design and detailing for eq loads seismic...
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SeismiCSeismiC DesignDesignIS 800 :2007IS 800 :2007
CE 627: Advanced Steel Design
Durgesh C RaiDepartment of Civil Engineering
Indian Institute of Technology KanpurKanpur 208016
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Section 12Section 12
• Design and Detailing for EQ Loads– Loads and load combinations– Response reduction factors– Connections, Joints and Fasteners– Column design– Storey drift– Lateral load resisting systems
• Moment Resisting Frames (OMRF, SMRF)• Concentric Brace Frames (OCBF, SCBF)• Eccentric Brace Frames (EBF)
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Loads and load combinationsLoads and load combinations
• EQ loads calculation– As per IS:1893(1)-2002
• Special load combinations– in addition to those given in Chapter 5
1.2 DL + 0.5 LL ± 2.5 EL0.9 DL ± 2.5 EL
NEW
Different from those given in the load combinations in Section 5
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• Special load combinations– Limit State Design
1.2 DL + 0.5 LL ± 2.5 EL0.9 DL ± 2.5 EL
– These are load combinations involving amplified earthquake loads for capacity design of members
Loads and load combinationsLoads and load combinations……
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• Earthquake resistant design– Reduction Factor R
Elastic Force reduced by R
Design Force
Actual
MaximumElastic Force
Elastic
0Deformability Δ
Lateral Load H
H, Δ
Loads and load combinationsLoads and load combinations……
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• Earthquake Behaviour– Displacement demand depends on shaking intensity
0Δ
H
Minor Shaking
Moderate Shaking
Strong Shaking
Loads and load combinationsLoads and load combinations……
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• Impact of displacement-based loading– Overstrength Ω
Design Force
0Δ
H
Overstrength ≈ 2.5
Ductility
H, Δ
Loads and load combinationsLoads and load combinations……
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Response reduction factorResponse reduction factor
• Design Seismic Force– Response reduction factor R
– Values of R for OCBF and OMRF are relatively higher compared to their RC counterparts
• Making steel buildings more popular
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IS:1893
5Special Moment Frames (SMRF)4Ordinary Moment Frames (OMRF)
Moment Resisting Frame System25Eccentrically Braced Frames (EBF)
4.5Special Concentric Braced Frames (SCBF)4Ordinary Concentric Braced Frames (OCBF)
Braced Frames Systems1IS:800Lateral Load Resisting SystemS.No.
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Connections, Joints & SplicesConnections, Joints & Splices
• All bolts used in frames resisting EQ loads– Standard holes– Fully tensioned– High strength friction-grip bolts
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• All welds used in frames resisting EQ loads– CJP groove welds
• Except in column splicesPartial JP groove welds
• Bolted joints – not to be used along with welds
on same faying surface
Loads and load combinationsLoads and load combinations……
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• If axial load demand P > 0.4 times compressive strength Pc– P (tensile or compressive, with M=0) shall be
determined by special load combinations• 1.2 DL + 0.5 LL ± 2.5 EL• 0.9 DL ± 2.5 EL
– P shall not be more than maximum load transferred to the column considering
• 1.2 times nominal strength of connecting beam/brace• Resistance of foundation to uplift
ColumnsColumns
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• Column splice– Partial JP groove welds– 200% of required strength – Minimum required strength of each flange splice
shall be 0.6 times fyAf
ColumnsColumns……
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Storey driftStorey drift
• As per IS:1893(1)-2002
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Concentric Brace Frames
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CBF ConfigurationsCBF Configurations
• Some basic types of CBFsMany configuration to choose fromMost efficient system for resisting lateral loadsProvide complete truss action
Diagonalbracing X-bracing
V-bracing Inverted(Chevron)V-bracing K-bracing
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• Choice of configuration– Chevron bracing– Chevron bracing imposes large flexural demand on floor
beams when compression brace buckles
CBF ConfigurationsCBF Configurations……
Tensionbrace
Compressionbrace
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• Choice of configuration…– CBF with unequal braces in two directions can also behave
like Chevron bracing• Poor practice to have unequal braces in same plane
CBF ConfigurationsCBF Configurations……
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• Choice of configuration…– K-bracing not suitable
for resisting EQ loads
CBF ConfigurationsCBF Configurations……
K-bracing
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• Choice of configuration…– K-bracing, when buckled,
cause column to deform horizontally leading to buckling and collapse
CBF ConfigurationsCBF Configurations……
Tensionbrace
Compressionbrace
