Transcript No Slide Title

IS 800: 2007 SECTION 12
DESIGN AND DETAILING FOR
EARTHQUAKE LOADS
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Steel Buildings with no damage after Kobe earthquake
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ADVANTAGES OF USING STEEL
FOR EARTHQUAKE RESISTANCE
Neither Corrosion nor Cost can be a factor against Steel Structures
Steel …
• is Ductile and gives robust structures
• can withstand reversal of stresses
• can dissipate considerable energy under cyclic loading
• is produced with quality control and suits Capacity Design
• gives light and flexible structures – reduction in seismic load
• can be easily retrofitted, repaired and rehabilitated with speed
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SECTION 12
DESIGN AND DETAILING FOR EARTHQUAKE LOADS
CONTENTS
12. 1 General
12. 2 Load and Load Combinations
12. 3 Response Reduction Factors
12. 4 Connections, Joints and Fasteners
12. 5 Columns and column splice
12. 6 Story drift
12. 7 Ordinary Concentric Braced Frames
(OCBF)
12. 8 Special Concentric Braced Frames
(SCBF)
12. 9 Eccentrically Braced Frames (EBF)
12.10 Ordinary Moment Frames (OMF)
12.11 Special Moment Frames (SMF)
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SECTION 12
DESIGN AND DETAILING FOR EARTHQUAKE LOADS
12.1 Scope: Design and Detailing for earthquake resisting frames
only
12.2 Load and Load Combinations
Earthquake Loads as per IS 1893 – 2002 except for Rfactors
load combinations for limit state design
1.5 (DL+ LL)
1.2 (DL + LL + EL)
0.9 DL + 1.50EL
– Special requirements to avoid instabilities like
buckling & over-turning
a) 1.2 DL +0.5 LL + 2.5 EL
b) 0.9 DL + 2.5 EL
(Conti….)
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SECTION 12
DESIGN AND DETAILING FOR EARTHQUAKE LOADS
12.3
SI.No
1
2
Response Reduction Factors
Lateral Load Resisting System
R
Braced frame systems:
a)Ordinary Concentrically Braced Frames (OCBF)
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b) Special Concentrically Braced Frame (SCBF)
4.5
c) Eccentrically Braced Frame (EBF)
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Moment Frame System:
a) Ordinary moment frame (OMF)
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b) Special moment frame (SMF)
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(Conti….)
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CONNECTIONS, JOINTS AND FASTENERS
• All bolts used in frames resisting earthquake loads shall be
fully tensioned, High Strength Friction Grip (HSFG) bolts, in
standard holes.
•
All welds used in frame resisting earthquake loads shall be
complete penetration butt welds, except in column splice
•
Bolted joints shall not be designed to share load in
combination with welds on the same faying surface
The black bolts or fillet welds may be used
in frames not intended to resist earthquake
loads provided they can tolerate the deformation.
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BEAM-TO-COLUMN CONNECTIONS
(a) Simple – transfer only shear at nominal eccentricity
Used in non-sway frames with bracings etc.
Used in frames upto 5 storeys
(b) Semi-rigid – model actual behaviour but make analysis
difficult (linear springs or Adv.Analysis). However lead
to economy in member designs.
(c) Rigid – transfer significant end-moments undergoing
negligible deformations. Used in sway frames for
stability and contribute in resisting lateral loads and
help control sway.
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BEAM-TO-COLUMN CONNECTIONS
column
web
stiffeners
(a)
diagonal
stiffener
(b)
web
plate
(c)
Rigid beam-to-column connections a) Short end plate
b) Extended end plate c) Haunched
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12.5 COLUMNS
Column strength
•
When Pr/Pc is greater than 0.4…..
Pr is required compressive strength of the member
Pc is actual compressive strength of the member
…… the required axial compressive in the absence of moment to be
determined from load combinations
a) 1.2 DL +0.5 LL + 2.5 EL
b) 0.9 DL + 2.5 EL
The required strength determined in above need not exceed
1.2 times the connecting beam or brace nominal strength
the resistance of the foundation to uplift.
• Actual load on column during severe earthquake could be higher due to
system over-strength, frame action, strain hardening & vertical motion
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COLUMN SPLICE
• Partial-joint penetration groove welds, provided in columns
splice, shall be designed for 200% of the required strength.
