EN 1993-1-8

Verification Summary

Utilisation ratios for all limit state checks

SIZE
BOLT CLASS

🔩

M20 - Grade 8.8

f_yb = 640 MPa · f_ub = 800 MPa · γ_M2 = 1.25

📐 Geometry

Nominal diameter	20	mm
Gross area A_g	314	mm²
Stress area A_s	245	mm²

💪 Material

f_yb	640	MPa
f_ub	800	MPa
f_yb/f_ub	0.80

↕️ Tension

F_t,Rd

141.1

k₂ = 0.9

✂️ Shear

F_v,Rd (single)	94.1	kN	Tensile stress area
F_v,Rd (double)	188.2	kN

🔧 Bearing

S235	14.40	kN/mm
S275	16.00	kN/mm
S355	17.20	kN/mm

⭕ Holes

Normal clearance hole d₀	22	mm
Oversize clearance hole d₀	24	mm

📏 Min. edge distances

e₁	27	mm	= 1.2 d₀
e₂	27	mm	= 1.2 d₀
p₁	49	mm	= 2.2 d₀
p₂	53	mm	= 2.4 d₀

📏 Max. edge distances

e₁	66	mm	= 3 d₀
e₂	44	mm	= 1.5 d₀
p₁	88	mm	= 3.75 d₀
p₂	66	mm	= 3 d₀

✅ ChecksM20 · 8.8

F_t,Ed

≤ 141.1 kN

F_v,Ed

≤ 94.1 kN

e₁

≥ 27 mm (min)

e₂

≥ 27 mm (min)

p₁

≥ 49 mm (min)

p₂

≥ 53 mm (min)

Using this bolt resistance calculator

What it calculates

This tool returns the design resistances of a structural bolt to Eurocode 3 (EN 1993-1-8): shear per shear plane, bearing on the connected plate, tension, punching shear through the plate, the combined shear-and-tension interaction, and slip resistance for a preloaded connection. It also gives the geometry a detailer needs - hole diameter, minimum and maximum spacing, edge and end distances.

Pick a bolt size and property class and every value follows. The resistances that depend only on the bolt are fixed; the ones that depend on the connected material - bearing above all - change with the plate you enter.

Worked example - M20 grade 8.8

These are the tool defaults, and they are worth knowing by heart because M20 8.8 is the workhorse of ordinary steelwork. Tensile stress area As = 245 mm2, gross area 314 mm2, ultimate strength f_ub = 800 N/mm2, gamma_M2 = 1.25.

Shear, threads in the shear plane: 0.6 x 800 x 245 / 1.25 = 94.1 kN per plane
Shear, shank in the shear plane: 0.6 x 800 x 314 / 1.25 = 120.6 kN per plane
Tension: 0.9 x 800 x 245 / 1.25 = 141.1 kN
Bearing, 10 mm S275 plate, end bolt at e1 = 1.5 d0: 86.0 kN
Hole diameter d0 (normal clearance): 22 mm

Two things in that table are worth pausing on. First, putting the shank rather than the threads in the shear plane gains 28 per cent of shear capacity for free - but only if the detail actually guarantees it, which needs the grip length checked, so most designers conservatively assume threads in the plane.

Second, and more important: the bearing resistance of 86 kN is LOWER than the 94.1 kN shear resistance. On a thin plate with modest end distance the connection is governed by the plate tearing, not the bolt shearing, so specifying a higher grade bolt would buy nothing at all. Bearing scales with plate thickness and end distance; bolt shear does not.

Why bearing so often governs

Bearing resistance is k1 alpha_b f_u d t / gamma_M2 - it depends on the plate ultimate strength f_u and thickness t, and on the end and edge distances through alpha_b and k1. The bolt grade appears only as an upper limit. That is why a connection can be improved far more cheaply by increasing the end distance or the plate thickness than by upgrading from 8.8 to 10.9.

The end distance term is the one people underestimate: at the code minimum of 1.2 d0 the factor alpha_d is 0.4, so the bearing resistance is 40 per cent of what a generous end distance would give. Moving from the minimum to 3 d0 roughly doubles it, usually at no cost.

Formula reference

Shear per plane: $F_{v,Rd} = \dfrac{\alpha_v\,f_{ub}\,A}{\gamma_{M2}}$
Bearing on the plate: $F_{b,Rd} = \dfrac{k_1\,\alpha_b\,f_u\,d\,t}{\gamma_{M2}}$
Tension: $F_{t,Rd} = \dfrac{k_2\,f_{ub}\,A_s}{\gamma_{M2}}$
Punching shear: $B_{p,Rd} = \dfrac{0.6\,\pi\,d_m\,t_p\,f_u}{\gamma_{M2}}$
Combined shear and tension: $\dfrac{F_{v,Ed}}{F_{v,Rd}} + \dfrac{F_{t,Ed}}{1.4\,F_{t,Rd}} \le 1.0$
Slip resistance, preloaded: $F_{s,Rd} = \dfrac{k_s\,n\,\mu}{\gamma_{M3}}\,F_{p,C}$
Preload force: $F_{p,C} = 0.7\,f_{ub}\,A_s$

alpha_v is 0.6 for classes 4.6, 5.6 and 8.8 and 0.5 for 10.9 when the threads are in the shear plane; k2 is 0.9 for tension, or 0.63 for a countersunk bolt. The theory page derives each expression and tabulates the factors.

Assumptions and limits

Resistances are for bolts in normal clearance holes to EN 1993-1-8 Table 3.4, with gamma_M2 = 1.25 as the recommended value - check your National Annex, which may differ. Oversized and slotted holes reduce bearing and require the k1 and alpha_b factors to be taken accordingly.

Bearing values assume the plate is the weaker element and that end and edge distances meet at least the code minima. The tool checks the individual bolt: block tearing of the group, net-section failure of the connected member, prying action in a tension connection, and the overall connection ductility are separate checks that a bolt resistance alone does not cover.

Slip resistance depends on the friction surface class, which is a specification and workmanship matter rather than a calculation - class A requires blasted surfaces and the value is void if the faying surface is painted or contaminated. Fatigue, fire and bolts in shear-tension reversal are outside the scope.

EC3 Bolt Design Data