Eurocode 3 Connection Design — Automated EN 1993-1-8 Checks
FrameAI reads your structural PDF, extracts every bolted and welded connection through GPT-4o vision,
and runs the full EN 1993-1-8 check suite clause-by-clause — bolt shear, bearing, tension with prying,
end-plate yield lines, fillet weld resistance — without a CAD operator.
Read the full clause-by-clause explainer →
Reviewed by Alex Kampstra, 3D Tekla Modelleur · Published 2026-06-06 · Last reviewed 2026-06-07
EN 1993-1-8:2005Bolt shear §3.6.1Prying §6.2.4T-stub yield linesFillet welds §4.5.3.3Component method §6S355 · M24 gr 8.8
The connection is a flush end-plate moment connection: an IPE400 beam frames into the flange of
an HEA300 column. A 12 mm S355 end plate is shop-welded to the beam web and flanges with 6 mm
fillet welds, then site-bolted to the column flange with four M24 grade 8.8 bolts in two rows of two.
HEA300 Column (S355)
h290 mm
b300 mm
t_f (flange)14.0 mm
t_w (web)8.5 mm
r (root)27 mm
f_y355 N/mm²
f_u510 N/mm²
IPE400 Beam (S355)
h400 mm
b180 mm
t_f (flange)13.5 mm
t_w (web)8.6 mm
r (root)21 mm
f_y355 N/mm²
f_u510 N/mm²
Connection Detail
BoltM24 gr 8.8
Bolt layout2 rows × 2
End plate t12 mm S355
e₁ (end dist.)55 mm
e₂ (edge dist.)50 mm
p (bolt pitch)90 mm
Hole d₀26 mm
Weld a6 mm fillet
Calculation Step 1
Bolt Shear Resistance — EN 1993-1-8 Table 3.4
Single-shear bolt in bearing-type connection. Shear plane passes through the threaded portion
(conservative per EN 1993-1-8 §3.6.1). Four M24 grade 8.8 bolts; γM2 = 1.25.
F_v,Rd — Design shear resistance per boltEN 1993-1-8 Table 3.4
Bearing on the 12 mm S355 end plate (thinner plate governs over the 14 mm column flange).
EN 1993-1-8 Table 3.4 uses the directional method: k₁ in the direction perpendicular to load,
α_b in the load direction.
α_b — bearing coefficient in load directionEN 1993-1-8 Table 3.4
F_b,Rd — Design bearing resistance per boltEN 1993-1-8 Table 3.4
F_b,Rd = k₁ · α_b · f_u · d · t / γ_M2 // f_u = 510 N/mm² (plate), d = 24 mm, t = 12 mm, γ_M2 = 1.25 F_b,Rd = 2.5 × 0.705 × 510 × 24 × 12 / (1.25 × 1000) = 208.0 kN
F_v,Ed / F_b,Rd (critical bolt)
0.29
Calculation Step 3
Bolt Tension and Prying — EN 1993-1-8 §6.2.4
Under moment loading, the top bolt row is in tension. Prying forces from end-plate bending
add to the bolt tension. EN 1993-1-8 §6.2.4 uses the T-stub model (Table 6.2) to account
for prying in three modes: Mode 1 (plate yielding), Mode 2 (plate yielding + bolt failure),
Mode 3 (bolt failure without plate yielding).
T-stub parameters — end plate in bendingEN 1993-1-8 §6.2.4
// m = distance from bolt centreline to weld toe on beam flange m = e₁_plate − t_fb/2 − a_w·√2 = 55 − 13.5/2 − 6·1.414 = 39.8 mm e = e₂ = 50 mm (edge distance to plate edge) n = min(e, 1.25·m) = min(50, 1.25×39.8) = min(50, 49.7) = 49.7 mm // Effective length l_eff for row group — non-circular pattern, Table 6.6 l_eff = p = 90 mm(bolt pitch governs interior row)
T-stub plastic moment M_pl,Rd (per row)EN 1993-1-8 §6.2.4, Table 6.2
Mode 1 governs — the 12 mm end plate yields before the bolts fail. Increasing plate thickness to 15 mm raises F_T,Rd to 181 kN and shifts the governing mode to Mode 3 (bolt failure). FrameAI flags this automatically and suggests t_ep = 15 mm.