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Basic Behaviour of XBasic Behaviour of X--BracesBraces
• Role of individual braces in EQ behaviour
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• Braced frame behaviour…Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Braced frame behaviour…– Single brace
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Braced frame behaviour…– Single brace
kL/r=30
kL/r=80
More energy dissipation
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Braced frame behaviour…– Double brace
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Braced frame behaviour…– Analytical models predict reasonably well
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Effects of Brace Buckling– Rapid loss of strength and tension brace overload– Excessive rotation of brace ends and local connection
failure– Local or torsional buckling at/near mid span– Out-of-plane deformation (bowing)– Energy dissipation is deficient
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
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• Effects of Brace Buckling…– Non-symmetrical deformation
• Induce large torsional response in the building during cyclic EQ shaking
Basic Behaviour of XBasic Behaviour of X--BracesBraces……
Post-buckling Strength
Py
-Pcr
-Py
u
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Design of Design of CBFCBFss
• Design Objectives– Hysteretic behaviour of CBFs characterized by
severely pinched hysteresis loops• However, reasonable stable deformation can be
achieved to protect against brittle failures
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• Approach– Strong-column weak-beam weaker-brace approach
• Use compact brace sections to avoid local instability
Design of Design of CBFCBFss……
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Design of XDesign of X--Braced Frame SystemsBraced Frame Systems
• Ordinary Concentric Braced Frame (OCBF)– Shall not be used in
• Seismic Zones IV and V• Seismic Zone III, if I>1.0
• Provisions for X-bracing only– Specialist literature for other V/Inverted-V braces– K-bracing not permitted
• kL/r shall not be more than 120• P shall not be more than 0.8Pc
• Braces shall be provided in either direction along any line of braces– Tension braces shall carry 30-70% of load in tension
Cl. 12.7.1.0
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• OCBF…– Brace section can be Plastic, Compact or Semi-compact
• Not Slender– In built-up braces, spacing of stitches shall be such that
• kl/r of individual element > 0.4 times kl/r of whole member
• Bolted stitches shall not be used in middle Lclear /4
– Connection shall be designed to withstand• 1.2 times Mp of brace section about buckling axis• Tensile force of 1.2fyAg
• Maximum force that can be transferred to the brace by the system
• Force in brace due to special load combinations
Design of XDesign of X--Braced Frame SystemsBraced Frame Systems……
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• Special Concentric Braced Frame (SCBF)– Provisions for X-bracing only
• Specialist literature for other V/Inverted-V braces• K-bracing not permitted
– kL/r shall not be more than 160 for hangars• Not given for others
– P shall not be more than Pc
– Braces shall be provided in either direction along any line of braces• Tension braces shall carry 30-70% of load in tension
– Brace section shall be Plastic
Design of XDesign of X--Braced Frame SystemsBraced Frame Systems……
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• SCBF…– In built-up braces, spacing of stitches shall be such that
• Similar to OCBF
– Connection shall be designed to withstand• 1.2 times Mp of brace section about buckling axis• Tensile force of 1.1fyAg
• Maximum force that can be transferred to the brace by the system
• Force in brace due to special load combinations
Design of XDesign of X--Braced Frame SystemsBraced Frame Systems……
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• Effective slenderness– K value as observed from experiments for various
connections
Design of XDesign of X--Braced Frame SystemsBraced Frame Systems……
0.5
0.4
In-plane
1.0Others
0.5Single gusset plate
Out-of-plane
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• Tension only X-bracingEconomical for low lateral load and long braces
Limited energy dissipation capacity
– Resist 100% of same load in tension and connect to beam-column connection
– Not exceed 4-storey
– Fully continuous column and constant cross-section
– Columns spliced for
Full moment resistance of cross-section
Shear force
– Must meet ductile brace requirement
Design of XDesign of X--Braced Frame SystemsBraced Frame Systems……
2 p y
s
Z fh
=
Canadian Code
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• Chevron-braces– Behaviour and force levels
• Lower tendency to distribute inelasticity
• Special care for industrial building• Low reduction factor due to ductility
and energy dissipation capacity, e.g. Eurocode 8 : q-factor for Chevronbraces are 2.5 for High Ductility and 2.0 for Medium Ductility when for X-and diagonal bracings q-factors are 4.5 and 4, repectively.AISC: Increases design forces for Chevron by 150%.
