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The Structural Engineer

SIR,-The letter in the February issue by Mr. P. E. Robertson, A.M.Inst.C.E., criticises my paper which was read in November last, and in particular takes exception to:- (a) That they possess physical properties identical with those of the base metal, and (b) That, presumably as a consequence, the elongation developed is equal to the minimum usually specified for mild steel.

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The Structural Engineer

During a summary of the paper the following additional points were mentioned by the author :- In several places in the paper attention has been drawn to the possibility that large masses of concrete may not behave in precisely the same manner as small specimens, and, on page 60, in discussing the application of residual coefficients, and the experimental evidence in support of their use, this point is particularly emphasised since in this case the specimens were smaller than usual. This problem of the effect of the mass of the concrete on its behaviour is receiving considerable attention in various laboratories throughout the world, and I propose to show two interesting photographs illustrating ways in which large masses of concrete behave differently from small masses. It is well known that when cement sets a considerable amount of heat is evolved. In concrete this heat results in a rise in temperature which depends on the mass of the concrete, the mix proportions, the insulation afforded by the shuttering, and the external conditions. My colleague, Mr. Norman Davey, has been carrying out experiments on this heat evolution and its effect on the strength and other properties of the concrete, and the two photographs illustrate results he has obtained on a mass of concrete measuring 3 ft. by 4 ft. by 2 ft. 6 ins. high. The first photograph is of a model which was constructed to represent the temperatures at various points across a horizontal mid-section of the concrete. The height of the model at each point represents the temperature rise at that point above the external air temperature. You will notice that the temperature increase is higher at the centre than at the edges where the loss of heat through the shuttering occurred. The second photograph represents the effect of this temperature gradient on the strength of the concrete at various points across the same section of the concrete. The strengths were obtained from cubes of concrete which were cast in the mass of the concrete in specially prepared moulds. You will see that in the same way as the temperature was higher at the centre of the mass the strength is also higher, and is over 50 per cent. greater at the centre than at the corners. Since both shrinkage and creep vary with strength, it is reasonable to suppose that they also will vary throughout the mass of the concrete. In addition concrete in large masses hardens at a time when the temperature increase due to heat evolution is considerable and consequently the return of the concrete to normal air temperature, which may in some cases take a matter of months, must be accompanied by a heat contraction which is additional to the normal shrinkage. As a matter of interest it may be mentioned that as a result of these effects very great precautions are being taken in the construction of modern concrete dams. In America, in particular, every effort is being made to keep the temperature of the concrete down by special cooling devices, and by

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The Structural Engineer

The question of determining camber allowances, discussed in general terms would not convey much real information, as each different type of bridge requires special consideration. This applies particularly to the two-hinged spandrel-braced steel arch which introduces problems peculiar to itself. Consequently this type is taken as an example and considered in det'ail, in the hope that in this way the various principles and processes will be clearly demonstrated, so that suitably modified they may be of use when considering similar complex structures. B.G.R. Holloway

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Author – Holloway, B G R

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The Structural Engineer

In most calculations of foundations and bases it has been assumed that the pressure under the foundation has been uniform in the case of concentric bases or has varied uniformly in the case of bases eccentrically loaded, and the bending moments on bases have been arrived at on this assumption. Oscar Faber

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Author – Faber, Oscar

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The Structural Engineer

In his paper The Handling and Storing of Grain with special reference to Canadian Methods, read at the Institution of Mechanical Engineers, London, on January 27th last, Mr. H. H. Broughton, M.I.Mech.E., emphasised he importance of grain silos by reminding us that the wheat of which the bread is made comes from the ends of the earth into this country at the rate of about 120,000 tons per week, and that, if the machinery responsible for such movement had to fail for any cause for a time reckoned in weeks, famine would inevitably result in this country because Britain has safe storage accommodation for only a small fraction of the wheat she consumes. T.J. Gueritte

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Author – Gueritte, T J

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The Structural Engineer

Mr. A. R. WARNES (Associate) said:-"That the subject of stone decay is of importance to the structural engineer is obvious, and Mr. Power is to be congratulated in bringing this subject before the members.

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The Structural Engineer

We regret to announce the death, which occurred on the 6th February, of M. Augustin Mesnager, who was an Honorary Member of the Institution since 1913. M. Mesnager was born in Paris in 1862, and studied civil engineering at the Ecole Polytechnique. He rapidly rose to the highest positions, and in 1900 was appointed Professor at the Ecole des Ponts et Chausskes, being given charge of the testing of structural material used in civil engineering work throughout France.

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The Structural Engineer

As announced in our last issue, the awards in connection with the Portland House Scholarship for 1932 have been made as follows :- Winner of the Scholurship and Gold Medal: Mr. Walter M. Goodesmith, Graduate. Winner of the Silver Medal: Mr. D. Bethune-Williams, Graduate. Winner of the Bronze Medal: Mr. C. Wright, Associate-Member.

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