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The effect of Eurocode 5 on timber structure design in Norway

TEMTIS Seminar in Horsens September 11, 2008. The effect of Eurocode 5 on timber structure design in Norway. Kolbein Bell and Kjell Arne Malo NTNU, Norway. Objective. To compare some important passages in the current Norwegian timber code NS 3470-1 (5th ed. July 1999) with

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The effect of Eurocode 5 on timber structure design in Norway

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  1. TEMTIS Seminar in Horsens September 11, 2008 The effect of Eurocode 5on timber structure design in Norway Kolbein Bell and Kjell Arne MaloNTNU, Norway Department of structural engineering

  2. Objective To compare some important passages in the current Norwegian timber code NS 3470-1 (5th ed. July 1999) with the current version of Eurocode 5 (EC5) EN 1995-1-1 including EN 1995-1-1:2004/A1 and to point out some problem areas Department of structural engineering

  3. Important NOTES • NS 3470-1 was drafted on the basis of the first draft of EC5 – the ”philosophy” is therefore much the same • A corrigendum to NS 3470-1 which will bring theNorwegian code closer to EC5 is about to be made an official part of NS 3470-1 – this in order to ”soften” the effects of the transition to EC5.This corrigendum is disregarded in this presentation Department of structural engineering

  4. Department of structural engineering

  5. Load duration EC5 NS 3470-1 Service class Department of structural engineering

  6. NS 3470-1 (from 1,1 to 1,32) 1,0 or 1,1 1,1 or 1,2 for solid timber EC5 for glulam Department of structural engineering

  7. Stability - NS 3470-1 Stability - NS 3470-1 Combined axial compression and bending: Bending: Department of structural engineering

  8. Stability - EC5 Combined axial compression and bending: Bending: Department of structural engineering

  9. The factor km Rectangle: (bending about two axes) this factor is not present in NS 3470-1 Department of structural engineering

  10. Comparison – simple column – P only p = 0 NS 3470-1 GL36c C/2 Pu = 188,2 kN A/3 Pu = 125,5 kN EC5 Pu = 178,3 kN C/2 A/3 Pu = 111,5 kN Department of structural engineering

  11. Comparison – simple column – P & p p = const. = 3,0 kN/m NS 3470-1 GL36c C/2 Pu = 120 kN A/3 Pu = 60,6 kN EC5 Pu = 139,3 kN C/2 A/3 Pu = 72,5 kN Department of structural engineering

  12. Compression perpendicular to grain The characteristic strength in NS 3470-1is more than twice that of EC5 (for all strength classes), but NS3470-1 uses the actual contact area in the calculation of The formula and the values of factor are not all that different (providing the requirements of “supplement” A1 are used).More about this later. Department of structural engineering

  13. Shear The introduction of an effective width by EC5 (”supplement” A1), may, dependig on the national choice for the value of , have a significant influence on shear design. Department of structural engineering

  14. Special glulam components For curved and pitched cambered beams EC5 has the following formula for combined shear and tension perpendicular to grain: (6.53) where For glulam: Department of structural engineering

  15. NS 3470-1 has no such formula, nor does it have the two factors and in particular is troublesome; it is both difficult to determine and it seems to have a very significant (adverse) effect for large components). Formula (6.53) will have a detrimental effect on typical Norwegian arch bridge designs. Department of structural engineering

  16. Example: Glulam arch bridge - loading Department of structural engineering

  17. Bending moment (M) and shear force (V) M problem V V V Department of structural engineering

  18. Prior to formula (6.53) EC5 had the formulation: …. shall be satisfied. In the recently approved ”supplement” A1 this has been changed to: …. should be satisfied. Exactly how should the designer interpret this? Department of structural engineering

  19. Connections • EC5 has similar, but more complex formulas thanNS 3470-1. • The most noticeable differences are: • in NS 3470-1 the first 6 fasteners are effective • NS 3470-1 does not take account of the rope effect • NS 3470-1 does not recognize block or plug shear • Our experiences so far suggest minor differences, but no systematic bias either way. Department of structural engineering

