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What would be a smart and solid way to fasten the beams to the posts in this image. The beams will have swings for 2-3 adults hanging down from them. The posts and the beams are 4" x 4" (95mm) pine and the posts are about 10 feet tall.

Posts waiting for beams

I don't want them coming crashing down on our head while swinging. Note that this is a pentagon and the beams will form a horizontal pentagon.

(note that I posted an answer below how I ended up solving it)

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  • I hope your posts aren't securely fastened to their concrete footers yet! That could make it difficult to make the joins, depending on how you plan on going about it. – FreeMan Jun 28 '16 at 16:01
  • The concrete has dried in the footers, so I'll have to bite that bullet. Although I'm not sure it makes it any harder anyway. – AtliB Jun 28 '16 at 16:11
  • That may have an impact on your choice of fastening methods - it will be fairly straight forward to drill holes & drive bolts. It will be quite a bit more difficult to accurately cut notches/half laps/tenons, etc up in the air. – FreeMan Jun 28 '16 at 16:17
  • True... but it makes it easier to level the posts afterwards. – AtliB Jun 28 '16 at 16:19
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    Thanks for the update! Ideally you should post your solution as an answer, separate from the question. – rob♦ Jul 20 '16 at 1:45
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There are many ways this could be done, including multiple pure framing/joinery options some of which wouldn't even require any metal fasteners. These kinds of all-wood joints are best done by pros though since even though this would now fall under "rough carpentry" the fit would have to be at a level far in excess of what's normal for that, approaching that of cabinet work with tolerances well under 1/64" ideally.

A more modern joinery option that first occurred to me is to cut off the ends of the five beams at the correct angle (72°) and then form half-laps on each of them so that they can interlock. The ends would look somewhat like this:

Half-lap ends

Source: Hexabana

Then drill vertically through the joint and into the top of the post for a bolt which will tie the structure together.

I'd be tempted to just used metal reinforcement plates myself though, with plenty of screws, or bolts for maximum security. Much faster and possibly more secure at the end of the day. If you use a basic T-shaped connector on the inside angles for example these can be easily bent to suit the interior angle. For the outside a basic steel band can be used, again custom bent to fit.

Regardless of the main joining option used some diagonal bracing I think is a must-do, although joining a pentagonal structure to square-section beams at each corner will require some tricky fitting work.

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  • 1) How would you bend the T connector on the inside angle? Both physically and around which axis. Are you thinking about connecting both beams AND the post at once with a single T connector? Do you mean something like this: cdn1.bigcommerce.com/n-ww20x/2454y/product_images/… – AtliB Jun 28 '16 at 16:07
  • 2) Would the diagonal bracing be needed on all posts/beams? Or should I retrofit it afterwards once I see it needs further stabilization? – AtliB Jun 28 '16 at 16:21
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    @AtliB, 1) although that could be bent (with the help of scoring from a cold chisel) that seems beefier than you'd need here. I was thinking of a much more modest T connector that could be bent by clamping between two offcuts and being bent with gloved hands, or with careful hammer blows (hit low down, not at the projecting end). 2) I don't know, but I think for greatest peace of mind it would be wise to equally support all five of the corners — a decent rule of thumb with this sort of thing is you'll rarely regret over-engineering for strength, especially if it costs very little. – Graphus Jun 29 '16 at 6:57
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    @AtliB, T-shaped connector of the type I was thinking of here. The bend I was thinking of would extend up from one side of the vertical portion. – Graphus Jun 29 '16 at 7:00
  • I just added pics of how I ended up doing this. Would you mind taking a look? – AtliB Jul 19 '16 at 11:39
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This is classic post and beam construction and there are several ways to approach the connections.

One way is with mortise and tenon connections. Note the short diagonal braces connecting the horizontal and vertical parts. These braces should be applied in both directions to stiffen the assembly.

Post and beam detail)

A second approach would be to use steel plates on each side of the joints. The photo is taken from www.forestryforum.com for a custom steel plate. Your application does not have nearly the same loading requirements and could be much thinner steel. There are many commercial options found at lumberyards or home improvement stores that would work. If you do not like the appearance of the steel plates you can do something similar with wood plates applied to the sides of the posts and beams.

Steel plate connector

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  • Thank you Ashlar, since the structure is a pentagon, it's a bit hard to find the proper connections since the posts aren't perpendicular/parallel. Thus I'm not sure if suggestions 1 or 2 apply – AtliB Jun 28 '16 at 13:47
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This is how I ended up doing it. I even put some glue for good measure (although I'm not sure how effective it is). To my surprise, I don't notice any racking while swinging so I'm pretty happy with it.

Constructive criticism surely accepted.

Solution 1 Solution 2 Solution 3

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