What Is Post-Installed Anchor Design?
Post-installed anchors (mechanical expansion anchors, adhesive/epoxy anchors, screw anchors) fasten to existing concrete — the retrofit connection for everything from equipment anchorage to structural strengthening. Their design per ACI 318 Chapter 17 considers concrete breakout, pullout, side-face blowout, and steel failure — with seismic and cracked-concrete reductions. The wrong anchor in the wrong concrete fails; the design selects the anchor for the application.
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Anchor types
Expansion anchors (torque-controlled or displacement-controlled — the mechanical wedge gripping the drilled hole; quick, economical, sensitive to hole quality), adhesive anchors (threaded rod or rebar bonded with epoxy or hybrid mortar — the highest capacities, the most installation-sensitive: hole cleaning determines the bond), and screw anchors (hardened screws cutting threads into the concrete — removable, good for temporary or adjustable connections). Each has its tested capacities, approved uses, and limitations (fire exposure, seismic categories, cracked vs. uncracked concrete assumptions).
ACI 318 Chapter 17 design
The engineer checks every failure mode: steel strength (the anchor's tensile/shear capacity), concrete breakout (the cone of concrete pulling out — edge distances and spacing control it), pullout/pull-through (the anchor slipping — expansion anchors), bond failure (adhesive anchors — the characteristic bond stress from testing), and side-face blowout (near edges in thin members). Seismic design adds the ductility requirements (the anchor must yield before the concrete fails — or the connection gets the overstrength factor). Cracked-concrete assumptions apply wherever the concrete may crack in service.
Installation: where anchors fail
Hole cleaning (adhesive anchors — the dust left in the hole destroys the bond; the specified brushing/blowing sequence is mandatory), installation torque (expansion anchors — under-torqued anchors never set), embedment depth (the design's assumed depth — shallow anchors fail in breakout), and installer certification (ACI/CRSI certified installers for structural adhesive anchors — the code increasingly requires it). Special inspection verifies the installation matches the design — the anchor's capacity exists only as installed.
The bottom line
Design post-installed anchors per ACI 318 Ch. 17 for all failure modes (breakout, pullout, bond, steel) — and inspect the installation (hole cleaning, torque, embedment) because capacity is as-installed.
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