# Strength of a 4x4



## Alexp08 (Apr 22, 2016)

Hello, so im here for a quick question. Im looking to build a hammock stand. Im hoping to use pressured treated pine 4×4s and im wondering if anyone know the breakibg point of such wood. It will be like the one in the picture. And im trying to determine the weight capacity of it.


Thanks.


----------



## conifur (Apr 1, 2015)

Way too many variables, the longer/higher you would put the 45* supports easier to break at that point, any flaws in the 4×4 supports, ect. How much weight do you want it to hold?


----------



## Alexp08 (Apr 22, 2016)

The 45* supports will be almost right where theyre at in the picture unless someone can recommend a better place for them. Ideally i would like it to hold 500lbs.


----------



## BurlyBob (Mar 13, 2012)

Alex, Pressure treated has only one useful purpose, being in contact with ground or bare concrete. PT lumber is general the poorest quality lumber milled. It's stored wet and once dry will assume all sorts of wonderful contortions. Perhaps adding a cross piece at the top would be the best solution for this project. Just looking at the design there seems to be a lot of weak points that will give with time, weight and stress.


----------



## Alexp08 (Apr 22, 2016)

The only reason i was looking at pressure treated was due to the fact that this stand will be outside. I can easily go with another. And realistically it only has to last me for a year or two till I can build a more permanent one or spend a few hundred on a metal one. Right now im just looking to see if it'll hold up to the weight


----------



## conifur (Apr 1, 2015)

Will the hammock netting hold 500lb?


----------



## Alexp08 (Apr 22, 2016)

The hammock netting will hold 650lbs


----------



## alittleoff (Nov 27, 2014)

It might work better if you were to raise the two end braces up as much as possible without getting in the way. You could also run a small aircraft cabler over the top and down the back of the 4×4, then anchor it on the bottom. 
Gerald


----------



## DirtyMike (Dec 6, 2015)

alex, i have worked with pt pine for similar outdoor projects and construction. If you were to follow that design i would use 4×6 's for the main supports. 4×4's are really meant to support vertical weight as opposed to being fulcrumed. a 4×6 should give your more support and not twist and bow after drying. proper bracing is the most vital part of the whole build for rigidity. good luck


----------



## wormil (Nov 19, 2011)

The wood will hold, the joinery is the question. The posted picture appears to be bolted together, so that or another strong form of joinery. If you don't want to use bolts maybe look at timber framing joinery.


----------



## Alexp08 (Apr 22, 2016)

Im fine with bolts, that was ths initial plan. As far as wood should i not use PT?


----------



## wormil (Nov 19, 2011)

The problem with pressure treated is it is very wet when you buy it and tends to warp, even more than regular construction pine; otherwise, it's fine.


----------



## Alexp08 (Apr 22, 2016)

So should i buy nonPT lumber? And how well will it then stand up to the elements?


----------



## Lazyman (Aug 8, 2014)

Personally, I would use PT only if it is in contract with the ground.

If you find after building this that the 4×4s have more flex that you feel comfortable with you could always add a 2×4 brace between the tops of the 2 4×4s to add more strength. Also, moving the angle braces higher up will also help. Because the weight will be partially directed downward in compression, it seems like it should be plenty strong.


----------



## clin (Sep 3, 2015)

I think 4×4 would be plenty strong in this application. They aren't going to break. And a little bow isn't going to hurt anything. If it bowed too much, the person in the hammock will start dragging they're hind end on the ground, but it's not going to fail in some spectacular fashion.

Look at that photo. I'd estimate that 2/3 of the force on the uprights is straight down ( compression), leaving about 1/3 pulling inward (bending load). Assume 500 lbs (3 adults?), that's 250 lbs per side and perhaps 80 to 100 lbs pulling inward. A 4×4 about 8 ft long could hold 100 lbs at the end of it with no issue at all.

Keep in mind that if the net were strung very tightly across (like a tight rope), then you could create bending forces greater than the applied weight. Point being you do want the netting to sag so the net is pulling down a lot (as seen in the photo). In other words keep the angle between the net and upright as small as possible.

The forces at the joint will be significant. But as mentioned, with strong joints (like the bolted one in the photo) you won't have any issues.

Like any outdoor wooden furniture or structure, you have to keep an eye out for rot that will weaken it. But that won't happen fast.

I also agree to avoid pressure treated wood, except perhaps the two bottom pieces that might touch the ground. Put a good exterior paint on it or other finish like Penofin, and it will last for years.


