Presentaion-AISC Seismic Design-Module2-Moment Resisting Frames

7/27/2019 Presentaion-AISC Seismic Design-Module2-Moment Resisting Frames

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Design of Seismic-Resistant Steel

Building Structures

Prepared by:

Michael D. EngelhardtUniversity of Texas at Austin

with the support of the American Institute of Steel Construction.

Version 1 - March 2007

2. Moment Resisting Frames


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2 - Moment Resisting Frames

Definition and Basic Behavior of Moment ResistingFrames

Beam-to-Column Connections: Before and After Northridge

Panel-Zone Behavior

AISC Seismic Provisions for Special Moment Frames


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Moment Resisting Frames



Panel-Zone Behavior

AISC Seismic Provisions for Special Moment Frames


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MOMENT RESISTING FRAME (MRF)

Advantages

Architectural Versatility High Ductility and Safety

Disadvantages

Low Elastic Stiffness

Beams and columns with moment resistingconnections; resist lateral forces by flexure andshear in beams and columns

Develop ductility by:- flexural yielding of beams

- shear yielding of column panel zones- flexural yielding of columns


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Moment Resisting Frame


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Achieving Ductile Behavior:

Choose frame elements ("fuses") that willyield in an earthquake, i.e, choose plastic

hinge locations. Detail plastic hinge regions to sustain

large inelastic rotations prior to the onsetof fracture or instability.

Design all other frame elements to bestronger than the plastic hinge regions.

Understand and Control Inelastic Behavior:


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Behavior of an MRF Under Lateral Load:Internal Forces and Possible Plastic Hinge Locations


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Possible Plastic Hinge Locations

Beam(Flexural Yielding)

Panel Zone(Shear Yielding)

Column(Flexural & Axial

Yielding)


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Plastic HingesIn Beams


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Plastic HingesIn Column Panel Zones


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Plastic HingesIn Columns:

Potential for SoftStory Collapse


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Critical Detailing Area for Moment Resisting Frames: Beam-to-Column Connections

Design Requirement:Frame must develop large ductilitywithout failure of beam-to-columnconnection.


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Moment Connection Design Practice Prior to1994 Northridge Earthquake:

Welded flange-boltedweb moment connectionwidely used from early1970s to 1994


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Pre-NorthridgeWelded Flange Bolted Web Moment Connection

Backup Bar

Beam Flange

Column FlangeStiffener

Weld Access Hole


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Experimental Data on Pre -NorthridgeMoment Connection

Typical ExperimentalSetup:


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Initial Tests on Large Scale Specimens:

Tests conducted at UC Berkeley ~1970 Tests on W18x50 and W24x76 beams Tests compared all-welded connections

with welded flange-bolted web connections


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Welded Flange Bolted Web Detail


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Observations from Initial UC Berkeley Tests:

Large ductility developed by all-weldedconnections.

Welded flange-bolted web connections developedless ductility, but were viewed as still acceptable.


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Subsequent Test Programs:

Welded flange-bolted web connections showedhighly variable performance.

Typical failure modes: fracture at or near beamflange groove welds. A large number of laboratory tested connections

did not develop adequate ductility in the beam

prior to connection failure.


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-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

-0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04

Drift Angle (rad)

B e n

d i n g

M o m

e n

t ( k N - m

)

Brittle Fracture at BottomFlange Weld

Mp

Mp

Pre-Northridge Connection


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Summary of Testing Prior toNorthridge Earthquake

Welded flange bolted web connection showedhighly variable performance

Many connections failed in laboratory with littleor no ductility


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1994 Northridge Earthquake

Widespread failure of welded flange - boltedweb momentconnections


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1994 Northridge Earthquake

January 17, 1994 Magnitude = 6.8

Epicenter at Northridge - San Fernando Valley(Los Angeles area)

Fatalities: 58

Estimated Damage Cost: $20 Billion


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Northridge - Ground Accelerations

