance. Under static loading, even with pre-damage, the ultimate load capacity increased by more than 38.0%, and the deflection decreased by more than 35.0%, with the strengthening effect becoming more pronounced with increasing overload ratio. More importantly, CFSTTs successfully shifted the failure mode from local damage to global failure, achieving full coordination and utilization of all girders. To overcome the limitations of conventional evaluation methods for the transverse connection strength (TCS), a new TCS index was proposed, CFSTTs increased the TCS under eccentric loading by 46.2%, effectively mitigating eccentric load effects. Dynamically, under random traffic flow loads, the peak stress and deflection of the critical girder were reduced by 25.3% and 20.2%, respectively, with fatigue damage plummeting by 63.7%. Finally, a simplified CFSTT calculation model was developed to provide a practical tool for engineering applications.