There's some benefit to having horizontal supports holding a piece from several sides, but in general beams provide no real support to anything above them. It's all about vertical support to the nearest solid ground. You need foundations of the strongest material you have touching the world surface, embedded in it where possible. Iron > Stone > Core Wood > Basic Wood

Note that core wood is confusing to work with. The horizontal beams behave like short, wide sections of wall and have no central snap points.