Commercial Influences

The principal commercial consideration that will affect the layout of a bridge is the amount and character of the traffic of which it will have to take care. If there is a variety of traffic, such as steam railway, electric railway, wagon, and pedestrian, considerable attention must be paid to the question of how best to take care of all probable combinations of the different kinds. Much money can be saved for a client by a bridge engineer who knows how to handle the question; and much can be wasted by one who is not properly posted on this important subject. An indisputable proof of the correctness of the latter statement is furnished by the notorious case of a proposed bridge to cross the Second Narrows at Vancouver, B. C. In that layout three railway tracks were adopted where two would have served the purpose equally well, with the result that the estimated cost of the structure was increased about seven hundred and fifty thousand dollars, and the project, in consequence, was either killed or relegated for consummation to the dim and distant future.

Property Considerations

Property considerations sometimes have a far greater effect on the layout of a structure than is at all legitimate. For instance, in the case of the Northwestern Elevated Railroad of Chicago, engineered by the author in the early nineties, certain high prices for land caused the company to lay out such a crooked line as to interfere materially with the attainment of a satisfactory train velocity. Refusal of property owners to allow the construction of piers or pedestals on their land will often oblige an engineer to adopt an unduly long span, or even an entirely different type of construction from the ordinary. Again, the necessity for occupying a certain city street will sometimes change entirely the character and layout of an approach to a bridge, and it might affect even the layout of the bridge itself. The method of crossing a railroad track at the entrance to a bridge might alter fundamentally the type of structure, a low bridge with an opening span being adopted if the crossing be at grade, and a high bridge with fixed spans if it be overhead. Public improvements sometimes cause material modifications of plans for proposed bridges; and even projected improvements with prior rights are liable to cause troublesome interference. The author has lately encountered obstructive opposition of this nature on a big bridge project.

General Features of Structure

The question of whether through, deck, or half-through truss spans or girders are adopted is one that will radically affect the layout, but mainly in the line of economies, because deck structures in most cases involve a saving of expense in both substructure and superstructure, in that the