I have bought 2 second hand box steel gates each 8ft high and 9ft wide for the entrance to my horses field. I estimate they weigh about 200kg and 250kg (one has a substantial letterbox welded on). I am thinking of 4.5m long box steel 100mm x 100mm x 5mm putting 1.5m into the ground and then filling the box with concrete before capping. No one at the steel merchants can tell me if this will support the weight, anyone any ideas? I have thought of a gate wheel however the drive slopes both inwards (gates will open in) and from right to left so I don't think I can use them.
It would seem to me that the 100mm square tube posts would be a bit chincy for holding the hinges for gate of that weight. If it was me I would probably be looking for something a bit beefier like a 5"x5" or a 6"x6" steel tube with a wall thickness of at least 0.25".
Gates will place a tremendous torque on the posts and I also have my doubts that a burial depth of 1.5m would be enough to prevent post movement above the ground when there is a huge cantilever weight hung on the post. This is the reason that some of the following ideas are applied to gate posts:
Overhead post tie:
Substantial support structure:
Swinging gate support:
Back bracing into adjacent fence: