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PD 6695 — UK Supplementary Recommendations for Design of Steel Structures and Bridges to BS EN 1993 (Toughness, Fatigue, and Global Stability)
Recommendations for the Design of Structures to BS EN 1993 (Steel) and Design of Bridges to BS EN 1993
🏗️ PD 6695 (a UK “Published Document”) provides non-contradictory complementary information and background for use in the UK with BS EN 1993 (Eurocode 3, Design of steel structures) and its UK National Annex. It was prepared by Subcommittees B/525/10 (Bridges) and B/525/31 (Structural use of steel) under Technical Committee B/525 (Building and civil engineering structures). PD documents are not formal British Standards; their recommendations use “should” and are non-contradictory guidance helping designers translate Eurocode principles into UK engineering practice.
💡 The PD 6695 series is a key “landing” document for the Eurocodes: the Eurocode gives principles and rules, while the PD provides UK background, supplementary tables, and more specific application advice, giving designers an actionable basis for toughness selection, fatigue verification, and global stability.
1. Series Structure 📌
| Part | Title | Corresponding BS EN | Subject |
|---|---|---|---|
| PD 6695-1-10:2009 | Recommendations for the design of structures to BS EN 1993-1-10 | BS EN 1993-1-10 | Material toughness (Charpy impact) and lamellar tearing prevention |
| PD 6695-1-9:2008 | Recommendations for the design of structures to BS EN 1993-1-9 | BS EN 1993-1-9 | Fatigue design (detail categories, stress ranges, fatigue models) |
| PD 6695-2:2008 (incl. A1:2012, C1:2013) | Recommendations for the design of bridges to BS EN 1993 | BS EN 1993 (bridge-related parts) | Global analysis, lateral torsional buckling, stiffeners, box girders, webs |
PD 6695-2 is the most extensive, covering 26 topics from global analysis to box-girder diaphragms and web breathing.
2. Material Toughness and Lamellar Tearing (PD 6695-1-10) 🧊
PD 6695-1-10 provides recommendations, per BS EN 1993-1-10, for selecting Charpy impact sub-grades when the reference temperature is outside the range given in the standard (e.g. above 10 °C or below −50 °C), and covers prevention of lamellar tearing in welded structural steelwork. Core content includes:
- Maximum permissible element thickness for steel grades (e.g. S235, S275) and sub-grades (JR, J0, J2) at reference temperatures (e.g. at σEd=0.75fy(t));
- Maximum thickness tables for internal steelwork in buildings (Tmd=−5 °C), external steelwork (−15 °C), and external steelwork in bridges (−20 °C);
- Prevention of lamellar tearing in tee/cruciform joints and corner joints.
⚠️ Impact toughness is the key to low-temperature brittle fracture. Selecting a steel sub-grade with insufficient toughness per the Eurocode may cause brittle fracture at low temperature — always check the maximum thickness against the design reference temperature.
3. Fatigue Design (PD 6695-1-9) 🔁
PD 6695-1-9 is the background and recommendation for fatigue design to BS EN 1993-1-9, covering:
- Effect of material and execution tolerances and inspection on fatigue (in relation to BS EN 1090-2 execution);
- Deriving fatigue strengths from tests;
- Assessment methods (nominal, modified nominal, and geometric stress ranges);
- Stress concentration factors (SCF) and determination of fatigue load parameters and verification formats;
- Mapping between fatigue detail categories and the older BS 5400-6 fatigue classes.
4. Bridge Global Stability and Box Girders (PD 6695-2) 🌉
PD 6695-2 is the comprehensive recommendation for designing bridges to BS EN 1993. Main topics include:
- Global analysis and non-dimensional slenderness (beam segments, beams without intermediate restraints, cantilevers, beams with torsional restraints);
- Simplified verification of lateral buckling of truss chords and compression flanges, and restraint effects at supports and intermediate points on main beams;
- Buckling resistance of plates with out-of-plane loading, design of flanges/webs with large openings, intermediate transverse and bearing stiffeners;
- Connections (beam splices, gusset plates, bolted and welded connections);
- Box girders — effects of torsion and distortion, plated diaphragms, and internal cross frames;
- Web breathing verification.
✅ Lateral torsional buckling and box-girder distortion are among the most easily overlooked yet critical stability problems in plate-girder/box-girder bridge design; PD 6695-2 gives more specific verification methods and restraint assumptions than the Eurocode clauses, ready for designers to adopt.
5. Engineering Practice Essentials 🛠️
- 🔍 Toughness selection: select the steel sub-grade per PD 6695-1-10 against the design reference temperature and thickness, and watch for lamellar tearing in welded joints.
- 🔧 Fatigue details: determine detail categories and stress ranges per PD 6695-1-9, noting the significant effect of execution tolerances and inspection on fatigue life.
- 📊 Stability verification: verify lateral torsional buckling, support restraint, and box-girder distortion per PD 6695-2, avoiding over-simplification.
- 📋 Execution interface: link fatigue and construction quality requirements to BS EN 1090-2 execution, and mark detail requirements clearly on drawings.
6. Frequently Asked Questions ❓
❓ Q: Is PD 6695 a formal British Standard?
A: No. It is a Published Document providing non-contradictory complementary advice for use with BS EN 1993 and its UK National Annex; it does not constitute a normative requirement.
A: No. It is a Published Document providing non-contradictory complementary advice for use with BS EN 1993 and its UK National Annex; it does not constitute a normative requirement.
❓ Q: Why select steel toughness by reference temperature?
A: Low temperature raises the risk of brittle fracture. The Charpy impact sub-grade (JR/J0/J2) determines the allowable thickness at low temperature; the lower the design temperature, the higher the required toughness.
A: Low temperature raises the risk of brittle fracture. The Charpy impact sub-grade (JR/J0/J2) determines the allowable thickness at low temperature; the lower the design temperature, the higher the required toughness.
❓ Q: Why is execution tolerance so important in fatigue design?
A: Fatigue is extremely sensitive to geometric imperfections, weld quality, and stress concentrations; detail categories are built on specific execution quality, and non-conforming tolerances or inspection significantly reduce fatigue strength.
A: Fatigue is extremely sensitive to geometric imperfections, weld quality, and stress concentrations; detail categories are built on specific execution quality, and non-conforming tolerances or inspection significantly reduce fatigue strength.
🔍 Q: Why is box-girder design special?
A: Box girders are affected by torsion and distortion, requiring dedicated design of plated diaphragms and internal cross frames, plus web breathing verification; PD 6695-2 provides specific design methods for these issues.
A: Box girders are affected by torsion and distortion, requiring dedicated design of plated diaphragms and internal cross frames, plus web breathing verification; PD 6695-2 provides specific design methods for these issues.