September 20081Department of structural engineering The effect of Eurocode 5 on timber structure design in Norway Kolbein Bell and Kjell Arne Malo NTNU, Norway TEMTIS Seminar in Horsens September 11, 2008

September 2008Department of structural engineering2 Objective To compare some important passages in the current Norwegian timber code NS (5th ed. July 1999) with the current version of Eurocode 5 (EC5) EN including EN :2004/A1 and to point out some problem areas

September 2008Department of structural engineering3 Important NOTES NS was drafted on the basis of the first draft of EC5 – the ”philosophy” is therefore much the same A corrigendum to NS which will bring the Norwegian code closer to EC5 is about to be made an official part of NS – this in order to ”soften” the effects of the transition to EC5. This corrigendum is disregarded in this presentation

September 2008Department of structural engineering4

September 2008Department of structural engineering5 P perm. A long t. B med. t. C short t. I inst. 1 0,6 0,7 0,8 0,9 1,0 1,1 2 0,6 0,7 0,8 0,9 1,0 1,1 3 0,5 0,6 0,55 0,65 0,7 0,8 0,9 Load duration Service class EC5 NS

September 2008Department of structural engineering6 NS ,1 or 1,2 1,0 or 1,1 (from 1,1 to 1,32) EC5 for solid timber for glulam

September 2008Department of structural engineering7 Stability - NS Combined axial compression and bending: Stability - NS Bending:

September 2008Department of structural engineering8 Combined axial compression and bending: Stability - EC5 Bending:

September 2008Department of structural engineering9 The factor k m (bending about two axes) Rectangle: this factor is not present in NS

September 2008Department of structural engineering10 Comparison – simple column – P only NS p = 0 C/2C/2P u = 188,2 kN A/3A/3P u = 125,5 kN EC5 C/2C/2 P u = 178,3 kN A/3A/3P u = 111,5 kN GL36c

September 2008Department of structural engineering11 Comparison – simple column – P & p NS p = const. = 3,0 kN/m C/2C/2P u = 120 kN A/3A/3P u = 60,6 kN EC5 C/2C/2 P u = 139,3 kN A/3A/3P u = 72,5 kN GL36c

September 2008Department of structural engineering12 Compression perpendicular to grain The characteristic strength in NS is more than twice that of EC5 (for all strength classes), but NS 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.

September 2008Department of structural engineering13 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.

September 2008Department of structural engineering14 Special glulam components For glulam: For curved and pitched cambered beams EC5 has the following formula for combined shear and tension perpendicular to grain: where (6.53)

September 2008Department of structural engineering15 NS 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.

September 2008Department of structural engineering16 Example: Glulam arch bridge - loading

September 2008Department of structural engineering17 V Bending moment ( M ) and shear force ( V ) V V M problem

September 2008Department of structural engineering18 Prior to formula (6.53) EC5 had the formulation: …. shall be satisfied. Exactly how should the designer interpret this? In the recently approved ”supplement” A1 this has been changed to: …. should be satisfied.

September 2008Department of structural engineering19 Connections EC5 has similar, but more complex formulas than NS The most noticeable differences are: in NS the first 6 fasteners are effective NS does not take account of the rope effect NS does not recognize block or plug shear Our experiences so far suggest minor differences, but no systematic bias either way.

September 2008Department of structural engineering20 More about compression perpendicular to grain Comparison EC5(A1) vs NS

September 2008Department of structural engineering21 geometric parameters: design parameters (ULS):

September 2008Department of structural engineering22 Strength ultimate limit state (ULS)

September 2008Department of structural engineering23 NS EC5/A1(2004)Solid Wood ???Parameters area 0.5 – factor C strength C C C (1.21)1.3material factor SC2, medium term Design: NS & EC5/A1(2004)

September 2008Department of structural engineering24 Solid wood, C24:

September 2008Department of structural engineering25 CASEVALUEMATERIALREQUIRED General1,0 Special a) cont.support 1,25 1,50 Cxx GL Special b) discr. support 1,5 1,75 Cxx GL EC 5/A1(2004)

September 2008Department of structural engineering26 For is where: while for is NS

September 2008Department of structural engineering27 Solid wood, Case 1: C24 Continous support Vertical column on end of beam

September 2008Department of structural engineering28 EC5 / A1(2004) Capacity pr unit width [ N/mm ]

September 2008Department of structural engineering29 NS Capacity pr unit width [ N/mm ]

September 2008Department of structural engineering30 Strength ratio: EC5(A1) / NS

September 2008Department of structural engineering31 Solid wood, Case 2: C24 Continous support Vertical column on continuous beam

September 2008Department of structural engineering32 EC5 (A1) Capacity pr unit width [ N/mm ]

September 2008Department of structural engineering33 NS Capacity pr unit width [ N/mm ]

September 2008Department of structural engineering34 Ratio EC5(A1) / NS Colums internal on cont. sup. beam

September 2008Department of structural engineering35 Solid wood (C24), Case 3: Vertical load transfer through beam section (h > a) Beam continuous EC5: (?)

