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


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• Earthquake Behaviour– Displacement demand depends on shaking intensity

0Δ

H

Minor Shaking

Moderate Shaking

Strong Shaking


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• Impact of displacement-based loading– Overstrength Ω

Design Force

0Δ

H

Overstrength ≈ 2.5

Ductility

H, Δ


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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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544

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


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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


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• Choice of configuration…– K-bracing not suitable

for resisting EQ loads


K-bracing

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• Choice of configuration…– K-bracing, when buckled,

cause column to deform horizontally leading to buckling and collapse


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


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• Braced frame behaviour…– Double brace


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• Braced frame behaviour…– Analytical models predict reasonably well


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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


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• Effects of Brace Buckling…– Non-symmetrical deformation

• Induce large torsional response in the building during cyclic EQ shaking


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


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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


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• Effective slenderness– K value as observed from experiments for various

connections


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


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%.


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


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• 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


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


1.2fyAg 0.24fyAg

Unbalanced vertical force

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• Top and bottom flanges of the beam at the point of intersection shall be designed to support 2% of beam flexural strength fybftf


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• Brace Connections– Connection should be adequate against out-of-plane

failure of gusset plate and brittle fracture


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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


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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


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• Brace Connections…– Brace too close to beam/column members


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


BeamBeam

Brace Brace

Column Column46

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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


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


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• OMRF…– In rigid fully-welded connections

• Provide continuity plate tcp≥tbf

Welded to column flange and web


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


• 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


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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


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


21MM

pb

pc .≥∑∑

Beam

ColumnStrong-Column Weak-Beam

Philosophy58

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design and detailing for eq loads seismic...

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