In the designing of suspension bridges there is still much to be learned, because so few of them have been built; and this is as it should be, because they are not an economic type of structure, excepting for exceedingly long spans, or in the case of a very light highway bridge at the crossing of a gorge or a river of great depth and swift current where it would be too expensive to build piers in the stream. As shown in Chapter XIII, for steam-railway structures suspension bridges are more expensive than cantilever bridges up to the practicable limiting length of the latter; and, moreover, in respect to the important element of rigidity the former are certainly inferior. But the suspension bridge has its legitimate place in engineering construction, and that is for long-span highway bridges pure and simple, and sometimes when they carry also electric railways. There are crossings, like that of the North River at New York City, where the conditions are such as to make the use of the suspension bridge obligatory.

It is, therefore, well worth while to study the economics of the type, even to the extent of making an exceedingly elaborate investigation, as the author did lately in his memoir on "Comparative Economics of Wire Cables and High-Alloy-Steel Eye-bar-Cables for Long-Span Suspension Bridges," presented in May, 1920, to the Engineers' Society of Western Pennsylvania, of which memoir more anon.

In designing a highway suspension bridge, the first economic point to consider is that of the deck and floor-system, both of which should always be made as light as the ruling conditions will allow, because the heavier the floor the greater the load on the cables. This general question of economics in deck and floor-system has been treated in Chapter XXI, to which the reader is referred. While it is certainly advisable to cut down the dead load to a minimum, it would be anything but economic to adopt a plank base for the pavement, on account of the great danger from fire which that type of construction involves. Modern highway bridges call for a reinforced-concrete base for pavement, and there is no dodging this issue; but in suspension bridges it should be made as light as practicable by using closely-spaced stringers and thus reducing the thickness of slab to a minimum. It is true that a buckled-plate floor is lighter than a reinforced-concrete slab, but, until quite lately, as indicated in Chapter XXI, experience has shown it to be so lacking in rigidity that under the passage

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