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

Design of Buildings for Seismic Action reduced regularity different structural systems for lateral bracing discontinuous bracing systems Diagonal bracing frame structure Ungleichmäßige Beanspruchung wegen verschiedener Massen, Steifigkeitssprünge etc. Nicht berücksichtigte Torsionseffekte bei L-förmigen Bauwerken Diagonal bracing

Design of Steel Structures for Seismic Action Ductility Sudden or brittle failure shall not occur Examples: Buckling Connection failure Load Deformation Ductility bedeutet de Deformationsfähigkeit eines Tragwerks, bei der kein plötzliches Versagen von vitalen Membersn auftritt

Design of Steel Structures for Seismic Action Ductility Examples: Typische Examples, die sich immer wiederholen, sind hier zu sehen

Design of Steel Structures for Seismic Action Ductility Specially endangered: Corner Columns Typische Examples, die sich immer wiederholen, sind hier zu sehen most endangered column

Design of Steel Structures for Seismic Action Ductility Examples: Typische Examples, die sich immer wiederholen, sind hier zu sehen

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 Dissipative Behaviour ist neben der Ductility die Kerneigenschaft von erdbebenresistenten Bauwerken. Die Dissipation geht von plastisch verformbaren Elementen aus, wie hier am Example eines Kragarms dargestellt.

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

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

Design of Steel Structures for Seismic Action Dissipative Mechanisms Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings) Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of Steel Structures for Seismic Action Dissipative Mechanisms Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings) Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Successive Formation of Plastic HInges Load Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Deformation

Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Load Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Deformation

Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Load Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Deformation

Design of Steel Structures for Seismic Action Dissipative Behaviour – Global System Succesive Formation of Plastic Hinges Load Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Deformation

Design of Steel Structures for Seismic Action Dissipative Behaviour – cyclic Experimental Investigations on Frame Structures Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of Steel Structures for Seismic Action Functioning dissipative Mechanisms Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of Steel Structures for Seismic Action Inadequate Dissipation Capacity Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members „Overstrength“ of Material Example S 235, nominal Yield Strength fy,k = 235 N/mm² Consequences: in the dissipative member the forces will become bigger than intended Failure of connections (e.g. bolts) Stability failure (e.g. columns) Stress 235 Overstrength Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Strain

Design of dissipative Members „Overstrength“ of Material how to ensure dissipative behaviour 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?) Stress 235 Overstrength Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Strain

Design of dissipative Members Plastic Fatigue of Materials Elastic Fatigue Strength Plastic Fatigue (Low Cycle Fatigue) Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members Plastische Ermüdung des Werkstoffs Elastic Fatigue Strength Plastic Fatigue (Low Cycle Fatigue) Δσ 104 5·106 >108 N 1 100 N ΔRpl Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members Toughness of Material Toughness of material – basic requirement for dissipation Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members Zähigkeit des Werkstoffs Toughness of material – basic requirement for dissipation Mesures: Selection of material quality / grade (sufficient toughness even for low temperatures) Dissipative zones outside the heat influence zones due to welding Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members Stability of cross sections Slender cross section show premature local buckling: dissipation will be less premature damage Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design of dissipative Members Stability of cross sections Slender cross section show premature local buckling: dissipation will be less premature damage Measures: Compact Cross Sections (Cross sectional class 1) For thin walled Structures design for elastic behaviour consider stability aspects (e.g. fluid tanks) Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Design für Dissipative Behaviour Global capacity design g+q Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Vanchor Vanchor Nanchor Ncolumn

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 weld net-section Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks. Bearing resistance Bolts

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: Rbolt > Rbearing > Rcross-section,pl > Eseismic/q for comparison: static design verification: (Rbolt , Rbearing , Rcross-section) > Ed Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.

Flow chart for design (1) 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 yes No further checks Wind > Earthquake Possible behaviour factors (system topology, regularity) no yes ductility class L Exploitation < 150 % no Natural Ductility q = 1,5 Ductility class M or H (q >1,5) Selection of behaviour factor q = max. exploitation [%] / 100

Flow chart for design (2) Selection of behaviour factor q = max. exploitation [%] / 100 Calculation using design spectrum Ed = Eelast / q member forces 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 Ed local capacity design (connection of dissipative elements)

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

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

Assuming an Elastic system 1 g horizontal = ......

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

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

First possible solution

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

Second possible solution Vertical Shear links

Biggest possible ductility in shear Avoid flexural failure mode 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

Calculate system again and diagonals for this load Capacity Design: 2nd loop of calculation from shear link: Vpl Vpl * γRd Calculate system again with Vpl * γRd ! Design columns, beams and diagonals for this load

Spacing of stiffener plates, type of link Plastic deformability θ= 0.02 .. 0.08 rad

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 Die Dissipationsfähigkeit einzelner Elemente ergibt das Dissipative Behaviour eines Bauwerks.