Help with sway in a tall and long deck.

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Old 03-18-13, 08:21 AM
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Help with sway in a tall and long deck.

First, thank you for taking the time to read this post. My project: I spent all of last summer building my first deck. I thought I had researched it enough to do the labor on my own. I did get all my permits and spent a lot of time at the county office talking through and reviewing my plans with the inspectors office. My backyard is tricky, it is not wide but it is long. My deck specs are:
28' long x 18' wide. SYP frame. 6x6 Cedar Posts. 2x12 Joists 12 inches o.c.
16' from house to first beam. beams are 2 2x12 on 3 notched 6x6 posts.
9' to second beam. 3' cantilever. Almost entirely all Strong-Tie hardware. The back posts are 11'3'' in height.

I am 100% certified for occupancy. But my problem is this, there is still too much sway on top of the deck for my comfort. You feel it move too much when people are walking. Not so much when everyone is standing, but when you sit and people walk around, it feels like I'm on a dock on water.

How do I remove this sway? I've put in blocking @ 8ft and 20ft. I've run a diagonal 2x4 across the bottom of the joists (done after pics below taken). I have two more things to try but wanted to ask pros if they are worth it or if I should hire help. First, my blocking was done with finish nails. It was a last minute thing and I was tired. I need to go back and do it with real hardware but not sure that will make that big a difference. Two, I've seen basements where the joists are sandwhich'd between 2x4s. I could do the same on the backside of the beams if that would help?

Finally, I'm 100% ok with hiring help to find a solution. In your opinion would it be best to go with a deck builder? architect? or engineer for this kind of problem?

Help would most certainly be greatly appreciated!
 
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Old 03-18-13, 09:20 AM
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How are the bottom of those diagonal 6x6's fastened to the posts? If they are simply lagged into the side then that is likely one weak spot as you are relying totally on the shear strength of the bolt. Ideally they should be knotched into the post.

Obviously.....something is moving......the connections are not solid at all the necessary locations. With that height.....the lateral loads are huge.....and the diagonal bracing needs to be quite substantial. You would likely need longer braces that start lower on the posts and go further along the beam. The bigger the triangle.....the better.....even 2x6's with multiple through bolts would help. Perhaps even consider some x's of steel cable tensioned with turnbuckles.


.
 
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Old 03-18-13, 09:28 AM
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The bracing you see currently was my second attempt actually. Originally I did have just one 6x6 brace from each outside beam going about 3/4 the way to the inside beam. It was less stable than the inside/out combination in these pics. But I agree, I need to figure out a way to get lower bracing. Currently the bracing is attached to the posts with 2 3/4inch 10 inch long lag screws at a slight angle upwards (so they didn't go through the post). Behind the post, i attached them using metal L brackets and strongtie screws. They are attached to the beam with 2 staggered 10inch lag bolts.
 
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Old 03-18-13, 09:38 AM
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You need cross bracing to stop the swaying. If the cross braces do not extend all the way from base to top the unsupported sections of column can flex. Since your cross bracing will be tall and thin you can break it up into two "X", one on top of the other and with their ends connected.

Good solid connections where the cross braces meet the columns is also important. I hate to say that 2x4 or 2x6 through bolted to the side of the column works pretty well. It's not as fancy or pretty as your current mitered braces but the through bolt makes it easy to get a good, solid joint.

Take a look at photos of wooden roller coasters for some inspiration. You'll see various cross bracing techniques used to support the legs from the ground all the way up.

Another option is to use cables for the bracing. It's visually less obtrusive but that can be a double edged sword as it's easier to accidentally walk into a cable. Cable only only works in tension versus a wooden brace that can work both in compression and tension so get more bang for your buck with wood but since you have a quite nice looking deck cables may fit the style better.
 
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Old 03-18-13, 09:41 AM
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Welcome to the forums.

I don't build decks but I'm just making some observations.

That is one large tall deck

First thing that comes to mind..... is your ledger system bulletproof. A tremendous strain pulling away from the house.

Halton is definitely right about the triangles.......they need to be much bigger.

I would think that two steel cables installed corner to corner with turnbuckles would be helpful.
 
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Old 03-18-13, 09:42 AM
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Not sure how effective this would be but my thought is to create a 2x6 long enough to span the width of your deck (I'm thinking a lap joint across the middle post so 2 10' 2x6s would be needed to create each one) and then carriage bolt it to the three posts across with two bolts per post. For the length of the deck, you would be able to span the two posts with one 2x6 on each side, so only four bolts in that one on each side.
 
