TORSIONAL AND LOCAL BUCKLING OF THIN WALLED SECTIONS

1.1       Introduction

It is desirable to keep the cross sectional area as small as possible and the radius of gyration as large as possible. This leads to the use of thin walled sections with wide flanges or webs.

There are two types of instability that might occur of strut of open sections or closed sections. The first is torsional buckling where the middle part of strut rotates bodily relative to the ends. It is linked to low torsional rigidity, so hollow sections are preferred. The second mode is local buckling, which occurs in struts composed of a number of long thin flat plates. It is generally appears as a series of waves along the length of the member where the junction between web and flange remaining without deformation.    

1.2       Pure Torsion of Thin-Walled Open Sections

Pure torsion means that the length of the open section is twisted by equal and opposite couples applied to its ends, acting in planes normal to the longitudinal axis and the ends are free to warp .

In pure torsion only shearing stresses, constant along the length, are produced. In members such as thin walled channels, made from flat elements, this means that the shear stresses lie along the middle line of the cross section.

If a transverse load is applied to a thin walled open section beam, it will cause it to twist as well as bend unless the line of action of the load passes through the shear center. When the load acts through the shear center, its moment about the centroid of the section balances the moment due to the distribution of shear stresses along the middle line of the cross section . If a section has two axes of symmetry, the shear center coincides with the centroid.