  20. More aboutcompression perpendicular to grain Comparison EC5(A1) vs NS 3470-1 Department of structural engineering

  21. geometric parameters: design parameters (ULS): Department of structural engineering

  22. Strength ultimate limit state (ULS) Department of structural engineering

  23. Design: NS 3470-1 & EC5/A1(2004) Department of structural engineering

  24. Solid wood, C24: Department of structural engineering

  25. EC 5/A1(2004) Department of structural engineering

  26. NS 3470-1 • For • is • where: • while for • is Department of structural engineering

  27. Solid wood, Case 1: • C24 • Continous support • Vertical column on end of beam Department of structural engineering

  28. EC5 / A1(2004) Capacity pr unit width [ N/mm ] Department of structural engineering

  29. NS 3470-1 Capacity pr unit width [ N/mm ] Department of structural engineering

  30. Strength ratio: EC5(A1) / NS 3470-1 Department of structural engineering

  31. Solid wood, Case 2: • C24 • Continous support • Vertical column on continuous beam Department of structural engineering

  32. EC5 (A1) Capacity pr unit width [ N/mm ] Department of structural engineering

  33. NS 3470-1 Capacity pr unit width [ N/mm ] Department of structural engineering

  34. Ratio EC5(A1) / NS 3470-1Colums internal on cont. sup. beam Department of structural engineering

  35. Solid wood (C24), Case 3: • Vertical load transfer through beam section (h > a) • Beam continuous • EC5: (?) Department of structural engineering

  36. EC5 (A1) NS 3470-1 [ N/mm ] [ N/mm ] Department of structural engineering

  37. Ratio EC5(A1) / NS 3470-1 (C24)Colums internal on beam Department of structural engineering

  38. Solid wood (C24), Case 4: • Vertical load transfer through beam section at the beam end • EC5?(cover this case?) Department of structural engineering

  39. Ratio EC5 (A1) / NS 3470-1Columns at beam end Department of structural engineering

  40. EC5 (A1):Better capacity for eccentric load transfer (C24) Department of structural engineering

  41. GLULAM Examples: GL32c Department of structural engineering

  42. Ratio EC5 / NS 3470-1 GL32c”Column at beam end, cont. sup.” Department of structural engineering

  43. Ratio EC5 / NS 3470-1 GL32c”Column internal on beam, cont. sup.” Department of structural engineering

  44. Ratio EC5 / NS 3470-1 GL32c”Column connections at beam end” Department of structural engineering

  45. Ratio EC5 / NS 3470-1 GL32c”Column connections internal on beam” Department of structural engineering

  46. Ratio EC5 / NS3 470-1 GL32c”Columns eccentric internal on beam, discrete. sup.” Department of structural engineering

  47. Compression perpendicular to grainConcluding remarks • EC5 compared to NS 3470-1 gives for: • Solid wood: • roughly only 2/3 of the capacities • small capacities for vertical load transfer through horisontal beams • enhanced capacities for colums at beam ends • highest capacities for small contact length due to the effektiv length concept • GLULAM: • Overall similiar to solid wood, but the difference is smaller • Higher capacities for small contact length (< 30 mm) • Smaller capacity for vertical load transfer through continuous beams Department of structural engineering

  48. Summary – ultimate limit state Generally speaking, ULS-design of timber structures by EC5 will result in somewhat more conservative designs than NS 3470-1. We are talking about 5 to 25 %, most of which is caused by and . In some special cases the effect can be much higher. Our experiences over the past couple of decades do not seem to warrant this ”extra safety”. Department of structural engineering

  49. Some additional problems Serviceability limit state design, as specified by EC5, is rather complex and error prone. EC5 is not particularly well suited for more accurate, nonlinear static analyses as basis for design (nor is NS 3470-1). Major issues are:- stiffness parameters (E and G)- shape and size of geometric imperfections- modelling of joints- failure criteria Department of structural engineering

  50. Consider • E (G) as “computational” parameter(s), accounting for all factors influencing the stiffness of the structural members • ultimate load design • solid timber and glulam of softwood Department of structural engineering

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