Calculation Step 4
Fillet Weld Resistance — EN 1993-1-8 §4.5.3.3
The end plate is shop-welded to the beam with 6 mm fillet welds along both flanges and the web.
The directional method (§4.5.3.3) checks the weld throat stress against the combined normal
and shear components. For structural steel S355, the correlation factor β_w = 0.90.
F_w,Rd — Design weld resistance per unit lengthEN 1993-1-8 §4.5.3.3, Table 4.1
Summary of all EN 1993-1-8 checks. Design loads: V_Ed = 120 kN, M_Ed = 45 kN·m.
FrameAI generates this table automatically for every connection detected in the PDF.
Check
Resistance (kN)
Demand
Utilisation
Status
Clause
Bolt shear F_v,Rd
4 × 135.6 = 542.4
V_Ed = 120 kN
0.22
PASS
Table 3.4
Bolt bearing F_b,Rd
4 × 208.0 = 832.0
V_Ed = 120 kN
0.14
PASS
Table 3.4
Bolt tension F_t,Rd
2 × 203.7 = 407.4
F_t,Ed = 90 kN
0.22
PASS
Table 3.4
T-stub (end plate, prying) Mode 1
F_T,Rd = 116.1
F_t,Ed = 90 kN
0.78
REVIEW
§6.2.4 Table 6.2
Fillet weld (flange)
528.4 kN
N_fl = 189 kN
0.36
PASS
§4.5.3.3 Table 4.1
Block tearing (bolt group)
V_eff,1,Rd = 542 kN
V_Ed = 120 kN
0.22
PASS
§3.10.2
The T-stub end-plate check at 0.78 is the critical path. FrameAI flags the utilisation amber
and recommends increasing the end-plate thickness from 12 mm to 15 mm, which reduces utilisation
to 0.49 and shifts the governing mode to Mode 3 (bolt failure) — the preferred failure sequence.
Worked Example — M20 gr 8.8 Bolt Group
M20 Grade 8.8 Bolt Group in Shear and Tension — EN 1993-1-8 §3.6, §3.10, §4.5
A secondary beam fin-plate connection: 4 × M20 grade 8.8 bolts in a single vertical line,
5 mm clearance holes (d₀ = 22 mm), 10 mm S355 fin plate. Design shear V_Ed = 95 kN.
All calculations per EN 1993-1-8:2005.
All four M20 grade 8.8 bolt checks pass with low utilisation — the fin-plate geometry governs
over the bolt capacity. FrameAI reports these numerical results per bolt and per-group in the
Connection Details tab with clause references §3.6.1, §3.10.2.
Automation
How FrameAI Runs These Checks Automatically
Upload a structural PDF. FrameAI does the rest in under 90 seconds.
Step 1 — GPT-4o Vision Extraction
InputStructural PDF
ExtractsMembers, bolts, welds
OutputStructured JSON geometry
LanguagesEN, NL, DE, FR, SV
Step 2 — EN 1993-1-8 Check Engine
Bolt shearTable 3.4
Bolt bearingTable 3.4
Prying§6.2.4 T-stub M1/2/3
Weld resistance§4.5.3.3 directional
Moment M_j,Rd§6.2.7 bolt-row sum
Stiffness S_j,ini§6.3 component springs
Step 3 — Fabrication Outputs
PDF drawingConnection Details tab
DXFR12 shop drawings
DSTV NC1Pro/Studio
IFC 4BIM model
BOM ExcelEN 10365 weights
Supported Connection Types
End-plate moment✓ flush + extended
Base plate✓ EN 1993-1-8 §6.2.8
Fin plate (shear)✓
Cleat (angle)✓
Hollow sectionChapter 7
References
Normative References
EN 1993-1-8:2005 §3.6.1 — Bolt shear resistance, Table 3.4 bearing coefficients k₁ and α_b
EN 1993-1-8:2005 §6.2.4 — T-stub in tension, prying force model, Mode 1/2/3 failure modes, Table 6.2
EN 1993-1-8:2005 §6.2.4 Table 6.6 — Effective lengths for end plate in bending (individual rows + groups)
EN 1993-1-8:2005 §4.5.3.3 — Fillet weld resistance per unit length, directional method
EN 1993-1-8:2005 Table 4.1 — Correlation factor β_w for fillet welds by steel grade
EN 1993-1-8:2005 §6.2.7 — Moment resistance M_j,Rd via bolt-row summation, lever arms h_r
EN 1993-1-8:2005 §3.10.2 — Block tearing failure modes VB_Rd effective net area
SCI P398 — Joints in Steel Construction: Moment-Resisting Joints to Eurocode 3
EN 10365:2017 — Hot-rolled steel channels, I and H sections; section properties used in geometry extraction
FAQ
Frequently Asked Questions
What is EN 1993-1-8?
