My bridge design is perfectly symmetrical, but the load test results on one side are different from those on the other side. Is this a bug in the software? If the bridge is symmetrical, shouldn’t the load test results be symmetrical too?

This is not a bug in the software. In order to have symmetrical load test results, three conditions must be present: (1) the structure and all its members must be perfectly symmetrical, (2) the supports must be symmetrical, and (3) the loading must be symmetrical. In the Bridge Designer, the supports are symmetrical for some, but not all, of the 54 site configurations. And the loading is never symmetrical when the AASHTO H25 truck loading is used. The H25 loading has a heavy axle (the rear one) and a light axle (the front one). Since the truck only crosses the bridge in one direction, the asymmetrical loading causes small differences in the load test results for identical members on opposite sides of a symmetrical bridge. For more information, click the Help menu in the Bridge Designer, then choose “Help Topics” and look up the topic, “What is Not Realistic about the Bridge Designer.”

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