Design of Design of CBFCBFss……
Chevronbracing
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• Some design guidelines…– Chevron-braces
• Beam intersected by Chevron braces shall be continuous between columns
• Beam shall be capable of supporting all tributary gravity load combinations assuming that bracing does not exist
Design of Design of CBFCBFss……
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• Some design guidelines…– Chevron-braces
• Beam shall be designed to support the following gravity loads and unbalanced brace forcecombinations
1.2 DL + 0.5 LL + Pb
0.9 DL - Pb
Design of Design of CBFCBFss……
Post-buckling Strength = 0.24 Py
Py
-Pcr
-Py
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• Some design guidelines…– Unbalanced Force (Bending moment and axial force) induced
by forces of : • 1.2fyAg from brace in tension• and 0.24fyAs from brace in compression
Design of Design of CBFCBFss……
1.2fyAg 0.24fyAg
Unbalanced vertical force
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• Some design guidelines…– Chevron-braces
• Top and bottom flanges of the beam at the point of intersection shall be designed to support 2% of beam flexural strength fybftf
Design of Design of CBFCBFss……
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• Brace Connections– Connection should be adequate against out-of-plane
failure of gusset plate and brittle fracture
Design of Design of CBFCBFss……
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• Brace Connections…– Connection should be designed for lesser of
• Tensile strength of the bracing• Maximum force that can be transferred to the brace
by the system
Design of Design of CBFCBFss……
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• Brace Connections…– Gusset Plate is most critical component of connection
• Enough strength required when brace buckles in plane of frame
• Provide for formation of hinge line, if brace buckles out-of-plane
Design of Design of CBFCBFss……
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• Brace Connections…– Brace too close to beam/column members
Design of Design of CBFCBFss……
Axis about which the brace rotates
out of plane
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• Brace Connections…– Brace should extend from the gusset plate from where
it can bend out of plane about the gusset plate
Design of Design of CBFCBFss……
BeamBeam
Brace Brace
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Diagonal BracingsDiagonal Bracings……
• Test at IIT Kanpur :: Cyclic Inelastic Buckling of angle section
Pcyclic
– Material: Nominal yield strength, fy = 250 MPaActual yield strength, fy = 375 MPa
– Section: ISA 35 × 35 × 5 mm, Class C; α = 0.49Area, A = 327 mm2; Minimum radius of gyration, rvv = 6.77 mm
– Unsupported length of the member, L = 1220 mm
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35
5
x x
v
v
u
u
y
y
Gusset Plate
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Brace BucklingBrace Buckling……
• Test at IIT Kanpur :: Cyclic Inelastic Buckling of angle section…
-40
-20
0
20
40
60
80
100
-60 -40 -20 0 20 40 60Displacement (mm)
Load
(kN
)
-Push
+Pull
Load-Displacement and Hysteresis plot
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Brace BucklingBrace Buckling……
• Test at IIT Kanpur :: Cyclic Inelastic Buckling of angle section…
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OCBF and SCBFOCBF and SCBF……
• OCBF vs SCBF
May be used in any seismic zone for any type of building
Should not be used in seismic zone IV and V and for building with importance factor > 1
Special concentrically braced frameOrdinary concentrically braced frame
Design of Bracing Member• Slenderness of bracing member <160• P (required) < 1.0 P (actual)• Bracing cross section plastic ( b/t <
9.4)
Design of Bracing Member• Slenderness of bracing members <120• P (required) < 0.8 P (design)• Bracing cross section not slender ( b/t
<15.7)
Should withstand inelastic deformation to a joint rotation of 0.04
Should withstand inelastic deformation to a joint rotation of at least 0.02