Sylmar: 0.91g H 0.60g V Sherman Oaks: 0.46g H 0.18g V Granada Hills: 0.62g H 0.40g V Santa Monica: 0.93g H 0.25g V North Hollywood: 0.33g H 0.15g V


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Damage Observations:Steel MomentConnections

Pre-Northridge


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Backup Bar

Beam Flange

Column FlangeStiffener

Weld Access Hole

Pre-NorthridgeWelded Flange Bolted Web Moment Connection


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Damage Observations

A large number of steel moment frame buildingssuffered connection damage

No steel moment frame buildings collapsed Typical Damage: fracture of groove weld

divot fracture within column flange fracture across column flange and web


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Response to Northridge Moment ConnectionDamage

Nearly immediate elimination of welded flange -bolted web connection from US building codes anddesign practice

Intensive research and testing efforts to understandcauses of damage and to develop improvedconnections AISC, NIST, NSF, etc. SAC Program (FEMA)


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Causes of Moment ConnectionDamage in Northridge

Welding

Connection Design

Materials

Causes of Northridge Moment Connection


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Damage:

Welding Factors

Low Fracture Toughness of Weld Metal

Poor Quality Effect of Backing Bars and Weld Tabs


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Weld Metal Toughness

Most common Pre-Northridge welding electrode(E70T-4) had very low fracture toughness.

Typical Charpy V-Notch: < 5 ft.-lbs at 70 0F(7 J at 21 0C)


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Welding Quality

Many failed connections showed evidence of poor weld quality

Many fractures initiated at root defects in bottomflange weld, in vicinity of weld access hole


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Causes of Northridge Moment ConnectionD


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Design Factors:

Stress/Strain Too High at Beam Flange Groove Weld

Inadequate Participation of Beam Web Connection inTransferring Moment and Shear

Effect of Weld Access Hole Effect of Column Flange Bending Other Factors

Damage:

s s

Fu


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Mp

Increase in Flange Stress Due toInadequate Moment Transfer Through Web Connection

F l a n g e

S t r e s

Fy


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Vflange

Increase in Flange Stress Due to Shear in Flange


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Strategies for Improved Performanceof Moment Connections

Welding

Materials

Connection Design and Detailing

Strategies for Improved Performance of Moment


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Connections:

WELDING

Required minimum toughness for weld metal: Required CVN for all welds in SLRS:

20 ft.-lbs at 0 0 F

Required CVN for Demand Critical welds:20 ft.-lbs at -20 0 F and 40 ft.-lbs at 70 0 F


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Strategies for Improved Performance of MomentConnections:


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Materials (Structural Steel)

Introduction of expected yield stress into designcodes

Fy = minimum specified yield strength

R y = 1.5 for ASTM A36

= 1.1 for A572 Gr. 50 and A992(See AISC Seismic Provisions - Section 6 for other values of R y )

Expected Yield Stress = R y Fy



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Materials (Structural Steel)

Introduction of ASTM A992 steel for wide flangeshapes

ASTM A992Minimum F y = 50 ksi

Maximum F y = 65 ksi

Minimum F u = 65 ksi

Maximum F y / Fu = 0.85



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Connections:

Connection Design

Improved Weld Access Hole Geometry


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Improved Weld AccessHole

See Figure 11-1 in the2005 AISC Seismic Provisions for dimensionsand finish requirements


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Connections:

Connection Design

Development of Improved Connection Designsand Design Procedures

Reinforced Connections Proprietary Connections Reduced Beam Section (Dogbone)

Connections Other SAC Investigated Connections


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Proprietary Connections

SIDE PLATE


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SIDE PLATECONNECTION

SLOTTED WEB


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SLOTTED WEBCONNECTION


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Connections Investigated ThroughSAC-FEMA Research Program

Reduced BeamSection


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Section


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WeldedUnreinforcedFlange - BoltedWeb

