The choice between bolted and welded connections is one of the most consequential decisions in structural steel design. Both methods can achieve the same structural capacity, but they differ fundamentally in behaviour, fabrication, and inspection. This guide covers the practical engineering considerations that drive the decision.
Fundamental Differences
A bolted connection transfers load through mechanical bearing and friction via a fastener. A welded connection fuses parent metals together using heat, creating a metallurgical bond. Both are addressed in EN 1993-1-8 (Design of Joints) for Eurocode 3.
| Property | Bolted | Welded |
|---|---|---|
| Ductility | High — slip and bearing provide ductility | Can be brittle if weld quality or detailing is poor |
| Stiffness | Lower (slip in holes); preloaded bolts improve this | Very high — effectively rigid at the joint |
| Site erection | Fast and safe — no hot work, no fire risk | Requires certified welders, preheat, weather protection |
| Inspection | Visual + torque wrench; straightforward | Requires NDT (UT, MPI, radiography); specialist needed |
| Fatigue | Generally better — less stress concentration | Weld quality class determines fatigue life per EN 1993-1-9 |
| Disassembly | Reversible — can be dismantled and reused | Permanent — requires cutting to separate |
| Appearance | Bolt heads visible; less clean finish | Can be ground flush — cleaner, architectural finish |
| Cost model | Higher material cost; lower skilled labour | Lower material cost; higher skilled labour and QC cost |
Bolted Connection Design (EN 1993-1-8)
Bolted connections transfer force through bearing (shank contact with hole edge) or friction (preloaded bolt clamping plates). The choice affects capacity and deformation behaviour significantly.
Shear Resistance per Bolt
αv = 0.6 for grades 4.6, 5.6, 8.8; αv = 0.5 for grade 10.9. γM2 = 1.25. A is the tensile stress area As when threads intercept the shear plane, or the gross shank area otherwise. Use the bolt calculator for any grade and size.
Bearing Resistance
Where αb = min(e1/3d0, p1/3d0 − 1/4, fub/fu, 1.0) and k1 = min(2.8e2/d0 − 1.7, 2.5). End and edge distances must satisfy EN 1993-1-8 Table 3.3 minimum spacing.
Preloaded Bolts (Slip-Resistant)
Grade 8.8 and 10.9 bolts can be preloaded to the design preload:
Slip resistance: Fs,Rd = ks · n · μ · Fp,C / γM3 where μ = 0.2–0.5 depending on surface preparation and γM3 = 1.25 at ULS.
Welded Connection Design
Fillet Weld Capacity (Simplified Method)
βw correlation factor: 0.80 (S235), 0.90 (S275), 1.00 (S355, S420, S460). a is the throat thickness, lw is the effective length. Minimum weld size is governed by the thicker plate being joined and the preheat requirements of EN 1011-2.
Weld Quality Classes (EN ISO 5817)
Quality is classified B (stringent), C (intermediate), or D (moderate). For fatigue-critical applications, class B is typically required and the fatigue detail category from EN 1993-1-9 Table B.1 must be checked. Always specify the required quality class on drawings and the inspection plan.
Practical Decision Guide
Choose Bolted When…
- Connections are made on site (field bolting is safer than field welding)
- The structure must be disassembled or relocated
- Fatigue governs — bolted details often have better fatigue category
- Quality control is difficult — bolts are far easier to inspect
- The contractor lacks a certified welding procedure
- Modular or pre-engineered construction is used
Choose Welded When…
- Maximum stiffness and rigidity is needed (moment frames)
- Aesthetics matter — no visible bolt heads on exposed steelwork
- Section size is small and bolt layout would be impractical
- Full-penetration butt welds can develop the full section capacity
- Connections are made in a fabrication shop under controlled conditions
- Hollow sections (CHS/SHS/RHS) are used — bolting through tubes is impractical
Combining Bolts and Welds
Shop-welded connection plates with site-bolted field connections are common and efficient. However, EN 1993-1-8 Clause 2.5 states that non-preloaded bolts and welds should not be assumed to share load in the same connection plane. Preloaded (Category C) bolts may be combined with welds if long-term deformation is checked. Violating this rule is a common and dangerous design error.
Inspection Requirements
Bolted: Torque wrench verification or DTI washers. Visual inspection confirms correct grade, nut position, and washers. Preloaded bolts: EN 1090-2 requires k-class certification or torque method per NA.
Welded: EN 1090-2 Execution Class (EXC1–EXC4) defines the NDT scope. EXC3 (typical CC2 buildings) requires 10–25% UT of butt welds and 5% of fillet welds. NDT must be performed by Level 2 personnel certified under EN ISO 9712.
Cost note: NDT for welded connections is consistently underestimated in preliminary budgets. For EXC3 projects, budget 8–15% of fabrication cost for inspection and quality assurance. Changing the weld quality class after fabrication begins is extremely expensive.
Key Takeaways
- Both methods can achieve the same structural capacity — the decision is driven by fabrication, erection, inspection, and lifecycle factors.
- Field connections should almost always be bolted unless a specific justification exists for field welding.
- Never mix non-preloaded bolts with welds in the same load path without engineering justification.
- For fatigue-critical structures, check EN 1993-1-9 detail categories for both options before deciding.
- Specify weld quality class and NDT scope on drawings from the start — not as an afterthought.
References: IS 800:2007, AISC 360-22, EN 1993-1-8. For reference only — verify against current editions before use in design.