Spent the weekend at a rental in Sevierville and learned a lot about deck safety

I had a similar scare with a 450-gallon Bullfrog spa we installed on our back porch last summer. everythign looked solid during the build, but the second we filled it up, I noticed a distinct lateral shimmy whenever someone climbed the steps. It’s nerve-wracking feeling that much weight move even a fraction of an inch under your feet. To fix it, I ended up having to crawl under there and double up the joists to 12-inch centers and install some heavy-duty Simpson Strong-Tie tension ties to the house rim joist. We also added 4x4 diagonal knee bracing on the corner posts to stop that side-to-side sway. It took an entire weekend of extra work, but the difference in stability was night and day.

People realy underestimate that a gallon of water is about 8.3 pounds. By the time you add the tub itself and four adults, you're looking at nearly 5,000 pounds concentrated in a small footprint. If those 6x6 posts aren't notched correctly or if they’re just relying on a few lag bolts to hold the weight, you’re basially sitting on a ticking clock.

I acutally had the opposite experiance when I built my platform for an Arctic Spas Kodiak. I was so paranoid about the sway people describe that I went overboard with the framing, using 2x12 joists spaced 8 inches on center. I thougt for sure it whould be as stiff as a concrete pad, but I still felt a tiny bit of vibration when the jets were on high. It drove me nuts for a week untill I realized it wasn't a structural failure, just the harmonic resonance from the pumps transferring trough the wood. Instead of more bracing, I ended up using thick rubber isolation pads under the cabinet corners. It made a world of difference. Sometimes what we feel as a 'shift' is just the energy of the water movement interacting with the wood's natural flexibility. That said, if you’re seeing the actual posts move, that’s a whole diffrent ballgame. I’ve seen some rentals in Gatlinburg where they just notched 4x4s and called it a day, wich is terrifying given that a full tub can easily hit 5,000 pounds.

While many focus on joist spacing, I find it alarming how often the shear capacity of the ledger fasteners is overlooked, especially considering that a concentrated load of that magnitude requires a specific staggered bolt pattern to prevent the rim joist from splitting under tension. If you felt a distinct sway, it's highly probable the builder neglected to install lateral load devices or proper diagonal blocking, wich is essential to prevent the 'racking' effect that occurs when the center of gravity is elevated. I always insist on using SDS heavy-duty connector screws for any structural hanger because the margin for error with standard hardware is virtually zero once you factor in the hydraulic resonance.

That sway is definitly unsettling, but have you ruled out the decking material itself just flexing under your weight? Sometimes it’s not even a structural failure in the joists, but just thinner 5/4 boards or even composite decking that isn't rated for that kind of point load at the steps. I've seen builds that were rock solid underneath, but the person getting in feels a 'dip' and assumes the whole deck is going down. Was the actual post moving at the base, or was it just some surface-level bounce?

You're spot on about the weight distribution. I've seen so many of these 'aesthetic' rentals skip out on the most basic hardware. One thing people often miss when they're beefing up those posts is the actual base of the support. If those 6x6s are just resting on pre-cast pier blocks sitting on top of the soil rather than being poured in deep concrete footings, all that extra bracing won't do a lick of good once the ground softens up from splash-over or rain. That sway usally starts right at the dirt.