While I am not sure how to fix it (I am new to Sketchup) I can tell you that you have non manifold edges. Use Meshlab to show them. See the attached jpg and all the red areas show issues. I think it's because your bridge is curved but is one single face. It needs to be made up of flat faces so many flat faces hooked together can make a curved surface.
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Screen shot 2012-11-11 at 12.12.51 AM.jpg
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