The bridge engineer’s work is very diversified, since no two bridges are alike.  At one time he might be ordered to span a stream in the midst of a populous country where every aid is at hand, and his next commission might be the building of a difficult bridge in a foreign wilderness far beyond the edge of civilisation.

Bridge-building is really divided into four parts, and each part requires a different kind of knowledge and experience.

First, the designer has to have the imagination to see the bridge as it will be when it is completed, and then he must be able to lay it out on paper section by section, estimating the size of the parts necessary for the stress they will have to bear, the weight of the load they will have to carry, the effect of the wind, the contraction and expansion of cold and heat, and vibration; all these things must be thought of and considered in planning every part and determining the size of each.  Also he must know what kind of material to use that is best fitted to stand each strain, whether to use steel that is rigid or that which is so flexible that it can be tied in a knot.  On the designer depends the price asked for the work, and so it is his business to invent, for each bridge is a separate problem in invention, a bridge that will carry the required weight with the least expenditure of material and labour and at the same time be strong enough to carry very much greater loads than it is ever likely to be called upon to sustain.  The designer is often the constructor as well, and he is always a man of great practical experience.  He has in his time stepped out on a foot-wide girder over a rushing stream, directing his men, and he has floundered in the mud of a river bottom in a caisson far below the surface of the stream, while the compressed air kept the ooze from flowing in and drowning him and his workmen.

The second operation of making the pieces that go into the structure is simply the following out of the clearly drawn plans furnished by the designing engineers.  Different grades of steel and iron are moulded or forged into shape and riveted together, each part being made the exact size and shape required, even the position of the holes through which the bolts or rivets are to go that are to secure it to the neighbouring section being marked on the plan.

The foundations for bridges are not always put down by the builders of the bridge proper; that is a work by itself and requires special experience.  On the strength and permanency of the foundation depends the life of the bridge.  While the foundries and steel mills are making the metal-work the foundations are being laid.  If the bridge is to cross a valley, or carry the roadway on the level across a depression, the placing of the foundations is a simple matter of digging or blasting out a big hole and laying courses of masonry; but if a pier is to be built in water, or the land on which the towers are to stand is unstable, then the problem is much more difficult.