Engineering Wonders of the World
Volume I

BY E. LANCASTER BURNE, A.M.I.C.E. This short article will help the reader to understand why Bridges vary so greatly in design, and what are some of the problems that the Bridge engineer has to solve. THE art of bridge construction was prob- ably the genesis of civil engineering, the principles of the beam and the arch being understood and made use of by the ancients, particularly the Romans, some of whose bridge structures survive to this day. But the greater capacity of iron and steel, combined with the knowledge gained since their introduction, has enabled us to achieve results that would have been impossible in the days of stone and timber. As the wooden joist was the precursor of the iron girder, a short examination of the effect of a load upon a simple rectangular beam will be a fitting introduction to the subject of the modern steel bridge. If a beam rest upon supports at each end, and a weight be placed upon it, a bending action is set up which will tend to stretch the fibres in the lower portion of the beam and compress those in the upper part (Fig. 1). The line of demarcation between these two {Fig-1.) COM PRESSION opposite stresses of tension and compression is known as the neutral axis, and it is evident that the greater the distance of the fibres from the neutral axis, the more advan- tageously is their strength applied. Given three beams a, b, and c, b having twics the width of a, and c twice the depth, but being in all other respects similar, their relative strengths will be as 1:2:4, although b and c contain the same amount of material. From this it follows that a solid beam signifies an uneconomical distribution of material. There-