EN 1993-1-8 Connection Design — Bolts, Welds, Joint Classification

Quick Reference: EN 1993-1-8 governs the design of steel connections under Eurocode 3. Bolt shear resistance Fv,Rd = alpha*v * fub _ A / gamma_M2. Fillet weld resistance Fw,Rd = fvw,d _ a where fvw,d = fu/(sqrt(3) _ beta_w * gamma_M2). Joints are classified as simple (pinned), continuous (rigid), or semi-continuous based on stiffness and strength.

Bolt Categories per EN 1993-1-8

EN 1993-1-8 Table 3.1 defines five bolt categories:

Shear Connections

Tension Connections

For typical building connections (shear tabs, fin plates, end plates), Category A (bearing type) is the default.

Bolt Grades

Grade 8.8 is the default for structural steel connections in the UK and Europe. Grade 10.9 is used where higher strength is needed or for slip-resistant connections. Grade 4.6 is for non-structural or lightly loaded applications.

Bolt Shear Resistance

For Category A (bearing type), the design shear resistance per shear plane:

Fv,Rd = alpha*v * fub _ A / gamma_M2

where:

• alpha_v = 0.6 for grades 4.6, 5.6, 8.8 (shear plane passes through threads)
• alpha_v = 0.5 for grade 10.9 (shear plane through threads)
• alpha_v = 0.6 for all grades if shear plane passes through unthreaded shank
• A = tensile stress area As (threads) or gross shank area (unthreaded)
• gamma_M2 = 1.25

For an M20 Grade 8.8 bolt (As = 245 mm^2): Fv,Rd = 0.6 _ 800 _ 245 / 1.25 = 94.1 kN (single shear, threads in shear plane)

For double shear: Fv,Rd = 2 * 94.1 = 188.2 kN.

Bolt Bearing Resistance

Fb,Rd = k1 _ alpha_b _ fu _ d _ t / gamma_M2

where:

• alpha_b = min(e1/3d0, p1/3d0 - 1/4, fub/fu, 1.0)
• k1 = min(2.8e2/d0 - 1.7, 1.4p2/d0 - 1.7, 2.5) for edge bolts
• k1 = min(1.4*p2/d0 - 1.7, 2.5) for inner bolts
• d = bolt diameter, t = plate thickness, d0 = hole diameter

For an M20 bolt (d0 = 22 mm) in 10 mm S355 plate, e1 = 40 mm, e2 = 35 mm:

• alpha_b = min(40/66, fub/fu, 1.0) = 0.61
• k1 = min(2.8*35/22 - 1.7, 2.5) = 2.5
• Fb,Rd = 2.5 _ 0.61 _ 470 _ 20 _ 10 / 1.25 = 114.7 kN per bolt

Fillet Weld Design (Cl. 4.5.3)

The design resistance of a fillet weld per unit length:

Fw,Rd = fvw,d * a

where:

• a = throat thickness (leg length / sqrt(2) for equal-leg fillets)
• fvw,d = fu / (sqrt(3) _ beta_w _ gamma_M2)
• beta_w = correlation factor (0.80 for S235, 0.85 for S275, 0.90 for S355, 1.00 for S460)
• gamma_M2 = 1.25

For a 6 mm fillet weld on S355 plate:

• a = 6 / sqrt(2) = 4.24 mm
• fvw,d = 470 / (sqrt(3) _ 0.90 _ 1.25) = 241.4 MPa
• Fw,Rd = 241.4 * 4.24 = 1,024 N/mm = 1.02 kN/mm per mm of weld length

A 100 mm long 6 mm fillet weld on S355 resists approximately 102 kN — exceeding the capacity of an M20 Grade 8.8 bolt in single shear. This is why "match the weld to the bolt" is good practice: the weld rarely governs.

Simplified Method (Cl. 4.5.3.3)

The directional method (Annex A) gives higher capacities by resolving forces into components, but the simplified method above is conservative and easier for hand calculations.

