Specifications for the Design of Bridges and Subways

In connection with his duties as Consulting Engineer to the Department of Bridges, New York City, in 1906, the writer found occasion to take up the revision of the specifications for the design of bridges, in order that a single specification might cover the general work of the Department. The bridges of the city varied in length of span from 16 to 1 600 ft., and were classed as either “long-span” or “short-span,” there being none of such intermediate length as would question the class to which it belonged. It had been the practice of the Department, as it had been the general practice elsewhere, to use a special specification for each bridge, conforming to the particular conditions of span and loading. For short-span bridges, where the live load was heavy and frequent, the allowable unit strains on the material were comparatively small, while on the longer spans, where the live load was more diffused and less frequent, the allowable strains were correspondingly increased. In spans of extreme length, where live-load congestion rarely occurs and is slowly applied, the allowable live-load strains may equal those permitted for dead load.