How can I reproduce the functionality of these 3-Arm pergola corner brackets for 4×4 wood posts? My concern is that it will need to securely hold the weight of the treated 4×4.
They would be used for this:
Wood – 3-Arm pergola corner bracket for 4x4s
deckoutdoorpergolawood
Related Solutions
You need to install knee braces every where you can. (The diagonal piece)
Ideally, the ends should be tightly dadoed a small amount into the beam and post to minimize the inevitable play that develops in connections made solely of mechanical fasteners. The bigger the brace the better, within reason. The best bracing would be the grand-daddy of knee braces-- X-bracing.
To accomplish the join I'd acquire some 1/4" by 3-1/2" by 24" steel plates and let them into opposing sides of the posts to act as gussets. I'd use countersunk flat-head sleeve bolts, piloted accurately and minimally, to sandwich the whole works.
Be sure to consider whether the existing posts are adequately anchored, as there will be substantial torque applied to them at their bases. It doesn't matter how well you attach the extensions to floppy posts.
That said, I think your plan seems flawed. A pergola creates some fairly unique structural concerns, namely the lack of diagonal bracing that a conventional roof would provide, as well as increased wind drag.
I'd be replacing your 4x4 posts with 6x6 posts that run all the way through. They'll do the job much better than spliced 4x4s, and they'll probably be more appropriate from an aesthetic standpoint, depending on your design. You don't want chicken legs under your nice new pergola.
If you elect not to do that, consider substantial diagonal bracing below the ceiling line connecting the posts to the primary horizontal members of your design in appropriate directions (where connection to a fixed structure doesn't provide support).
Best Answer
You'd reproduce that function with knee braces, which is what nearly everyone else does. Such hardware would need to be very robust to do the job on a larger pergola, making it cost-prohibitive (and somewhat ugly). You can find heavy-duty pergola hardware kits online, though they won't be exactly what you've shown.
However, three simple L-brackets on each corner (two on the faces and one on top), properly sized and with adequate bolts, would probably do. You'd need at least four bolts through each bracket, for a total of 12 at each corner. You can see how this gets messy in a hurry.
To rectify your apparent misconception, the challenge isn't carrying the weight of your beam. It's dealing with the racking forces applied to the corner joints. There can be a ton of torque on those points in the wind.