On Some Common Errors in Iron Bridge Design

be reasonably objected to. ■ From this point of view also, the magnificent 350 foot span girders at Indooroopilly, Queensland are justifiable. 13. Eccentricity.—The condition laid down on page 12 that a properly designed framed structure should consist of a series of triangles having common angles, involves the necessity of any three or more bars meeting at a point being so arranged that all their mean fibres pass accurately through that point, the mean fibre being defined as the line passing through the centre of gravity of the cross section of each bar. If these mean fibres or gravity lines, as they are sometimes called, do not meet truly, bending moments and shears are set up in the bars, and loss of strength ensues, unless obviated by the introduction of additional material. Very great laxity is sometimes shown in this respect, as may be seen from Fig. 17, representing portion of a large gilder recently erected over Primrose Street, in connection with the enlargement of the Liverpool Street Railway Station, London, illustrated in the <£ Engineer,” of 21st August, 1896. Heie it is to be noted that the end pillar, instead of being placed centrally over the rocker support, is placed most eccentrically so as to concentrate the stress on one edge instead of distributing it equally, and further, that the mean fibres of the last diagonal and end pillar meet far above the upper surface of the top chord, instead of as they ought in its mean fibre. Another glaring case is shown in Fig. 18, representing part of a bridge near Windsor, N.S.W., inspected some years ago by the writer. Recent Australian, practice appears fairly free from these inexcusable faults, which is more than can be said for English practice. A most disastrous accident took place ten years ago near Boston, U.S.A., causing the destruction of a train, the loss of twenty-five lives, and injury to about one hundred persons. This all started from the failure of the improperly designed bridge hanger shown in Fig. 19. This hanger had ample sectional area for the direct pull, but that pull was imposed so eccentrically as to give rise to a bending moment which increased the stress several times. This and other cases show the necessity of guarding most carefully against this fault. Few persons realise that in a rectangular section a deviation of the centre of stress