# 4x4 post bending - HELP!!



## jamisonlm (Mar 28, 2021)

I am working on a project for my son's 5th birthday - hanging a ninja warrior type obstacle course in the backyard. Here is a link to the product. 








Amazon.com: Ninja Obstacle Course - 2X42 FT Ninja Kit with Most Complete Accessories for Kids, 2 Slacklines, Trapeze Swing, Web Ladder, Ninja Wheel : Toys & Games


Amazon.com: Ninja Obstacle Course - 2X42 FT Ninja Kit with Most Complete Accessories for Kids, 2 Slacklines, Trapeze Swing, Web Ladder, Ninja Wheel : Toys & Games



www.amazon.com





I don't have trees that would work, so I set two 8ft 4x4 pressure treated posts in concrete to use. When we hooked up the ropes and ratcheted them tight, the posts appear to be bowing inward. They seem stable at the bottom. Is there anything I can do to remedy this? I'm worried that it will not be safe for son to use as it is now.

I am not a handy person AT ALL  and I'm afraid I have completely screwed this up.


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## Bud9051 (Nov 11, 2015)

Pictures might help with advice, but bending is not good. Others will be along.

Bud


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## Bud9051 (Nov 11, 2015)

Just looked at the link and wonder if a board could be added between the tops of those posts

Bud


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## huesmann (Aug 18, 2011)

Well, yes, once you ratchet the line between the tops of the posts tight, it will pull the posts together.

I doubt you could put a 42' board between them...

If you look at the Amazon questions:

*



Does this have to be attached to a tree or can you attach it to a strong wood beam?

Click to expand...

*


> This can definitely be attached to a strong wooden beam. Just make sure the beam can support 1000kg of force and is firmly cemented to the ground. Thanks
> By BRANTON SELLER  on September 4, 2020


I doubt a 4x4 can handle a metric ton of force without bending.

At this point your best bet may be to brace them on the inside, like so:


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## jamisonlm (Mar 28, 2021)

huesmann said:


> Well, yes, once you ratchet the line between the tops of the posts tight, it will pull the posts together.
> 
> I doubt you could put a 42' board between them...
> 
> ...





huesmann said:


> Well, yes, once you ratchet the line between the tops of the posts tight, it will pull the posts together.
> 
> I doubt you could put a 42' board between them...
> 
> ...


Thought I had done enough research on the posts, but obviously not. what would be best to use to brace it?


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## Bud9051 (Nov 11, 2015)

42', I didn't see that. Could also be braced from the outside like anchoring a telephone pole.

Bud


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## jamisonlm (Mar 28, 2021)

Bud9051 said:


> Pictures might help with advice, but bending is not good. Others will be along.
> 
> Bud











And this is with it not ratcheted tight


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## Half-fast eddie (Sep 19, 2020)

8 ft post is not nearly long enough. It needs to be at least 4 ft in the ground.
Easiest solution at this point would be a back brace, like the sketch in post 4, but going to the outside so you dont interfere with the play area.


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## huesmann (Aug 18, 2011)

Bud9051 said:


> 42', I didn't see that. Could also be braced from the outside like anchoring a telephone pole.


You'd need something to anchor any guy wires. I guess you could try a bigass steel stake (like one of those fence stakes) pounded into the ground as an anchor.


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## Half-fast eddie (Sep 19, 2020)

These anchors from home depot will work, but you need to sink them at least half way into the ground.


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## Domo (Nov 9, 2018)

PT wood is wet. You have to buy lion dried to have any hope it will stay stable. They also have specific grades that will stay out.


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## jamisonlm (Mar 28, 2021)

Half-fast eddie said:


> These anchors from home depot will work, but you need to sink them at least half way into the ground.
> 
> View attachment 647665


How exactly would I do this? 

I tried to avoid hiring someone to do this, and now I'm kicking myself for that.


