A STUDY ON THE STABIRITY OF CIRCULAR SECTION STEEL STUB-COLUMNS IN THE PLASTIC-FLOW RANGE

以往的圆截面钢柱在塑性区象脚模式屈曲承载力计算公式的解中含有切线模量。因此，无论圆截面圆柱的径厚比如何，圆柱均在屈服平台上屈曲。平板在屈服平台上不会失去扭转刚度。因此，平板可以被压缩到应变硬化范围内，而不会在屈服平台上发生过早屈曲。但试验结果表明，在塑性范围内，从屈曲强度的角度来看，圆形截面比箱形截面具有优势。本研究旨在解决这一矛盾。明确了以下几点：1.在以往的理论中，变形理论在屈服平台以外的区域似乎与试验结果相吻合.小径厚比圆截面压杆在应变硬化区之前的屈服平台上不会发生过早屈曲.象足屈曲时，由于环向曲率随直径的变化而变化，出现马鞍形屈曲。这提供了低屈曲应力。但对于弹性解，其影响小于1%.效果3。在塑性屈曲中，特别是在屈服平台上，可能以不同的形式出现。

Past equations to give the buckling strength of elephant foot mode of circular section steel stub-column in the plastic range contain tangent modulus in their solutions. Therefore all circular section stub-column buckle on the yield plateau despite of their diameter- to- thickness ratio. Flat plates does not lose their torsional rigidity on the yield plateau. Therefore flat plates can be compressed into strain-hardening range without experience of premature buckling on yield plateau. But the experimental results show that circular section have an advantage over box section from the view point of buckling strength in the plastic range. This research intended to solve this contradiction. The followings were made clear.1. Among past theories, deformation theory seems to coincide with test results in the region beyond yield plateau.2. It seems that circular section stub-columns with small diameter-to-thickness ratio does not experience premature buckling on the yield plateau before strain-hardening range.3. Buckling mode like a saddle take place for elephant foot buckling because the curvature in circumferential direction occur depending on the change of diameter. This gives low buckling stress. But the effect is less than 1% for elastic solution.4. The effect 3. may appear in different form for plastic buckling especially on the yield plateau.