When Beam Splices Are Required

A beam splice is necessary when:

1. Transport limits: UK road transport restricts single lengths to approximately 13.5 m on standard articulated trailers. Beams longer than this require a field splice.
2. Erection constraints: Crane capacity or site logistics may limit individual lift weights. A 15 m UKB 610x229x140 weighs approximately 2.1 tonnes.
3. Section change: The beam depth or mass may reduce in regions of lower bending moment, with a splice providing the transition.

BS EN 1993-1-8 Clause 6.2.7.2 requires the splice to develop the full forces at the splice location from the governing ULS load combination. The splice must not be the weak link in the load path.

Design Principles — Flange and Web Splice Plates

The bending moment is resisted by a force couple in the flange cover plates. The shear force is resisted by web cover plates. This separation of actions is the standard UK design assumption.

Flange force from bending moment:

[ Nf = \frac{M{Ed}}{h - t_f} ]

Where (h) is the section depth and (t_f) is the flange thickness. The lever arm (h - t_f) is the distance between flange centroids.

Flange cover plate sizing: The gross cross-sectional area of the cover plate must equal or exceed the beam flange area: [ A_p \ge A_f = b_f \times t_f ]

The cover plate width is typically equal to the beam flange width for visual uniformity. Plate thickness is then: [ t_p \ge \frac{A_f}{b_p} ]

Cover plate steel grade should match or exceed the beam steel grade. For S355 beams, use S355 plate (BS EN 10025-2).

Bolt Design — EN 1993-1-8 Clause 3.6.1

Bolts in flange cover plates are loaded in single shear (one shear plane per bolt at the flange-to-cover-plate interface). Web cover plates typically provide double shear (two shear planes).

Bolt shear resistance (Clause 3.6.1, Table 3.4):

[ F*{v,Rd} = \frac{\alpha_v f*{ub} A}{\gamma_{M2}} ]

Where:

• (\alpha_v = 0.6) for grades 4.6, 5.6, 8.8; (\alpha_v = 0.5) for grade 10.9
• (f_{ub}) = bolt ultimate tensile strength (800 MPa for 8.8, 1000 MPa for 10.9)
• (A) = tensile stress area (A_s) if threads in shear plane; gross area (A) if threads excluded
• (\gamma_{M2} = 1.25) (UK NA)

Bolt bearing resistance (Table 3.4):

[ F*{b,Rd} = \frac{k_1 \alpha_b f_u d t}{\gamma*{M2}} ]

Where (\alphab = \min(\frac{e_1}{3d_0}, \frac{p_1}{3d_0} - \frac{1}{4}, \frac{f{ub}}{f_u}, 1.0)) and (k_1) accounts for edge distance perpendicular to load.

Number of bolts per flange (each side of splice):

[ n \ge \frac{Nf}{F{v,Rd}} ]

A minimum of 4 bolts per flange per side (2 rows x 2 columns) is recommended in UK practice for redundancy and rotational restraint.

Web Plate Splice — Shear Transfer

The web cover plates (one each side of the web, or a single thicker plate) transfer the design shear (V_{Ed}) through bolts in double shear.

Web cover plate thickness: Per SCI P398, each web cover plate should be at least the beam web thickness (t_w). Where two plates are used (one each side), each should be at least (t_w/2) but never less than 6 mm for practical fabrication.

Bolts in the web splice: Bolts through the web cover plates are in double shear (two shear planes). For M20 Grade 8.8 bolts with threads excluded: [ F_{v,Rd} = \frac{0.6 \times 800 \times 245 \times 2}{1.25} = 188.2 \text{ kN per bolt} ]

Worked Example — UKB 533x210x92 Beam Splice

Problem: Design a bolted beam splice at approximately one-third span for a UKB 533x210x92 beam in S355. The splice must transfer (M*{Ed} = 950) kN.m and (V*{Ed} = 280) kN.

Section properties — UKB 533x210x92: (h = 533.1) mm, (b_f = 209.3) mm, (t_f = 15.6) mm, (t_w = 10.1) mm, (r = 12.7) mm (A_f = 209.3 \times 15.6 = 3,265) mm(^2) Steel grade: S355JR, (f_y = 355) MPa (flange (t_f \le 16) mm)

Step 1 — Flange force: Lever arm (= h - tf = 533.1 - 15.6 = 517.5) mm (N_f = M{Ed} / 517.5 = 950 \times 10^6 / 517.5 = 1,836) kN per flange

Step 2 — Flange cover plates: Required area (= 3,265) mm(^2) Use 210 x 16 mm plate in S355: (A_p = 210 \times 16 = 3,360) mm(^2 > 3,265) mm(^2). OK.

Step 3 — Flange bolts (M24 Grade 8.8, threads excluded from shear plane): (F_{v,Rd} = 0.6 \times 800 \times 353 / 1.25 = 135.6) kN per bolt (single shear) Number required (= 1,836 / 135.6 = 13.5 \rightarrow) use 14 bolts per flange per side. Arrange in 2 rows of 7 bolts at 70 mm pitch ((> 2.2 \times 24 = 52.8) mm minimum per Table 3.3).

