The Component Method — EN 1993-1-8 Clause 6.3

The component method decomposes a joint into basic components, each characterised by a force-deformation curve (initial stiffness k_i, design resistance F_Rd, and deformation capacity delta_Cd). The joint rotational stiffness S_j and moment resistance M_j,Rd are assembled from:

Tension zone components: Column flange in bending, end plate in bending, bolts in tension, column web in transverse tension.

Compression zone components: Column web in transverse compression, beam flange in compression.

Shear zone component: Column web panel in shear.

The initial rotational stiffness is:

S_j,ini = E * z^2 / (sum(1/k_i))

Where z is the lever arm between the compression centre and the tension bolt row centroid, and k_i are the stiffness coefficients from EN 1993-1-8 Table 6.11.

Column Web Panel in Shear — EN 1993-1-8 Clause 6.2.6.1

The shear resistance of the unstiffened column web panel:

V_wp,Rd = 0.9 * f_y,wc * A_vc / (sqrt(3) * gamma_M0)

Where Avc = A_c - 2 * bc * t_fc + (t_wc + 2r_c) * t_fc for a rolled I/H section.

For a 203x203x52 UC (S355, A*vc = 2458 mm^2): V_wp,Rd = 0.9 * 355 _ 2458 / (1.732 * 1.00) / 1000 = 454 kN.

If supplementary web plates are added, increase Avc by b_s * ts * n (n = number of plates, typically 1 or 2). A single 10 mm supplementary web plate on a 203x203x52 UC column increases A_vc by approximately 203 * 10 = 2030 mm^2, roughly doubling the shear resistance.

Column Web in Transverse Compression — EN 1993-1-8 Clause 6.2.6.2

F_c,wc,Rd = omega * k_wc * b_eff,c,wc * t_wc * f_y,wc / gamma_M0

Where b_eff,c,wc = t_fb + 2*sqrt(2) * a_p + 5(t_fc + s) + s_p (for end plate connections).

For a 356x171x45 UB beam (t_fb = 9.7 mm) on a 203x203x52 UC column (t_wc = 7.9, t_fc = 12.5): b_eff,c,wc = 9.7 + 2.83 * 6 + 5(12.5 + 10.2) + 6 = 9.7 + 17 + 113.5 + 6 = 146.2 mm.

With omega = 1.0 (no interaction with shear for lower utilisation): F*c,wc,Rd = 1.0 * 1.0 _ 146.2 _ 7.9 _ 355 / (1.00 * 1000) = 410 kN.

Key insight: The web in compression rarely governs for UK/European beam-to-column joints with typical section proportions. The column web panel in shear is usually the critical component, followed by the tension bolt row.

Column Web in Transverse Tension — EN 1993-1-8 Clause 6.2.6.3

F_t,wc,Rd = omega * b_eff,t,wc * t_wc * f_y,wc / gamma_M0

Where b_eff,t,wc is the effective width of the column web in tension, taken as the effective length of the equivalent T-stub representing the column flange (for a bolted connection). This component only governs when the column flange is thick relative to the web — typically when t_fc / t_wc > 2.0. For standard UC sections (t_fc / t_wc typically 1.3-1.8), web tension rarely governs.

Column Flange in Bending — Equivalent T-Stub per EN 1993-1-8 Clause 6.2.4

The column flange and end plate are both modelled as equivalent T-stubs. Three failure modes are checked:

Mode 1: Complete yielding of the flange (ductile, no bolt failure). Mode 2: Bolt failure with flange yielding (mixed mode). Mode 3: Bolt failure (brittle — design should avoid this mode governing without prying).

The T-stub resistance is the minimum of the three modes:

Where m is the distance from the bolt centre to the plastic hinge at the web-to-flange junction, and n = min(e, 1.25m) where e is the edge distance.

Worked Example 1 — Bolted Extended End Plate Joint

Problem: Design a bolted extended end plate moment connection between 356x171x45 UB beam (S355) and 203x203x52 UC column (S355). Design moment M_Ed = 110 kN*m, design shear V_Ed = 160 kN.

