structural dynamics and earthquake engineering design concepts

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Structural Dynamics and Earthquake Engineering

Course 12

Seismic design of steel structures

Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/

Design concepts

Low-dissipative structural behaviour

Dissipative structural behaviour

Design concept Range of thereference values ofthe behaviour factor q

Structural ductilityclass

High dissipative structural behaviour

only limited by structural type

H (high)

Medium -dissipative structural behaviour

q < 4.0, also limited by structural type

M (medium)

Low dissipative structural behaviour

q = 1.0-1.5 L (low)

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Structural types: behaviour factors q (tab 6.3)

Structural types: behaviour factors q (tab 6.3)

3

Ductility of steel structures

Steel - ductile material ductile steel structures???

Ductile steel structure:

– Ductile material (steel)

– Ductile cross-section

– Ductile elements

– Appropriate connections

– Structural ductility

Material and cross-section ductility

Material ductility

– fu/fy>1.2

– elongation at rupture > 20%

– elongation at the end of the yield plateau > 1.5%

Cross-section ductility

– elements in tensions: cross-section ductility = material ductility

– elements in compression: local buckling reduced strength and ductility

– compression: due to axial forces or due to bending

– Eurocode 3: four cross-section classes

M

Mpl

Clasa 4

Clasa 3

Clasa 2 Clasa 1

Mel

Ductility class

Behaviour factor q

Cross-section class

DCH Acc. tab. 6.3 1

DCM Acc. tab. 6.3 1 or 2

DCL1,0 q 1,5 1, 2 or 3

q = 1.0 1, 2, 3 or 4

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Element ductility

Buckling reduces both strength and ductility

Compression elements: flexural buckling

Elements subjected to bending: lateral-torsional buckling

Buckling should be prevented for dissipative elements by limiting element slenderness

– stockier elements

– lateral restraints

Connections

Complex behaviour and design: validation through tests

Dissipative connections: plastic deformations in connections

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Connections


Dissipative connections: plastic deformations in connections

Connections


Non-dissipative connections: overstrength with respect to the connected dissipative elements

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Connections

Non-dissipative connections: designed with an overstrength with respect to the connected dissipative elements

fyovd R1,1R

DISPLACEMENT

FO

RC

E

expected strength ofdissipative member

Rfy

ovR fy

1.1ovR fy

Rd

non dissipative connections

nominal strength ofdissipative member

Structural ductility

Strength hierarchy in order to promote a global plastic mechanism

– maximum possible number of plastic zones

– uniform distribution of ductility demands in the structure

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Moment-resisting frames

Horizontal forces are mainly resisted by members acting in an essentially flexural manner

Dissipative zones located in plastic hinges in the beams (or the beam-column joints)

The dissipative zones may also be located in columns:

– at the base of the frame;

– at the top of the columns in the upper storey of multi-storey buildings;

– at the top and bottom of columns in single storey buildings in which NEd / Npl,Rd < 0.3


Dissipative zones in beams:

Ved,G - shear force due to gravity loading

– VEd,M= (Mpl,Rd,A+Mpl,Rd,B) / L

– Lateral supports at dissipative zones

0,1M

M

Rd,pl

Ed

15,0N

N

Rd,pl

Ed

5,0V

V

Rd,pl

Ed VEd=VEd,G+ VEd,M

VEd,G

Mpl,Rd,A M

pl,Rd,B

VEd,G VEd,M VEd,M VEd

Mpl,Rd,A

Mpl,Rd,B

VEdL

L L

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Columns:

5,0V

V

Rd,pl

Ed pl,Rd,i Ed,iM /MM

i

ov1,1 MT


Dissipative connections

– experimental proven rotation capacity

– connection flexibility accounted for in analysis

Non-dissipative connections: overstrength over connected elements

– reduce beam strength

– increase connection strength

Rotation capacity of beam-column connections:

– 0.04 rad for DCH

– 0.03 rad for DCM

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Frames with concentric bracings

Horizontal forces are mainly resisted by members subjected to axial forces

Dissipative zones should be mainly located in the tensile diagonals

Type of bracings :

– active tension diagonal bracings, in which the horizontal forces can be resisted by the tension diagonals only, neglecting the compression diagonals;

– V bracings, in which the horizontal forces can be resisted by taking into account both tension and compression diagonals

– K bracings, in which the intersection of the diagonals lies on a column may not be used


Braces shall be placed in such a way that the structure exhibits similar stiffness and strength in opposite senses

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Analysis:

– under gravity load conditions, only beams and columns shall be considered to resist such loads

– in frames with diagonal bracings, only the tension diagonals shall be taken into account

– in frames with V bracings, both the tension and compression diagonals shall be taken into account

Brace design:

– slenderness limitation X braces

– slenderness limitation V braces

– strength:

0,23,1

2,0

EdRd,pl NN


Design of beams and columns

Beams in V-braced frames:

– all non-seismic actions without considering the intermediate support given by the diagonals

– the unbalanced vertical seismic action effect applied to the beam by the braces after buckling of the compression diagonal

Npl,Rd

0.3Npl,Rd

i,dE i,Rd,plN

i N/N

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Frames with eccentric bracings

Horizontal forces are mainly resisted by axially loaded members,

But the eccentricity of the beam-brace connections is such that energy can be dissipated in seismic links by means of either cyclic bending or cyclic shear


Seismic links:

– short links (plastic deformations in shear) - e<1.6Mpl,link/Vpl,link

– long links (plastic deformations in bending) - e>3.0Mpl,link/Vpl,link

– intermediate links (plastic deformations in shear + bending)

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Detailing:

– stiffeners

– lateral supports


Elements not containing seismic links (columns, braces, beams):

Short links:

Intermediate and long links:

i,Edi,link,plVi V/V5,1

i,Edi,link,plM

i M/M5,1

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Buckling restrained braced frames

Horizontal forces are mainly resisted by members subjected to axial forces

Dissipative zones: buckling restrained braces (BRBs)


BRBs are composed of a steel core encased in a steel tube filled with mortar, which prevents buckling of the steel core.

Stable hysteretic response

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Design of braces:

– Check for axial force strength

– Experimental tests to prove a corresponding behaviour of the system.

0

yEd Rd

M

A fN N

Fo

rța

axi

ală

, kN

Deformația specifică, %


Beams and columns: non-dissipative elements.

Forces in the seismic design situation correspond to attainment of corrected strength in compression and are determined using the following formulas:

structural dynamics and earthquake engineering design concepts

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