Some Points Concerning the Design of Concrete Tanks

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.

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

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