Post-Installed vs Cast-in-Place Anchors: Choosing the Right Anchor for Seismic Anchorage

Roughly 90% of nonstructural seismic anchorage in California is post-installed because the equipment arrives long after the slab is poured. The remaining 10% — primarily heavy floor-mounted equipment designed in coordination with the structural drawings — uses cast-in-place headed bolts. Both are governed by ACI 318-19 Chapter 17 and both must satisfy the seismic provisions of §17.10. The choice between them changes capacity, qualification paperwork, plan-check risk, and constructability.

Cast-in-place headed anchors

Cast-in-place anchors — typically ASTM F1554 Grade 36/55/105 headed bolts or A325/A490 high-strength bolts — are set in formwork and encased in fresh concrete. Capacity is calculated directly from first-principles equations in ACI 318-19 §17.6 (tension) and §17.7 (shear): A_se,N · f_uta for steel, k_c · √f'_c · h_ef^1.5 for concrete breakout.

• Pros: highest published capacities; simplest qualification (the headed bolt is its own ESR); easy to detail with anchor reinforcement to shift the limit state from concrete to steel.
• Cons: requires coordination at the slab pour; survey of bolt pattern is rarely tight enough for tight equipment templates; impossible to retrofit.

Post-installed mechanical anchors (expansion, sleeve, undercut)

Mechanical post-installed anchors expand against drilled hole walls when torqued or set. Capacity is taken from the manufacturer's ICC-ES Evaluation Service Report (ESR), which lists tabulated values of N_p, V_sa, and the required Ψ factors for cracked and uncracked concrete.

• Pros: install through the equipment baseplate after the slab is set; field-adjustable bolt pattern; the ESR has already done the qualification work for you.
• Cons: capacities are 30–60% lower than equivalent cast-in-place; expansion anchors are sensitive to edge distance and spacing (see edge distance & spacing); some products are not seismic-qualified for cracked concrete.

Post-installed adhesive (epoxy) anchors

Adhesive anchors bond a threaded rod into a drilled and cleaned hole with a structural epoxy or vinyl ester. Per ACI 318-19 §17.10.5, adhesive anchors used in seismic applications must be qualified per ACI 355.4 with the seismic test category — confirm this in the ESR.

• Pros: high tension capacity in deep embedments; useful when edge distances are tight; widely available.
• Cons: sensitive to overhead installation, hole cleaning, and elevated temperatures (sustained tension at >100 °F can cause creep); requires a certified installer for many ESRs (ACI/CRSI installer certification).

Screw anchors (Titen HD, KH-EZ, etc.)

Self-tapping concrete screws have largely replaced expansion anchors for medium-duty equipment in California. They are easy to install, removable, and have ESRs covering cracked-concrete seismic use.

• Pros: fast install, no setting tool, removable for re-positioning, predictable performance.
• Cons: capacity per anchor is moderate; tight torque control needed; some manufacturers limit re-use.

Capacity comparison — headed bolt vs ½″ post-installed

For a ½″ anchor at h_ef = 4 in. in 4,000 psi cracked concrete, far from any edge:

• Cast-in-place ASTM F1554 Gr 36 headed bolt — N_cb ≈ 8,500 lb (concrete breakout); φN_cb = 0.65 · 0.75 · 8,500 ≈ 4,150 lb.
• Post-installed expansion (typical ESR ½″, h_ef = 4 in., cracked, seismic) — published φN_n,seismic ≈ 2,300 lb.
• Post-installed adhesive (typical ESR, ½″ rod, h_ef = 4 in., cracked, seismic) — published φN_n,seismic ≈ 3,800 lb.

The differential is real, and it is why a chiller anchor pattern that works with cast-in-place may need a deeper embedment or larger diameter when redesigned with post-installed anchors.

ACI 318-19 §17.10 seismic implications by anchor type

• All anchors in SDC C–F must satisfy §17.10 — the 0.75 reduction on concrete-controlled φN/φV and the Ω_0p amplification per ASCE 7-22 §13.4.2.
• Adhesive anchors additionally require the ACI 355.4 seismic qualification (§17.10.5).
• Screw anchors and expansion anchors must be qualified per ACI 355.2 with seismic Category C2 testing — verify in the ESR's "seismic" section.

How to choose on a real project

1. Is the equipment installed during the pour (e.g., generators on a podium pad cast around them)? Cast-in-place headed bolts.
2. Edge distance < 1.5 h_ef? Adhesive (or anchor reinforcement on a cast-in-place).
3. Cracked-concrete seismic, dense rebar, mid-size equipment? Screw anchor or qualified expansion anchor.
4. Heavy equipment, generous edges, retrofit? Adhesive with deep embedment.
5. Sensitive electronics with low tolerance for vibration during install? Adhesive (no impact wrench; gentle install).

Plan-check landmines specific to post-installed

• Wrong column of the ESR (uncracked vs cracked) — see our cracked vs uncracked guide.
• ESR specifies "for non-seismic only" — disqualifies the anchor in SDC C–F.
• Adhesive used overhead without ACI 355.4 horizontal/upward orientation qualification.
• Edge distance below the ESR's c_ac (critical edge distance) — capacity drops more than the linear Ψ_ed,N reduction suggests.
• Expansion anchor in lightweight concrete without the lightweight λ adjustment.

How PANACHE ENGINEERING handles this

Our calculator selects the anchor type from a curated library of seismic-qualified ESRs and flags any combination that violates §17.10. For project-specific guidance, see our Equipment Anchorage Design workflow or request a stamped calculation.