Normally this happens when your SupportLine is smaller than the width of the bridge. One should always make the SupportLine longer than the width of the bridge.
Here is an example illustrate the issue and resolution. Shorter SupportLine will cause the deck to draw properly and the beams will not get placed correctly as you notice:
To resolve the issue extend the SupportLine more than the width of the bridge the deck will get fixed.
To fix the beam placement - If you have already placed the beams them reopen the Beam Definition dialog and click on OK to place the beams again: