Issue 5

This paper was prepared as part of the work of the Environmental Wind Committee of the Institution, and the author was assisted by other members of the Committee and by the representatives of the Institution in many countries, as well as by the national standards organizations concerned. T.A. Wyatt

Dr. G. S. Pandit: The tests reported by the authors have made an invaluable contribution to the understanding of the problem of combined bending and torsion of prestressed concrete beams. The tests have further corroborated the generally accepted view that the bending moment can increase the torsional strength only if the prestressing is eccentric and that the effect of bending moment of whatever magnitude is to reduce the torsional strength of concentrically prestressed beams. The line of thrust (or the centre of compression) of an unloaded prestressed concrete beam coincides with the centroid of the prestressing steel. The effect of bending moment is only to shift the line of thrust through a distance em = Mb / P where P is the total effective prestressing force. If the bending moment Mb is of such magnitude that em equals the eccentricity of prestressing force e, then the line of thrust coincides with the centroid of the cross-section producing uniform compressive stress over the entire cross-section. Hence the optimum bending moment for maximum torsional strength would appear to be Mb, opt = Pe. For the Series E,P =1/2 x 1820 x 5 x 8 = 36400 Ib and e = 8/6 = 1.33 in. Hence Mb,opt = 36 400 x 1.33 = 48 533 Ib in or 48.5 kips in. This value of optimum bending moment is in close agreement with authors' test results for beams of Series EW of Part 2 with p = 1.0 per cent and 1.6 per cent as can be seen from the interaction diagrams of Fig 18 by scaling out the bending moment corresponding to the maximum torque. The agreement is not so good for the beams of Series E, Fig 9, in which the optimum bending moment appears to be about 80 kips in. The shape of the interaction diagram for Series E in Fig 9 does not agree with the theoretical shape and I wonder, therefore, whether this could be attributed to the usual scatter and the possible errors of observation. It may be pointed out that the theoretical interaction diagram of Fig 8 is in contradiction with authors' own observations regarding the increase and decrease in torsional strength due to bending moment for eccentrically and concentrically prestressed beams respectively. Thus the authors' theory would appear to be conservative for eccentrically prestressed beams and errs on the unsafe side for concentrically prestressed beams in the range Mb < Mboc.

A simple method is presented for fhe calculation of the creep buckling load of long columns on the basis of strain distribution in concrete under a sustained load and the consequent change in the moment of resistance of the critical secfion in the column. W. Dilger and A.M. Neville

Mr. W. G. Cantlay (F): Although I am a partner in the author's firm I had no involvement in this project and would, therefore, ask a simple question. You have stated that you chose steel to achieve speed of erection but you have not stated whether composite construction was adopted forthefloors. I would ask you to make some comment as to whether your choice of steel met the requirement for speed of erection.

In a previous paper, full-scale and model tests on trough-shaped bunkers, rectangular in plan, were described, and experimental stresses were compared with those from conventional design and from an improved design method {which requires empirical allowances for the contribution of the plating to the longitudinal bending strength). This paper explains two theoretical solutions, the first allowing for shear panel action in the plating and overall hipped-plate behaviour in the bunker, the second utilizing a commercial space frame computer program allowing for in-filled panels. The results obtained compare favourably with those obtained previously by experiment and by the improved design method; either method may now be utilized to check and modify designs for this type of structure. E. Lightfoot and J.K. Withrington

A description is given of a system of interconnected beams representing a right simply supported multiple web cellular deck with no intermediate diaphragms, followed by an outline of a method of analysing the response of such a system to any externally applied loading. A short computer program using this method of analysis is described. Results produced by this program are compared with experimental results of a model test and show agreement to within a few per cent. Finally it is shown how this theory was applied to the design of the New Cattle Market Bridge now under construction which when completed will carry the new Derby Inner Ring Road over the River Derwent. J.G. Parkhouse

In 1969, a study was made of the results of tests on eighty steel-concrete composite beams in which longitudinal shear stress was high, and a new design method for transverse reinforcement in the slab was deduced. This gave a more uniform margin of safety than the method of CP 117 Part 1: 1965, and showed that the amount of reinforcement could be reduced by about 35 per cent.