• Lower required base shear • Ensure high ductility • Greater magnitude of response
modification factor is used
• Designed for large base shear • Low ductility demands • Low response modification factor
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OCBF and SCBFOCBF and SCBF……
Design of Bracing Member• Bracing slopes in both directions• Tensile braces carry 30-70% of load• Built-up braces: local slenderness <
0.4 overall slenderness
Design of Bracing Member• Bracing slopes in both directions• Tensile braces carry 30-70% of load• Built-up braces: local slenderness <
0.4 overall slenderness
Special concentrically braced frameOrdinary concentrically braced frame
Design for Connection Member• Here the minimum tensile force in
the bracing is 1.1 fyAg
• For the rest IS code doesn’t give any special provisions for SCBF
Design for Connection Member• Connection should be designed for
minimum of the tensile force in the bracing 1.2 fyAg , force under additional load combination and maximum possible force
• The connection should be designed to withstand a moment 1.2 times the full plastic moment of the braced section
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• SCBF…– Columns
• Section shall be Plastic• Splices in columns shall be designed to develop
at least Nominal Shear Strength of smaller connected member and50% of nominal flexural strength of smaller connected sectionSplices shall be located in the middle-third of clear column height
Braced Frame SystemsBraced Frame Systems……
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• OMRF…– Beam-to-Column Joints and Connections
• Rigid connections shall be designed to withstand smaller of
1.2Mp of connected beamMaximum moment that can be delivered by the system
• Semi-Rigid connections shall be designed to withstand smaller of
0.5Mp of connected beamMaximum moment that can be delivered by the system
This moment shall be developed within a rotation of 0.01 radians
Moment Frame SystemsMoment Frame Systems……
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• OMRF…– Rigid or semi-rigid connections shall be designed to
withstand a shear resulting from • 1.2DL+0.5LL• Plus that resulting from
1.2 DL + 0.5 LL ± 2.5 EL0.9 DL ± 2.5 EL
– Stiffness of connections shall be included in analysis
Moment Frame SystemsMoment Frame Systems……
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• OMRF…– In rigid fully-welded connections
• Provide continuity plate tcp≥tbf
Welded to column flange and web
Moment Frame SystemsMoment Frame Systems……
Column
Beam
Continuity Plate
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• Special Moment Frame (SMRF)– Beam-to-Column Joints and Connections
• Shall be rigid connections • Shall be designed to withstand smaller of
1.2Mp of connected beamMaximum moment that can be delivered by the system
• Where a reduced beam section is used, connection shall be designed for at least 0.8Mp of unreduced section
• Shall be designed to withstand a shear resulting from 1.2DL+0.5LL+ that resulting from 1.2Mp applied at each end in opposite direction but, need not exceed that obtained from
1.2 DL + 0.5 LL ± 2.5 EL0.9 DL ± 2.5 EL
Moment Frame SystemsMoment Frame Systems……
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• SMRF…– In strong-axis beam-to-column connection
• Panel Zone shall be checked for shear buckling at the shear force obtained as above
Doubler plates may be used if requiredIndividual thickness t of column web or doubler plate
shall be more than (dp+bp)/90
Moment Frame SystemsMoment Frame Systems……
Column
DoublerPlate
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• SMRF…– Beam-Column Limitation
• Beam and column sections shall be either plastic or compact
At plastic hinge locations, they shall be plastic
• Beam and column sections shall satisfy the relation
Moment Frame SystemsMoment Frame Systems……
21MM
pb
pc .≥∑∑
Beam
ColumnStrong-Column Weak-Beam
Philosophy58
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