If i draw a beam in plan view en define the height, the beam disepare. Is there a posibility that the beam is in place and you can see a dash dot line and two dashed lines in plan view?
Most likely the beam level is above the view cut plane level. If you put in a plan region where the beam is located ( View tab / plan views / plan region ) and set this cut level to be through the beam, you can then see the beam. You can then use the linework tool to get a dash dot line or however you would like it to display.