a gateway that is 24' wide with 6 x 6 posts supporting two 12' gates. I want to install a top beam between them, using steel connectors on top of the posts.

That is a very long span for any type of beam without any center support! But if I were to do it, I would probably use 3 paralam beams sandwiched together with diagnal bracing. But truthful, there is no beam that spans that distance by code

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hkstroud... if those boards were connect that way and laid flat, they would sag like crazy. An engineered beam would work. Or built this one by connecting and mounting on the edge not the face. But 24' is an awful long span. I wouldn't do it.