I’ve installed enough public benches to say the cleanest, strongest method is not just one bolt—it’s the combination of the right anchor, the right fastener, and making the hardware hard to attack. For concrete, I’d spec 1/2-inch or 5/8-inch stainless steel wedge anchors, grade 316 if you are near salt or coastal weather. Each leg should get at least two anchors if the base plate allows it. Embed them a minimum of 3-1/2 to 4 inches into solid, cured concrete—not into cracks, joints, or shallow slab edges.
Drill with a hammer drill and a carbide-tipped bit matching the anchor diameter. Drill about 1/4 inch deeper than the anchor embedment, clean the dust out with a blow bulb or vacuum, and tap the anchor through the bench base plate into the hole. Tighten gradually with a torque wrench or ratchet until the wedge sets. Don’t over-tighten; you can crack the concrete or strip the anchor.
The real theft deterrent is the fastener head. Standard hex bolts are easy to remove. Use tamper-resistant security bolts: pin-in-torx, tri-groove nuts, or shear-head bolts that snap off at a set torque. If the bench has exposed nuts, use tack welds or center-punch the threads to destroy them after install. You can also fill the bolt head recesses with epoxy or stainless ball bearings, but make sure you never need to remove the bench for maintenance.
For mounting plates that don’t sit flat, add stainless washers or shims, and leave a small gap or use rubber spacers so water doesn’t pool against the steel. If the concrete is old or weak, use an epoxy anchor system instead of wedge anchors: drill, clean, inject, and set threaded rod, then use structural nuts. It’s more work but holds better in questionable slab edges.
Check local code before drilling into a public sidewalk or plaza. You may need a permit, and you don’t want to hit conduit, rebar, or utilities. If you hit rebar while drilling, stop and shift the hole slightly; do not cut reinforcement.
Short answer: use 1/2-inch stainless wedge anchors, two per leg, drilled 4 inches deep, paired with tamper-resistant bolt heads or welded nuts. That’s the best balance of strength, corrosion resistance, and theft deterrence.