考虑二次防御设计的钢框架抗连续倒塌研究

Research on progressive collapse resistance of steel frame considering secondary defense design

各种偶然荷载作用可能引起结构局部破坏,甚至导致结构连续倒塌。采用二次防御设计理念,改善钢框架结构的抗连续倒塌性能,提出在梁柱节点区域设置弯起钢筋形成新的替代路径,并对梁-柱子结构进行Pushdown加载试验和数值模拟研究,分析钢筋直径、弯起高度和钢筋长度对子结构的作用,对比各子结构的荷载-位移曲线、破坏模式、变形能力,以及悬链线效应发展程度。研究结果表明:钢筋使子结构变形发展均匀,左右两侧梁对称变形,容易形成二力拉杆的效果,提高承载力和延性的同时,不改变塑性铰外移等构造特性;钢筋在弹性阶段基本没有参与钢梁的弯曲效应,随着加载位移增大,钢筋由弯折状态逐渐被拉直参与子结构的受力和变形,较大提高了悬链线抗力提供的内能,使悬链线贡献系数提高了20%左右;子结构的承载力随钢筋弯起高度和钢筋长度的变化趋势为先增大后减小,破坏模式也随之改变,根据钢筋和钢梁破坏顺序将子结构分为钢筋首先发生断裂、随后钢梁开裂和钢梁首先开裂、随后钢筋断裂两种破坏模式;弯起高度接近30 mm使钢筋充分发挥拉结作用,钢筋直径为12 mm易与常用弯起高度匹配,并避免损伤导致结构承载力大幅减小。

Various accidental loads may cause local damage to the structure and even lead to progressive collapse.Using the concept of secondary defense to improve the progressive collapse resistance of steel frame structure,a novel principle was proposed.The new concept involves installation of kinked reinforced bars in the beam-column joint area to realize alternative path.Pushdown loading test and numerical simulation were carried out on beam-column frame substructure with the kinked reinforced bars,taking into account the diameter,kinked height and length of the reinforced bars.The load-displacement curve,failure mode,deformation capacity and catenary development of the specimens were compared.The results show that the reinforced bars result in even deformation of the substructure,and encourage symmetrical deformation of the left and right beams to facilitate two-force tie rods,a case which improve the bearing capacity and ductility without changing the plastic hinge movement and other structural characteristics.The reinforced bars participate little in the bending action of the steel beam in the elastic state.With the increase of displacement,the reinforced bars gradually change from the bending state to the straightening state and participate in the stress and deformation of the specimen,which greatly improves the internal energy provided by the catenery resistance and increases the catenary contribution coefficient by about 20%.It is also found that the ultimate bearing capacity of the specimen increases and then decreases with the increase of kinked height and length of the reinforced bars,and the failure mode also changes accordingly.According to the sequence of reinforced bar fracture and steel beam cracking,the substructure is divided into two failure modes:steel bars first fracture,followed by steel beam cracking,and steel beam first cracking,followed by steel bars fracture.When the kinked height is close to 30 mm,the reinforced bars can play a better role in its tensile function and the diameter d=12 mm can better match the commonly used kinked height,and avoid reduction in the bearing capacity of the specimens caused by damage.