Full Description

Concerned primarily with the stability of "as-delivered," rolled, structural steel shapes of the I or WF type, this discussion considers the influence of residual stresses on the load carrying capacity of compressed members. Both the pure axial load case and that of combined thrust and bending due to end moments, eccentric or lateral loads, etc. have been included. While the procedures presented could be extended to include as a basic material property the influence of a curved type of of stress-strain relationship (in addition to the apparent nonlinearity due to residual stresses) the numerical work involved in obtaining equations or graphs similar to those shown herein would be much greater.

It should be noted at the onset that the problem under consideration (that is, the condition of stability being discussed) is that of "excessive bending." For the axially loaded member, a limiting case of the generalized beam-column, the first possibility of lateral deformation from the straight equilibrium position will be considered. As has been demonstrated both analytically and experimentally, this initiation of bending at the tangent modulus load is a lower limit to the true carrying capacity of the member. Further, for most "as-delivered" rolled shapes it represents a good approximation to the actual maximum load the member can sustain.

For the case of combined bending and thrust, the problem is somewhat different in that it is necessary to determine the maximum load in a bent configuration. Indifference with regard to equilibrium in the deformed position (as load is held constant) is then the criterion of the solution. Because of the manner in which loads "get into a structure," bending moments are present in most members. The problem of importance then is more often the second of the two

The problem of lateral torsional types of instability is not included in this presentation. These types of problems are considered by a different research committee of the Council. It should be pointed out, however, that for a majority of cases encountered in building construction the members are laterally restrained by wall systems, etc. For these and other cases where adequate lateral support is provided, the results presented herein directly apply.

As pointed out by the first speaker this morning, when discussing problems of stability (or more often-lack of stability) one must talk about ultimate loads rather than working loads. Correspondingly, when considering such members in actual structures, the ultimate loading behavior of the structure as a whole must be investigated. A column may be elastically restrained by adjoining members at the working load, but these restrains (due to yielding) may he absent as the ultimate load of the structure is approached. Such a member is then effectively pin-ended at this ultimate load situation and should be considered as such when determining the carrying capacity of the structure.

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Published: 1958 Number of Pages: 17 File Size: 1 file , 660 KB