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How can I reproduce the functionality of these 3-Arm pergola corner brackets for 4x4 wood posts? My concern is that it will need to securely hold the weight of the treated 4x4.

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They would be used for this: enter image description here

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    I'm not sure what you mean by "reproduce the functionality". Are you asking how tops are normally joined to pergolas support brackets? Also, there are enough typos in your 2nd sentence that I'm not sure how to correct it. – FreeMan Nov 23 '20 at 17:23
  • Are you saying that the ones you found are the wrong size? Such hardware would need to be very robust to do the job on a larger pergola. – isherwood Nov 23 '20 at 17:32
  • Sorry about the typos. I'm usually very careful with that. – rbhat Nov 23 '20 at 18:23
  • No worries! As isherwood noted in his answer, I still think you're thinking about it the wrong way - more than the weight of the horizontals, it's the racking forces that you have to worry about. These would need to extend a fair bit down and across and be pretty darn thick to resist enough racking to keep your pergola upright in a strong wind or earthquake. – FreeMan Nov 23 '20 at 18:28
  • The ones I found are steel with minimal rust protection. I'm sure they'll show some rust after a few weeks. They're also $80 each. – rbhat Nov 23 '20 at 18:29
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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.

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