I don't get the bad idea. This is done all the time for support columns. Yeah they support a lot more weight, but they are not subjected to the same torque that a 2 post lift would have.
You are right on. It's all about that moment around the base with regard to limiting design factors. The side-to-side resistance is important, but the front to back moments are potentially greater. (Vehicles are typically balanced well side-to-side but not front to back. Likewise, its more challenging to balance the load on a two post lift front-to-back.
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^ The flat slab, bolted to the posts, creates an inverted "T", which gives you the resistance to the tipping forces. (The slab acts in the same way side-to-side.)
A shallow footer (a few feet) would give you much less resistance to that tipping, depending on the base/soil properties and how deep the footing is. (This is why engineers and the manufacturers will specify that any footings need to be pinned or keyed to the slab.)
You could do a deep footing to resist the tipping, but you'd have to sink the thing deep like a telephone pole.
I looked at the mfg spec page and it is a joke. They don't talk about torque, they think it is all dead weight. They talk about installation and how they calculated it but no mention of actual dimensions. A 6" tapcon is going to be able to hold more than a 3" tapcon. What happens when you drop a large weight off of one side, like removing the engine/trans assy.? The few lifts I know had bolts buried in the wet concrete with a piece of plywood that matched the lift base plate to make sure it was secure.
Yea, all the ones I've seen give a few spec's for a flat slab (which has a lot of capacity to resist the tipping loads) and then pages on C-Y-A information related to what the actual lift can handle. Very little attention seems to be paid to what-if's around attachment or if slabs are too thin. I think I did see a bunch of disclaimers in some Bend Pak instructions regarding shifting loads, removing engines, etc. I recall they also suggested that the ends of the car be supported by posts, transmission jacks, etc. - that's what I would do for sure on a 2 post lift. (I'm allergic to cars falling on me.)
This is essentially true for a 4-post lift but for a 2 post lift without a stablizing beam across the top or bottom this not true.
I expect the tipping moments front-to-back for an off-balanced load would be a lot greater than the side-to-side ones.
My comments around not needing a footer are aimed at new construction. When a slab is properly poured, footers don't buy us anything and can lead to more cracking in the slab. If done improperly, they can also make the situation unsafe.
Lots of folks have a need for footers in a remodeling situation to address attachment/depth issues, but when done properly, they are typically pinned or keyed to the larger slab. (Some unkeyed/unpinned shallow or small footprint footers could resist the tipping moments, but that depends a lot on the base/soil properties. At that point you'd spend crazy amounts of money on soil testing and engineering to know if the arrangement would act properly.)
