一、梁的整体失稳现象 The overall instability of the beam 钢梁常采用工字形截面，截面高度比较大而宽度比较小，这样，在刚度大的平面内承受荷载可节省材料。设截面对主轴x轴的惯性矩为Ix，对y轴的惯性矩为Iy，Ix＞Iy。当梁两端分别承受对x轴的弯矩Mx时，梁处于纯弯曲状态。此时，梁在弯矩作用平面内（即YOZ平面）将发生弯曲变形，当弯矩值增加到一定数值时，受压区的翼缘及与它相连的部分腹板在压应力作用下，有可能压曲，从而引起整个梁突然发生侧向弯曲变形，并伴随扭转变形，以致丧失继续承载能力。这种现象称为梁的弯曲扭转屈曲（简称弯扭屈曲），或称梁丧失整体稳定。当梁即将失去整体稳定时，它所能承受的最大弯矩称为临界弯矩，相应弯曲应力称为临界应力。 Steel beams are often used in zigzag section, which is large and small in width, which can save the material in the flat surface of the stiffness. Let's say the moment of inertia of the X-axis of the main axis is Ix, and the moment of inertia of the Y-axis is Iy. The beam is pure bending when the two sides of the beam are subjected to the X-axis bending moment. At this point, beams in bending moments plane (i.e., YOZ plane) bending deformation will occur, when the bending moment value increases to a certain value, compressive OuDeYi margins and connected to its part of the web under the effect of compressive stress, the possible buckling, causing the whole beam sudden lateral bending deformation, and with torsion deformation, so that the loss of continue carrying capacity. This phenomenon is called the bending torsional buckling of the beam, or the loss of the overall stability of the beam. When the beam is about to lose its overall stability, the maximum bending moment that it can bear is called the critical bending moment, and the corresponding bending stress is called the critical stress. 二、临界弯矩和临界应力 Second, critical bending moment and critical stress 根据弹性稳定理论可导出双轴对称工字形截面的简支梁在纯弯曲受力状态下临界弯矩Mcr，并可看出梁的临界弯矩与许多因素有关，其中受压翼缘侧向自由长度l1影响较大。当l1减小时，可使临界弯矩显著提高，增强梁的整体稳定性。此外，提高梁的侧向抗弯刚度EIy及自由扭转刚度GIt均使梁的整体稳定性提高。 Derived according to the theory of elastic stability the biaxial symmetrical i-section section of the beam under pure bending stress the critical bending moment Mcr, and can see the critical beam bending moment is associated with many factors, including compressive flange lateral free length l1 influence. When l1 is reduced by an hour, the critical bending moment can be greatly improved and the overall stability of the beam is enhanced. In addition, the overall stability of the beam has been improved by improving the lateral bending stiffness EIy and the free torsional stiffness. 梁的临界应力σcr的计算公式：σcr＝Mcr/Wx The calculation formula of the critical stress sigma cr for the beam is the sigma cr = Mcr/Wx Wx为受压最大纤维确定的梁毛截面抵抗矩。 Wx is the resistance moment of the girder cross section determined by the maximum fiber.