September 2008Department of structural engineering36 EC5 (A1) NS [ N/mm ]

September 2008Department of structural engineering37 Ratio EC5(A1) / NS (C24) Colums internal on beam

September 2008Department of structural engineering38 Solid wood (C24), Case 4: Vertical load transfer through beam section at the beam end EC5? (cover this case?)

September 2008Department of structural engineering39 Ratio EC5 (A1) / NS Columns at beam end

September 2008Department of structural engineering40 EC5 (A1): Better capacity for eccentric load transfer (C24)

September 2008Department of structural engineering41 GLULAM Examples: GL32c

September 2008Department of structural engineering42 Ratio EC5 / NS GL32c ”Column at beam end, cont. sup.”

September 2008Department of structural engineering43 Ratio EC5 / NS GL32c ”Column internal on beam, cont. sup.”

September 2008Department of structural engineering44 Ratio EC5 / NS GL32c ”Column connections at beam end”

September 2008Department of structural engineering45 Ratio EC5 / NS GL32c ”Column connections internal on beam”

September 2008Department of structural engineering46 Ratio EC5 / NS GL32c ”Columns eccentric internal on beam, discrete. sup.”

September 2008Department of structural engineering47 Compression perpendicular to grain Concluding remarks EC5 compared to NS 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

September 2008Department of structural engineering48 Summary – ultimate limit state Generally speaking, ULS-design of timber structures by EC5 will result in somewhat more conservative designs than NS 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”.

September 2008Department of structural engineering49 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 ). Major issues are: - stiffness parameters (E and G) - shape and size of geometric imperfections - modelling of joints - failure criteria

September 2008Department of structural engineering50 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

September 2008Department of structural engineering51 EC5 - some definitions Mean value: Characteristic (fifth percentile) value: Final mean value: Design value: - partial factor for a material property - factor for quasi-permanent value of an action - factor for the evaluation of creep deformation

September 2008Department of structural engineering52 NOTE: EC5 specifies: for solid timber for glulam for solid timber for glulam for solid timber for glulam

September 2008Department of structural engineering53 EC5 states that the analysis of a structure shall be carried out using the following values for stiffness properties: 1st order linear elastic analysis of structure whose distribution of internal forces is insensitive to stiffness distribution 1st order linear elastic analysis of structure whose distribution of internal forces is affected by the stiffness distribution 2nd order linear elastic analysis of structure

September 2008Department of structural engineering54 Case - Simply supported column Glulam GL36c strong axis

September 2008Department of structural engineering55 EC5 - nonlinear approach L e Geometric imperfection in the shape of the 1st buckling mode e = L / 400 (EC5 suggests L / 500) For E = Mpa P E = 304,7 kN for L = 4 m P E = 135,4 kN for L = 6 m

September 2008Department of structural engineering56 q = 0 L = 4 m e = 10 mm medium slenderness: Capacity (P ult ) for long-term load in service class 3 Linear analysis: P ult = 122,7 kN Nonlinear analysis ( k c,y = k c,z = 1,0 ):

September 2008Department of structural engineering57 q = 0 L = 6 m e = 15 mm large slenderness: Capacity (P ult ) for long-term load in service class 3 Linear analysis: P ult = 56,5 kN Nonlinear analysis ( k c,y = k c,z = 1,0 ):

September 2008Department of structural engineering58 q = 1 kN/m L = 6 m e = 15 mm large slenderness: Capacity (P ult ) for long-term load in service class 3 Linear analysis: P ult = 43,0 kN Nonlinear analysis ( k c,y = k c,z = 1,0 ):

September 2008Department of structural engineering59 q = 0 L = 6 m e = 15 mm Long-term load in service class 3 q = 0 L = 6 m e = 15 mm

September 2008Department of structural engineering60 L / 3 S / 1 L / 3 q = 0 L = 6 m e = 15 mm

September 2008Department of structural engineering61 This simple example seems to leave a few questions: - size of imperfection? - what is the appropriate stiffness? and are factors, and and parameter really independent of load duration and service class, or should they be functions of in some ways?

September 2008Department of structural engineering62 Thank you