First, use the biggest, beefiest hinge hardware you can find. Second, build the gate doors with ample bracing. For large doors, skip the "Z" shaped bracing in favor of a "double Z" for each door. In other words, each door should utilize three 2-by-4 cross braces perpendicular to the fencing and two diagonal braces connecting them, with the fence boards driven into each brace they cross. Correctly constructing your support posts is also critical. You'll get more stability from 6-by-6 posts rather than 4-by-4's. Each post should be anchored with a concrete footing and be buried in the ground a third of its height. Plus, at the base of the two posts, install a horizontal post flush between the two, or use a poured bridge of concrete, to prevent the posts from pulling towards each other at the bottom, due to the weight of the gate doors.
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