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EN 1993-1-8 is Part 1-8 of Eurocode 3 (Design of steel structures) — the normative standard for design of joints in steel construction. It covers bolted connections (shear, bearing, tension, prying via the T-stub model), welded connections (fillet and butt welds), base plates, and the component method for calculating moment resistance and rotational stiffness of beam-to-column joints. It is applicable throughout the EU and in countries that have adopted the Eurocodes.
How does FrameAI verify prying forces per EN 1993-1-8 §6.2.4?
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FrameAI implements the T-stub in tension model from EN 1993-1-8 §6.2.4 (Table 6.2). For each bolt row it calculates the plastic moment of the end plate or column flange (M_pl,Rd), the bolt centreline-to-weld-toe distance m, and the edge distance n = min(e, 1.25·m). It then evaluates all three failure modes — Mode 1 (complete flange yielding with full prying), Mode 2 (bolt failure with partial prying), and Mode 3 (bolt failure without prying) — and takes the minimum as the governing row resistance. Prying is implicit in Mode 1 and Mode 2: the plastic hinge mechanism in the plate concentrates additional load into the bolts beyond the applied tension.
Which connection types does FrameAI check under EN 1993-1-8?
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FrameAI checks: (1) End-plate moment connections — flush and extended, using the component method (§6) for M_j,Rd and S_j,ini; (2) Base plates — bearing pressure distribution, anchor bolt tension, and weld to column (§6.2.8); (3) Fin plate (shear tab) connections — bolt shear, bearing, block tearing (§3.10.2); (4) Angle cleat connections — double-angle shear connections with bolt group checks; (5) Hollow section connections — Chapter 7 formulae for CHS/RHS chord and brace joints. Bolted connections use Table 3.4 for shear, bearing, and tension resistances.
What partial factors does FrameAI use for EN 1993-1-8?
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By default FrameAI uses the recommended EN 1993-1-8 partial factors: γ_M0 = 1.0 (cross-section resistance), γ_M1 = 1.0 (member stability), γ_M2 = 1.25 (net section fracture and bolt/weld failure). National Annex overrides are supported for countries that prescribe different values — for example, the UK NA keeps γ_M0 = 1.0 but increases γ_M2 to 1.25 for bolts in tension (same as EN). The national annex is set at the project level and propagates to all connection checks in that project.
How accurate is the GPT-4o extraction for connection geometry?
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For standard hot-rolled sections in structural PDFs with readable dimensions, FrameAI achieves >95% extraction accuracy on bolt diameter, bolt grade, edge distances, and profile sizes. The model is trained on structural drawings in English, Dutch, German, French, and Swedish. When a dimension is ambiguous or missing, FrameAI flags the field as "extracted with low confidence" in the results JSON and in the PDF report, prompting the engineer to verify. Critical dimensions for connection design — end distances, bolt pitch, plate thickness — are always flagged if they cannot be resolved unambiguously.
Does FrameAI classify the connection as rigid, semi-rigid, or pinned?
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Yes. After computing M_j,Rd and S_j,ini via the component method (EN 1993-1-8 §6.3), FrameAI classifies the joint using the stiffness boundaries from EN 1993-1-8 §5.2.2 — sway vs. non-sway frames, and braced vs. unbraced. The classification (rigid / semi-rigid / nominally pinned) is shown in the Connection Details tab of the pipeline and in the export PDF. For semi-rigid connections, S_j,ini is passed to the member check module so that frame analysis uses the correct boundary conditions.
Run This Check in FrameAI
Upload any structural PDF and get the full EN 1993-1-8 connection report — bolt shear, bearing,
prying, weld, block tearing — with utilisation ratios, fabrication drawings, and DXF output.
No CAD licence needed. No setup.