Most park benches are anchored one of two ways: they are either bolted to a concrete pad or their legs are set directly into the ground. When a bench sits on concrete, the most common fasteners are 1/2-inch or 5/8-inch hot-dip galvanized or stainless steel wedge anchors, sleeve anchors, or drop-in anchors. You drill a hole into the concrete, tap the anchor into place, set the bench foot over it, and tighten the nut to expand the anchor. If the underside is accessible, some installers use through-bolts with large fender washers and nyloc nuts, but that only works well when the underside cannot be reached easily.
Are they theft-proof? No, not completely. A standard hex-head anchor bolt is definitely not theft-proof. Someone with a socket set, adjustable wrench, or cordless impact driver can remove ordinary bolts in minutes. That is why parks and public spaces usually switch to tamper-resistant hardware once theft becomes an issue.
The best anti-theft options include one-way clutch-head bolts, penta-head bolts, pin-in-Torx security bolts, and Tri-groove security bolts. These require special driver bits that most opportunistic thieves do not carry. Another very effective choice is a shear-head breakaway bolt. When it is tightened to the correct torque, the hex head snaps off and leaves a smooth round or slightly conical head that cannot be gripped with pliers or a normal socket. Some crews also hammer a ball bearing into the hex socket after tightening, tack-weld the nut to the stud, or cover the bolt head with grout or a welded cap. For concrete applications, you can epoxy the threaded rod into the hole or use security nuts such as cone nuts or shear nuts to further slow down removal.
Sizing depends on the bench, but common anchor sizes are 1/2 inch x 3-1/2 inches or 5/8 inch x 4 inches. In coastal or salt-treated areas, use 304 or 316 stainless steel so the anchors do not rust and fail later.
Realistically, no bolt is 100% theft-proof. A determined thief with an angle grinder, pry bar, or heavy bolt cutters can defeat almost any anchor. The goal is to make theft slow, noisy, and inconvenient. Use a thick concrete pad, at least four anchors per bench, recessed or shrouded bolt heads, and inspect the hardware regularly. That combination will stop most casual theft and make the bench a much harder target.