Joint Classification (Cl. 5.2)

EN 1993-1-8 classifies joints by stiffness and strength:

For a nominally pinned connection (like a fin plate), the joint must have sufficient rotation capacity to accommodate the beam end rotation without developing significant moment.

Bolt Tension Resistance

For bolts in tension (Category D or E), the design tension resistance is:

Ft,Rd = k2 _ fub _ As / gamma_M2

where k2 = 0.63 for countersunk bolts, and k2 = 0.90 for all other bolts.

For an M20 Grade 8.8 bolt (As = 245 mm^2): Ft,Rd = 0.90 _ 800 _ 245 / 1.25 = 141.1 kN per bolt.

Preloaded bolts (Category E) provide additional stiffness and fatigue resistance but have the same static tension capacity as non-preloaded bolts. The preload force Fp,Cd = 0.7 _ fub _ As = 0.7 _ 800 _ 245 = 137.2 kN for an M20 Grade 8.8.

Combined Shear and Tension (Table 3.4)

When a bolt is subjected to both shear and tension, the interaction check is:

Fv,Ed / Fv,Rd + Ft,Ed / (1.4 * Ft,Rd) <= 1.0

For an M20 Grade 8.8 bolt with Fv,Ed = 50 kN and Ft,Ed = 40 kN:

• Fv,Rd = 94.1 kN, Ft,Rd = 141.1 kN
• 50/94.1 + 40/(1.4*141.1) = 0.531 + 0.203 = 0.734 < 1.0 → OK.

This is the common case in moment-resisting end plates where the top row of bolts carries both tension (from bending) and shear.

Block Tearing (Cl. 3.10.2)

Block tearing checks a group of bolt holes where a block of material may tear out. The design resistance is:

Veff,1,Rd = fu _ Ant / gamma_M2 + (1/sqrt(3)) _ fy * Anv / gamma_M0

for a concentrically loaded bolt group with a single row of bolts:

Where Ant is the net area in tension and Anv is the net area in shear. For the bolt group:

Veff,2,Rd = (1/sqrt(3)) _ fu _ Anv / gamma_M2 + fy * Ant / gamma_M0

The governing value is the smaller of Veff,1,Rd and Veff,2,Rd.

Block Tearing Worked Example

Fin plate connection, S275 plate 10 mm thick, single row of 3 M20 bolts (d0 = 22 mm), e1 = 40 mm, e2 = 35 mm, p1 = 60 mm:

• Ant = (35 - 22/2) _ 10 = (35 - 11) _ 10 = 240 mm^2
• Anv = (40 + 260 - 2.522) _ 10 = (40 + 120 - 55) _ 10 = 1,050 mm^2

Veff,1,Rd = 410 _ 240 / 1.25 + (1/1.732) _ 275 _ 1,050 / 1.00 = 78,720 + 166,600 = 245.3 kN Veff,2,Rd = (1/1.732) _ 410 _ 1,050 / 1.25 + 275 _ 240 / 1.00 = 198,800 + 66,000 = 264.8 kN

Block tearing resistance = min(245.3, 264.8) = 245.3 kN.

Check against the applied shear V_Ed = 150 kN: 150 < 245.3 → OK.

Prying Action in T-Stubs (Cl. 6.2.4)

In bolted end plates and T-stub connections, prying forces amplify the bolt tension. The effective T-stub method models the plate as a series of equivalent T-stubs in bending.

The design resistance of a T-stub flange is:

FT,Rd = min(FT,1-1,Rd, FT,2-2,Rd, FT,3-3,Rd)

where the three modes correspond to:

• Mode 1: Complete yielding of the flange (plastic hinge mechanism)
• Mode 2: Bolt failure with yielding of the flange
• Mode 3: Bolt failure alone (thick stiff flange)

For a thick end plate (tp >= 25 mm for M20 bolts in S275), Mode 3 typically governs and prying is negligible. For thin end plates (tp <= 12 mm), prying can increase bolt force by 50-100%. Always check the plate thickness against the minimum to avoid prying, or use the T-stub method to quantify it.

