On Some Common Errors in Iron Bridge Design

23 from the centre of figure by one-sixth of the width of the bar, doubles the stress on. one side and reduces that on the other to zero. Eccentricity should be most carefully avoided in designing new structures, and when it exists in old ones, special local strengthening should be applied, unless computation shows the parts to contain a sufficient excess of material to resist the moments and shears involved as well as the direct stresses. 14. Unscientific and. wasteful end pillars.-—There are multitudes of girders in existence which are reasonably and consistently designed as far as chords and web members are concerned, but which have the terminal verticals of excessive size, and most complicated construction involving great and altogether unnecessary increase in the weight and cost of the girder. No defence or excuse for this anomaly has ever readied the writer’s ears, but its vitality is remarkable, as it appears not only on bridges built thirty or forty years ago, but even in quite recent structures, including some of those crossing the Manchester Ship Canal. One example is shown in Fig. 14, where the encl pillar or box contains at least three times as much material as would be needed for a proper terminal diagonal or “ batter brace,” to use the American term. A second example is shown in Fig. 20, representing part of a bridge over the Rio Verde, South America, the work of a late president of the Institution of Civil Engineers. Here the encl pillar AB has 98 square inches sectional area against 21 square inches in the adjoining diagonal BC, which bears 40 per cent, more stress, and 20 square inches in the vertical CD, which endures a compression three-fourths of that on AB. Were AB reduced to 25 square inches area, which would be most abundant, 11 per cent, of the weight and cost of the whole girder would have been saved. There is, however, a still better arrangement originated by Whipple, an American Engineer, and used in America for nearly half a century past. It is to abolish the bars AB, BC, BD, and insert a compression diagonal AD as shown by dotted lines. This will leave the compression on. the top chord unchanged, and the stress on all except the encl panel of the bottom chord, which at present is unstressed, though of massive section, and will reduce the stress on CD largely, thus saving no less than 17 per cent, of the