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Old 03-18-13, 10:00 AM
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I definitely got carried away with the size ! The ledger is pretty secure to the house. Ledger lok screws every 6 inch with 2x12 wood in the unfinished basement between the house floor joists filling the gaps. PJ, that is interesting, the cables running under the deck joists? I had assumed the cables would run post foot to head? Mitch/Pilot, would it make more sense to have One Giant X on the front and back posts? So bottom right all the way to top left? Or have 2 2x6 xs on each set of posts. Meaning Bottom Right to Middle, bottom left to middle...etc. option 1 or 2? I'm thinking 2x6 cedar?
 
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Old 03-18-13, 10:08 AM
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What kind of board would it take to make such an X? By my math, the back posts being about 11' tall would mean the boards in the big X would be 21' or more in length.
 
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Old 03-18-13, 10:28 AM
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I think a major problem is the long span from the house to the first beam (16'). Even though 2 x 12 joists at 12" centers may be good for it, deflection is always directly proportional to span length, and yours is definitely out there at the upper limits, asking for problems. If it were mine, I'd throw in another beam at the mid-point of that span, 6' from the house. Properly constructed, with tight fit at all of the joists (steel shims, if necessary), will make a big difference.

Another potential problem area I see is the use of cedar columns. Your deck, and especially the connections between columns and beams/stiffening diagonals, would be much stronger and less likely to allow movement if the connections consisted of all through bolts (not lag bolts), assisted by the use of steel plate (2" x 2") washers bearing against the cedar. Doing so will prevent the cedar from crushing when the bolts are tightened enough to make an effective, friction connection.

Speaking of diagonals--I disagree that the diagonals should span the full columns' height. Doing so enables the members used to flex too much because of excessive length, and actually defeats the purpose of shortening the effective span length of both column and beam by moving the connection points to the ends of the members. I think your current location is just about right, but you should also add diagonal braces oriented at 90-degrees to those already in place, and possibly a foot or two longer. Tall columns (especially soft cedar) like to deflect and buckle in both major directions, not just one, when loaded vertically.
 
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Old 03-18-13, 10:47 AM
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Post Footings

How deep are the post footings below grade?
 
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Old 03-18-13, 10:56 AM
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My thought on the long one was to meet on the middle post on a 45 degree cut, secure then continue the line all the way though to the next post. It would be 4 pieces of wood, but make one continuous X?
 
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Old 03-18-13, 10:59 AM
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Wirepuller, code here requires I be 30'' below grade. I'm currently @ 32 I think. I also bought the Bigfoot system, so I have a 12'' column that bells out at the bottom to 28''.
 
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Old 03-18-13, 11:08 AM
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BridgeMan, I can definitely add lag bolts w/ washers to strengthen my connections. What do you mean by the 90-degree braces, a foot or two longer? I'm having trouble imagining that one. Clarification would be appreciated! I did not make the decision to be 16' lightly. I have a retaining wall that goes out about 13' that caused me to go out so far. The city engineer said I could take the 2x12 out to as much as 18' without it being a problem. I didn't question that. I could add a Post on one side @ 6' and secure it to the framing directly, think that would help at all?
 
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Old 03-18-13, 11:16 AM
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By the way, I very much appreciate all the feedback! Two quick questions.

1. The sway is disconcerting, but does anyone think it creates an unsafe condition?
2. If I sought professional, local help, would it be smarter to use a contractor, architect or engineer?

Thanks!
 
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Old 03-18-13, 11:21 AM
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#1 - can't say for sure but if it scares me - I consider it unsafe
#2 - a contractor can do the work and hopefully know enough as to what should be done to fix the sway. An engineer would know what needs to be done and who you could hire but wouldn't do the work himself. An architect can usually but not always come up with the same info as the engineer.

btw - welcome to the forums!
 
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Old 03-18-13, 11:37 AM
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Eh, we like to do work ourselves, don't go hiring anyone yet
 
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Old 03-18-13, 11:41 AM
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Ya, after re reading my post it does kind of sound like I was recommending to hire the job out
I always figured doing the work myself let me keep my hard earned money in MY pocket
 
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Old 03-18-13, 11:44 AM
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Agreed! I would love to do the work myself, if I could only figure out what the work is! My this has to work last ditch effort solution was thinking I could add 6 new posts, 2 on each beam in between the existing, 2 at the very end of the deck bolted directly into the rim joists. If I knew that would work I would start digging tomorrow.
 
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Old 03-18-13, 11:54 AM
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I don't know that more posts is the answer, I would think cross bracing is the way to go ......... but I know more about staining decks than I do about building them
 
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Old 03-18-13, 12:08 PM
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I think the sway can be a hazard while any bounce in the 16' span is more a cosmetic annoyance so I would first address bracing the legs and would do two X's like you showed.



