structural dynamics and earthquake engineering design concepts
TRANSCRIPT
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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)
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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
Complex behaviour and design: validation through tests
Dissipative connections: plastic deformations in connections
Connections
Complex behaviour and design: validation through tests
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
Moment-resisting frames
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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Moment-resisting frames
Columns:
5,0V
V
Rd,pl
Ed pl,Rd,i Ed,iM /MM
i
ov1,1 MT
Moment-resisting frames
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
Frames with concentric bracings
Braces shall be placed in such a way that the structure exhibits similar stiffness and strength in opposite senses
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Frames with concentric bracings
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
Frames with concentric bracings
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
Frames with eccentric bracings
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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Frames with eccentric bracings
Detailing:
– stiffeners
– lateral supports
Frames with eccentric bracings
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)
Buckling restrained braced frames
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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Buckling restrained braced frames
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, %
Buckling restrained braced frames
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: