Notes on the National Insurance Position of the Self-employed as from the 6 April 1975

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

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

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