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Design of Steel and Composite-Structures for Seismic Loading – Safety Requirements, Concepts and Methods – Prof. Dr.-Ing. Ekkehard Fehling, University.

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Presentation on theme: "Design of Steel and Composite-Structures for Seismic Loading – Safety Requirements, Concepts and Methods – Prof. Dr.-Ing. Ekkehard Fehling, University."— Presentation transcript:

1 Design of Steel and Composite-Structures for Seismic Loading – Safety Requirements, Concepts and Methods – Prof. Dr.-Ing. Ekkehard Fehling, University Kassel Dr.-Ing. Benno Hoffmeister, University / RWTH Aachen

2 Design of Buildings for Seismic Action reduced regularity different structural systems for lateral bracing discontinuous bracing systems Diagonal bracing frame structure Diagonal bracing

3 Design of Steel Structures for Seismic Action Ductility Sudden or brittle failure shall not occur Examples: Buckling Connection failure Load Deformation

4 Design of Steel Structures for Seismic Action Ductility Examples:

5 Design of Steel Structures for Seismic Action Ductility Specially endangered: Corner Columns most endangered column

6 Design of Steel Structures for Seismic Action Ductility Examples:

7 Design of Steel Structures for Seismic Action Dissipative Behaviour Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle: Elastic behaviour Load Deformation

8 Design of Steel Structures for Seismic Action Dissipative Behaviour Load Deformation Plastification Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:

9 Design of Steel Structures for Seismic Action Dissipative Behaviour Plastification Load Deformation Plastification dissipated energy Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:

10 Design of Steel Structures for Seismic Action Dissipative Mechanisms Bending (Frame)Normal Force (Bracings)Shear (ecc. Bracings)

11 Design of Steel Structures for Seismic Action Dissipative Mechanisms Bending (Frame)Normal Force (Bracings)Shear (ecc. Bracings)

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16 Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Successive Formation of Plastic HInges Load Deformation

17 Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Deformation Load

18 Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Deformation Load

19 Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Deformation Load

20 Design of Steel Structures for Seismic Action Dissipative Behaviour – cyclic Experimental Investigations on Frame Structures

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23 Design of Steel Structures for Seismic Action Functioning dissipative Mechanisms

24 Design of Steel Structures for Seismic Action Inadequate Dissipation Capacity

25 Design of dissipative Members Overstrength of Material Example S 235, nominal Yield Strength f y,k = 235 N/mm² Stress Strain 235 Overstrength Consequences: in the dissipative member the forces will become bigger than intended Failure of connections (e.g. bolts) Stability failure (e.g. columns) Consequences: in the dissipative member the forces will become bigger than intended Failure of connections (e.g. bolts) Stability failure (e.g. columns)

26 Design of dissipative Members Overstrength of Material how to ensure dissipative behaviour Stress Strain 235 Overstrength Measures: –Capacity Design (design of critical members and connections with overstrength) –Limitation of maximum yield strength in dissipative Members –Control of execution (strength as ordered = delivered strength?) Measures: –Capacity Design (design of critical members and connections with overstrength) –Limitation of maximum yield strength in dissipative Members –Control of execution (strength as ordered = delivered strength?)

27 Design of dissipative Members Plastic Fatigue of Materials Elastic Fatigue Strength Plastic Fatigue (Low Cycle Fatigue)

28 Design of dissipative Members Plastische Ermüdung des Werkstoffs Elastic Fatigue Strength Plastic Fatigue (Low Cycle Fatigue) Δσ ·10 6 >10 8 N 1100 N ΔR pl

29 Design of dissipative Members Toughness of Material Toughness of material – basic requirement for dissipation

30 Design of dissipative Members Zähigkeit des Werkstoffs Mesures: –Selection of material quality / grade (sufficient toughness even for low temperatures) –Dissipative zones outside the heat influence zones due to welding Mesures: –Selection of material quality / grade (sufficient toughness even for low temperatures) –Dissipative zones outside the heat influence zones due to welding Toughness of material – basic requirement for dissipation

31 Design of dissipative Members Stability of cross sections Slender cross section show premature local buckling: –dissipation will be less –premature damage

32 Design of dissipative Members Stability of cross sections Measures: –Compact Cross Sections (Cross sectional class 1) –For thin walled Structures design for elastic behaviour consider stability aspects (e.g. fluid tanks) Measures: –Compact Cross Sections (Cross sectional class 1) –For thin walled Structures design for elastic behaviour consider stability aspects (e.g. fluid tanks) Slender cross section show premature local buckling: –dissipation will be less –premature damage

33 Design für Dissipative Behaviour Global capacity design g+q N column N anchor V anchor

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38 Design für Dissipative Behaviour local capacity design Measures: –avoid premature brittle failure of non dissipative connections for bolted / or welded connections: design with overstrength for bolted connections: bearing stresses should be more critical than shear in bolt Measures: –avoid premature brittle failure of non dissipative connections for bolted / or welded connections: design with overstrength for bolted connections: bearing stresses should be more critical than shear in bolt weld net-section Bolts Bearing resistance

39 Seismic Design of Steel Structures Codes: –EN 1998 (or: DIN 4149 = EN 1998 simplified) –codes for steel structures and materials Seismic Design: –Make use of dissipation, assuming behaviour factor q (Reduction of elastic action) –Application of capacity design e.g. for bolted connections: R bolt > R bearing > R cross-section,pl > E seismic /q for comparison: static design verification: (R bolt, R bearing, R cross-section ) > E d

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41 Flow chart for design (1) Natural Ductility q = 1,5 Preliminary design of building (e.g. for wind loads) Result: dimensions, topology, permanent and variable loads Decision about conceivable dissipation mechanisms Combination of actions for earthquake Calculation using response spectrum Comparison of actions due to wind and earthquake Wind > Earthquake No further checks ductility class L Exploitation < 150 % Possible behaviour factors (system topology, regularity) Ductility class M or H (q >1,5) Selection of behaviour factor q = max. exploitation [%] / 100 yes no

42 Flow chart for design (2) Selection of behaviour factor q = max. exploitation [%] / 100 Calculation using design spectrum E d = E elast / q Check of degree of exploitation (dissipative members) usually max. exploitation 100 % min. exploitation 80 % Inverse degree of exploitation Ω = 1 / 0,80 = 1,25 global capacity design with g + q and 1,2 Ω E d local capacity design (connection of dissipative elements) member forces

43 Application Example: Reactor- and Heater Towers for a steel producing direct reduction plant in Indonesia

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48 Assuming an Elastic system a top = 0,5 … 1,0 g a g = 0,2 … 0,4 g Ground and Response Acceleration a top

49 1 g horizontal = Assuming an Elastic system

50 Ductility: where to get it from? not o.k. ! buckling = failure

51 Ductility: where to get it from? o.k. ! Buckling o.k.

52 First possible solution

53 Dissipative Elements Example: Shear –Link in Eccentrically Braced Frame (EBF) V pl V

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55 Second possible solution Vertical Shear links

56 Design of Shear Links Biggest possible ductility in shear Avoid flexural failure mode Web buckling should occur at large deformations only Ensure lateral stability of flanges

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58 Capacity Design: 2 nd loop of calculation from shear link: V pl Calculate system again with V pl * γ Rd ! Design columns, beams and diagonals for this load V pl * γ Rd

59 Spacing of stiffener plates, type of link Plastic deformability θ= rad

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61 Conclusions Design for Earthquake requires different way of thinking: verification of behaviour rather than verification of strength The behaviour of a structure under seismic loading is mainly determined by: –Regularity – avoid extreme straining/ loading of certain members –Redundancy – enable reserves of saftey –Ductility – plastic deformations without premature failure –Dissipation – from formation of cyclic plastic hystereses –Quality and Control of Execution – too much of strength may be dangerous

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