Issue 7

The empirical design method recommended by design Codes for flat slab structures assumes that ribbed and plain panels behave in the same manner under load. However, there are significant differences between the behaviour of ribbed and plain plates, the increased ratio of flexural to torsional rigidities in a ribbed plate resulting in a substantial reduction in the load that is transmitted by torsion of the plate elements. In the existing design method, the total ultimate load design moment for each panel, which is based on thequilibrium requirement of the panel, is distributed according to specified coefficients derived from an elastic analysis of square plain panels. A theoretical study of the moment distributions within plain and ribbed flat slab panels has been carried out, and the theoretical moment coefficients for internal, edge, and corner panels are presented. It is concluded that the recommended coefficients should be modified for the design of ribbed panels. Suitable alternative coefficients for internal and external ribbed panels are presented. The effect of solid regions around columns is considered, and it is shown that solid regions attract higher negative moments in the column strip than are allowed for in existing design coefficients. An additional design rule that allows for these effects is proposed. I.E. Tebbett and J.E. Harrop

I comment on the paper 'Structural behaviour of reinforced concrete columns in service'. I would make it clear that I make no claim 'to be competent to judge the quality of the authors' work and, as is my usual practice, I first read only the synopsis. This included the comment that the creep effects of load transfer from concrete to steel should be considered at the design stage. Being a design engineer, I concluded I ,must read the full paper. Having done so, I was not able to recognise any part of the contents that appeared to support this particular conclusion. There appeared to be no consideration of the effect on the ultimate capacity or serviceability of the column, which I understand to be the material criteria for design. There was a reference to the relationship between observed steel stresses and the CP 114 value of 'working stress', but I respectfully submit that the authors misunderstand this Code, as it is not actually based on working stresses but on factored ultimate stresses-a fundamentally different concept that makes the comparison meaningless. J.A. Tanner

Mr. M. G. Stevens: The layout and load capacity of a typical king pile is given in Fig 1.

The authors have, for 'many years, been involved in leading multidisciplinary engineering groups, working on the design of buildings with other firms of architects and quantity surveyors. This paper reviews the experience gained by examining the different stages of the design process and commenting on the lessons learned by working in this way. This review is illustrated by referring to some relevant projects. The paper concludes by suggesting that much can be done to encourage effective interdisciplinary collaboration by means of training and by improving the ways in which the 'normal duties' of the building design professions relate to each other. J.N. Martin, R.F. Emmerson and J.S.A. Berry

These trees We remarked in March that an arboriculturist friend considered that oak trees in this country were regenerating satisfactorily and their conservation was not at risk. That comment was engendered by a correspondent's concern over the felling of six oak trees; since then we have heard that some experts are seriously concerned about the possible invasion of oak wilt which is endemic in America and which can be carried in the American oak sapwood imported into this country for its highly decorative value. The disease apparently spreads with great rapidity and, like Dutch elm disease, there is no treatment for it. The suggested precaution is to ban the importation of American oak wood. Verulam