Steel Connection Design: Eurocode 3 Rules & Calculator

Steel Connection Design Rules: Eurocode 3 Reference

Quick lookup and full reference for designing bolted and welded steel connections to BS EN 1993-1-8:2005 with the UK National Annex. Covers bolt spacings, hole clearances, weld sizes, block shear and plate bearing resistance, and connection stiffness classification. All formulas are taken directly from the standard. Use the lookup tool below for the most-asked spacing rules, or jump to a tab for the underlying formulas and worked examples.

Quick lookup

Bolt size:

Plate thickness (mm):

Bolt grade:

Quick lookup combines values from the spacing, bearing and weld tables below. For shear-and-tension interaction, edge effects (αb capped at 1.0), or combined connection types, refer to BS EN 1993-1-8 directly.

Bolt spacings and edge distances

Standard: BS EN 1993-1-8:2005 Table 3.3. All distances measured to the centre of the bolt hole. Variables: e₁ = end distance (in direction of force), e₂ = edge distance (perpendicular to force), p₁ = pitch (in direction of force), p₂ = spacing (perpendicular). d₀ = hole diameter.

Minimum distances

Distance	Formula	Notes
End distance e₁	≥ 1.2 × d₀	In the direction of load transfer
Edge distance e₂	≥ 1.2 × d₀	Perpendicular to load
Pitch p₁	≥ 2.2 × d₀	Bolts in line with load
Spacing p₂	≥ 2.4 × d₀	Bolts perpendicular to load

Maximum distances (corrosion / local buckling control)

Distance	Maximum
End distance e₁ (general)	4t + 40 mm (t = thinner ply thickness)
Edge distance e₂ (general)	4t + 40 mm
Pitch p₁ (compression)	min(9t, 14t, 200 mm)
Pitch p₁ (tension)	min(14t, 200 mm)
Spacing p₂	min(14t, 200 mm)

Pre-computed minimum distances (mm)

Bolt size	d₀ (mm)	min e₁	min e₂	min p₁	min p₂

UK practice starting values (M20 bolts): e₁ = e₂ = 40 mm, p₁ = p₂ = 70 mm. Then verify against the minimum and maximum rules above. SCI P398 tabulates standard bolt group arrangements for common connection types.

Weld design reference

Standard: BS EN 1993-1-8:2005 § 4.5 (resistance), BS EN ISO 5817 (quality), BS EN 1090-2 (execution).

Minimum throat thickness by plate thickness

The throat of a fillet weld must not be less than 3 mm for thin plates. For thicker plates, minimum throat is set to avoid excessive cooling rate and hydrogen cracking.

Thinnest plate (mm)	Minimum throat a (mm)

Common UK fillet weld sizes

Practical fillet weld throats used in UK fabrication. Larger welds slow production and concentrate heat distortion; smaller welds may not meet the minimum rule above.

Throat a (mm)	Leg length (mm)	Typical use

Weld types

Type	Description

Weld quality categories (BS EN ISO 5817)

Category	Description	Typical execution class

Block shear (block tearing) resistance

Standard: BS EN 1993-1-8:2005 § 3.10.2. Block shear is the failure mode where a group of bolts at the end of a connected element tears out a block of material from the connected ply. Always check this on fin plates, gusset plates and cleat connections.

Concentric loading

Symmetrical bolt group with concentric loading (e.g. tension splice on a plate):

V_eff,1,Rd = (f_u · A_nt) / γ_M2 + (f_y · A_nv / √3) / γ_M0

Eccentric loading

Bolt group with eccentric loading (e.g. fin plates connecting beam to column web):

V_eff,2,Rd = 0.5 · (f_u · A_nt) / γ_M2 + (f_y · A_nv / √3) / γ_M0

The factor 0.5 on the tension term accounts for non-uniform tension stress distribution under eccentric loading.

Net area calculations

Variable	Formula	Description
A_nt	(e₂ − 0.5 × d₀) × t	Net area in tension (single bolt column). For double columns, sum across columns and subtract bolt holes from the gross tension area.
A_nv	(e₁ + (n − 1) × p₁ − (n − 0.5) × d₀) × t	Net area in shear, where n = number of bolt rows in shear.

Partial safety factors

Factor	Value	Applies to
γ_M0	1.00	Resistance of cross-sections
γ_M2	1.25	Resistance of net section in tension and bolts

For component design: when sizing a fin plate or cleat, block shear is one of four required checks alongside bolt shear, bolt bearing, and weld. Iterate plate dimensions to ensure V_eff,Rd ≥ design force.

Plate bearing resistance

Standard: BS EN 1993-1-8:2005 § 3.6.1(4) Table 3.4. Bearing resistance per bolt depends on plate thickness, edge distance, hole spacing and bolt-to-plate strength ratio.

Bearing formula

F_b,Rd = (k₁ · α_b · f_u · d · t) / γ_M2

Where:

f_u = ultimate tensile strength of plate (470 N/mm² for S275, 510 N/mm² for S355)
d = bolt diameter (mm)
t = thickness of connected plate (mm)
γ_M2 = 1.25

k₁ factor (edge effect)

Bolt position	k₁ formula	Cap
Edge bolt	min(2.8 × e₂/d₀ − 1.7, 2.5)	Capped at 2.5
Inner bolt	min(1.4 × p₂/d₀ − 1.7, 2.5)	Capped at 2.5

α_b factor (end effect and bolt-to-plate strength)

Bolt position	α_b formula
End bolt	min(e₁/(3 × d₀), f_ub/f_u, 1.0)
Inner bolt	min(p₁/(3 × d₀) − 0.25, f_ub/f_u, 1.0)

Worked example: M20 grade 8.8 in 10 mm S275 fin plate, UK practice spacings

Variable	Value
k₁ (edge & inner)	2.5 (capped, since e₂ = 40, p₂ = 70 give k₁ > 2.5)
α_b (end bolt)	0.61 (e₁ = 40, so 40 / (3 × 22) = 0.61)
α_b (inner bolt)	0.81 (p₁ = 70, so 70 / (3 × 22) − 0.25 = 0.81)
f_u (S275 plate)	470 N/mm²
F_b,Rd (end bolt)	(2.5 × 0.61 × 470 × 20 × 10) / 1.25 / 1000 = 114.7 kN
F_b,Rd (inner bolt)	(2.5 × 0.81 × 470 × 20 × 10) / 1.25 / 1000 = 152.3 kN

Compare to bolt shear: M20 grade 8.8 with threads in shear, F_v,Rd = 94.1 kN. Bolt shear governs over bearing on a 10 mm S275 plate. Plate thickness can be reduced until bearing becomes the governing check, then sized for the design force. Reducing plate thickness saves embodied carbon proportionally — see the embodied carbon calculator.

Connection stiffness classification

Standard: BS EN 1993-1-8:2005 § 5.2. Connections are classified as nominally pinned, semi-rigid or rigid based on initial rotational stiffness relative to the connected beam stiffness. The classification determines how the connection is modelled in the global frame analysis.

Stiffness boundaries (braced frames)

Classification	Boundary	Modelling
Nominally pinned	S_j,ini ≤ 0.5 × E·I_b/L_b	Idealised as a hinge in the analysis
Semi-rigid	0.5 × E·I_b/L_b < S_j,ini < 8 × E·I_b/L_b	Modelled with rotational spring
Rigid	S_j,ini ≥ 8 × E·I_b/L_b	Idealised as a rigid joint

For unbraced (sway) frames, the rigid boundary is 25 × E·I_b/L_b.

UK practice connection types

Connection type	Typical classification	Common use