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The Structural Engineer

Test results of nine stiffened panels loaded axially show that the Perry-Robertson formula accurately predicts the collapse load and the direction of collapse provided the imperfection term ©¯ is interpreted in a logical manner. A new expression for ©¯ is derived. Four other panels were tested in bending or with combined bending-axial loadings. Simple rules are also presented for the design of stiffened panels with these loadings. N.W. Murray

The Structural Engineer

This paper discusses in summary form the forthcoming changes in National Insurance as particularly affecting the self-employed, but including those with other occupations as well. It is written to provide a quick resume of the position as it is known, which can be assimilated at one reading. It aims to provide an awareness so that those affected can see what further action is required and what further areas need to be studied. It must be emphasized, however, that it is not prepared as a substitute for official publications, nor for the Statutes.

The Structural Engineer

When specimens of wood with seemingly similar characteristics are tested, the strength results usually show high variability. This is attributed to two major sources: (a) Primary sources or the material factors. (b) Secondary sources or the human factors. Daniel N. Nwokoye

The Structural Engineer

Shells constructed by lifting a flat square lattice into a doubly curved shape are a recent form of construction. Such a shell of four times greater span than any previous examples had to be completed in 18 months for an exhibition. This paper describes the engineering design. It attempts to show how the loads were defined, how the structure was modelled and tested both physically and mathematically, and how these models were used to determine the construction details. The design process had to be evolved to derive sufficient understanding so that the decisions could be made by the critical dates. E. Happold and W.I. Liddell

The Structural Engineer

The interest in improving fire escape facilities continues unabated and tends to centre on the helter-skelter solution. We may have treated this somewhat light-heartedly in the January issue: not because we found it risible, but because it is pleasant to think that a concept normally associated with children's pleasure may solve a horrifying hazard. Verulam

The Structural Engineer

A plastic method of analysis is applied to determine the collapse load for double layered flat reticular grids. Two grid types are considered, namely orthogonal and diagonal systems. The plastic analysis is performed by rotational considerations as conventionally applied to concrete slabs and from individual member deformations. The collapse load is also determined from a generalized computer displacement analysis that permits member yield and where necessary incorporates post buckling behaviour of individual members. The collapse load factors obtained from the different analyses compare favourably. J.F. Dickie and D.J. Dunn