I DEEPLY recommend you watch any of the numerous youtube videos on how building stability works in Valheim. Short answer, it doesn't work like you think it might. Crossbeams don't directly equate to stability. Stability has to do with distance to the ground, straight down, from that piece and the route that takes. The more pieces in that route the less stable things become. Seriously watch a few vids and it'll make some sense, trust me. I had the same question when I first started.