The determination of the live loading for any proposed structure is an economic problem of prime importance; and it often involves the employment of engineering talent of the highest order. As a rule, that loading should be made large enough to take care of the greatest moving loads that may reasonably be expected to come often upon the structure during its lifetime; and as the latter for a well-designed, modern bridge is of indefinitely great length, the problem is by no means an easy one to solve. It must be recognized that the occasional application of a load exceeding by 25 per cent, or even more, the one used in making the design, will do no harm to the structure, but that when the excess reaches 50 per cent and is applied often the condition begins to become serious.

While it is right and proper for an engineer to act with precaution by assuming the live load great enough to meet all likely contingencies, it would be uneconomic to attempt to provide for improbable or impossible loadings. Again, while it is feasible to subject a short span to a very heavy loading, it is not so in the case of a long one; and the longer the span the smaller is the chance of its ever having to carry an abnormally great live load. This remark applies more forcibly to highway bridges than to steam-railway structures; although it undoubtedly holds good to a certain extent for the latter, because it is very seldom that long trains are composed entirely of the heaviest kinds of loaded cars. About the only exception to this rule is in the case of a structure on a railroad carrying long trains of fully-loaded ore-cars or coal-cars.

There is one point in connection with highway live loadings that is of importance, viz., that all new highway bridges should be proportioned to carry heavy trucks. The concentrated loads caused by these are so great that the old-fashioned wooden-joist floors will not be able to carry them. There is no telling how far from home these exceedingly heavy auto-trucks will travel, so that no out-of-the-way highway bridge will be safe from their invasion. Again, it must be remembered that auto-truck loads are rapidly on the increase; hence one should be liberal in his assumptions for this feature of the live loading.

In respect to live loading in general, it seems almost unnecessary to mention that the amount thereof decreases slowly with the span-length. In steam-railway structures this is primarily because the effect on the total span-length of the greater weight per foot of the coupled standard locomo-

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