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 Actionreduced regularity

different structural systems for lateral bracing

Diagonal-verband

Diagonal-verband

Rahmen-tragwerk

discontinuous bracing systems

Diagonal bracing

frame structure

Diagonal bracing

Design of Steel Structures for Seismic ActionDuctility

Sudden or brittle failure shall not occur Examples:

Buckling Connection failureLoad

Deformation

Design of Steel Structures for Seismic ActionDuctility

Examples:

Design of Steel Structures for Seismic ActionDuctility

Specially endangered: Corner Columns

most endangered column

Design of Steel Structures for Seismic ActionDuctility

Examples:

Design of Steel Structures for Seismic ActionDissipative Behaviour

Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:

Elasticbehaviour

Load

Deformation

Design of Steel Structures for Seismic ActionDissipative Behaviour

Load

Deformation

PlastificationPlastification

Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:

Design of Steel Structures for Seismic ActionDissipative Behaviour

PlastificationLoad

Deformation

Plastification

Plastification

dissipatedenergy

Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:

MN

Q

Design of Steel Structures for Seismic ActionDissipative Mechanisms

Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings)

Design of Steel Structures for Seismic ActionDissipative Mechanisms

Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings)

Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System

Successive Formation of Plastic HInges

Load

Deformation

Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System

Succesive Formation of Plastic Hinges

Deformation

Load

Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System

Succesive Formation of Plastic Hinges

Deformation

Load

Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System

Succesive Formation of Plastic Hinges

Deformation

Load

Design of Steel Structures for Seismic ActionDissipative Behaviour – cyclic

Experimental Investigations on Frame Structures

F - ID

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

-7.00 -6.00 -5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

ID [%]

F [k

N]

F - ID

-80.00

-60.00

-40.00

-20.00

0.00

20.00

40.00

60.00

80.00

-7.00 -6.00 -5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

ID [%]

F [k

N]

Design of Steel Structures for Seismic ActionFunctioning dissipative Mechanisms

Design of Steel Structures for Seismic ActionInadequate Dissipation Capacity

Design of dissipative Members„Overstrength“ of Material

Example S 235, nominal Yield Strength fy,k = 235 N/mm²

Stress

Strain

235Overstrength

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)

Design of dissipative Members„Overstrength“ of Material

how to ensure dissipative behaviour

Stress

Strain

235Overstrength

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?)

Design of dissipative MembersPlastic Fatigue of Materials

Elastic Fatigue Strength Plastic Fatigue(Low Cycle Fatigue)

Design of dissipative MembersPlastische Ermüdung des Werkstoffs

Elastic Fatigue Strength Plastic Fatigue(Low Cycle Fatigue)

Δσ

104 5·106

>108

N1 100

N

ΔRpl

RplRpl

Design of dissipative MembersToughness of Material

Toughness of material – basic requirement for dissipation

Design of dissipative MembersZähigkeit 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

Design of dissipative MembersStability of cross sections

Slender cross section show premature local buckling:– dissipation will be less – premature damage

Design of dissipative MembersStability 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

Design für Dissipative BehaviourGlobal capacity design

Npl

N

NN

V V

pl

StützeAnker

Anker Anker

g+q

NcolumnNanchor

VanchorVanchor

Design für Dissipative Behaviourlocal capacity design

Npl

Schweißnaht

Netto-querschnitt

Schrauben

Lochleibung

Measures:– avoid premature

brittle failureof 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 failureof 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

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

Flow chart for design (1)

Erster Bauwerksentwurf (z.B. für Windlasten)Ergebnis: Abmessungen, Topologie, ständige und veränderliche Lasten

Entscheidung über mögliche Dissipationsmechanismen

Lastkombination für Erdbeben

Berechnung mit elastischem AntwortspektrumVergleich der Beanspruchungen aus Wind und Erdbeben

Wind > ErdbebenJA keine weiteren

Nachweise

Ausnutzung < 150%

NEIN

JADuktilitätsklasse 1

NEIN

Duktilitätsklasse 2 oder 3 (qerf > 1,5)

Wahl des Verhaltensbeiwertsq = max. Ausnutzung [%] / 100

möglicheVerhaltensbeiwerte(Systemtopologie,Regelmäßigkeit)

Natural Ductilityq = 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 spectrumComparison of actions due to wind and earthquake

Wind > EarthquakeNo 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

yes

no

no

Flow chart for design (2)

Wahl des Verhaltensbeiwertsq = max. Ausnutzung [%] / 100

Berechnung mit reduziertem AntwortspektrumEd = Eelast / q

Überprüfung der Ausnutzungen (dissipative Bauteile)i.d.R. max. Ausnutzung ~ 100 %

min. Ausnutzung ~ 80%Inverser Ausnutzungsgrad = 1 / 0,80 = 1,25

Globale Kapazitätsbemessung mitg + p und 1,2 Ed

lokale Kapazitätsbemessung (Anschlüsse dissipativer Bauteile) mit1,2 Rk,plast

Schnittgrößen

Selection of behaviour factor q = max. exploitation [%] / 100

Calculation using design spectrum Ed = Eelast / 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 Ed

local capacity design (connection of dissipative elements)

member forces

Application Example:

Reactor- and Heater Towersfor a steel producing direct reduction plant

in Indonesia

Assuming an Elastic system

atop = 0,5 … 1,0 g

ag = 0,2 … 0,4 g

Ground and Response Acceleration

atop

1 g horizontal = ......

Assuming an Elastic system

Ductility: where to get it from?

not o.k. ! not o.k. !

buckling = failure

Ductility: where to get it from?

o.k. ! o.k. !

Buckling o.k.

First possible solution

Dissipative Elements

Example: Shear –Link in Eccentrically Braced Frame (EBF)

Vpl

V

Second possible solution Vertical Shear links

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

Capacity Design: 2nd loop of calculation

from shear link: Vpl

Calculate system again with

Vpl * γRd !Design columns, beams

and diagonals for this load

Vpl * γRd

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