Experimental Investigation of Hotspot Stress Characteristics of Joints in Rectangular Concrete-Filled Steel Tube Composite Truss Girders

This study investigates the hotspot stress concentration characteristics of rectangular concrete-filled steel tube (CFST) truss joints under bridge deck composite action. Two truss specimens, with and without a reinforced concrete deck, were tested under midspan vertical loading. Nominal stresses and hotspot stresses were measured at typical welded joint details, and finite element models were developed to examine the effect of deck thickness on hotspot stress distribution and potential fatigue-vulnerable regions. The results show that the chord-to-brace intersection corners exhibit much higher hotspot stresses than other locations, indicating that these regions are potential fatigue-vulnerable details requiring attention in fatigue evaluation. Bridge deck composite action significantly changes the load-transfer path and local stress distribution. Compared with the specimen without a deck, the hotspot stresses in the upper joint are markedly reduced, with the maximum reduction at key measurement points reaching approximately 85%. Meanwhile, the controlling hotspot stress location shifts from the chord corner of the upper joint to the brace corner of the lower joint. Increasing deck thickness further reduces the hotspot stresses in the upper joint, whereas the lower joint shows weaker and nonuniform variations. These findings indicate that deck composite action and deck thickness should be considered in the fatigue assessment of rectangular CFST composite truss bridges.