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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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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

ADVANTAGES OF USING STEEL FOR EARTHQUAKE RESISTANCE

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SECTION 12 DESIGN AND DETAILING FOR EARTHQUAKE LOADS

CONTENTS

12. 1 General12. 2 Load and Load Combinations12. 3 Response Reduction Factors12. 4 Connections, Joints and Fasteners12. 5 Columns and column splice12. 6 Story drift12. 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 R-factors

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 Response Reduction Factors

(Conti….)

SI.No Lateral Load Resisting System R

1 Braced frame systems: a)Ordinary Concentrically Braced Frames (OCBF) 4

b) Special Concentrically Braced Frame (SCBF) 4.5

c) Eccentrically Braced Frame (EBF) 5

2 Moment Frame System: a)   Ordinary moment frame (OMF) 4 b)   Special moment frame (SMF) 5

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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

Rigid beam-to-column connections a) Short end plateb) Extended end plate c) Haunched

column webstiffeners

diagonalstiffener

web plate

(a) (b) (c)

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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.

12.5 COLUMNS

• 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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Sway frames and non-say framesBraced and un-braced frames

Concentrically Braced Frames (CBF)Eccentrically Braced Frames (EBF)

(a)Diagonal bracing

(b)Cross or X-bracing

(c )Chevron bracing

(d)Eccentric bracing

Link Beams

Bracing systems in Steel Frames

SEISMIC BEHAVIOUR OF 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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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

12.7 ORDINARY CONCENTRICALLY BRACED FRAMES (OCBF)

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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

12.8 SPECIAL CONCENTRICALLY BRACED FRAMES (SCBF)

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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Shear buckling of a plate

BUCKLING OF WEB PLATES IN SHEAR

cr

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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

dp

bp

Continuity plate

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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.

SPECIAL MOMENT FRAMES (SMF)

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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 KumarDept. of Civil Engineering

IIT MadrasEmail: sr.satishkumar@gmail.com