This would require Beams to have cover fills. Unfortunately, they do not have them and this is a long-standing, highly supported wish:
http://archicad-talk.graphisoft.com/viewtopic.php?t=15891
You could draw fills on top of them but that is not very BIM-like.
You could also use GDL Objects found in the default ARCHICAD library to place those Beams. GDL Objects usually have fills.
You could convert those Beam to Morphs, which have Cover Fills.
You could try to use the RoofMaker Add-On from the Design\Design Extras menu to construct those Beams.
When such situations arise it is always good to try to think outside a box: not in terms of what Tool something is supposed to be modeled by, but what Tool is capable of producing the exact 2D and 3D representation you need.
With the Element Classification field in the Tags and Categories panel of element Settings Dialogs any element can be classified as any other element so you can use a Roof to model a Slab, a Wall to model a Beam, a Morph to model a Column, etc., if that gives you the result you need.
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