The trunnion type is comparatively simple, but has not the advantage of the retreating span possessed by the rolling lift, and hence often necessitates a rather long leaf for a fixed clear opening.

By the adoption of the Waddell and Harrington detail for bearings, the ambiguity of stress distribution and the secondary stresses, involved through the bending of the ordinary trunnion-axle or trunnion-girder, are entirely avoided, thus rendering the design of structure decidedly more scientific and cutting out some abnormally high intensities of working stresses.

The roller-bearing type has not been much used. When properly designed, it is neat, scientific, and in every essential way excellent, but is not pre-eminently economic.

There is considerable rivalry between the patentees of the various standard types of bascule. Each one seems convinced not only of the superiority of his own type but also of its greater economy; consequently it is not an easy matter to draw conclusions on bascule economics that will satisfy all concerned. This much, however, can be said—when the bascule is entirely a deck structure, the most economic type will depend greatly upon the governing conditions; but when the counterweights, towers, machinery, etc., are above the deck, the Strauss and the Brown types appear to have an advantage over all the others. Until a short time ago the Strauss heel-trunnion type held the record for economy, but the lately-developed Brown Balance-Beam type appears to be slightly more economical than any other bascule in the overhead-counter-weight class. At present there is no example of this new bascule in existence, and the only one yet designed in detail is that for a proposed crossing of the Mystic River at Mystic, Conn. This was designed by the firm of Thos. E. Brown and Son, Consulting Engineers. Estimates of quantities of materials made from their finished drawings indicate that the Browns have succeeded in producing the most economic bascule with overhead counterweight yet evolved.

Economics of Vertical-Lift Spans

The governing conditions which prove economic for the vertical lift, in comparison with the other types of movable span, are as follows:

First. Low vertical clearance.

Second. Large horizontal clearance.

Third. Heavy moving span.

Fourth. Existence of fairly-long flanking-spans.

Fifth. Deep foundations, especially when the flanking-spans are long.

Sixth. Expensive piers, when flanking-spans are long.

Seventh. Skewed crossings.

Eighth. Concrete deck desired.

Ninth. Other first-class deck, especially if heavy.