Tuesday, January 11, 2022

1902+1938 Ashland Avenue Bridges over South Branch

The 1902 bridge was a rare Page & Shnable trunnion design.

MWRD posted
A view to the northeast showing the nearly complete Ashland Avenue bridge over the South Branch of the Chicago River on October 6, 1902. The bridge was built by the Sanitary District (now MWRD) and was open to traffic in January 1903.
Ray Harstick: Unique design. Was this the only one?
MWRD shared
Michael Madden: Those two guys standing on the top of the right bridge had a view many hadn’t seen of the city at that point.

CDOT via Chicago River Bridges (Patrick T. Mcbrarity) via BridgeHunter-1902, License: Creative Commons Attribution (CC BY)

1909 Photo via HistoricBridges

The bridge was designed by Page & Shnable and they evidently drew two different designs. I think this through truss design is the one described by the text in the digitized resource.
Digitized by Google, p393

But this low-truss design is obviously the one that got built. I can't determine if it uses a rack-and-pinion or a long screw rod to raise the bridge.
Digitized by Google, p393 @ 125% magnification

When I read "the rack is curved so that it is always tangent to the driving pinion at the point of contact," I was reminded that Page & Shnable also designed the rare CN/GM&O/Chicago & Alton bridge over the South Fork because it also has a curved rack.

I'm thankful that the Chicago City Engineers constructed a much more elegant and simple design in 1902 for the Cortland Street Bridge. They probably saved the city a lot of maintenance cost and has passed the test of time. In fact, the 1938 replacement bridge at Ashland is the same basic design.

This is the post that motivated the research for these notes.
MWRD posted
Construction of the north abutment for the Ashland Street bridge over the South Branch of the Chicago River on September 24, 1901.

Street View

"This is rare example... indeed one of only three... examples of very wide pony truss bascule bridges in Chicago that feature three pony truss lines to carry a wide roadway." [HistoricBridges]

1 comment: