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MR. M. GREGORY commented that the slow convergence or divergence of the moment distribution process at loads near the critical load had tended to make the calculation of critical loads for elastic rigidly jointed frames by this method rather tedious. He quoted the reason Dr. Bolton had given: “Basically this difficulty arises because the testing distortion used is not the critical mode, but merely the rotation of one joint. If the critical mode were to be used as the testing distortion, one cycle would be sufficient to decide whether the calculations were converging or not.” Mr. Gregory thought this had become clear to most people who had used the Hoff moment distribution convergence criterion. It took many distributions for the effect of a single disturbance to be felt throughout the whole structure, and many more for the carry-over to reflect back to the originally disturbed joint, and the first step towards reducing the length of the calculation was obviously to apply disturbing moments at several joints rather than at one joint, particularly if the disturbances could be given the correct sign, which was often the case if the desired buckling mode could be pictured. This method had previously been used in the University of Tasmania,
but it was interesting to see it carefully worked out in detail in Dr. Bolton’s paper, particularly with regard to the information which could be gained by keeping running totals of the disturbing moments after each distribution, and he complimented Dr.
Bolton on this advance.
The flexibility which is inherent in nominally rigid joints and footings in plane frames is known not to affect the value of the plastic collapse load. The object of the present paper is to show that substantial amounts of flexibility can often occur without affecting the deflections at the point of collapse, that is when the plastic collapse load has just been attained but no motion of the collapse mechanism has taken place. This is in striking contrast to the obvious increase in the deflections in the elastic range of behaviour when flexibility of any kind is present.
Professor G. Neal
The problems connected with the design of pile groups are often the subject of lengthy and complicated analyses. This applies particularly to pile groups which have to withstand relatively large horizontal thrusts, where the use of raking piles would be advantageous. Because of the indeterminate nature of piled foundation structures, an exact analysis of structural members has not yet been possible; and this presents a difficulty which is quite apart from the problems of soil mechanics involved. Investigations into the action of earth mass around piles and attempts to utilize the shear strength or the modulus of elasticity of soil for defining the ground resistance to horizontal thrust, do not appear to have yielded conclusive results of great practical value; and easily applied design methods have certainly not yet been derived