I would also consider adding some horizontal ties from the middle of your outer posts back to the middle legs. Then, if you want a belt and suspenders and can bear the appearance add cross bracing to prevent the flex of one leg from being transmitted to another. I know this will make the under side of your deck more like a wooden coaster but it will do a lot to stiffen up those legs.
 
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Old 03-18-13, 12:55 PM
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I think the bracing is necessary. The manner in which it is applied is still up in the air as far as having a definitive answer goes. I do like PJ's comment and suggestion regarding cabling with turn buckles diagonally around the perimeter, in addition to the wood bracing suggested. We did this on a garage for an engineer buddy of mine, and he had no sway at all. He had to tighten up the turnbuckles occasionally as the wood shrank and cabling stretched I guess.
 
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Old 03-19-13, 06:34 AM
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One thing not mentioned that I think is a weak link is the attachment of the 6x6's to the concrete pier. It provides little stability other than it will keep the lumber centered on the footings. You have what you call a 5/8" by 8" retrofit bolt holding your 6x6's lumber on top of the concrete. While a no-no for cedar, contrast that with a double poured footer with a PT 6x6 buried in 30" of concrete inside say a 14" tube. Now you are forcing the actual 6x6 to bend and not relying on a single 5/8" bolt to stabilize everything above.

I also have been trying to come up with a solution that harmonizes the smooth and sleek look of a very handsome deck. As mentioned, the solutions offered would make your deck look like a coney island roller coaster. Not the nicest of looks unless you live near the ocean and are stilted against hurricane forced winds and storm surge.

Front to back the house provides stability, its the left to right sway that needs to be addressed. While the cross bracing could have been larger, I think we can shore it up in a smoother way without replacement. I would lock both the top and bottoms of the cross bracing to allow zero movement except as a group. This would be similar to blocking put in between your floor joists. a 2x8 mounted horizontal to the bracing on both sides of the 6x6 and top and bottom of the existing bracing should lock everything from moving.
 
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Old 03-19-13, 07:19 AM
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czizzi, that is a terrific look. After all of the other thoughtful recommendations on here, I was going to go with the attached design with either 2x4 or 2x6 I wasn't entirely sure which yet. I was going to do this to both sets of beams. After seeing your recommendation, I do think I want to try it first. It is definitely the cleanest look. The only question I have is, can I do these spamming 2x8 supports in cedar as well, or should they be in a stronger syp?
 
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Old 03-19-13, 07:25 AM
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I agree chandler. After I lock down the wood bracing on the posts, I still think I will do an underside cabling to the ledger. I know all of this may be a bit excessive but I'm ok with the extra costs on materials considering I had no actual labor costs! I've never looked at cabling so not sure how expensive it is, but it is next on my list.
 
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Old 03-19-13, 09:41 AM
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Based on what I displayed in my earlier post. The corner to corner cabling method is very effective.

The mounting points need to be solid as does the cable. A break would be a disaster. If one side broke the other side would be pulling.
 
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Old 03-19-13, 05:30 PM
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I have to admit, that I don't know the structural strength of cedar vs. Pressure treated. If you choose to test the design I have offered, then from a cost perspective, I would go with Pressure treated for the test. If it gives you the desired results, then switch to cedar and see. Having done some work with cedar, I know how pricey it can get. You can always replicate the look of cedar with semi-transparent stains.

PJ - Trying to figure out how your recommendation of cross cables will prevent side to side sway? It will hold the unit together, but help me with the lateral movement if you could... To me it will help with Racking or torquing but I am not seeing the lateral. Always willing to learn
 
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Old 03-19-13, 09:30 PM
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The cables need to be attached in the four corners and in the middle.

It would seem to me that racking is the swaying the OP is feeling.
 
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Old 03-19-13, 10:32 PM
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As I mentioned in an earlier post, I think his "swaying" is a combination of the cedar columns buckling slightly under load, and the slightly over-spanned joists deflecting.

Just for kicks, using an assumed load per joist of just 50 lb. per foot, and a "guesstimated" modulus of elasticity (E) of 1.2 million, the maximum theoretical joist deflection works out to more than 1/3 inch (0.345") for the 16' span. If he were to shorten the span by one-half, installing an additional beam at the midpoint of the present 16' span, the maximum deflection would only be 0.022".

Significant difference, yes?
 
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Old 03-20-13, 04:29 AM
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Yes, it is significant but the OP's main concern is sway not bounce in the 16' span.
 
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Old 03-20-13, 06:41 AM
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Significant difference yes. The only reason I am leaning toward bracing vs post support now though is because the sway is more significant on the outer half of the deck, less so "almost non-noticeable" in the 1st 3rd of the deck.
 
