Column Splice Types per EN 1993-1-8

BS EN 1993-1-8 Clause 6.2.7.1(14) distinguishes two principal column splice arrangements:

Type 1 — Bearing splice (direct contact): The column ends are machined flat and bear directly against each other. Compressive axial load transfers through direct bearing between the prepared ends. The splice plates and bolts serve three purposes:

• Provide continuity for any tension (uplift or reversal)
• Provide nominal bending continuity
• Restrain the column laterally at the splice location

The ends must be machined or ground to achieve full contact. BS EN 1090-2 requires a bearing surface flatness tolerance of 0.25 mm over the contact area. A division plate (end plate) is sometimes inserted between the column ends to distribute load evenly.

Type 2 — Non-bearing splice (flange and web cover plates): The column sections are separated by a gap and connected through bolted flange and web cover plates. All forces (axial, moment, shear) transfer through the bolts and cover plates. This type is used when:

• Machining column ends is not practical or economical
• The splice must develop significant bending moment
• A section change occurs at the splice location (upper column is smaller than lower column)

Non-bearing splices are more common in UK practice for typical multi-storey braced frames where column sections reduce every 2-3 storeys.

Splice Capacity — Axial Load

Bearing splice: The compressive resistance is governed by the bearing area of the column ends. For a machined-end splice with full contact: [ N*{c,Rd} = \frac{A \times f_y}{\gamma*{M0}} ]

The splice plates and bolts carry only the tension component and nominal moment. Per BS EN 1993-1-8 Clause 6.2.7.1(14), the splice must be capable of transferring a nominal moment of at least 25% of the column plastic moment capacity in both axes: [ M*{nom} \ge 0.25 \times M*{pl,Rd} ]

Where (M*{pl,Rd} = W*{pl} fy / \gamma{M0}). This nominal moment accounts for accidental eccentricity and frame imperfections. For columns in simple construction (nominally pinned connections), the 25% rule can be reduced to a practical minimum of 50 kN.m for most UC sections.

Non-bearing splice: The full axial force transfers through the flange cover plates and bolts. For a concentrically loaded column, the flange cover plates carry the axial force in proportion to their area relative to the total column area.

Splice Capacity — Moment Transfer

For the 25% nominal moment requirement from Clause 6.2.7.1(14), the bolt group in the flange cover plates must resist the flange force from the nominal moment:

[ N*{f} = \frac{M*{nom}}{h - tf} \pm \frac{N{Ed} \times A_f}{A} ]

The plus sign applies to the compression flange; the minus sign to the tension flange. For bearing splices, the compression flange force from axial load is transferred through direct bearing, and the bolts carry only the moment-induced forces and any net tension.

Bolt group capacity check: The bolt group is designed per EN 1993-1-8 Clause 3.6 for shear and bearing on the flange cover plates. Each bolt must satisfy: [ F*{v,Ed} \le F*{v,Rd} \quad \text{and} \quad F*{v,Ed} \le F*{b,Rd} ]

For preloaded connections (Category C — slip-resistant at ULS), add: [ F*{s,Rd} = \frac{k_s n \mu}{\gamma*{M3}} F_{p,C} ]

Where:

• (k_s = 1.0) for normal holes
• (n = 1) (single shear plane per bolt)
• (\mu = 0.5) for Class A surface (grit blast)
• (\gamma_{M3} = 1.25) (UK NA)
• (F*{p,C} = 0.7 f*{ub} A_s) (preload force per Clause 3.9)

Worked Example — UKC 254x254x107 Column Splice

Problem: Design a bearing-type column splice at the 3rd-to-4th storey level for a UKC 254x254x107 column in S355. The splice must transfer (N_{Ed} = 2,400) kN (compression) plus the nominal 25% bending moment. Column ends are machined for full bearing contact.

Section properties — UKC 254x254x107: (h = 266.7) mm, (bf = 258.8) mm, (t_f = 20.5) mm, (t_w = 12.8) mm, (r = 12.7) mm (A = 136) cm(^2) = 13,600 mm(^2) (W{pl,y} = 1,480) cm(^3) = 1,480 (\times 10^3) mm(^3) (f_y = 355) MPa (flange (t_f > 16) mm but (\le 40) mm) (A_f = 258.8 \times 20.5 = 5,305) mm(^2)

Step 1 — Axial bearing check (machined ends): (N_{c,Rd} = 13,600 \times 355 / 1.00 = 4,828) kN (> 2,400) kN. OK. Bearing at the column ends governs. The splice plates and bolts carry only the 25% nominal moment and incidental tension.

Step 2 — Nominal moment (25% of Mpl,Rd): (M*{pl,Rd} = 1,480 \times 10^3 \times 355 / 1.00 = 525.4) kN.m (M*{nom} = 0.25 \times 525.4 = 131.4) kN.m

Step 3 — Flange force from nominal moment: Lever arm (= h - tf = 266.7 - 20.5 = 246.2) mm (N_f = M{nom} / 246.2 = 131.4 \times 10^6 / 246.2 = 534) kN per flange

Step 4 — Flange cover plates: Required plate area: (A_f = 5,305) mm(^2) Use 260 x 20 mm plate in S355: (A_p = 260 \times 20 = 5,200) mm(^2) (98% of (A_f) — acceptable per SCI P398 for bearing splices where direct bearing carries axial load). Alternatively use 260 x 22 mm for full area match.

Step 5 — Flange bolts (M24 Grade 8.8, threads excluded): (F_{v,Rd} = 0.6 \times 800 \times 353 / 1.25 = 135.6) kN per bolt (single shear) Number required (= 534 / 135.6 = 3.9 \rightarrow) use 4 bolts per flange per side (2 rows x 2 columns). Pitch = 80 mm, end distance (e_1 = 45) mm, edge distance (e_2 = 45) mm.

