Spent the afternoon reinforcing the porch for my daughter's sandbox

I ran into a similar headache last summer when my wife wanted one of those Coleman SaluSpas on our 10x12 side deck. People relly underestimate how quickly weight adds up. A full hot tub is way worse than your sandbox, but the principle is the same. I ended up sistering the 2x8 joists and throwing in a few extra Simpson Strong-Tie hangers becuase the original builder just used toe-nails.

Good call on the vertical 4x4s. I did the same directly under the tub's footprint, resting them on concrete pier blocks. One thing to watch out for with the dog pool is moisture trapped underneath. Sand gets wet, and if that plastic sits there all season, it'll rot your deck boards faster than you can beleive. I had to replace three cedar planks after just one season because I didn't leave any room for airflow. You might want to slide a few composite shims under the pool edges just to let the wood breathe.

I acutally looked into the load tables for my own 12x12 pressure-treated deck when we were thinking about a large planter setup. Most decks built to code are rated for about 40 pounds per square foot live load. If that dog pool is about 5 feet wide, 600 pounds spread out over that area is actually less than having four grown men standing around a grill in the same spot. I ussually worry more about the "dead load" aspect where that weight never moves, wich can cause the wood to bow over time if it's not seasoned well. My bigger concern with the plastic pool approach is the moisture trapped underneath. I put a heavy sandbox on my old cedar deck years ago and within two seasons, the boards underneath were constant mush beacuse they couldn't breathe. I ended up putting my sandbox on a platform with some 1-inch spacers to keep the airflow moving. If you're using those 4x4 posts now, you've definitly got the support, but keep an eye on those floorboards once the kid starts splashing water into that sand.

Oh man, seeing those 4x4 posts go in must have been so satisfying! I’m just starting to learn about deck framing and this is exactly the kind of project that gets me pumped to start my own. Since you guys are talking about moisture and sand, has anyone ever tried using those outdoor interlocking deck tiles on top of the wood? I'm wondering if those woud help with the airflow under a heavy pool or if they'd just trap more gunk and make the rot even worse!

Glad you cut the sand in half 'cause wet sand's way heavier than folks think. If it rains or the kid splashes, that 300lbs turns into 500lbs real quick. I'd keep an eye on those concrete blocks too. If they're just sitting on top of the dirt, they're gonna sink once that weight settles in. You might wanna check if they've shifted after the first few weeks of use.