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## Nealtw (Jun 22, 2017)




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## jc27310 (Sep 12, 2020)

huesmann said:


> At this point your best bet may be to brace them on the inside, like so:
> View attachment 647663


I would do this... based on the effort to properly install either a cable in tension or a beam on compression, the post in compression seems like the best bet.

kind of like this:









I'd do 2 per post, on the inside of the post, each 20 to 30 degrees offset from the wire:
(pardon the lousy drawing)


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## Nealtw (Jun 22, 2017)




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## jc27310 (Sep 12, 2020)

Nealtw said:


> View attachment 647676
> 
> View attachment 647677


 the tension bothers me... how about adding a second post with a cable and brace:









all posts set in concrete with gravel below like Neal shows...


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## Nealtw (Jun 22, 2017)

I don't see the difference but if you like that it should work.


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## jc27310 (Sep 12, 2020)

Nealtw said:


> I don't see the difference but if you like that it should work.


I think it all depends on skill level and effort, then safety.
I'd personally notice a deflection in a beam before a slipping u-shackle. They can look tight and still slip.
Personally, I'd just go with a beam brace (old school fencing technique (and grape vine trellis). 
your mileage may vary...

imagine this:










doubling up is the safest , but may be overkill. belt and suspenders perhaps?


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## Nealtw (Jun 22, 2017)

jc27310 said:


> I think it all depends on skill level and effort, then safety.
> I'd personally notice a deflection in a beam before a slipping u-shackle. They can look tight and still slip.
> Personally, I'd just go with a beam brace (old school fencing technique (and grape vine trellis).
> your mileage may vary...
> ...


Sure if you are building a fence but I thought it was more like a zip line? I am sure fence bracing will work.


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## Half-fast eddie (Sep 19, 2020)

All you guys who keep suggesting a brace in compression ... you need to step back from the keyboard for a minute and remember what the application is. There is a rope stretched between the tops of the posts, with kiddie swings and such hanging down. Anything on the inside of the posts is going to interfere with the play area.


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## DexterII (Jul 14, 2010)

I don't know that I've ever seen the dynamics discussed on this, and I'm not an engineer, but it sure seems to me that 4x4's are undersized for this application. They'll support a lot of weight, vertically, but this isn't a vertical load and it's not static. Go to 6x6's and incorporate guys at each end as suggested? It sounds closer, but, again, I'm not an engineer. Any chance that you know someone at work, church, or wherever who might be willing to share an educated guess? At a minimum, assuming that you stick with what you have, I think that a guy like Neal suggested in post 15 is your best bet. I would through drill the posts, not use lags, and use a steel plate on the inside so that the wood immediately around the hole is not bearing all of the weight. And the closer that you can get to a straight hold the better, so I would figure 45 degrees the minimum and try to get farther out than that even.


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## huesmann (Aug 18, 2011)

Half-fast eddie said:


> All you guys who keep suggesting a brace in compression ... you need to step back from the keyboard for a minute and remember what the application is. There is a rope stretched between the tops of the posts, with kiddie swings and such hanging down. Anything on the inside of the posts is going to interfere with the play area.


Yeah, we get that, but it's 42 ft. I suppose it could be installed shorter, but the OP hasn't indicated a spacing.


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## SpentPenny (Dec 15, 2020)

A 4 inch post will not put up with much lateral load, and being pressure treated it will crawl all over as it dries out anyway. Do as others have suggested and brace as best you can.


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## HotRodx10 (Aug 24, 2017)

Take a look at the guy wires for overhead lines. The anchor as shown in Neal's post #15 would be the most straightforward approach. You could add a turnbuckle in place of the light chain section, to adjust the tension.


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## jc27310 (Sep 12, 2020)

jamisonlm said:


> I am working on a project for my son's 5th birthday - hanging a ninja warrior type obstacle course in the backyard. Here is a link to the product.
> 
> 
> 
> ...



Hey *jamisonlm *
Your son has to love this gift! I would have ben psyched to play on it as a kid (or even now!). You haven’t screwed anything up, the post bending is not the worst thing and can be worked on. 

Everyone here is trying to help and offering suggestions to “do it right”. This forum and its members are pretty resourceful and a diverse background so you are getting diverse ideas. Whatever you decide to do, we all hope the kids have a great time playing on the set (deflection on the posts is not a terrible thing… just go ahead and keep playing!)

