饶芝英    童根树
    （浙江大学土木系  杭州 310027）

[提要]  在材料和截面层次上刚度达到零定义了传统的强度极限状态，而在构件和结构（子结构）层次上的刚度等于零即达到了稳定极限状态。这种层次的区分清晰地表达了强度问题和稳定问题的区别和内在联系，解释了具有正刚度的构件或结构在荷载作用下产生失稳的原因，引入了荷载的负刚度概念，并提出了荷载负刚度的计算公式，有助于工程师理解稳定的基本概念和掌握结构稳定的计算方法。此外，还提出了为使厂房柱平面外计算长度达到侧向支承点之间的距离而必须的支撑刚度的最低要求。
[关键词]  钢结构  构件  稳定  刚度  强度

Limit states are reached when the stiffnesses in material cross-section member, subassemblage and overall structure levels become zero. This classification shows clearly the difference and their relationship between the strength and the stability problems. Authors also provide an explanation of why compressed members become unstable under vertical loads. The concept of negative stiffness of the vertical load is introduced, and an expression for determining this negative stiffness is also provided. The minimum stiffness of longitudinal braces in industrial building is also suggested in order to make the out-of-plane effective length of the column reach the distance between the braced points.
Keywords:  steel structure; section; subassemblage; stability; stiffness; strength