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承受梯度弯矩的工字型钢梁的侧向—扭转屈曲(LTB)强度取决于弯矩梯度系数Cb。而Cb取决于弯矩图的非均匀性、无支撑长度内所施加的横向荷载的大小和钢梁支座类型。一般地,Cb由规范根据弹性LTB分析理论给出。然而,同样的Cb被用在梁的非弹性屈曲分析中。提出1个三维有限元ANSYS模型用于工字型钢梁的非线弹性弯-扭分析,并用此模型研究了无支撑长度和偏剪心荷载(分别位于中心、上翼缘和下翼缘)对非弹性性能区域弯矩梯度的影响。研究发现AISC-LRFD的钢结构规范(AISC360-05)和结构稳定研究委员会导则给出的Cb对加载点在非弹性屈曲工字型钢梁的中心和下翼缘的情况并不准确。AISC-LRFD的抗弯公式过高评估了梯度弯矩作用下非弹性工字型钢梁的实际抗弯承载力。因此,提出了一个适用于该工况下非弹性区域的简单公式,用于替代规范给出的公式。
The lateral-torsional buckling (LTB) strength of I-beams subjected to the gradient moment depends on the moment-gradient coefficient Cb. Cb depends on the non-uniformity of the moment diagram, the magnitude of the lateral load applied over the unsupported length, and the type of steel beam support. In general, Cb is given by the specification based on the theory of elastic LTB analysis. However, the same Cb is used in inelastic buckling analysis of beams. A three-dimensional finite element ANSYS model was proposed for the nonlinear elastic bending-torsion analysis of I-shaped steel beams. The unsupported length and the shear-center load (located at the center, the upper flange and the lower flange, respectively) Effect on bending moment gradient in inelastic region. The study found that the AISC-LRFD code for steel structure (AISC360-05) and the CST guidelines give the fact that loading points are not accurate at the center and bottom flanges of inelastic buckling I-beams. The flexural resistance of AISC-LRFD is too high to evaluate the actual flexural capacity of inelastic I-beam under the action of gradient moment. Therefore, a simple formula that is suitable for the inelastic region in this condition is proposed to replace the formula given in the specification.