690 likes | 1.21k Views
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
E N D
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
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
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
Load duration EC5 NS 3470-1 Service class Department of structural engineering
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
Stability - NS 3470-1 Stability - NS 3470-1 Combined axial compression and bending: Bending: Department of structural engineering
Stability - EC5 Combined axial compression and bending: Bending: Department of structural engineering
The factor km Rectangle: (bending about two axes) this factor is not present in NS 3470-1 Department of structural engineering
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
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
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
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
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
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
Example: Glulam arch bridge - loading Department of structural engineering
Bending moment (M) and shear force (V) M problem V V V Department of structural engineering
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
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
More aboutcompression perpendicular to grain Comparison EC5(A1) vs NS 3470-1 Department of structural engineering
geometric parameters: design parameters (ULS): Department of structural engineering
Strength ultimate limit state (ULS) Department of structural engineering
Design: NS 3470-1 & EC5/A1(2004) Department of structural engineering
Solid wood, C24: Department of structural engineering
EC 5/A1(2004) Department of structural engineering
NS 3470-1 • For • is • where: • while for • is Department of structural engineering
Solid wood, Case 1: • C24 • Continous support • Vertical column on end of beam Department of structural engineering
EC5 / A1(2004) Capacity pr unit width [ N/mm ] Department of structural engineering
NS 3470-1 Capacity pr unit width [ N/mm ] Department of structural engineering
Strength ratio: EC5(A1) / NS 3470-1 Department of structural engineering
Solid wood, Case 2: • C24 • Continous support • Vertical column on continuous beam Department of structural engineering
EC5 (A1) Capacity pr unit width [ N/mm ] Department of structural engineering
NS 3470-1 Capacity pr unit width [ N/mm ] Department of structural engineering
Ratio EC5(A1) / NS 3470-1Colums internal on cont. sup. beam Department of structural engineering
Solid wood (C24), Case 3: • Vertical load transfer through beam section (h > a) • Beam continuous • EC5: (?) Department of structural engineering
EC5 (A1) NS 3470-1 [ N/mm ] [ N/mm ] Department of structural engineering
Ratio EC5(A1) / NS 3470-1 (C24)Colums internal on beam Department of structural engineering
Solid wood (C24), Case 4: • Vertical load transfer through beam section at the beam end • EC5?(cover this case?) Department of structural engineering
Ratio EC5 (A1) / NS 3470-1Columns at beam end Department of structural engineering
EC5 (A1):Better capacity for eccentric load transfer (C24) Department of structural engineering
GLULAM Examples: GL32c Department of structural engineering
Ratio EC5 / NS 3470-1 GL32c”Column at beam end, cont. sup.” Department of structural engineering
Ratio EC5 / NS 3470-1 GL32c”Column internal on beam, cont. sup.” Department of structural engineering
Ratio EC5 / NS 3470-1 GL32c”Column connections at beam end” Department of structural engineering
Ratio EC5 / NS 3470-1 GL32c”Column connections internal on beam” Department of structural engineering
Ratio EC5 / NS3 470-1 GL32c”Columns eccentric internal on beam, discrete. sup.” Department of structural engineering
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
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
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
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