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No more responsible task can fall to a structural engineer than that of designing bridges. One reason for this is that bridge design may entail the most difficult problems of construction which engineers have to solve, while the other is that the bridge holds a very important place in architecture and people expect it to assume a dignified and imposing shape. In the series of articles entitled " Great Engineers " which appeared in the pages of this Journal last year I had occasion to refer to some of the most famous bridges of the past. The works of the two Rennies, Telford, the Brunels, Robert Stephenson, Sir John Fowler and Sir John Wolfe Barry were passed under review. Not only were famous stone bridges such as Waterloo Bridge, London Bridge, Dunkeld Bridge and the Royal Border Bridge described and analysed but examples belonging to " the steel age " of bridgebuilding, such as the Forth Bridge, the Tower Bridge, Saltash Bridge and the suspension bridges at the Menai Straits and at Clifton were the subject of detailed comment. I do not propose, therefore, to traverse this same ground again, but shall devote my attention entirely to the more modern developments of bridge-building, and especially to those which exemplify the new ferro-concrete construction.
A. Trystan Edwards
Referring to Fig. 25, the dimensions are closely those of a typical through span. When one track only is loaded, in round figures, 20/29 of the panel load W is applied to the near side main girder A and 9/29 of W to the far side girder B. Girder A will deflect by an amount C, and girder B by a smaller amount D. These deflections will be proportionate to the loads producing them, and the difference in deflection which is proportional to the shear strain on the sway bracing is due to the unbalanced vertical shear of 11/29 W or the difference of the loads applied to the two main girders. Referring to the lower diagram of Fig. 25, A and B represent in an exaggerated form the relative deflections of the girders in the upper sketch. Under uniform load the deflection will be sensibly parabolic, and the maximum deflection sensibly 1 1/2 times the mean deflection. The strains on the sm7ay frames will be proportional to the differences d1, d2, etc., of deflection of the main girders, and as, these differences are also sensibly parabolic, the strain on the central sway frame will be closely 1 1/2 times that which would obtain if all the frames resisted equally. The unbalanced shear of 11/29 W represents the mean unbalanced shear per panel and by multiplying this by l 1/2 we obtain C.57 W. The above argument assumes the main girders to remain in truly vertical planes, and that no lateral deflection takes place. Actually the unequal horizontal deflections of the upper and lower lateral systems permits the main girders to twist and to take up positions slightly inclined to the vertical. This departure from the vertical plane relieves the sway frames of a considerable amount of the shear stress which they would otherwise suffer. The amount of this twisting action is greatest near the centre of span, diminishing towards the ends and so tends more to the relief of those sway frames which would otherwise be the more heavily stressed. Moreover, this relief is greater in through spans than in deck spans, since in the former the sway bracing may only be brought down to the clearance line leaving it considerable length of relatively flexible post below. The lateral bending of the posts
still further contributes to the relief of the vertical shear stress in the sway frames. In deck bridges with full depth cross bracing the transverse rigidity is proportionately greater, and the relief of the vertical shear stress somewhat less. A detailed investigation shows that not more than 20 per cent. of the load on one track is likely to be transferred as vertical shear through the transverse bracing from the near to the far side main girder, in the case of deck bridges, and less than 10 per cent. in the case of through bridges.
Professor J. Husband
Sir,-I wish to draw attention to an erroneous method of calculation which is customary
in providing for eccentric loads upon stanchions.
George H. Blagrove