Hi all.

 

I just signed up here hoping to get some information on a project I'm currently working on. I would consider myself a 100% untrained newbie when it comes to woodworking but I am eager to learn more.

 

So I am currently building a pergola / swing in my backyard and although my "design" has gone up fairly straightforward, I do wonder about the structural stability of my main 4x4 beam which is supposed to hold the swing. My idea was to add a notch on each side of the pergola's upper 4x4 support beam for the main 4x4 (let's call it the "swing beam") to fit into (please see picture). This was mostly done for cosmetic reasons, but as I progressed in my build I started wondering if I have compromised the strength of the structure ultimately holding the swing up. 

 

If is helps, some rough numbers on the build are:

 

4 main posts: 4x4 12 foot posts, buried ~ 3 feet deep with each post having 100lbs of quickrete anchoring it all down.

 

Overall footprint is roughly 91"(od) x 48" (od).

 

Each side of the pergola have a 4x4x48" beam placed on top and secured with a 7" lag screw going straight down into the post. Both of these posts have a notch cut out in the middle for the main "swing beam" to sit inside.

 

The swing beam is a 4x4x91" post.

 

Can anyone share some insight on if I have drastically weakened the 2 4x4x48" side support posts that are ultimately responsible for holding the "swing beam", swing, myself and family? Thanks in advance 👍

 

One last note, the build is not yet finished, yet the only remaining bits are all cosmetic. I didn't really want to move forward until I had a better idea if the bones of my structure would be sound. Thanks

Welcome aboard, Cliff.

 

I'll probably get out-experienced by someone else here, but I would recommend angled support braces (4x4 at 45*) inside the upright legs that come together near the center of the notched 4x4. That should carry the load down to the legs and support the 'compromised' 4x4.

 

I'm sure you'll get more advice but that's how I would do it.

 

Good luck with whatever you finally do and please post pics of the finished swing.

Welcome to the forum. I am not a structural engineer but i do not think a single 4x4 is enough. By notching the support beam you effectively have only a 2x4 supporting the sing beam. to support sing beam 4x4 will work but I think 2x6 doubled would be better for sing beam as it will not sag. For connection bolts will work as we are not dealing with furniture a fancy joint is not needed. I cannot tell what you connected the supports to posts with but dropping this down and bolting thru would be better support. Nails are not good for this application as the forces applied from swinging will loosen them DAMHINT

That notched 4x4 looks like it has a 2x? (6?) sistered to it on the outside of the frame (that's a question, actually). I would think that may give enough additional support.

 

Welcome to the forum......

Welcome. 

I'd second the angle bracing - you don't want it racking.  And I like those structural screws that use a torx bit over lag screws.

And you might want to consider a 4x6 PT for that bearing beam.  You could still notch it and have some beef left.  That size is readily available around here, but I don't know about your area.

Thanks for all the quick responses here. While my original design may hold, it seems pretty obvious that it is not certainly, without a doubt, going to hold. Considering this is for my family, including 2 y/o daughter, I definitely will be reinforcing some things.

 

I have already planned on using some 4x4 angled support braces on the interior corners (sorry, forgot to mention that earlier). And yes, I have some 2x6 attached to the outside of the 4x4s which I had hoped would add some strength but also they are there to keep the the top looking more flush once I attach the 2x2 decorative bits over the topside. 

 

I may go get a 4x6 as suggested for the "swing beam", but I think my first priority will be strengthening up those 2 side 4x4's since they are holding all the weight at their weakest point in addition to having a cutout at their weakest point.

 

Would it be ok if I secured (glued and screwed) a 2x4 on the underside of the notched 4x4s, then braced with angled 4x4s, and secured a 2x6 along the outside of the notched (4x4 (+) 2x4). I supposed that would be a poor man's 4x6, with a 1.75" notch in the center, braced with a 2x6 along the outside. I probably can just go buy some bigger pieces, but I figure try and use what I have first. 

2 hours ago, Fred W. Hargis Jr said:

That notched 4x4 looks like it has a 2x? (6?) sistered to it on the outside of the frame (that's a question, actually). I would think that may give enough additional support.

 

Welcome to the forum......

Looks like that to me, also.  I'd also agree on the diagonal supports.

 

ditto: Welcome to the forum......

 

Another option, one that's too late:   Let's say your notch in the support is 2" deep.  You could cut a 1" notch in it and a 1" notch in the "swing beam" making sort of a half lap.  What I do know is "stiffness" is proportional to the cube of the height.  So if you double the width of a beam, you've doubled the strength.  But if you double the height, you've increased the strength by a factor of 8.   So, for example instead of a 2" high spot in the middle, you have a 3" spot (8 vs. 27 stiffiples®.)

Hi and welcome Cliffe5. I am not the person who’s advice you would want to use on this issue . I am an electrician pretending to be a woodworker, sometimes. It is fun. Electrical questions are more up my alley. So good to see a new face here, and welcome. 

5 hours ago, Cliffe5 said:

 

Would it be ok if I secured (glued and screwed) a 2x4 on the underside of the notched 4x4s, then braced with angled 4x4s, and secured a 2x6 along the outside of the notched (4x4 (+) 2x4). I supposed that would be a poor man's 4x6, with a 1.75" notch in the center, braced with a 2x6 along the outside. I probably can just go buy some bigger pieces, but I figure try and use what I have first. 

Adding a 2x4 to underside of support should strengthen it. I would use thru bolts instead of screws. The bracing also sounds like the thing to reduce flex. I still would rather two 2x6 for the cross swing support, but since the weight is close to the ends the 4x4 may work.

Welcome to the forum ! I agree that the beam might need some beefing up. I would glue a 2X4 on the top using decking screws and PL400 or equivalent. With the addition of the diagonal braces you will be able to hang a truck engine from it. 
Paul

Welcome to The Patriot Woodworker @Cliffe5. Great to have you on board. As you can see a great group here always willing to help with ideas, suggestions and experience. Looking forward to your continued participation and especially seeing this project through completion.

 

With where you are currently rather than disassemble or re-construct, I would add a 2x4 across the bottom of the 4x4 "swing beam". In addition I would add a 2x6 to each side of the "swing beam", notching at each end so the 2x6's would be flush on top with the 4x4. Honestly should be stronger than a 4x6 swing beam and definitely less expensive. I agree with others, you need angled supports to minimize risk of the frame racking due to the force of the swing motion.

 

Anyways, my $.02. Keep us updated as the project progresses. We're in it with you and the project may help others in the future. Good luck!