Step 4 — Bolt bearing check on 16 mm cover plate: Assume end distance (e1 = 40) mm, pitch (p_1 = 70) mm, hole diameter (d_0 = 26) mm. (\alpha_b = \min(40/(3 \times 26), 70/(3 \times 26) - 0.25, 800/470, 1.0) = \min(0.513, 0.647, 1.70, 1.0) = 0.513) Assume (k_1 = 2.5) (adequate edge distance perpendicular to load). (F{b,Rd} = 2.5 \times 0.513 \times 470 \times 24 \times 16 / 1.25 = 185.3) kN (> 135.6) kN. OK.

Step 5 — Net section check (critical row with 2 holes): Net width (= 210 - 2 \times 26 = 158) mm (A*{net} = 16 \times 158 = 2,528) mm(^2) (N*{u,Rd} = 0.9 \times A*{net} \times f_u / \gamma*{M2} = 0.9 \times 2,528 \times 470 / 1.25 = 855) kN Flange force per bolt row (7 rows, 2 bolts in critical row): (1,836 \times 2 / 14 = 262) kN (< 855) kN. OK.

Step 6 — Web splice: (V*{Ed} = 280) kN. Use 2 x 8 mm web cover plates (S275, each side). M20 Grade 8.8 bolts in double shear (threads excluded): (F*{v,Rd} = 188.2) kN. Number required (= 280 / 188.2 = 1.5 \rightarrow) use 4 bolts per side (2 columns x 2 rows). Bolt spacing: 60 mm pitch, 8 mm plate thickness checks OK by bearing capacity verification.

Final specification:

• Flange cover plates: 2 x 210 x 16 mm in S355 (top and bottom)
• Flange bolts: 14-M24 Grade 8.8 per flange per side (2 rows x 7), threads excluded
• Web cover plates: 2 x 8 mm in S275 (each side)
• Web bolts: 4-M20 Grade 8.8 per side (2 rows x 2 columns)
• All bolts to be tightened to snug-tight minimum. Preloaded (Category B/C) if fatigue governs.

Simplified Capacity Table — UKB Beam Splices

Pre-calculated flange splices for common UKB sections. Values assume S355 cover plates, M24 Grade 8.8 bolts (threads excluded), and splice at approximately one-third span. Verify all values for your specific project.

Based on S355 steel, M24 Grade 8.8 bolts with threads excluded from shear plane. Cover plate width = flange width. Web splice designed separately for shear (V{Ed}). All values are preliminary — verify per project-specific loads and geometry._

Construction Considerations

Splice location: Position field splices at approximately one-quarter to one-third span where bending moments are lower than at mid-span. For continuous beams, splices near points of contraflexure minimise the moment to be transferred.

Bolt tightening: Snug-tight as a minimum. Specify preloaded bolts (Category B or C per EN 1993-1-8 Clause 3.4) for: fatigue loading, stress reversal, or where bolt slip would affect structural performance. Preload to (F*{p,Cd} = 0.7 f*{ub} A_s) per Clause 3.9.

Erection alignment: Provide a temporary erection cleat or backing bar to align the beam sections before bolting. The cleat can be shop-welded to one beam and field-bolted to the other.

Surface preparation: Contact surfaces within the bolt group should be clean and free of paint, oil, and mill scale. For preloaded slip-resistant connections, specify surface preparation Class A (grit blast to Sa 2.5 per BS EN ISO 8501-1) or equivalent.

Frequently Asked Questions

When should a UK beam splice be welded rather than bolted?

Welded splices are used for shop-fabricated connections where access, weather, and inspection are not constraints. For field splices, bolted connections are strongly preferred in UK practice because they are faster to erect, require less skilled site labour, and are easier to inspect. BS EN 1090-2 requires full inspection and procedure qualification for site welding, which adds cost and programme time. Bolted splices are the default for UK field connections.

What governs the number of bolts in a flange splice — shear or bearing?

For typical UK splice geometries with standard end distances ((e_1 \ge 1.2d_0)), bolt shear typically governs for M24 Grade 8.8 bolts in S355 plate. Bearing governs when edge distances are tight (less than (1.5d_0)), when plate thickness is less than (0.6d), or when higher-grade bolts (10.9) are used. Always check both failure modes; the lower capacity governs.

How do you design a beam splice for a section with unequal flanges?

For asymmetric sections (e.g., plate girders with a larger bottom flange), design each flange splice independently for the force in that flange. Use the actual lever arm between flange centroids. The difference between top and bottom flange forces generates a small axial force in the web splice plate, which must be included in the web bolt group design. This is uncommon for standard rolled UKB sections but relevant for fabricated plate girders.

Reference only. Verify all values against the current edition of BS EN 1993-1-8:2005 Clause 6.2.7.2 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.