Torsion and Buckling of Open Sections

Herbert Wagner
naca-tm-807
October 1936


Open Section members are made of rolled or drawn sheet metal and do not, like the closed or tubular sections, enclose any area. Open sections are applied a gread deal in metal constructions because they can be so easily joined to one another or to other plates; in addition, they are accessible at all positions and so lend themselves to easy maintenance and repair. In contrast to closed sections, however, open sections possess very small torsional rigidity. Thus it is known that the torsional rigidity of an open member whose cross section is not prevented from warping, is only as great as that of the flat metal strip from which it is made. If, however, warping of the section when the member is twisted is prevented, for example, at one end of the section (at least for a relatively short member), then longitudal stresses arise which offer a considerable resistance to torsion. The computations of this effect have already been carried out for certain types of sections, especially I beams. In this paper we shall discuss the general principles for open sections of any shape. Open sections are usually so designed that they are not subject to any torsional stresses. But even where they are applied as compression members, such sections often give way by twisting or tilting long before the Eulerian buckling load or the yield point is reached. Particularly do the compression members members used in airplane structures whose ratio of load to length of member (reference 2) is in general small and which are therefore made with very thin walls, have a tendency to twist. In what follows this torsion will be computed and on the basis of the results obtained it will be possible to obtain a proper design of section in each case. The torsion buckling members for the case where they are centrally loaded, leads to a problem in pure stability and is similar to that of the bending of stressed beams.

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