Sparked by this excellent high resolution photo from the Shorpy collection, posted at Eng-Tips, and a comment that it was a three-pinned arch (coming from Wikipedia), I thought I would investigate further.

Zooming in on the crown shows that there is indeed something different happening at the arch crown:

Hinge detail

… and this is shown more clearly in this detail taken from a collection of high resolution images at The Library of Congress.

Hinge Detail

The Library of Congress collection also includes a written report, and three detailed drawings, prepared at the time of the bridge’s demolition in 1982, from which excerpts are shown below (click on any image for a full screen view, or visit the Library of Congress site for full high resolution files:

Plan, section and notes (visit Library site for high resolution version)

Hinge Detail Drawing

Design notes