Experimental research on mechanical behavior of RCFST composite truss beam under negative bending

The rectangular concrete-filled steel tube (RCFST) composite truss beam consists of concrete slab and RCFST truss. Subjected to the action of vertical loads, the composite effect of concrete slab and truss can be fully achieved in positive bending area, but its mechanical properties are weak in negative bending area as well as the concrete slab in tension tends to craze more easily. In response to this problem, a new type of composite truss beam with prestressed concrete slab and shear studs to release the shear action in local was presented. Two RCFST composite truss beams subjected to negative bending moments were tested by applying reverse concentrated loads at mid-span to simulate support reaction forces of continuous beams. The load versus displacement relationships, characteristics of development of cracks, strain distribution of concrete slab, load versus strain relationships of steel truss beam, the interface slip between steel truss and concrete slab and the bearing capacity of the RCFST composite truss were all analyzed. The structural characteristic loads at different loading stages were discussed according to the simplified mechanical model of composite truss beam. The results show that the new type composite truss beam with prestressed concrete slab and shear studs to release the shear studs to release shear action in local can improve the crack resistance capacity of structure, but it has limited influence on flexural capacity. The structural plastic deformation increases because of the cracking of concrete slab and yielding of truss members at first, then the composite truss reaches ultimate state after the failure of welds at the joint. The structural characteristic loads calculated from the simplified mechanical model agree well with the measured values, which can provide references for the design and calculation of RCFST composite truss beam under negative bending. © 2019, Editorial Office of Journal of Building Structures. All right reserved.