Ottawa's Illinois River Bridge (IL-23,71)

20150809 4056, west elevation from the south shore

(IDOT: simple detailed, John Weeks III; Satellite)

There have been quite a few predecessor bridges here. This 4-lane continuous steel plate girder bridge was built in 1981 for $11 million. It is 1,317' long with a 510' longest span. The navigation channel is 466' wide with 47' of clearance. The other two spans are 385' and 305'. "When it was built, it was the longest plate girder bridge in the world." [Weeks]

east side from the south shore

South span

The third span is mostly over land. Since it is over land, why didn't they add some piers and use shorter spans, which would be cheaper? I decided that the design of the bridge is essentially a cantilever bridge and that they needed this big approach span to balance the south half of the center span.

I have yet to find a resource that lets me determine how deep the bedrock is at different places in the river. I suspect that there is a thick layer of sand here and that the piers are very expensive to build. That would explain why they spent so much money on steel to make such long spans. Note that the center span is about 50' longer than the navigation channel width. I tried checking the width of the navigation channel for the Seneca and Marseilles bridges, but the IDOT database is blank for that entry for those bridges. John has a number of 354' for the Seneca Bridge. So the center span is more than 150' longer than what is really needed for navigation.

I assume these piers go down to bedrock and that bedrock is significantly deeper in the middle of the Fox and Illinois River flows.

Satellite

Since the waters of the Illinois are muddier than the waters of the Fox river, it looks like there might be a less-eroded ridge in the bedrock between the confluence of the two streams. This seems to be about where the pier in the river is located. The fact that the 1855 bridge failed because the piers began to sink and the 1885 bridge had an emergency condemnation in 1906 is more evidence that building piers in this location is difficult (expensive).

As would be expected for such long spans, the girders are deep. I waited a while for a semi-truck to cross the bridge so that I could take a picture with one on the bridge to help give scale to the depth of the girders. But none arrived. In fact, I didn't see any cars either. So I took a picture with a jogger on the pedestrian path. I believe it was a woman, so I'll estimate she is 5'6" tall. That would make the beams about 11' deep. I wonder if they can roll plates that wide or if a couple of plates are butt welded along their long edge to make those beams.

I included the plate on the bottom in the lower-left corner of the picture to show how I can find the field joints in the pictures. I accidentally left the camera on ISO 800 after I took some pictures in a museum in La Salle so the resolution of the pictures is not good enough to see the bolts of the field joints among all of the rust. But the resolution is good enough to see the bottom plates of the field joints.

Because of the uncertainty of  the jogger's height and the location of the sidewalk, I took this picture to include the span in almost the same plane as the depth chart.This view indicates the depth is about 10'. I'll go with this value since the jogger-based estimate had a couple of uncertainties.





Because I had easy access, I took pictures from underneath. If I can fill most of the camera frame with beams instead of sky, I can easily get a good exposure of the steel work.

Update: a couple of more thoughts concerning the plate girders.

Lloyd Scott Hardin posted two photos with the comment: "n/b 12 pk."

1

2