Let us know how comfortable you are with working with cable vs wood. This will help focus our efforts and suggestions.

---

My personal opinion is to keep it simple. Reinforce the post you have with more wood. Just to sum it up on my perspective:

TLDR:

Installing a post/gusset is simpler (See post #4/ Huesman), less expensive and more durable than adding cables. 
Adding the additional posts and gusset outside the assembly on each end allows the full length of the kit to be used, but if you install on the inside, you can use the gusset as an additional climbing point, plus you dont really loos much length.
(ladder rails or bolt on climbing holds are easy to add making this a feature, not a problem)

At the end of the day, either solution will work, one is more complicated and expensive to implement.
pick an approach that works with your skills and knowledge
—

Want the details, here we go…

Unless you have a specific vision in mind, I would build more structure with posts and gussets vs “cable” suspension… there’s a reason. Weight bearing cables are highly engineered and require load rated connections. 

A simple approach is to use more nylon straps like the set:
HomeDepot 20-ft-Tow-Strap

These have stretch and are not meant to live outside exposed to UV light. They will weaken a degrade in short order.
Something suited for outdoor use would be full chain (over any cable with manual added connectors). Something with welded links and corrosion resistance
HomeDepot Chain example

—

Lets do some math:

Your kit weighs 18 lb, (from Amazons description) add a couple kids at 75 lb each (or 3x 50 lb). This equates to ~180 lb “total load”. They will have friends and get older, so add some weight, call it 200 or 250 lb. (not saying you'd get on it, but I would) Let’s apply a minimum safety factor of 1.5x, meaning we use 375 lb total static load.

This is Ninja Warriors, so multiply for dynamic changes. 3g’s is not unrealistic when jumping and swinging, so 3x375= 1125 lb “dynamic load”

We can use a calculator to do the math for us:
Cable Load Calculator











We divide the total load (1125) over the 42 feet of length getting 26.8 lb/ foot along the length (yes its not really distributed evenly, the endpoints don’t care)
Let’s allow 2-feet of sag when loaded. (Allowing less = more tension weight and more structure) We are ignoring the stretch of the nylon strap.
We have identified ~3000 lbs of pull at the top of the post, (creating deflection). You can see just by loading the tension with straps how this works. Right here is where some readers will eye roll… its all about leverage. Yes, a bunch of little kids jumping in unison, will exert enough force to move a small car.
—

Let’s look at how to address the post deflection: Triangles are our structural friend, you can put them on the inside or the outside of the assembly to create added structure.

A) IF you want to reinforce the post using tension (several are saying “cable”, I say “chain or tow strap), spot a footing directly inline with the posts. Assume a 45 degree angle and install a concrete footing on each side to manage the 3000 lbs of force. *** 
See Nealtws post #15, just use all chain...

Here is where I think you have more work than needed:
Concrete is about 150 lb per cu-ft. A 3000 lb footing is (3000/150) = 20 cu-ft of concrete (or a volume of ~32”x32”x32”). Again, use a technique like Nealtw shows in post #15 with rebar, but I would use chain all the way up. (See the HomeDepot link above)

*** I am simplifying the math and geometry adding to the safety factor. You can always make the concrete footing lighter and if needed, add weight later.

B) IF you use a beam structure (in compression): You can easily build something that can withstand the 3000 lb. A 4x4 diagonal or 2 would cover this. (I believe #2 PT supports over 4000 lbs each in compression)

—

Attached are some images to help understand how you can make adding wood work:

I don’t think you can go wrong with whatever you decide to do, even if you let the posts sag.
Good luck with however you decide to go! Go outside and play.
-jc


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## jamisonlm (Mar 28, 2021)

jc27310 said:


> Hey *jamisonlm *
> Your son has to love this gift! I would have ben psyched to play on it as a kid (or even now!). You haven’t screwed anything up, the post bending is not the worst thing and can be worked on.
> 
> Everyone here is trying to help and offering suggestions to “do it right”. This forum and its members are pretty resourceful and a diverse background so you are getting diverse ideas. Whatever you decide to do, we all hope the kids have a great time playing on the set (deflection on the posts is not a terrible thing… just go ahead and keep playing!)
> ...


