Anchor Bolt / Embed Plate Calculator

Frequently Asked Questions

• What is the minimum embedment depth for anchor bolts per ACI 318? +
  ACI 318-19 §17.9.5 sets absolute minimum embedment depths based on anchor type: cast-in headed bolts require hef ≥ 4da (4 times nominal bolt diameter), and hooked bolts require hef ≥ 3da. However, the formula-based embedment from the concrete breakout equation (back-solving Nb ≥ Nua/φ) typically governs and produces a larger value. Always take the greater of the code minimums and the formula-required value. A ¾″ bolt minimum absolute is 3 inches, but a loaded ¾″ bolt in a column base often requires 7–10 inches or more depending on applied load and concrete strength.
• What is the difference between a cast-in anchor and a post-installed anchor? +
  Cast-in anchors (headed bolts, J-bolts, L-bolts, headed studs) are placed in the formwork before concrete is poured and become mechanically locked in place as the concrete cures around the head or hook. They typically achieve higher concrete breakout capacity because the head provides direct bearing. Post-installed anchors are placed in holes drilled after the concrete has hardened, using either adhesive/chemical grout (epoxy, vinylester) or mechanical expansion. Post-installed anchors are more flexible for retrofit work but require stricter installation procedures, are sensitive to temperature and moisture (for adhesives), and generally require a lower breakout coefficient (kc = 17 vs. 24) unless qualified per ACI 355.4.
• How do I size an embed plate for a structural steel column? +
  Embed plate sizing follows AISC Design Guide 1 (Column Base Plates). The plate must be large enough to (1) accommodate the bolt group with proper edge distances, (2) distribute bearing stress to the concrete within allowable limits, and (3) resist bending from the column base moment. The minimum plate width is typically the column flange width plus 2–3 inches per side for weld clearance. Plate thickness is designed for bending — the column flange force creates a cantilever moment in the plate past the column profile. A minimum plate thickness of 0.5 × bolt diameter is a practical starting point for lightly loaded connections, but heavily loaded moment bases may require 1.5–3″ thick plates with stiffeners.
• What does the concrete breakout capacity (Ncbg) mean and when does it govern? +
  Concrete breakout capacity (Ncbg) is the load at which the anchor pulls out a cone of concrete — a roughly 35-degree cone radiating from the embedded head to the concrete surface. It is the most common governing failure mode for anchor bolts in tension, especially in lower-strength concrete or with shorter embedment depths. The basic single-anchor breakout capacity is Nb = kc × λ × √f'c × hef^1.5. The breakout capacity increases as hef^1.5, so doubling the embedment depth gives roughly 2.8 times more breakout capacity. Steel fracture capacity increases with bolt area, so in high-f'c concrete with large bolts, steel fracture may govern instead — that is generally the preferred ductile failure mode.
• What is the minimum edge distance for anchor bolts? +
  ACI 318-19 §17.9.4 requires a minimum edge distance of 6 times the nominal bolt diameter (ca,min = 6da) for cast-in anchors without special cover requirements. For a ¾″ bolt, that's 4.5 inches from the bolt centerline to the concrete edge. When edge distance is less than 6da, the breakout capacity must be modified using the ψed,N factor (edge distance modification), which can significantly reduce the effective anchor capacity. In seismic design categories C, D, E, and F, additional edge distance requirements apply. Anchors within 1.5 × hef of an edge are also at risk for side-face blowout failure, which requires separate calculation per §17.6.4.
• Can I use expansion anchors for sustained tensile loads? +
  No. ACI 318-19 §17.2.3.1 explicitly prohibits the use of mechanical expansion anchors and undercut anchors for sustained tensile loads — they are only permitted for overhead applications or sustained loads when classified as a special (undercut) anchor with documented test data. Mechanical expansion anchors (wedge anchors, sleeve anchors) can creep under sustained tension, gradually losing clamping force and capacity. For sustained tension, you must use cast-in headed anchors or qualified adhesive (chemical) anchors with approved test data under ACI 355.4. This restriction applies regardless of how the anchor is sized or what the published manufacturer capacity tables show.
• What is F1554 and which grade should I specify? +
  ASTM F1554 is the standard specification for headed anchor bolts used in structural steel construction. It has three grades: Grade 36 (Fy = 36 ksi, weldable, not heat-treated), Grade 55 (Fy = 55 ksi, weldable on request), and Grade 105 (Fy = 105 ksi, not weldable). Grade 36 is used when the bolts must be field-welded to templates or shear plates. Grade 105 is the most common specification for high-capacity column bases because it provides the most tensile capacity per bolt cross-section area. Do not attempt to weld Grade 105 — the high carbon equivalent makes it susceptible to hydrogen cracking under weld thermal cycles. If a weldable high-strength option is needed, use Grade 55 with the weldability supplement.
• How does a group of anchors affect the capacity calculation? +
  When anchors are spaced closely (within 3 × hef of each other), their concrete breakout cones overlap, reducing the total group breakout capacity below the sum of individual capacities. ACI 318 handles this with the ANc / ANco ratio — the projected area of the actual group's breakout cone divided by the projected area of a single anchor's idealized cone. Two ¾″ bolts spaced 6 inches apart on a 10″ embedment do not develop twice the single-anchor breakout capacity; the overlapping cones mean the group capacity may be only 1.4–1.6 times the single-anchor value. Steel tensile capacity (φNsa) does add directly: two bolts deliver twice the steel capacity. This is why groups at close spacing are not proportionally stronger in concrete breakout — spacing anchors farther apart (ideally > 3 × hef) eliminates cone overlap and maximizes group efficiency.
• Do I need a licensed engineer to design anchor bolts? +
  For structural connections — column bases, equipment anchorage, overhead supports, seismic anchorage — yes, a licensed structural engineer is required in virtually all US jurisdictions under the International Building Code. Anchor bolt design for code-regulated structures must be sealed by a licensed professional engineer. This calculator is intended as a preliminary sizing tool and educational resource. The interaction of all failure modes (steel fracture, concrete breakout, pullout, side-face blowout, pryout) and seismic ductility requirements per ACI 318 Chapter 17 require engineering judgment that goes beyond any online tool.
• What is the combined tension and shear interaction check? +
  When an anchor is subjected to both tension and shear simultaneously, ACI 318 §17.8.3 requires a combined interaction check. If Nua exceeds 20% of the tensile capacity (φNn) AND Vua exceeds 20% of the shear capacity (φVn), the interaction must satisfy: Nua/(φNn) + Vua/(φVn) ≤ 1.2. The capacity used (φNn and φVn) must be the lowest applicable capacity from all failure modes — including concrete breakout, steel strength, and pullout. In practice this means a bolt carrying both a significant uplift load and lateral wind shear needs careful sizing; each load individually might be within capacity, but combined they can exceed the limit. Shear lugs welded to the baseplate are often added to transfer shear directly to concrete, freeing the anchor bolts to resist tension only.