Issue 6

To the Editor of The Structural Engineer. Sir,-I have read with considerable interest the paper by Lt.-Col. J. Mitchell Moncrieff relating to earth pressure theories, published in the Journal dated March 28. I beg to refer to his remarks on page 71 concerning the road bridge over the River Tyne at Newburn. Col. Moncrieff would like to know why the average frictional resistance was less at 474 ft., 62 ft. and 65 ft., than at 22 ft., 20 ft. and 21 ft. I venture to suggest that lateral pressure does not always increase with depth as commonly supposed, except in the case of a perfectly dry granular material, and that to a certain degree the reverse is found when there is cohesion; in other words it is cohesion that increases with the depth, due probably to the greater compactness of the material, brought about by the increased weight, and also to the fact that the portion closer to the surface is subject to atmospheric changes, ground water.

THE PRESIDENT said Mr. Snelgrove had given them a very interesting paper, as all would agree; more particularly with regard to that remarkable tank they had seen practically on the ground. It reminded him rather of an interesting little bath he had himself had something to do with, and which had failed for variousreasons. It was not a concrete tank, but a brick-walled bath or tank with a concrete bottom. The owners had endeavoured to make it water-tight by simply concreting up the cracks, and the remarkable thing was that these repairs were quite satisfactory for a few months of the year, and then the tank inevitably gave out when they wanted to use it for a swimming festival. The reason was that the bath was sunk entirely below the ground level, and on two sides there was a little stream from which they derived their water higher up. The stream ran at no great distance from the wall, and in the wet weather the earth was thoroughly saturated, and full pressure was produced against the sides; but when the summer came the earth dried out. You could pnt your hand down between the backing and the side of the tank, and, of course, the pressure of the water did not rest.

After a period of activity and rising prices extending over many months, the industrial market in the Stock Exchange had a decided relapse last month. The inevitable reaction in quotations was delayed much longer than many observers had anticipated, and when it came was mild in comparison with the extensive advance which had preceded it. This ma: be accounted for by the restriction on one type of speculation which is now tacitly imposed by the Stock Exchange.

In investigating this case it will be useful to recapitulate the simpler formulae first. The bending moment on beams variously loaded and supported is B =Wl/n, where B = bending moment in lb. inches, W = load in lbs., l = span in inches, n = coefficient according to nature of loading and supporting. Professor Henry Adams

The Hudson River Bridge of 3,500 ft. span, is a stiffened cable suspension bridge which will provide communication between Upper Manhattan, New York City, and Fort Lee on the New Jersey side. The completion of this structure will constitute a remarkable advance on all previous bridging schemes. The span is double of that of the Philadelphia-Camden Bridge, which at present is only exceeded by the 1,800 ft. span of the Quebec Bridge. This undertaking is the more remarkable in that the increase of bridge spans has in the past been uniformly gradual for a period of more than 100 years, whereas in this instance an immediate increase of 100 per cent is being made. The unprecedented dimensions of this bridge have not unnaturally given rise to considerable criticism as to whether its successful construction is a feasible proposition. It is interesting, in this regard, to recall certain observations made by the late Sir Benjamin Baker, as far back as 1866. At that date, in discussing the suspension bridge with stiffening girders, he stated :- " The combined lightness and sustaining power of a festoon of rope stretched between two supports, could hardly have failed to attract, at a very early date, the attention of thoughtful practical men. The distance apart at which the points of support might be placed, would appear almost infinite as compared with what would have been the limit had the intervening space been spanned by a solid bar of the same size as the rope, merely resting on the supports. Then, therefore, the occasion to throw a light structure across a wide river or ravine first arose, the similarity of the conditions to the case of the rope with its two distant points of support must almost necessarily have suggested a similar mode of procedure; and knowing the great superiority of the tensional strength of iron over rope, it was only natural that the "suspension bridge," in its simplest form, should be evolved, and that, it should be the earliest form of the'long-span bridge'as understood in our definition." Professor J. Husband

Mr. President and Gentlemen, a word of apology is first necessary. It was only after the printing of the programme that I realised that the time allotted this evening would be too short for the purpose of developing the theme I had originally in mind and prepared under the title of this paper. The choice then lay between an abridgement and a selection. By your leave the latter is here chosen, and the intention is now to present brief observations upon certain details of tank design and to describe two examples, of recent date, which, it is hoped, may furnish material for discussion. E. Copeland Snelgrove

To The Editor of The Structural Engineer. Sir :- The May issue of The Structural Engineer contained a letter written to you by Mr. Godfrey, in which he attributed the failure of the St. Francis Dam in California to neglect to provide for upward pressure.