COST ACTION TU0601 Robustness of Structures Workshop Coimbra 02./03. March 2009

Transcription

1 COST ACTION TU0601 Robustness of Structures Workshop Coimbra 02./03. March 2009 Ductility of connections - a measure to improve redundancy

2 Redundancy within the structural system by alternate load paths Alternate load path method strengthening of one/more floors design of joints with sufficiently ductile behaviour and activation of membrane forces causes usually much extra costs less extra costs demands d for well-designed d connections 2

3 Alternate load path method alternate load paths may be achieved by various measures plastic reserves bearing-mechanism structural behaviour bending capacity σ σ y actual values 2D f y,c f y,d nominal values σ y uniaxial ε tension capacity material hardening 3D σ y spatial 3 requires sufficient available ductility of members and joints

4 Ductility demands to allow the activation of bearing reserves plastic material reserves plastic system reserves large plastic strain change of bearing mechanism catenary action large global and local deformations DUCTILITY ductility of members ductility of joints 4

5 Type of connections influence the global resistance of the structure joints are usually the weakest link in a framed structure special focus type of beam-to-column connection rigid partial-strength pinned M M M M M M M j M j M beam M beam M beam M j M beam M j 5 N Φ N Φ N Φ suitable but expensive suitable not suitable lack of resistance by adequate resistance and sufficient ductility

6 Why is sufficiently ductile joint behaviour so important? M M no redistribution possible plastic redistribution possible Φ 6 hinge is located in the joint high requirements concerning joint deformability Φ

7 Influence of over-strength effects Over-strength effects materials membrane effects Adjustment of single components weakest components always ductile oversizing of the bolts 7 avoiding premature brittle bolt failure with limited ductility of the joint

8 Influence to the relevant component Influence of over-strength effects M ductile component M brittle component M connection nominal values Φ 1 Φ 2 Φ 1 +Φ 2 + = Φ Φ Φ cd M ductile component M brittle component M connection actual values + = Φ Φ Φ Φ cd 8 actual values of material strength may limit the deformation capacity clearly

9 Influence of over-strength effects on resistance and ductility Influence on the moment-rotation-curve IPE 500, M , t=15 mm nmoment [knm] moment Knote ΔM (μ+σ) ΔM (μ) ΔM (μ-σ) f ub,(μ+σ) & f y,(μ+σ) Bol f ub,(μ+σ) & f y,(μ) Bol Boltmax+EPmax f ub,(μ+σ) & f y,(μ-σ) Boltmax+EPmit f ub,(μ) & f y,(μ+σ) Boltmax+EPmin Boltmit+EPmax Bol f ub,(μ) & f y,(μ) Boltmit+EPmit Bol f ub,(μ) & f y,(μ-σ) Boltmit+EPmin Bol Boltmin+EPmax f ub,(μ-σ) & f y,(μ+σ) f ub,(μ-σ) & f y,(μ) Bol Boltmin+EPmit f ub,(μ-σ) & f y,(μ-σ) Boltmin+EPmin 0 ΔΦ (μ-σ) ΔΦ (μ) ΔΦ (μ+σ) rotation Knotenrotation [mrad] 9 small influence on the resistance, clear influence on the rotation capacity

10 Influence of over-strength on the structural response structural response under service conditions Required rotation at support depends on bending resistance of member and joints that depend on material strength knm] jo oint moment [ 5% - value mean value 95% - value required rotation [mrad] 10 Variation of material strength influence also structural response

11 Probabilistic analysis to assess sufficient rotation capacity stochastic distribution of strength f f structural steel bolts f y M determintion of M- Φ-curve Φ avai f u,b Φ avai equation of limit state Φ avai - Φ req = Z Z < 1 stochastic distribution of loads f live load q M Φ Φ req rotation not sufficient 11 method to assess the geometrical criteria for sufficiently ductile joint behaviour

12 Redundancy by allowing force redistribution requires large deformations Φ demands for sufficient resistance (M-N), beside ductility N N 13 activation of alternate load paths requires joint ductility and sufficient M-N resistance

13 Experimental test simulation of steel and composite joints Steel Joint Tests Composite Joint Tests bending test bending and tension M M-Φ-curve M M-N-Interaction 14 Φ determination of requirements and design criteria for highly ductile behaviour M N M-Φ-curve Φ for hogging and sagging moment

14 Test results Institut Für Konstruktion und Entwurf Composite joints under bending and tension M [knm m] M-phi-curve 1 0,8 0,6 0,4 0, ,2-0,4-0,6 06-0,8-1 phi[mrad] Joint Rotation V1 Joint Rotation V2 Joint Rotation V3 Joint Rotation V4 Joint Rotation V5 Moment [k knm] M-N-interaction V1 V2 V3 V4 V Normal Force [kn] hogging g moment sag gging momen nt Joints have been able to follow the whole M-N-curve from pure bending state to pure tension state due to sufficient ductility 15

15 Joint tests confirmed by a Substructure test Φ avai [Substructure test Liége] 16 structural response allows force redistribution by activation of membrane effects

16 Outlook further numerical and experimental investigations are planned stress t 1t t 2t3 P(t) strain determination high stress-strain rates of plates and bolts P(t) component response under high velocity loading rotation capacity and strength? 17 including additional bearing effects 3D effects

17 Thanks for attention! Universität Stuttgart Institut für Konstruktion und Entwurf Schwerpunkte: Stahlbau, Holzbau und Verbundbau Prof. Dr. -Ing. Ulrike Kuhlmann Dipl.-Ing. Lars Rölle Pfaffenwaldring Stuttgart Germany Tel Fax sekretariat@ke.uni-stuttgart.de