Is my diagonal bracing overkill for an elevated deck?

Bracing a tall deck with 6x6 posts is rarely a mistake beacuse lateral movement is the most common cause of structural fatigue in high-elevation builds. When you have a crowd up there, the sheer weight is one thing, but the rhythmic movement of people walking or shifting can create a harmonic sway that eventually loosens your fasteners. Those diagonal braces are doing the heavy lifting by creating triangles that lock the geometry of the frame. You haven't gone overboard; you've built for longevity. Regarding the tension and wood expansion, you won't see the braces 'pulling' the frame apart. Pressure-treated timber mostly shrinks as it seasons rather than expanding significantly. As those 6x6s dry out over the next year, you might notice some checking or small gaps opening up at the shoulder of your diagonal cuts. This is completley normal. Just keep your impact driver handy and check the tension on your **thru-bolts** or structural screws after a full season of weather. If anything, the wood will pull away from the fasteners slightly as it loses moisture, so you might need to give them a quarter-turn later on. One thing to check while you still have your tools out is the connection point between the braces and the posts. If you used simple lag screws, make sure they are long enugh to bite deep into the heartwood. For a build this high, I always prefer using **structural timber screws** like TimberLoks or even galvanized bolts with washers. This prevents the brace from levering against the fastener head during high winds.

As for the concrete pad, as long as your post bases are elevated on stand-off brackets to keep the end grain out of standing water, you're in good shape. The weight is distributed vertically thru those 6x6s, and the bracing just ensures that load stays vertical. It feels rock solid now because it is, and you'll be glad you did the extra blocking when you have twenty people up there and the floor doesn't budge an inch.

I did a 12-foot high deck solo last year and had that same "is this too much?" feeling. It isn't. One thing I found was that while the posts were stiff, the actual floor had a bit of a drum effect when I walked on it. Since you still have the sawhorses out, try jumping around the middle of the deck. If it feels springy, throw some solid blocking between your joists every 4-6 feet. I used my offcuts for this and it turned a bouncy floor into somethig that feels like a concrete slab. It’s a small thing that makes a huge difference in how "pro" the final build feels.

While your diagonal bracing certainly mitigates lateral deflection, you ought to verify the orientation of your 6x6 heartwood relative to the prevailing wind loads, as the inherent stiffness of such substantial timber can occasionally mask minor eccentricities in the load path that only manifest during significant thermal cycling. I would also suggest inspecting the shear capacity of your ledger connection if the deck is attached to the rim joist of the house, because an overly rigid post-and-beam structure can sometimes concentrate stress at the house-to-deck interface if the ledger isn't adequately flashed and bolted to resist the cumulative tension of those solid braces.

Honestly, don't sweat the overbuilding. I've never seen a deck fail 'cause it was too stiff, but I've seen plenty that felt like a cruise ship in a storm. Since you're still set up, grab a level and double-check your ledger-to-house connection. If that's rock solid and you've got those 6x6s braced, you're golden. Did you end up using a hidden fastener system for the boards? Sometimes those can squeak a bit as the wood seasons, so that's the only real 'noise' you might gotta deal with later on.

Quick update — I took the advice and tried jumping around the middle. There’s definately a bit of that drum effect vibration happening even though the posts are solid. I also double-checked the ledger connection just to be sure. Since I’ve still got the miter saw out, do you think adding extra bridging between the joists woud kill that bounce, or am I overthinking the floor stiffness now?