Maintenance and Repairs

The cost of maintenance and repairs as well as that of operation may sometimes be a vital consideration affecting the layout of a structure. For instance, when the Jefferson City highway bridge over the Missouri River was about to be built, the bridge company, in spite of the author's forcible remonstrance, let the contract for the structure on the basis of a high bridge with a long and expensive timber trestle approach. Later they were convinced that the annual expense of maintaining the said trestle would be so great as to consume more than the total net income from traffic receipts; hence they had to change to a low bridge design.

Theoretical Economics

From time to time an engineer encounters a bridge problem in which the controlling factor in the layout determination is really that of economics, and then he is happy; for, comparatively speaking, the case is a simple one. A case of this kind occurred in the author's Canadian Northern Pacific Railway bridge across the North Thompson River, near Kamloops, B. C. The structure consists of a number of deck, plate-girder spans, one of which is lifted so as to permit of the passage of small river steamers at certain high stages of water.

The requirements of aesthetics often conflict with those of economics; for it would not look well to let the span lengths change backward and forward, perhaps, to suit the vagaries of an unusual bed-rock profile; hence it is best in many cases to compute the economic span length for average conditions of pier cost and to use one length instead of several. It will generally be found that such an arrangement does not involve any extra expenditure worth mentioning when the cost of structure for that layout is compared with that for the truly economic one. The question of economics, however, cannot be finally settled by adopting simply that structure for which the initial cost is a minimum; because, as pointed out previously, the truly economic bridge is the one for which the sum of the first cost and the capitalized annual cost of operation, maintenance, and repairs is a minimum.

As a conclusion to the general subject under discussion, in order not to discourage young engineers, it might be well to state that any designer who, when determining the layout for any large and important bridge, can and does give full and due consideration to all the factors treated in this chapter, is truly worthy to be termed an expert bridge engineer.