Pmin =0.6 fyAf
Pmin =0.6 fyAf
STORY DRIFT
• The Design Story Drift and story drift limits shall be confirm
to IS:1893-2002
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SEISMIC BEHAVIOUR OF FRAMES
Sway frames and non-say frames
Braced and un-braced frames
Concentrically Braced Frames (CBF)
Eccentrically Braced Frames (EBF)
(a)Diagonal
bracing
(b)Cross or
X-bracing
(c )Chevron
bracing
Link
Beams
(d)Eccentric
bracing
Bracing systems in Steel Frames
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GENERAL COMMENTS ON FRAME CLASSIFICATION
•
Structural steel frames classified as ordinary and special
depending upon their design ductility levels
•
High seismic zones and important frames, relying more on
ductility and use of higher response reduction factors would be
beneficial from economic and safety consideration
12.7.1.1
• Ordinary frames not permitted in seismic zones IV and V
• Also in Zone III for structures with I > 1.0
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12.7 ORDINARY CONCENTRICALLY BRACED FRAMES (OCBF)
Frame Configuration
• Provisions only for diagonal and X- bracing
• V and inverted-V to be designed as per specialist literature
• K-braced frames not permitted
12.7.2 Bracing Members
• Slenderness of bracing member < 120
• P(required) < 0.8 P(design)
• Bracing cross-section not slender (b/t < 15.7 )
• Bracing slopes in both directions
• Tensile braces carry 30-70% of load
• Built-up braces: local slenderness < 0.4 Overall slenderness
• Connection strength to withstand 1.2Agfy, force under
additional load combinations and maximum possible force
• Check for tension rupture, block shear and gusset local
buckling
• Connection to withstand 1.2 Mp of brace section
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12.8 SPECIAL CONCENTRICALLY BRACED FRAMES (SCBF)
Frame Configuration
• Provisions only for diagonal and X- bracing
• V and inverted-V to be designed as per specialist literature
• K-braced frames not permitted
Bracing Members
• Slenderness of bracing member < 160
• P(required) < 1.0 P(actual)
• Bracing cross-section plastic (b/t < 9.4 )
• Bracing slopes in both directions
• Tensile braces carry 30-70% of load
• Built-up braces: local slenderness < 0.4 Overall slenderness
• Connection as in OCBF
• Columns should have plastic cross-sections
• Splices to resist shear and 0.5Mp of smaller section
12.9 Eccentrically Braced Frames (EBF) as per specialist literature
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12.10 ORDINARY MOMENT FRAMES (OMF)
• Rigid or Semi-rigid moment Connections permitted.
• Rigid moment connection to withstand 1.2Mp of beam or the
maximum moment that can be delivered, whichever is less.
• Semi-rigid connections to withstand 0.5Mp of beam, or the
maximum moment that can be delivered, whichever is less .
The design moment shall be achieved within a rotation of 0.01 rad.
• The stiffness and strength of semi-rigid connection shall be
accounted for in the analysis and design, and the overall stability
of the frame ensured
• Both Rigid and Semi-Rigid connection, to withstand a shear
resulting from the load combination 1.5DL+1.5LL plus the shear
corresponding to the design moment defined above
(respectively).
• In Rigid fully welded connections, continuity plates (stiffener
plates) of thickness equal to or greater than the thickness of the
beam flange shall be provided and welded to the column flanges
and web.
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12.11 SPECIAL MOMENT FRAMES (SMF)
Beam-to-column joints and connections
• Rigid connections only, to withstand a moment of
1.2Mp of beam.
In case of a reduced beam section, 0.8Mp of unreduced
section.
• The connection to withstand a shear from the load
combination 1.2DL+ 0.5LL plus the shear from the
application of 1.2 Mp in the opposite sense, at each
end of the beam. The shear strength need not exceed
the value corresponding to additional load
combinations.
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BUCKLING OF WEB PLATES IN SHEAR
cr
Shear buckling of a plate
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Beam-to-column joints and connections (SMF)
• In column strong axis connections, the panel zone shall be
checked for shear buckling at the design shear defined
above. Doubler plates or diagonal stiffener may be used to
strengthen the web against shear buckling.
The individual thickness of the column webs and doubler
plates, shall exceed (dp+bp)/90.
• Continuity plates (stiffener plates) shall be provided in all
strong axis welded connections except in end plate
connection
Continuity
plate
dp
bp
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SPECIAL MOMENT FRAMES (SMF)
Beam and column limitation
• Beam and column sections shall be plastic or compact. At
potential plastic hinge locations, they shall be necessarily
plastic.
• The section selected for beams and columns shall satisfy the
following relation
 Mpc > 1.2  Mpb
• Lateral support to the column at both top and bottom beam
flange levels shall be provided so as to resist at least 2% of the
beam flange strength, except for the case described below.
• A Plane frame with support in the direction perpendicular to its
plane, shall be checked for buckling, also under the additional
load combination.
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12.12 Column Bases
• Fixed column bases and anchor bolts to withstand a
moment of 1.2 times Mp of column section
• All bases to withstand full shear under all load
combinations or 1.2 times shear capacity of column section
whichever is higher
Dr. S. R. Satish Kumar
Dept. of Civil Engineering
IIT Madras
Email: [email protected]
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