Issue 3

Oscillation of a structure induced by vortex excitation is likely to occur at frequencies quite different from the natural frequency of the structure because of the locking-in phenomenon. Effective damping by means of a passive mechanical system requires that the system be capable of operating over a large range of forcing frequencies under the exciting forces which may vary in direction and intensity. A damping device based on the principle of energy dissipation by impact has been developed for this purpose. It has a nonlinear response to displacements, and is suitable for operation at relatively low frequencies. Selected experimental results are presented in this paper. H.Y. Wong

Design curves and formulae are presented for the calculation of deflections in reinforced concrete flexural members. These are based on the procedure given in appendix A of CP110 for direct calculation of curvatures and deflections due to short-term and long-term effects. Using the design curves as the basis, an estimate of the errors introduced by the simplifying assumptions presently used in deflection calculation is obtained, and the area of validity of these assumptions is defined. The need for a more realistic assessment of deflections and the use of the design curves are illustrated by an optimum design example. The advantage of the design formulae and the associated curves in deflection calculations is illustrated by considering a doubly reinforced member. D.J. Gunaratnam

Whilst I am a firm believer in the economy obtained by the intelligent use of grade 50 steel, the submission by Mr. Needham that the present cost differential is an extra £3.50 per tonne compared with grade 43 steel is somewhat misleading. This figure is applicable only when comparing the cost of grade 43A steel with that of grade 50A steel, and is thus limited only to certain rolled sections. Significant economy by the use of grade 50 steel with rolled steel sections is doubtful, especially where deflection is the criterion. When using structural hollow sections in lattice construction, where the maximum economy in weight is possible, it is necessary to compare the cost differential between grade 43C and grade 40C. This is, I am given to understand, as much as 50 per tonne depending on the sections used. B.J. Jessup

Professor A. L. L. Baker (F): At one time it appeared that we might use a limited tensile strain normal to uniaxial load of cube strength x Poisson's ratio / E as the general criterion for ultimate strengths for all practical purposes. Indeed, if we assume that the ratio of stress over strain at the ultimate limit state is 2 x l0 to the power 6, and that the Poisson's ratio is 1 over 4.5, we can derive, for a multiaxial strained cube, an equation that fits the mean values shown in Fig A1 in the paper. It also fits the higher ranges, if E is reduced in all the terms of the basic strain equation to agree with low values indicated by the curves in Fig A4 and provided that secondary strains do not at some point reverse.

. . spare that tree Mr. C. G. H. Jofeh is clearly a man concerned about nature conservancy. He writes: I read with interest the letter from Mr. A. Billingham (November 78). What interested me was not the question of what the building regulations do or do not say, but the attitude of mind of the author that I inferred from what he said about the trees. Verulam