Sunday, August 16, 2015

Ottawa Illinois River Bridge, Revisited

The morning after I published the posting on the Ottawa Illinois River bridge, I woke up with a couple of questions on my mind:

  • What is the length of the plates used to construct the girders?
  • Why are the girders for the short spans as deep as the long span?
To try to answer the first question, I marked the four field joints in the long span with red lines.

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There are three girder segments between the two pier-based structures. My closeup of the pier structure is not wide enough to include the field joint on the near side. I cropped the following view out of a different photo. I included the car to show that they are visible if they are in the outside lane.

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I estimate that the pier structure contributes 36' to the 510' long span. So the three plate girders in the middle cover 510-2*36 feet or 438 feet. It appears the three plates are about the same length so the length of a plate is 438/3 = 146'. I have been wondering where there is a rolling mill that can roll plates at least 10' wide. Now I wonder if the rolling mill is near a river or canal so that they can be shipped by barge. That is one advantage of building long spans, they tend to be over navigation channels.

I have two thoughts as to why the girders under the short spans would be stronger (deeper) than necessary. One is that once they tooled up to fabricate the deep girders for the long span, it was cheaper to pay for extra steel and reuse that tooling to fabricate the girders for the shorter spans. Second is that the weight of the big girders in the short spans was needed to balance the weight of the center span. My current opinion favors the second possibility --- to provide weight and strength to cantilever half of the long span.


  1. I did a little bit of work on the parking lot design at the north end of the bridge. Normally a plate can't be rolled more than 12 feet deep. Greater depths can be pieced together with shop welds. I recall the girders were transported by barge by one of the USS plants. The design of the girders and haunches all works together - there is a cantilever balancing of the inner span with the two outer spans, as well as moment resistance by the deeper haunches.

  2. USS had a 96" plate mill at Chicago South Works, gone for about 30 years ,and a 160/210 Plate mill at Gary Works, still operating.