半开式桁架桥上弦杆平面外屈曲临界力研究

Research on out-of-plane critical buckling load of upper chord in half-through truss bridge

基于阶梯形分布的上弦杆轴力假定,考虑斜腹杆的抗弯刚度和横梁高度对上弦杆侧向弹性支撑的影响作用,采用能量法推导了上弦杆的平面外屈曲临界力计算公式,并以一座半开式桁架桥为例,对影响桁架桥上弦杆稳定性的因素进行了分析.结果表明：较之现有规范中的轴力分布假定,按照阶梯形轴力计算的上弦杆屈曲临界力更符合实际状态;桁架桥的跨径不变时,随节间数量增大,侧向弹性约束刚度与上弦杆的平面外屈曲临界力接近于线性增大,斜腹杆对侧向刚度的作用也随着节间数量的增多而增大,在计算当中节间数量较多时不应忽略斜腹杆的抗弯刚度;横梁高度增大则上弦杆的平面外屈曲临界力也相应增大,但对于本文算例,当横梁高度大于0.4m时继续增大横梁高度对侧向刚度的影响可以忽略.

Step-shaped axial force in the upper chord was assumed,and flexural rigidity of diagonal web member and height of the transverse beam were also taken into account to achieve the stiffness of lateral elastic supports in analysis.The out-of-plane critical buckling load was derived by using the energy method.A half-through truss bridge was taken as an example to analyze the factors influencing the stability in upper chord of half-through truss bridges.Analysis results show that buckling load of the upper chord with step-shaped axial force is closer to actual mechanical behavior than parabolic axial force distribution assumption in Chinese codes.As a truss bridge with the same span is considered,buckling load and lateral elastic restraint stiffness increase linearly with the number growth of internodes,and the effect of lateral stiffness of diagonal web members also rises as internodes increase.Diagonal web members should not be ignored in calculation of buckling load,especially when there are many internodes.Buckling load takes on ascend trend with the increase of transverse beams？ height,and in the proposed example,the effect of beam height can be neglected when it is greater than 0.4 m.