Step 1 — End Plate and Bolt Layout:

Extended end plate 480 x 160 x 15 mm, S275. Bolts: 2 rows of M20 Grade 8.8 in the tension zone, with 25 mm below the top flange and 85 mm between bolt rows. Bolt gauge 110 mm. Two additional M20 bolts in the compression zone (below bottom flange) for shear.

Step 2 — Tension Resistance per Bolt Row:

Row 1 (above top flange, stiffened by beam flange -- no prying considered here): FT,Rd = min(2 * Mpl,1,Rd / m, n * F_t,Rd) where n = number of bolts in row.

Mpl,1,Rd for 15 mm plate S275: 15^2 * 275 / (4 _ 1.00) = 15,469 Nmm/mm. Effective length l_eff of T-stub (circular pattern for bolt row outside tension flange): l_eff = min(2*pi*m, 4m+1.25e).

m = (110 - 8.5) / 2 - 0.8 _ 6 _ sqrt(2) = 50.75 - 6.8 = 44 mm, e = 35 mm. l_eff = min(276.5, 219.8) = 219.8 mm.

M_pl,Rd for T-stub = 219.8 * 15,469 = 3,400,000 N*mm = 3.40 kN*m.

Mode 1: FT,1,Rd = 4 * 3.40 _ 10^6 / 44 / 1000 = 309 kN. Mode 3 (bolt failure): 2 _ Ft,Rd = 2 * 0.9 _ 800 _ 245 / 1.25 / 1000 = 282 kN.

Tension resistance per row = 282 kN (bolt-limited -- Mode 3).

Step 3 — Moment Resistance Assembly:

Lever arm z = distance from compression centre (mid-thickness of beam compression flange) to each bolt row.

Row 1 (top bolts): z1 = (480/2) + (9.7/2) = 240 + 4.9 = 244.9 mm. Row 2: z2 = 244.9 - 85 = 159.9 mm.

Mj,Rd = sum(F_T,Rd,i * zi) = 282 * 0.245 + 282 * 0.160 = 69.1 + 45.1 = 114.2 kN*m.

M_Ed = 110 kNm < 114.2 kNm. OK (96% utilisation).

Step 4 — Column Web Panel Shear Check:

V_wp,Ed = M_Ed / z_eq - V_Ed ~ M_Ed / (0.5 * (z1 + z2)) - V_Ed.

V_wp,Ed ~ 110 / 0.202 - 160 = 545 - 160 = 385 kN < 454 kN. OK (85% utilisation).

Selected: 480x160x15 mm S275 end plate, 4+2 M20 Grade 8.8 bolts, 6 mm FW all round.

Worked Example 2 — Flush End Plate (Nominally Pinned)

Problem: A beam-to-column connection using a flush end plate is intended as a nominally pinned joint. Beam = 305x165x40 UB (S355), Column = 254x254x73 UC (S355). Design shear V_Ed = 120 kN. Verify the connection can be designed as nominally pinned per EN 1993-1-8 Clause 5.2.2.

Step 1 — Connection Detailing:

Flush end plate 260 x 160 x 10 mm, S275. Welded to beam web by 2 x 6 mm fillet welds. Bolts: 2 columns x 3 rows M20 Grade 8.8 at 70 mm pitch, 90 mm gauge, 40 mm end distance. Beam sits between column flanges; end plate bolted to column flange.

Step 2 — Verify Nominally Pinned Classification:

Per EN 1993-1-8 Clause 5.2.2, a connection may be classified as nominally pinned if S*j,ini <= 0.5 * E _ I_b / L_b (where L_b is the beam span). For a 6 m span beam:

Sj,ini,max = 0.5 * 210,000 _ 85.010^6 / 6000 = 1.49 * 10^9 N*mm/rad.

The initial stiffness of a flush end plate connection is typically 5-15% of this value for typical UK sections -- nominally pinned classification is readily satisfied.

Step 3 — Shear and Bearing (same method as EN 1993-1-8 Table 3.4):

Bolt shear (single shear, threads in plane): F_v,Rd per bolt = 47.0 kN. 6 bolts: V_Rd = 6 * 47.0 = 282 kN > 120 kN -- OK (43% utilisation).