The minimum end plate thickness to avoid prying for M20 8.8 bolts is approximately:

tp,min = sqrt( 4 _ Ft,Rd _ m / (fy * beff) )

where m is the distance from the bolt centre to the 20% line (for yielding line near the web/flange), and beff is the effective width of the T-stub per bolt.

Long Joint Reduction (Cl. 3.8)

When the distance Lj between the centres of the end fasteners in a joint exceeds 15*d (d = bolt diameter), the design shear resistance Fv,Rd must be reduced by the factor:

beta_Lf = 1 - (Lj - 15d) / (200d), but 0.75 <= beta_Lf <= 1.0

For M20 bolts (d = 20 mm), the threshold is 15 * 20 = 300 mm. For a joint with Lj = 500 mm:

beta_Lf = 1 - (500 - 300) / (200*20) = 1 - (200/4000) = 1 - 0.05 = 0.95

The reduction is modest (5%) at 500 mm. At Lj = 1,000 mm: beta_Lf = 1 - (700/4000) = 0.825. Long splices and heavy bracing connections can trigger this reduction.

For preloaded bolts (Category B or C), the long joint reduction does NOT apply to the slip resistance — it only applies to the bearing resistance.

Weld Design — Additional Checks

Butt Weld Capacity (Cl. 4.7.1)

Full penetration butt welds are designed as the parent metal: the weld strength equals the weaker connected part strength, so no explicit weld check is required. Partial penetration butt welds are designed as deep penetration fillet welds.

Long Weld Reduction (Cl. 4.5.3.2(6))

For fillet welds longer than 150*a (a = throat thickness), the design resistance is reduced:

beta_Lw = 1.2 - 0.2 * Lj / (150*a), but 0.6 <= beta_Lw <= 1.0

For a 6 mm fillet weld (a = 4.24 mm), the threshold is 150 _ 4.24 = 636 mm. For a weld 1,500 mm long: beta_Lw = 1.2 - 0.2 _ 1500/636 = 1.2 - 0.472 = 0.728.

Directional Method (Annex A — Alternative)

The simplified method is conservative. Annex A's directional method resolves the applied force into components perpendicular to the weld throat (sigma_perp, tau_perp) and parallel (tau_para), then checks:

sqrt( sigma_perp^2 + 3(tau_perp^2 + tau_para^2) ) <= fu / (beta_w * gamma_M2)* AND sigma_perp <= 0.9 * fu / gamma_M2

This can give up to 25% higher capacity for welds at 45 degrees to the applied load. Use this for optimization but the simplified method is adequate for standard design.

Worked Example: Fin Plate Connection

Given: 457x191x67 UKB beam, S355, connected to column web via 10 mm S275 fin plate with 3 M20 Grade 8.8 bolts. V_Ed = 150 kN, single shear, threads in shear plane. d0 = 22 mm, e1 = 40 mm, e2 = 35 mm, p1 = 60 mm, p2 = 70 mm.

Step 1 — Bolt Shear: Fv,Rd = 0.6 _ 800 _ 245 / 1.25 = 94.1 kN per bolt per shear plane. Total = 3 * 94.1 = 282.3 kN. Utilisation: 150/282.3 = 0.53 → OK.

Step 2 — Bolt Bearing on Fin Plate (S275, 10 mm): alpha_b = min(40/66, 60/66-0.25, 800/410, 1.0) = min(0.606, 0.659, 1.95, 1.0) = 0.606 k1 = min(2.835/22 - 1.7, 1.470/22 - 1.7, 2.5) = min(2.75, 2.76, 2.5) = 2.5

Fb,Rd = 2.5 _ 0.606 _ 410 _ 20 _ 10 / 1.25 = 99.4 kN per bolt. Total = 3 * 99.4 = 298.2 kN. Utilisation: 150/298.2 = 0.50 → OK.

