Under the term ordinary steel structures will be included only girder or simple-truss fixed spans of the following types:

   Deck, rolled-I-beam spans.

   Deck, plate-girder spans.

   Half-through, plate-girder spans.

   Deck, open-webbed, riveted spans.

   Through, riveted spans.

   Deck, pin-connected spans.

   Through, pin-connected spans.

The economics of all other types of steel bridges will be treated in other chapters.

Rolled-I Beam Spans

These are always deck structures because of the shortness of their limiting lengths. They are preferable to plate-girder bridges up to the limit which due consideration for the question of deflection sets, even if the economic limit for weight of metal be exceeded. This is because of their extreme simplicity for both manufacture and erection, and also at most times because the base prices for rolled I-beams are lower than those for plates and light shapes besides the cost of shopwork is less, owing to the small amount of detailing and the comparatively few rivets that have to be driven. In respect to the economics of erection, they are generally built of standard lengths and sizes, thus permitting the larger railroad systems to keep a supply of them in stock to be used for emergencies. Moreover, they are put in place very readily, and there are but few rivets to drive. Of course, plate-girder spans could be standardized and employed in the same manner, but not quite so conveniently.

For ordinary I-beams the longest steam-railway spans are about twenty (20) feet when four lines of stringers per track are adopted, but by employing the thirty (30) inch special sections of the Bethlehem Steel Company the limit can be increased to about thirty (30) feet for fairly-heavy engine-loads. By using six lines of beams per track the limit can be increased to about thirty-five  (35)  feet,  for  which  span  the  depth  is  only  one-fourteenth  (1/14)  of  the  length,  a  ratio  common  enough  in  England,  but  ob-

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