Step 6 — Bearing check on 20 mm cover plate (M24, hole (d_0 = 26) mm): (\alphad = e_1 / (3 d_0) = 45 / 78 = 0.577) (\alpha_b = \min(0.577, 800/470, 1.0) = 0.577) Assume adequate edge distance: (k_1 = \min(2.8 e_2 / d_0 - 1.7, 1.4 p_2 / d_0 - 1.7, 2.5) = 2.5) (F{b,Rd} = 2.5 \times 0.577 \times 470 \times 24 \times 20 / 1.25 = 260.4) kN (> 135.6) kN. OK.

Step 7 — Web splice plates: Web splice plates provide lateral restraint and carry any incidental shear. Use 2 x 10 mm plates (S275, each side of web), 4-M20 Grade 8.8 bolts per side. Web plate dimensions: 180 mm wide x 215 mm deep (to fit within the root radius).

Final specification for UKC 254x254x107 splice:

• Column ends machined flat for full bearing contact
• Flange cover plates: 4 x 260 x 20 mm in S355 (one inside, one outside each flange)
• Flange bolts: 8-M24 Grade 8.8 per flange per side (2 rows x 4 columns) — or 4-M24 per flange per side (2x2) for non-seismic applications
• Web cover plates: 2 x 10 mm in S275
• Web bolts: 4-M20 Grade 8.8 per side
• Nominal moment capacity: 0.25 (M_{pl,Rd}) satisfied per Clause 6.2.7.1(14)

Column Splice Bolt Sizing Table

Recommended bolt configurations for common UKC sections with bearing-type splices (machined ends). Values satisfy the 25% nominal moment requirement. All bolts Grade 8.8, threads excluded.

All values are preliminary. Cover plate width typically equals flange width plus 2 mm clearance. Bolts are Grade 8.8 in standard clearance holes ((\le 2) mm oversize). Verify end distances and bearing per EN 1993-1-8 Table 3.3.

Buckling at Splice Location — BS EN 1993-1-1 Clause 6.3

The column buckling check at the splice location must consider whether the splice provides full continuity of stiffness. BS EN 1993-1-1 Clause 6.3 requires:

1. Flexural buckling: The splice must not introduce a pin or partial restraint that reduces the effective length. For bearing splices with machined ends and adequate flange cover plates (minimum 4 bolts per flange), full flexural continuity is assumed. The column effective length factor (k) is unchanged through the splice.

2. Torsional-flexural buckling: For column sections where torsional-flexural buckling governs (typically UC sections in S355 with relatively thick flanges), the splice cover plates must provide sufficient warping restraint. The standard detail of 2 rows x 2 columns of bolts with full-depth flange cover plates satisfies this requirement for all standard UKC sections.

3. Section change at splice: Where the column section reduces at the splice (common in multi-storey construction), the buckling check uses the smaller section properties above the splice. The splice itself must develop the yield capacity of the smaller section. The bearing splice with machined ends achieves this directly through the machined contact area.

Erection Considerations

Temporary stability: During erection, the upper column must be temporarily stabilised before bolting is complete. Use erection cleats or guy lines. Per BS EN 1090-2, no more than 4 storeys of unbolted column splices may be left unsecured in any one erection sequence.

Division plate: For bearing splices, a 6-10 mm division plate (packing plate) is sometimes inserted between the machined column ends to accommodate fabrication tolerances. The division plate should be the same steel grade as the column and project 5 mm beyond the column footprint on all sides for visual alignment verification.

Seismic detailing: For columns in UK structures designed for seismic resistance (rare outside London and major city centres), the splice must develop the full plastic capacity of the smaller column section, including strain hardening. Consult BS EN 1998-1 for seismic-specific splice requirements.

Frequently Asked Questions

When should a UK column splice be preloaded (Category B or C)?

Preloaded column splices are required when: (a) the structure is subject to fatigue loading (e.g., crane columns, bridge columns), (b) stress reversal occurs at the splice (wind governed multi-storey), or (c) slip at the splice would affect the global frame stiffness (unbraced frames). For typical UK braced multi-storey frames in simple construction, non-preloaded bearing splices (Category A — bearing type) are standard and economical.

How is the 25% nominal moment applied in practice?

EN 1993-1-8 Clause 6.2.7.1(14) requires the splice to resist a nominal moment of 25% of the column plastic moment capacity. This is applied independently in both major and minor axes. The moment is not combined with the full axial load — it is a robustness check. Apply the 25% moment separately to each axis bolt group and check the bolts for the combined axial-plus-moment force. The 25% rule derives from frame imperfection effects (initial sway (\phi = 1/200)) and accidental eccentricity of loading.

What is the minimum number of bolts in a column splice per UK practice?

BS EN 1993-1-8 does not specify a minimum bolt count for column splices. UK industry practice (SCI P398) recommends a minimum of 4 bolts per flange per side (2 rows x 2 columns) for all column splices. This provides redundancy, rotational restraint, and resistance to accidental loads during construction. For columns with (b_f < 200) mm where 4-M24 bolts may not fit with adequate spacing, M20 bolts are acceptable with verification of the reduced capacity. For lightly loaded columns (domestic or low-rise), 4-M20 bolts per flange is a practical minimum.

Reference only. Verify all values against the current edition of BS EN 1993-1-8:2005 Clause 6.2.7.1(14), BS EN 1993-1-1:2005 Clause 6.3, and UK NA. This information does not constitute professional engineering advice. All structural designs must be independently verified by a Chartered Structural Engineer (MIStructE or MICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION.