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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

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

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

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Design of Steel Structures for Seismic Action Ductility Examples:

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

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Design of Steel Structures for Seismic Action Ductility Examples:

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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

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

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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:

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

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

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

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

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

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

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

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Design of Steel Structures for Seismic Action Functioning dissipative Mechanisms

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Design of Steel Structures for Seismic Action Inadequate Dissipation Capacity

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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)

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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?)

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

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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

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

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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

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

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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

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

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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

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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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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

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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

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

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

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1 g horizontal = Assuming an Elastic system

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

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

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First possible solution

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

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Second possible solution Vertical Shear links

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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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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

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

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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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