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Old 03-20-13, 06:44 AM
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And, I think the sway is a potentially more hazardous condition. If you have a party one good song could get everyone dancing and really set the deck swaying.
 
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Old 03-20-13, 07:10 AM
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LOL, not a problem. I'm going to ask people to wear climbing gear and strap on to the house before going outside!
 
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Old 06-19-14, 11:27 AM
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Was this issue resolved?

Sounds like this issue was resolved... I predict a similar issue for a deck 10 feet above grade similar to this case that I am finalizing. (im using pressure treated lumber, on a freestanding deck that can't be attached to my house because of property setback issues - I'm building under Accessory structure rules).

Any chance the original poster could post what his final solution was?

I'm finding most building codes are pushing to the "post on pier" method, and this stiffness issue is a pretty common one.

Thanks in advance!
 
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Old 06-19-14, 02:15 PM
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The OP hasn't frequented the forums since the thread ended. You may find it good to send him a Private Message. Most will have an alert when it is received.
 
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Old 07-25-14, 01:58 PM
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I hope you come back. I just came across this post while looking for something else. For all those with a similar problem I recommend they look to what bridge builders do. What you're looking to build is a WARREN TRUSS or a PRATT TRUSS.

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I think czizzi was almost there but this will make the deck rock solid and still leave the area under the deck clear.

If you can do this is in two horizontal directions then you could pretty much drive a drive a truck on it. (don't drive on it)

One more thing...No lag bolts! full nuts and bolts with big washers. You can save a few bucks on this by using hot dipped galvanized threaded rod cut to size then nut and washer on either end. Stainless is good too but cha-ching...
 
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Old 07-25-14, 02:32 PM
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Not many (if any) residential decks are loaded so heavily that they require bridge trusses to be built to stiffen their load-carrying members.

It could be done, I guess, if someone wanted to spend the time and money to do so. And howitzers could also be used to kill their mosquitoes.

Also, cutting galvanized threaded rod into shorter lengths will result in the ends having exposed steel, subject to corrosion, since they won't have galvanizing where they were cut. Not much savings when considering the cost of (correctly) applying zinc-rich paint to all of the cut ends.
 

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Old 07-25-14, 02:52 PM
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And howitzers could also be used to kill their mosquitoes
I would have been willing to use one against skeeters when I lived in fla they're thick enough down there for a big shell to take out a bunch with every shot

While I've stained a lot of decks over the years, I've never seen one that utilized a truss system like that.
 
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Old 07-25-14, 03:11 PM
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The standing joke when I worked in Alaska was that it was the only state that required its mosquitoes to have landing lights on them--they were big suckers!
 
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Old 07-30-14, 07:56 PM
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I've always hit the end of the cut rod with a spray of clear coat. Never seen a bit of rust in years of using this method. If it does rust the ends will not decay...Or whatever...buy bolts...that was just a suggestion that has worked for me for years.

I can't speak to the "residential requirements. I'm not suggesting bridge trusses or that you bring in iron workers...Just to copy the design that they use with timber.

....but we're talking about stabilizing a very tall deck that IS swaying. Something that I assume is going support friends and family and being people, these are not static loads. $200 worth of PT 2x4 and some fasteners, to provide you with a rock solid deck that will also leave you with all the space below to me seems to me like a bargain....And believe me, I'm cheap.

Say 12 people at a BBQ. each weigh 180lbs = 2160 lbs. Now add the weight of the BBQ, a keg of beer, 16oz burgers and the family dog....A dynamic load, all swaying side to side atop 11' posts....

If my family is going to be on it...I'm over engineering...I have been on a deck when it collapsed...put a real damper on the party....Thankfully it wasn't as tall as this deck or people would have been badly hurt.
 
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Old 08-25-14, 05:13 PM
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Resolution...ish

I wanted to update you all on the state of the deck. I apologize for the delay, a new baby put a halt to all of my diy! I'm attaching a picture of the current state of the deck. This is what I did.

I attached horizontal 2x6's first, front and back posts, 2 per set, so in all 4 horizontal 2x6's. those didn't really do much.

I added 2x6's in an X pattern to the back posts. those helped significantly, and had a noticeable impact on the sway. I believe doing the same thing to the front posts will complete my project.

I haven't don't the front posts yet, because, well, I don't love the look. It isn't nearly as clean as when I started. I am exploring alternate ways of getting the same results but with a cleaner presentation. The horizontal 2x6s will come down, they added no value.

Ultimately, looks aside, I believe the consensus was correct, long beams that attach from the bottom of 1 post to the top of the other is the way to go for tall decks. How to make it pretty....well...working on it.
 
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