Bearing on end plate (10 mm S275, fu = 410 MPa): alpha_b = min(40/(322), 70/(322)-0.25, 800/410, 1.0) = min(0.606, 0.811, 1.951, 1.0) = 0.606. k1 = min(2.835/22-1.7, 1.4*70/22-1.7, 2.5) = min(2.75, 2.75, 2.5) = 2.5. F_b,Rd = 2.5 * 0.606 _ 410 _ 20 _ 10 / 1.25 = 99.4 kN per bolt. Total V_Rd = 6 * 99.4 = 596 kN >> 120 kN -- OK.

Step 4 — Vertical Shear on End Plate (gross section):

Vpl,Rd = A_v * fy / (sqrt(3) * gamma*M0) = (260 * 10) _ 275 / (1.732 * 1.00) / 1000 = 413 kN > 120 kN -- OK.

Selected: 260x160x10 mm S275 flush end plate, 6-M20 Grade 8.8 bolts, 6 mm FW to beam web. Nominally pinned -- all good.

Joint Classification — EN 1993-1-8 Clause 5.2

Joints are classified by stiffness into three categories:

Classification Criterion Global Analysis Method
Rigid S_j,ini >= 25 E I_b / L_b (braced frames) Continuous (full moment transfer)
Semi-rigid Between rigid and nominally pinned Continuous with joint spring modelling
Nominally pinned S_j,ini <= 0.5 E I_b / L_b Simple (pin-ended)

Most flush end plate and fin plate connections in UK/European practice fall into the nominally pinned category. Extended end plates typically achieve semi-rigid classification, and can approach rigid for deep beams with short spans.

Tying Resistance — EN 1991-1-7 Structural Integrity

For structural integrity (progressive collapse prevention), beam-to-column joints must have minimum tying resistance:

F_tie = 75 kN (Class 2A buildings, lower group) or F_tie = 150 kN (Class 2B buildings, upper group).

The tying resistance of a flush end plate connection is the minimum of:

For the 6-M20 connection in Example 2: F_tie = 6 * 141.1 = 846.6 kN >> 75/150 kN -- easily adequate.

Frequently Asked Questions

What is the component method in EN 1993-1-8? The component method is a mechanical model that decomposes a joint into individual components (column web in shear, compression, tension; column flange in bending; end plate in bending; bolts in tension). Each component is assigned a force-deformation curve, and these are assembled to determine the joint's rotational stiffness and moment resistance. For a typical bolted end plate joint, 6-8 components are checked. The component method replaces the empirical joint models used in older codes (like BS 5950) with a physics-based approach.

Which component usually governs beam-to-column joint design? For UK/European practice with UC columns and UB beams: the column web panel in shear governs in 50-60% of cases for un-stiffened joints. The tension bolt row (end plate + column flange + bolts) governs in the remaining cases. Column web in compression almost never governs for standard beam-column proportions because the load-spreading effective width b_eff is generous. If supplementary web plates are added to the column web panel, the tension zone becomes the governing component.

When are column web stiffeners required? Supplementary web plates (transverse stiffeners) are required when: (a) V_wp,Ed exceeds the bare column web panel shear resistance; (b) F_c,wc,Ed exceeds the bare column web in compression resistance; or (c) to improve the joint classification from nominally pinned to semi-rigid/rigid by increasing the panel zone stiffness. Diagonal stiffeners (N-type or K-type) are occasionally used for heavily loaded beam-to-column joints in multi-storey frames, particularly at the ground-to-first-floor transfer level.

How does the component method differ from AISC design? AISC 360 uses a separate approach: column flange and end plate are checked for prying action (Part 9), the column web panel zone is checked for shear (Chapter J10), and stiffener requirements are derived from local flange bending and web yielding/crippling checks. The EN 1993-1-8 component method unifies all these checks into a single computational framework with consistent force-deformation relationships. The EN approach is more systematic but also more computationally intensive -- practical design relies heavily on standardised connections (SCI P398 Green Book for the UK).

Related Pages

Educational reference only. Joint design per EN 1993-1-8:2005 component method. Verify against SCI P398 and current Eurocodes. Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent Chartered Engineer verification.