A survey is made of the main structural design problems of thin-walled steel members; the structural components discussed are tensile members, columns, and beams. It is shown that most of the basic problems originate in the thinness of the material; thinness may give rise to twisting of tensile members, to local buckling where compressive stresses are high, and to torsional buckling effects in columns and beams.Cold-formed steel sections are made from hot-rolled steel strip; the shapes which can be formed are discussed briefly. The flexural and torsional properties of thin-walled open sections are reviewed; the shear-centre axis and the warping stifFness are defined.The design of columns requires a knowledge of both local and overall c o l u m n buckling. The strength of beams is governed by local buckling in compression flanges and webs, and, for narrow beams, by overall lateral buckling.Finally, a brief comparison is made of the relative structural efficiencies of cold-formed and hot-rolled steel sections; comparisons are given of channel columns, and weights of steelwork used in industrial sheds. ~NTRODUCTTON THIN-WALLED steel members have been used for many years in buildings bothas main structural components, and as secondary members; wall panels and floor units are used as well as structural sections. 2.During the past 15 years many studies have been made in this country and abroad of the design problems of light-gauge steel sections. Some of the earliest work on local buckling in thin steel members was carried out by Winter' in the United States; a design specification has been based on this work2. At about the same time studies were made in Britain by Moir and Kenedi3, and in 1951 Shearer Smith also proposed a design specification4. Further researches5.6.7, * have been made in the last 10 years, and it is now possible to formulate design methods for a number of simple problems; these researches form the basis of a recent British design specification* for the use of coldformed steel sections in building.

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DiscussionProfessor Sir Alfred Pugsley (University of Bristol) wished to begin by making the general observation that when new materials were introduced into civil engineering they were not only of immediate benefit to the profession but had also a much wider and in the long run more important effect: they gave rise to new structural problems, the solution of which caused a more general advancement of knowledge. Most new materials had had that effect, and this was very well exemplified by the Author's work. The advancement in this case had initially been of a detailed analytical character, exemplified by the Author's analysis of local buckling. But it had since led to a deepening of the understanding of the behaviour of structures generally; and the relevance of the local buckling problem to the behaviour of structures and structural components in general was illustrated in the Paper.57. There were two details in the Paper on which he wished to comment. There was an interesting little section on the tendency for a tension member, even when axially loaded in a particular distributed fashion, to twist when tension was applied. Did the Author regard this as important in practice? These light cold-rolled sections had very low torsional stsnesses; was it not very easy to suppress a twist of that sort by local connexions if it were desired to do so? 58. In another section of the Paper reference was made to the economy and efficiency of the use of cold-rolled sections and this was nicely illustrated by some curves of the permissible stresses for struts in relation to the structural loading coefficient (the end load divided by the length squared). In this graph, Fig. 22, the cross-over indicated the range over which cold-rolled sections as struts were particularly efficient. To avoid using 10-3 and the like, if one multiplied by 2,240 to bring the figures to lb/sq. in. it would be found that the cross-over occurred for a coe6cient between 2 and 4 Ib/sq. in.That was a pointer-and no doubt the Author meant it as such-to the sphere in which sections of this sort were specially valuable: lightly loaded structures where the members wererelativelylong. Therangeof structureloadingparameterincivilengineeringpractice was much wider and went up to 10 or 15 lb/sq. in., and at the upper end no one would wish to use cold-rolled sections; but that was no unusual disadvantage because all materials and all sections had such limited ranges of efficiency. 59. Sir Alfred had hoped that the Author would indicate more than he had in fact done the work to which he looked in the future. It might be that he felt that local buckling had had its day. He had done his work so well that he might not want to touch it up further, but what did he think was the next step? One possible step came to mind on reading the introduction to the Paper, in which it was pointed out that the process of forming cold-rolled sections cold-worked the material very heavily where the plate was bent most, at the "corners", so that the local hardness went up. One wondered ...

The Authors have presented a lucid description of buckling tests on nominally identical specimens at four different slenderness ratios. With the current move towards semiprobabilistic methods for assessing safety" it will become necessary to conduct series of tests of this nature where the maximum control is kept on some of the more obvious objective variables involved in order to assess the subjective uncertainties which reflect man's imperfect knowledge.la Only then will designers have the best bases for choosing characteristic values and partial safety factors for strength.46. It can be seen from Fig. 6(c) that the buckling strength coefficients of variation from the Author's tests (Fig. 5 ) and those of the European group* show a peak close to 10% at l/r= 78 which is the slenderness value closest to (f/r)T. However, it seems that it is at this slenderness value that the minimum value of do occurred. It would therefore appear that most of the variation in strength must be due to something other than geometrical imperfections which are k n o q to exert their greatest effect in the (I/r)T region. Likewise the excellence of the test arrangements would seem to discount the possibility that end constraints or eccentricities have any significant influence. If this is true then the uncertainties illustrated by Fig. 6(c) (Which average about 4%) are more likely to be subjective than objective. Can the Authors confirm these inferences and explain this scatter?47. In the same vein Fig. 11 may be of interest. It shows the result of compression tests on thirteen nominally identical uniaxially tee bar stiffened plate panels designed in the buckling-yield transition.18 The experimental conditions were carefully controlled to minimize their uncertainties. The panels were made from mild steel selected and constructed under normal shipyard conditions. The effect of stiffener shape imperfections was negligible but the effects of residual welding stresses and plate deflexions were apparent but moderate. The objective random uncertainties, which were dominated by variations in yield stress, had a coefficient of variation of about 6%. Subtracting this from the experimentally found total uncertainty of 10.9%, and assuming statistical independence, suggested a typical subjective random uncertainty for such structures of 9-10%. This is similar to that obtained by the Authors, although somewhat greater overall as one would expect for a more complex fabricated structure.48. I am surprised at the cusped nature of some of the figures at the transition slenderness. Of course many theoretical approaches show greatest sensitivity to shape and material imperfections at this transition, but it is my experience and belief that such discontinuities seldom appear in practical structures during test.