Step 3 — Bolt Bearing on Beam Web (S355, tw = 8.5 mm): alpha_b = min(40/66, 60/66-0.25, 800/470, 1.0) = min(0.606, 0.659, 1.70, 1.0) = 0.606 k1 = 2.5 (same geometry)

Fb,Rd = 2.5 _ 0.606 _ 470 _ 20 _ 8.5 / 1.25 = 96.9 kN per bolt. Total = 3 * 96.9 = 290.7 kN. Utilisation: 150/290.7 = 0.52 → OK.

Step 4 — Block Tearing of Beam Web: Ant = (35 - 22/2) _ 8.5 = 204 mm^2 Anv = (40 + 120 - 55) _ 8.5 = 893 mm^2

Veff,1,Rd = 470204/1.25 + (1/1.732)355893/1.00 = 76,704 + 182,800 = 259.5 kN Veff,2,Rd = (1/1.732)470893/1.25 + 355204/1.00 = 193,700 + 72,420 = 266.1 kN

Resistance = min(259.5, 266.1) = 259.5 kN. Utilisation: 150/259.5 = 0.58 → OK.

Step 5 — Fin Plate Weld (6 mm fillet to column): Weld length Lw = 220 mm (plate depth), E43XX electrode (fu = 430 MPa), S275 plate.

a = 6/sqrt(2) = 4.24 mm, fvw,d = 430/(sqrt(3) _ 0.85 _ 1.25) = 233.6 MPa

Fw,Rd per mm = 233.6 _ 4.24 = 0.99 kN/mm Total weld capacity = 0.99 _ 220 * 2 (two welds) = 435.6 kN. Utilisation: 150/435.6 = 0.34 → OK.

Summary — All checks pass. Bolt shear governs at 53% utilisation. The fin plate connection is adequate for 150 kN shear.

Joint Stiffness Classification — Application

The stiffness classification drives the global analysis model:

• Nominally pinned joints (Sj,ini < 0.5*EI/L): Analysis models the beam as simply supported. The connection must be detailed to allow rotation (e.g., fin plate with clearance holes, flexible end plate) without developing significant moment.
• Rigid joints (Sj,ini >= 25*EI/L): Analysis models the beam as fixed-ended. The connection must transfer moment (e.g., extended end plate, welded moment connection) without significant rotation.
• Semi-continuous joints: Require a second-order analysis that includes the joint moment-rotation characteristic explicitly.

A typical fin plate connection for a 6 m span 457x191x67 UKB has Sj,ini ~ 5,000 kN.m/rad, well below the rigid threshold of ~62,000 kN.m/rad. It is classified as nominally pinned — the global analysis may assume simple supports.

EN 1993-1-8 vs Other Codes — Connection Design

The EN 1993 approach to connections is the most prescriptive — bolt categories (A-E), explicit long joint reductions, the T-stub method for prying, and joint stiffness classification all have detailed code provisions. AISC relies more on the Manual (commentary) for specific connection types (shear tabs, end plates) while AS 4100 reference to both code and standardised connection designs (ASI Design Guide 1-4).

Related Pages

• EN 1993-1-1 Beam Design Guide — Full worked example: classification, Mc,Rd, LTB
• EN 1993 Column Buckling Guide — Curves a0-d, Perry-Robertson, Nb,Rd
• UK Steel Beam Sizes — UB, UC, PFC section tables
• Bolted Connections Calculator — Free EN 1993 bolt capacity calculator
• Welded Connections Calculator — Free EN 1993 weld capacity calculator

This page is for educational reference. All resistance formulae are per EN 1993-1-8:2005 + A1:2014. Verify against the applicable National Annex for your project jurisdiction. Connection design is a critical life-safety element — always have a qualified structural engineer review the final design. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.