矩形钢管混凝土组合桁梁负弯矩区受力性能试验研究

矩形钢管混凝土组合桁梁由混凝土板和矩形钢管混凝土桁架组成，在竖向荷载作用下，其正弯矩区可充分发挥混凝土板和桁架的组合作用，但负弯矩区的力学性能较为薄弱且受拉混凝土板容易开裂。针对这一问题，提出了在负弯矩区混凝土板施加预应力以及布置局部释放剪切作用的剪力钉相结合的组合桁梁结构形式。采用跨中施加反向集中荷载模拟连续梁支点反力的方法，对2榀承受负弯矩的矩形钢管混凝土组合桁梁进行了静力加载试验，对其荷载-位移关系、裂缝发展规律、混凝土板应变分布、桁梁荷载-应变关系、钢与混凝土界面滑移及承载力进行了分析。还根据组合桁梁的简化力学模型对不同加载阶段的结构特征荷载进行了讨论。结果表明：采用局部释放剪切作用的剪力钉和混凝土板施加预应力的组合桁梁结构形式可有效提高其抗裂性能，但对受弯承载力影响较小；在加载过程中混凝土板的开裂和杆件的屈服导致结构塑性变形增大，最终节点处焊缝撕裂，组合桁梁丧失承载力；由简化力学模型计算得到的结构特征内力与实测值吻合较好，可为矩形钢管混凝土组合桁梁负弯矩区的设计和计算提供参考。

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.