端板连接节点中抗拉承载力和撬力计算的探讨

Disscussion on the Tension Capacity with Prying Force of Bolted End-Plate Connections

弯矩作用下的高强度螺栓端板连接中,端板在拉力作用下会发生弯曲变形并产生撬力,相对刚度较弱的连接节点设计中,撬力作用不可忽视。按国内标准设计的端板连接节点普遍存在螺栓数量多、端板厚度大的问题,因而有必要分析撬力Q、节点承载力Nt、螺栓拉力Nb以及螺栓抗拉承载力Nbt之间的关系,并通过对现行标准算式及其参数进行对比研究,找出导致我国标准偏于保守的原因。受拉T型件模型可用于模拟高强度螺栓端板连接节点,国内外的受拉T型件节点试验和理论分析均表明:撬力效应明显的受拉T型件节点,其第一个塑性铰均首先出现在T型件翼板根部附近。随着外拉力Nt逐步增加,T型件翼板由弹性状态逐步进入弹塑性状态,撬力也从无到有,逐步增大,直至螺栓被拉断或T型件翼板完全破坏(在螺栓孔附近出现第二个塑性铰)。外拉力Nt分为Nt1和Nt2两部分,其中Nt1对应T型件翼板根部出现塑性铰时的外拉力,Nt2则与撬力Q在螺栓附近形成弯矩平衡,外拉力Nt的力臂为T型件根部至螺栓孔的距离,撬力Q的力臂为螺栓孔至T型件边缘的距离。根据节点破坏模式和破坏状态下螺栓孔处的弯矩平衡条件,得到节点抗拉承载力Nt、撬力Q、螺栓拉力Nb及破坏模式临界板厚的计算式,根据节点抗拉承载力算式可以绘制节点承载力曲线,并得到三种破坏模式的控制条件及三种破坏模式之间的关系。国内外对于考虑撬力效应的高强度螺栓连接设计方法存在差异,与国外标准的对比结果表明:1)中美标准均是已知外力Nt计算板厚;欧洲标准是已知板厚计算节点抗拉承载力Nt。三者虽然形式上有差别,但计算式原理相同,可以相互转换。2)GB 50017—2017《钢结构设计标准》中高强螺栓抗拉承载力Nbt已计入撬力效应;其他相关钢结构标准中有关节点承载力计算式仍以Nbt作为高强螺栓抗拉承载力上限值,将造成撬力效应的重复计算。3)同条件下,欧美标准计算结果与试验结果较为接近,中国标准计算结果则较保守。后者未对塑性铰位置进行修正是造成其偏于保守的重要因素。4)美国最新标准中进一步对受拉T型件翼板计算宽度进行了限制,欧洲标准中等效T型件的翼板有效计算长度lef是取各种可能破坏形式中的最小屈服线长度作为节点承载力计算依据。中国标准中对计算T型件翼板宽度的规定较为笼统,建议对其进行限制。

In the end plate connection with high-strength bolts subjected to bending moment,the end plate occurs bending deformation and produces prying force.In the design of connection joints with weak relative stiffness,the prying force can not be ignored.The end plate connection joints designed according to domestic specifications generally have the problems of large number of bolts and large thickness of end plate.It is necessary to analyze the relationship between prying force Q,joint bearing capacity Nt,bolt tension Nb and bolt tensile bearing capacity Nbt,and find out the reasons for the conservatism of China’s standards through the comparative study of the current specifications formulas and their parameters.The tensile T-stub joint model can be used to simulate the high-strength bolted end-plate connection joint.The tensile T-stub joint test and theoretical analysis at home and abroad show that the first plastic hinge of the tensile T-stub joint with obvious prying force effect first appears near the root of the wing plate of the T-stub.With the gradual increase of the external tension Nt,the wing plate of T-stub gradually enters the elastic-plastic state from the elastic state,and the prying force also gradually increases from nothing until the bolt is broken or the wing plate of T-stub is completely damaged(a second plastic hinge appears near the bolt hole).The external tension Nt is divided into two parts Nt1 and Nt2,where Nt1 corresponds to the external tension when plastic hinge occurs at the wing plate root of T-stub,Nt2 forms a moment balance with the prying force Q near the bolt.The force arm of the external tension Nt is the distance from the root of T-stub to the bolt hole,and the force arm of prying force Q is the distance from the bolt hole to the edge of T-stub.According to the joint failure mode and the bending moment balance condition at the bolt hole in the failure state,the calculation formulas of joint tensile bearing capacity Nt,prying capacity Q,bolt tension Nb and the critical plate thickness of the failure mode are obtained.According to the joint tensile bearing capacity formula,the joint bearing capacity curve can be drawn,and the control conditions of the three failure modes and the relationship between the three failure modes can be obtained.There are differences in the design methods of high-strength bolted connections considering prying force effect at home and abroad.The comparison results with foreign specifications show that:1)both Chinese and American specifications know the external force Nt and calculate the plate thickness;the European specification is to calculate the tensile bearing capacity Nt of the joint when the plate thickness is known.Although the three have different forms,the calculation formula has the same principle and can be converted to each other.2)The tensile bearing capacity of high-strength bolts in the Code for Design of Steel Structure(GB 50017-2017)has been included in the prying force effect;Nbt is still used as the upper limit of tensile bearing capacity of high-strength bolts in the calculation formula of relevant joint bearing capacity in other relevant steel structure specifications,which will cause repeated calculation of prying force effect.3)Under the same conditions,the calculation results of European and American specifications are close to the test results,while the calculation results of Chinese specifications are conservative.The failure to correct the position of plastic hinge is an important factor that leads to the conservatism of the current specification in China.4)The latest American specification further limits the calculated width of the flange of tension T-stub.In the European specification,the effective calculated length lef of the flange of equivalent T-stub is the minimum yield line length in various possible failure forms as the calculation basis of the joint bearing capacity.The provisions on calculating the wing width of T-stub in Chinese specifications are relatively general,and it is suggested to make them clear.