A set of swinging gates of the size you describe is a nice little engineering exercise. A major problem with a 12 foot long swinging gate is the fact it produces a heck of a moment (torque) about the base of the gate post it is hinged off of. On powerlines, we have right-of-way access gates which are often double-leaf gates to allow the passage of trucks. The design follows the practice the old farmers and ranchers used. That is, the gate posts are made quite a bit taller than the gates. Wire rope and turnbuckles are then run from the outer top corner of each gate to a pivoting plate at the top of the post, on something from a 30 to 45 degree angle. If you had a 12 foot gate leaf, allowing say 12" for a fabricated set of ears and kingpins (fancy way of making a heavy hinge), you would have the point of the gate 13 feet from the face of the post. If you had a gate leaf with its top say 5' above grade, then you would need a post 12'6" tall if you went up on a 30 degree angle with the cable brace. On the right-of-Way gates, the gate posts are something heavy like old 6" steel pipe or I or W section beam. The posts are set in a substantial concrete footing. The gates themselves are made from pipe and structural steel as people tend to try to trespass onto powerpline rights-of-ways. We initially used livestock gates bought at Agway, made of light gauge galvanized tubing, but people were hooking onto them with chains and pulling them off in some locations.
The idea of an end wheel on each gate leaf is good if you have hard, level pavement for the wheels to run upon. I have seen some security gates where this idea was done, but over time on macadam topping, the pavement got rutted from the caster wheel on each gate and then the gates began to sag.
If you make a set of heavy gates of ornamental ironwork, you might work a diagonal brace to a high post into your design. Here's another idea: If you don;t want to go for two (2) deep independent footings on the gate posts, or perhaps soil conditions work against it, you might consider a one-piece spread footing. If you design a reinforced concrete spread footing with imbedded anchor bolts for both gate posts, you can accomplish s few things:
-handle the moment the gates put onto the bases of the gate posts
-hold both posts in good alignment so differential movment due to soil settling or frost heaving will not be an issue.
If you go with steel gate posts rather than masonry piers, you can always lay masonry around them to conceal them or incorporate them into a masonry wall. The posts could project above the top of the masonry wall and be topped with ornamental lamps or anything else that suited your ideas. Steel posts also let you weld on pads at the right orientation to mount the hinges. You can use 6" x 6" square rectangular structural tube and it will take a heck of a bending moment and provides a nice flat surface for welding mounting plates for the hinges.
I would suggest you machine the hinges out of steel plate, use bronze bushings and make sure to put grease fittings on. Make the hinge pins out of stainless and you can incorporate a thrust collar on each one.
I would not use ball bearing pillow blocks as the gate only sees angular motion.
If you go with the idea of a monolithic foundation, go about 16" thick x 4 ft wide and extend it back beyond the posts by a couple of feet. The dead weight of this footing would then be (assuming 24' gate opening, 2 foot addtional for hinges/post clearance, 2 foot additional for posts/sole plates or about 30 ft overall length of footing) 24000 lbs. & would take 5.29 cubic yards of concrete. You will likely pay a minimum charge for a truckload of concrete, and a truck can hold 9-10 yards, so go for it and make the foundation as big and heavy as you can. I would suggest that it be reinforced (and I am guessing now) with some number 5 or number 6 rebar chords, and imbed some "L" anchor bolts of at least 5/8" diameter, 3/4" being better. Concrete and rebar are relatively cheap. If you don;t put a couple of concrete piers at each end down below the frost line, at least dig out and go with number two stone down below the frost line. This will give you a heck of a nice rigid foundation to anchor the posts to. By going with chords of number 5 or 6 rebar, the foundation then functions as a "beam" since the moments about the gate post bases tend to want to "bend" or "curl" the foundation ends upwards. You could make this fundation and set it down below the finished grade line by however deep your paving is going to be, and pour a couple of raised pads for the gate posts monolithic (engineering for "all in one pour") with the foundation This is all "pull it out of the air" kind of engineering (having practiced as an engineer for 32 years, you get a kind of sense about things) , and if you do design any sort of foundation system and posts, I would suggest you run some real numbers on it or get an engineer to check things for you. Once you get the foundation and posts in, if you do them heavy, you can be as creative as you like and won;t have to worry about a few hundred extra pounds of ornamental ironwork or wood slats or whatever you choose to finish the gates off with.
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