Help with joining a gate frame together

[clarke]

Hi everyone

I have a project to do tomorrow and wanted some advice on best procedure.

I have 3 3m lengths of tanalised 150x150mm softwood posts and am going to join them into a frame to hang new double gates on. The hinge posts will be sunk 600mm or so into ground and the top post between them will sit around 200mm from the top of the hinge posts. The top post will be around 2.5m long.

Obviously the top post is heavy and won't be sitting on top of the hinge posts so needs to be supported adequately. I plan on either 'notching' or housing the entire thickness of the top post into the hinge posts by around 2-3" or a mortise and tenon (but unsure whether to thicken up the tenon or even make it a full 150mm long tenon which may require a double tenon?). I'm a little unsure of dimensions for either joint given the scale of the frame.

What would be the best solution considering safety, resistance to weather ingress and the fact that once the hinge posts are cemented in (two weeks time) I won't be changing things very easily!

Many thanks

Clarke

[clarke]

In the end I went with a stopped mortise and tenon, 80mm into the hinge post and made the tenon width a third (50mm) and 90mm long so 30mm of shoulder top and bottom, 50mm of shoulder both sides of tenon.
I made the mortises by hand as I wasn't going to get a 150mm post under the morticer! I made the first tenon by hand and then after a very tight fit followed by one that was too loose I remembered that we don't live in caves anymore and the festool made me a perfect tenon in around 15 mins.
Going to use expanding exterior glue for the mortises and was considering using a line of silicon around the top and sides of the tenon shoulders to prevent water ingress