Anchor Bolt Design Calculator

What it checks

This anchor bolt calculator verifies a cast-in or post-installed anchor, single or in a group, to Eurocode EN 1992-4. It runs every failure mode the code requires and reports the governing one: steel failure in tension and shear, concrete cone breakout, pull-out, concrete splitting, concrete pry-out, concrete edge breakout, and the combined tension-shear interaction.

That list matters, because an anchor is only as strong as its weakest mode and the weakest mode is usually in the concrete rather than the steel. Checking the bolt alone - which is what a bolt-capacity table gives you - will overestimate the anchorage, often by a factor of two or more.

What you enter

Anchor type

Headed or bonded; cast-in or post-installed

Concrete condition

Cracked or uncracked - cracked is the safe default and changes the cone factor

Anchor size and steel

Diameter, tensile stress area As, f_uk and f_yk

Group layout

Number of anchors, nx by ny, and the spacings s1, s2

Embedment depth h_ef

The effective embedment, in mm - the single most influential input

Edge distances c1, c2 and member depth

In mm - these cut the projected cone area

Concrete strength f_ck

Characteristic cylinder strength, N/mm2

Design actions

N_Ed, V_Ed and M_Ed at the base plate

Embedment depth is the main lever

Concrete cone resistance goes with h_ef raised to the power 1.5, so it is superlinear in embedment. Increasing the embedment by 50 per cent raises the single-anchor cone resistance by about 84 per cent, whereas doubling the bolt diameter does nothing at all for the cone mode - it only helps steel failure, which is rarely the one that governs.

That is why anchorage design is normally driven by embedment and edge distance rather than by bolt grade. If a check fails on cone breakout, deepening the anchor or moving it away from the edge fixes it; specifying grade 10.9 instead of 8.8 does not.

Worked example - 2 by 2 group of M20 cast-in headed anchors

The tool defaults: four M20 headed cast-in anchors in a 2 by 2 pattern at 280 mm spacing both ways, 200 mm effective embedment, 400 mm from each edge, in cracked C30/37, carrying 100 kN tension and 45 kN shear. Every figure below is reproducible without entering anything.

Cone factor k1 (cracked, cast-in headed)

8.9

Single-anchor cone, k1 sqrt(f_ck) h_ef^1.5

137.9 kN

Characteristic spacing s_cr,N = 3 h_ef

600 mm

Characteristic edge distance c_cr,N = 1.5 h_ef

300 mm

Reference area A_c0 = s_cr^2

360 000 mm2

Actual projected area A_c,N

880 x 880 = 774 400 mm2

Area ratio A_c,N / A_c0

2.151

Edge factor psi_s (c = 400 > c_cr = 300)

1.00

Group cone resistance N_Rk,c

296.6 kN, design 197.7 kN

Cone utilisation vs 100 kN

0.506

Steel tension per anchor, A_s f_uk / gamma_Ms

130.7 kN, demand 25 kN

Steel utilisation

0.191

Concrete cone breakout governs at 0.506 against 0.191 for steel - the concrete is working two and a half times harder than the bolt. This is the normal result for a cast-in anchor group and it is the single most important thing to understand about anchorage: the bolt is almost never the weak link.

Note also the area ratio of 2.151 rather than 4.0. Four anchors do not give four times the cone resistance, because at 280 mm spacing the individual cones overlap - the spacing would need to reach s_cr,N = 600 mm for the anchors to act independently. Group efficiency is 2.151/4 = 54 per cent here, and closing the spacing further would reduce it more.

Formula reference

Steel tension

$N_{Rd,s} = \dfrac{A_s\,f_{uk}}{\gamma_{Ms}}$

Single-anchor cone

$N^{0}_{Rk,c} = k_1\,\sqrt{f_{ck}}\;h_{ef}^{1.5}$

Group cone

$N_{Rk,c} = N^{0}_{Rk,c}\,\dfrac{A_{c,N}}{A^{0}_{c,N}}\,\psi_{s,N}\,\psi_{re,N}\,\psi_{ec,N}$

Characteristic spacing / edge

$s_{cr,N} = 3\,h_{ef}, \quad c_{cr,N} = 1.5\,h_{ef}$

Edge factor

$\psi_{s,N} = 0.7 + 0.3\,\dfrac{c}{c_{cr,N}} \le 1.0$

Pull-out, headed

$N^{0}_{Rk,p} = k_2\,A_h\,f_{ck}$

Bond, bonded anchor

$N^{0}_{Rk,p} = \pi\,d\,h_{ef}\,\tau_{Rk}$

Combined tension and shear

$\left(\dfrac{N_{Ed}}{N_{Rd}}\right)^{1.5} + \left(\dfrac{V_{Ed}}{V_{Rd}}\right)^{1.5} \le 1.0$

The theory page derives the projected-area method and every psi-factor, and explains where the k-factors come from.

Assumptions and limits

Cracked concrete is assumed by default, which is the correct assumption unless you can demonstrate the anchorage zone stays uncracked under all relevant load combinations. Uncracked raises the cone factor substantially, so it is a claim worth being certain about rather than a default to switch on.

For a post-installed anchor the resistances here follow the EN 1992-4 expressions with generic factors. A real post-installed product is qualified by its ETA, and the manufacturer declared values in that approval take precedence over any generic calculation - including the bond strength tau_Rk, which is product-specific and temperature-dependent. Treat this tool as a sizing and sanity check, not as a substitute for the ETA.

The base-plate model assumes a rigid plate distributing N and M as a couple. Plate flexibility, prying, grout and shear transfer by friction or by a shear key are not modelled, nor is the reinforcement that would let you use the supplementary-reinforcement provisions of EN 1992-4 rather than plain concrete resistance. Seismic and fatigue design are outside the scope.

FAQ