I'm planning to build one of the loft beds for my 3.5-yr-old son, and I'm wondering about how much weight these beds can safely support. I know it would be safe for the little guy, but what about an adult or him plus some friends? I am not a tiny lady, and I know that some of the commercially available loft beds have fairly low limits - I'm imagining the whole thing falling down under me! Is there a way to calculate how much weight a plan will support? Suggestions for modifications that would increase strength if necessary?