----------



## Neilswoodcraft (Apr 18, 2016)

I think 4×4 is the bare minimum. I would do some jointery as suggested above.


----------



## WhyMe (Feb 15, 2014)

I built a hammock stand very similar to the one pictured using 4X4,2X8,2X4 PT lumber and it's been in use for many years. It's been loaded with at least three people at once (about 500lb) and I worried about the hammock failing before the stand.


----------



## oldnovice (Mar 7, 2009)

If you want a truly engineering answer, go to the Forest Products Laboratory Web site and ask them! 
A real nice Web and physical site in Madison Wisconsin and very knowledgeable people.


----------



## wormil (Nov 19, 2011)

With this design, most of the stress will be at the point circled (on each side) so that joint has to be very strong. In the picture they appear to have a bolt running all the way through and an angled washer to handle the stress. Something to consider.


----------



## ThomasChippendale (Nov 6, 2015)

This joint does not have to be strong, its the 4 X 4 that sees all the stress at this point and the joint is mostly under compression. Removing wood in the 4 X 4 to create a joint will weaken it at the most critical place when all you need is a way to keep the corner brace to slide upwards.

If the load in the middle is 500 lbs, and the angle of the downwards puling force on the 4 X 4 is 45 degrees from vertical, then you are pulling on each tie point with 350 lbs. Say the uprights are angled 30 degrees from vertical, there is 300 lbs in compression and 175 lbs pulling perpendicular to the upright at the tie point.

Since there is a lever effect, assuming the knee brace is at 1/6 the height, there will be 6 X 175 lbs puling outwards at the botom bolt, this has to be the strongest joint since it is holding 1050 lbs in horizontal force and 300 lbs in vertical force. Placing this corner brace higher diminishes all those stresses proportionaly.

There will be 875 lbs pushing on the corner brace at the point of contact and trying to bend the upright so this is a bad place to locate a knot or defect. All this calculated without safety factor, which should be at least 2 or 3 so you can easily double the loads.


----------



## bbc557ci (Sep 20, 2012)

> I think 4×4 would be plenty strong in this application. They aren t going to break. And a little bow isn t going to hurt anything. If it bowed too much, the person in the hammock will start dragging they re hind end on the ground, but it s not going to fail in some spectacular fashion.
> 
> Look at that photo. I d estimate that 2/3 of the force on the uprights is straight down ( compression), leaving about 1/3 pulling inward (bending load). Assume 500 lbs (3 adults?), that s 250 lbs per side and perhaps 80 to 100 lbs pulling inward. A 4×4 about 8 ft long could hold 100 lbs at the end of it with no issue at all.
> 
> ...


I agree. Most of the load is vertical, going with the 4X4s, they should hold up just fine. Could go with 4X6's if it makes you sleep better at night but X4's will hold up. Either way pick thru the pile and buy material with the least amount of knots. I also agree about the PT lumber likely "moving" as it dries out. I've heard that dried PT can be purchased. Never bought any or looked for it, but it could be available I guess.


----------



## wormil (Nov 19, 2011)

> This joint does not have to be strong … all you need is a way to keep the corner brace to slide upwards.
> - ThomasChippendale


Try nailing something like that together and see how long it lasts. For a few months it'll feel fine then the rocking motion and expansion and contraction of the wood will loosen the nails and before you realize you'll be on your ass because it fell apart. Using a bolt as in the original would be prudent.


----------



## kelvancra (May 4, 2010)

The four thousandth vote for avoiding pressure treated wood. Pressure treated wood is very much over used. In part, because people think of it as indestructible. Not only is it ugly, it will get even uglier, as others pointed out. Part of that includes shrinking, thus, cracking and splitting even more than regular wood because it, more often than not, has more moisture content.

The treatment used will not protect the wood from the effects of the sun's rays. It only fights rot. Even then, the wood is not permeated with the treatment so may begin failing from the center first. That said, your project shows little reason to be concerned with rot anytime soon.

With the unit off the ground, simple maintenance will do more for rot than using treated wood ever would.

As others pointed out, you can do other things to beef up the strength. For example, one method is, run a cable secured to the center line of the outside of the frame, from one end to the other, terminating at the tops. If you used turnbuckles on each termination point, they could be tightened to put more pressure on the two support posts being drawn apart and against the pull from the hammock.

Back to just adding a spreader on top, I would think five 2×4's (a single in the middle and two on each end, sandwiching the 4x and the middle one) would add more strength than even having a small herd of dogs, cats and kids would require.


----------