Thank you for very much for the info! Skill level is definitely low with either cable or wood haha. Single mom here, and I am not very handy. I will sit down and read through this again carefully before deciding how to proceed.


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## HotRodx10 (Aug 24, 2017)

After jc27310 ran the numbers, I withdraw my previous endorsement of suggestion of the outside guy wire with anchor block. It would probably be inadequate, unless the anchor block was fairly large and buried fairly deep. I like jc27310's configuration in Post #16, although I think the diagonal tension cable is an 'extra' that's just a backup for the wood diagonal compression strut.

Either way, the outer post will need to be set in about a 10" diameter by 24" deep concrete base if it needs to resist the uplift created by the 3000 lb tension on the line, although I doubt the strap supplied in the kit is that strong. They call it a "slackline" for a reason - It's not meant to hold 200lbs at the center of the span with only 2' of sag.


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## komondor (Feb 20, 2021)

Sorry but I have not seen anything showing how deep the posts is in the ground you said an 8' post and it is taller than you?
It should be 4' in the ground depending on your soil.


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## Oldmaster (Jan 14, 2020)

Probably should have used at least a 6x6.


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## SDkid605 (Oct 24, 2020)

jamisonlm said:


> I am working on a project for my son's 5th birthday - hanging a ninja warrior type obstacle course in the backyard. Here is a link to the product.
> 
> 
> 
> ...





jamisonlm said:


> I am working on a project for my son's 5th birthday - hanging a ninja warrior type obstacle course in the backyard. Here is a link to the product.
> 
> 
> 
> ...


Hello;
Best of luck on this; I live near someone here who was lucky to have two big trees for this.

Looks awesome, and a fun thing for kids.
I was just biking by, but had to stop and look it over, looks so cool for kids to play on, get some exercise & learning.

I don't have another solution other than what has already been said, I just wanted to wish you luck on your project.


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## azeotrope (Jun 3, 2015)

I vote for the guy wire pulling tension on the back of the post. 

You would want to cement in a block that you can attach a D Ring to. Use a strap to a through bolt at the top of the post and to the D ring on the block. As you tension the slack line you also tension the straps you are using as a guy wire. 

Both the guy wire and the forward braces present safety concerns. Considering that kids may be falling off this as they play it is best to keep anything hard away from heads reach. A forward post would get in the way and could become an issue if they can land on it when they fall. 

The guy wire also has a safety issue where it can clothes line someone if they don’t see it. The strap would be a lot softer if they run into it than cable or chain will be, it is also adjustable so you don’t need to get the length perfect in the first go. And it’s easy to hang red flags on it so people can see it easier when they are playing and looking past it, or just simply block it off so the area underneath can’t be accessed

The instructions in the video from your link say the support only needs to be capable of withstanding 2200 Lbs. You can calculate how much sag you will get by measuring the height of the line, then hanging a weight similar to your child’s weight, then measuring the height of the line again. The difference is your sag. Calculating the tension based off the free fall weight and the sag will produce a conservative working load to use. In actuality there will be flex and stretchy that changes your sag angle and that will have a reduced result after calculating your tension. Because you are stiffening the supports it’s best to calculate based off the sag. The point I’m getting at here is that there are a bunch of variables than need to be considered when calculating the forces and because there will still be movement the numbers will adjust a bit. If the manufacture says you only need to calculate your required hardware based on 2200 Lbs then I would think they have enough financial incentive to not under engineer that portion and that they have built in a buffer safe enough to trust for yourself. It costs then nothing to spec a higher rating so if they thought it was necessary then they would have just upped the numbers, if for nothing more than liability reasons. 

I think there was a good photo already posted that showed a block cemented into the ground for the guy wire. I would suggest using hardware that either bolts into the concrete or that folds down so that it doesn’t become a trip hasard when you are not using it. With preference to something that folds down because an open threaded hole hase a tendance to accumulate sand and dirt rather quickly and that may just become annoying.


