chrisp
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But whatever fastener is used, isn‘t the strength of the concrete itself the weakest link?
It’ll either be the concrete or how well the fastener is fitted to the concrete. I’ve seen quite a few expansion sleeve anchors (aka dynabolts) that are loose or come up pretty easily. The fastener itself doesn’t usually fail, but rather it was never fitted right or the concrete was weak.
The thing about concrete is it’s really strong for compression loads (when the forces are squeezing to together) but it’s quite weak for tensile loads (when it’s being pulled apart). Its tensile strength is only about 10% of its compressive strength (so 3.2 MPa using your example). Hence, steel reinforcements are used to overcome the tensile weakness.
The point that is the weak link in my view is the tensile forces developed on an unevenly loaded two-post hoist. The vertical upright and the support pad essentially form a lever a bit like a claw hammer being used to pull a nail. The front-back imbalance is amplified by this leverage so 100s of kg imbalance on the hoist might turn into several tonnes at the fasteners and this force will be compressive on one side (no problem for concrete) but tensile on the other side (which will soon pull out and poorly affixed fasteners). It’s quite scary to think that the fastener is hanging in purely by the huge expansive forces asserted on the walls of a small hole in the concrete.
I like those two-posters that have a elongated pad to reduce the tensile forces. I see that some manufacturers offer an extended base as an option. I reckon that would be a pretty good investment in the integrity of the safety of a two-post hoist. Or, consider a 4-post design where the tensile forces will be considerably lessened by the nature of the design.