'Continuous' tied arch bridge

[tmalik3156]

Good day all.
I was looking at the beautiful Bluewater International Bridge between Michigan and Ontario over St. Claire River. It's a twin bridge. The one shown on the near side of the photo is a tied arch bridge with continuous spans.

I was wondering how the structural system works. We know, in a tied-arch bridge the deck in tension counteracts the horizontal component of the arch in compression at point A. I tried to draw a sketch of tension (green) and compression (red). But I am getting a bit confused about the part AB (shown in yellow). Is this in tension as well? How do the forces act if we draw free body diagram at joints A and B?

 

[BridgeSmith]

The deck will obviously be in tension in the green section, and the arch will be in compression from point A to the interior piers. Beyond that, a structural model would be needed to determine whether the deck from A to B and the bottom chord from B to the interior pier is in tension or compression. Depending on the relative length of the spans, those members may see stress reversal for live loads in different positions.

 

[tmalik3156]

Thank you BridgeSmith.

the bottom chord from B to the interior pier is in tension or compression.

I think this half arch has to be in compression, so it can counteract the compression coming from the main arch at the interior pier (from point A to pier). The interior pier has no ability to carry a thrust, so the horizontal components of reaction at the interior pier have to cancel out.

By the way, I have drawn the supports at the interior pier incorrectly. One of them has to be a rocker type bearing to allow thermal expansion of the central span.

 

[BridgeSmith]

Under dead load, most likely you're correct about the bottom chord in the end spans being in compression. This would necessitate the top chord (the deck in yellow) being in tension. However, a large live load on the end span could reverse the net reactions producing a net tension in the bottom chord and throwing the deck into compression there. I don't know whether it would or could happen with a realistic live load on the end spans; just saying it's theoretically possible.

And yes, at least one of the interior pier bearings has to allow for movement, whether it's a rocker, sliding plate, elastomeric, or some other bearing type.

 

[bridgebuster]

The deck isn't in tension. The tie is in tension - the green line & yellow lines form the tie. It would always be in tension.