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## HotRodx10 (Aug 24, 2017)

azeotrope said:


> You would want to cement in a block that you can attach a D Ring to. Use a strap to a through bolt at the top of the post and to the D ring on the block. As you tension the slack line you also tension the straps you are using as a guy wire.


I originally thought that was a good idea, too. Then I thought about how big and how deep the block would have to be to resist about 3000 lbs (2200lbs / COS 45 degrees), and I rescinded that suggestion.


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## azeotrope (Jun 3, 2015)

HotRodx10 said:


> I originally thought that was a good idea, too. Then I thought about how big and how deep the block would have to be to resist about 3000 lbs (2200lbs / COS 45 degrees), and I rescinded that suggestion.


I suppose it depends on your soil quality as to how easy it would be. 

I don’t think it would be as bad as it sounds, the posts are already resisting a good portion of that load, and ground anchors can resist a surprising amount of load in a small footprint. It’s easy for me to dig into my clay soil, nice and deep to cement in a good anchor, it wouldn’t be the same effort or results if it was loose rock or sand. 

It takes little for me to dig down 3’ or more to make a hole so it seems like an easy task that would give clean results... just be sure to leave at least a foot of soil above any concrete if your want to grow grass over it again, if your soil is too thin then it dries out too quick and the grass won’t grow. 

I would think a pyramid shape deep down would be more than sufficient. It’s not the weight of the cement that is holding the load, it’s the dirt on top and it’s resistance to move that really holds it in place. Again, soil quality plays a role here.


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## HotRodx10 (Aug 24, 2017)

azeotrope said:


> It takes little for me to dig down 3’ or more to make a hole so it seems like an easy task


Either you have very unusual soil, or you and I have very different ideas of an "easy task".


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## azeotrope (Jun 3, 2015)

HotRodx10 said:


> Either you have very unusual soil, or you and I have very different ideas of an "easy task".


Yeh, I guess that is subject to individual opinion. I’ve never been one to shy away from a job and feel quite rewarded after completing a task like that. But I’ve also dug our 14’ trampoline into the ground and installed a sump pump and pit into it deep enough so adults can jump without hitting bottom (about 4 feet in the middle for those wondering). I don’t think a couple ground anchors would be anywhere near that scope of work and quite easy in comparison. Actually I would expect it to be not much worse than digging more post holes, just slightly different shape and one more foot deep, plus a little hardware.


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## markedenfield1 (Jun 1, 2018)

Didn't have time to read all the comments but I would avoid putting guide wires (anchors) on the outside of the play unit. Very dangerous - someone will clothesline themselves on them.


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## Nealtw (Jun 22, 2017)

HotRodx10 said:


> Either you have very unusual soil, or you and I have very different ideas of an "easy task".


I don't the weight has to match the load, there is the resistance in the dirt too,
Just looking at guys for telephone poles.


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## HotRodx10 (Aug 24, 2017)

Nealtw said:


> I don't the weight has to match the load, there is the resistance in the dirt too,
> Just looking at guys for telephone poles.
> View attachment 648103


Believe me, I know. I deal with these all the time. One of my specialties is analyzing span wire structures (temporary traffic signals hung from cables spanning between poles) The anchor you show in the diagram is typically buried at least 6'; usually 8'. It's not a linear relationship for depth to resistance, either - it's an exponential of 3, so cubed. IOW, the same size anchor buried 6' deep has 27 times the resistance of one buried 2' deep. The anchor in your diagram is buried 6' deep in order to resist a tension load of no more than 10,000 lbs (the tensile strength of the guy wire. So, that same anchor buried only 2' deep should only be expected to resist 370 lbs of tension. At a 3' depth, you could probably expect about 1250lbs of resistance. Obviously, a bigger anchor will help, but not as much as you'd think, since, as you pointed out, most of the resistance is because of the wedge (cone) of soil the anchor pushes against, so the volume of that wedge doesn't change significantly with changes to the anchor size.


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## huesmann (Aug 18, 2011)

And the soil resistance is dependent on the kind of compaction you get on the backfill.


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