WeldedUnreinforced


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Flange - WeldedWeb

Free Flange


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Free FlangeConnection

Welded FlangePlate Connection


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Plate Connection

Bolted UnstiffenedEnd Plate


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End Plate

Bolted StiffenedEnd Plate


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Bolted FlangePlate


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Double Split Tee


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Double Split Tee

Results of SAC-FEMA Research ProgramR d d S i i D i C it i


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Recommended Seismic Design Criteriafor Steel Moment Frames

FEMA 350Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings

FEMA 351Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings

FEMA 352Recommended Postearthquake Evaluation and Repair Criteriafor Welded Steel Moment-Frame Buildings

FEMA 353Recommended Specifications and Quality AssuranceGuidelines for Steel Moment-Frame Construction for Seismic

Applications


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FEMA 350



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Panel-Zone Behavior AISC Seismic Provisions for Special Moment Frames

Column Panel Zone


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Colum n Panel Zon e: - subject to high shear - shear yielding and large

shear deformations possible(forms shear hinge)

- provides alternate yieldingmechanism in a steel momentframe


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Joint deformationdue to panel zoneshear yielding


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Plastic Shear HingesIn Column Panel Zones


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"kink" at corners of panel zone

400C i RBS S i i h


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-400

-300

-200

-100

0

100

200

300

-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Story Drift Angle (rad)

C o

l u m n

T i p

L o a

d ( k i p s

)

Composite RBS Specimen withWeak Panel Zone

1200Composite RBS Specimen with


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-1200

-800

-400

0

400

800

-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Panel Zone g (rad)

P a n e

l Z o n e

S h e a r

F o r c e

( k i p s

)

Weak Panel Zone

g

Observations on Panel Zone Behavior


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Observations on Panel Zone Behavior

Very high ductility is possible. Localized deformations (kinking) at corners of panel

zone may increase likelihood of fracture in vicinity of beam flange groove welds.

Building code provisions have varied greatly on panelzone design.

Current AISC Seismic Provisions permits limitedyielding in panel zone.

Further research needed to better define acceptablelevel of panel zone yielding



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Panel-Zone Behavior AISC Seismic Provisions for Special Moment Frames


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2005 AISC Seismic Provisions

Section 9 Special Moment Frames (SMF)

Section 10 Intermediate Moment Frames (IMF)

Section 11 Ordinary Moment Frames (OMF)

Section 9Special Moment Frames (SMF)


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p ( )9.1 Scope

9.2 Beam-to-Column Joints and Connections

9.3 Panel Zone of Beam-to-Column Connections

9.4 Beam and Column Limitations

9.5 Continuity Plates

9.6 Column-Beam Moment Ratio

9.7 Lateral Bracing of at Beam-to-Column Connections

9.8 Lateral Bracing of Beams

9.9 Column Splices

AISC Seismic Provisions - SMF 9.1 Scope


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Special moment frames (SMF) are expected to withstandsignificant inelastic deformations when subjected to theforces resulting from the motions of the designearthquake.


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AISC Seismic Provisions - SMF - Beam-to-Column Connections9.2a Requirements


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Beam-to-column connections shall satisfy the following three

requirements:

1. The connection shall be capable of sustaining aninterstory drift angle of at least 0.04 radians.

2. The measured flexural resistance of theconnection, determined at the column face, shall

equal at least 0.80 M p of the connected beam atan interstory drift angle of 0.04 radians.

9.2a Requirements

Beam-to-column connections shall satisfy the following three


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requirements (cont):

3. The required shear strength of the connectionshall be determined using the following quantityfor the earthquake load effect E :

E = 2 [ 1.1R y

M p

] / Lh

(9-1)

where:

R y = ratio of the expected yield strength to theminimum specified yield strength

M p = nominal plastic flexural strength

Lh = distance between plastic hinge locations

Required Shear Strength of Beam-to-Column Connection


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Lh

(1.2 + 0.2S DS ) D + 0.5 L or (0.9-0.2S DS ) D1.1 R y M p 1.1 R y M p

V u = 2 [ 1.1 R y M p ] / L h + V gravity

V u V u

AISC Seismic Provisions - SMF - Beam-to-Column Connections9.2b Conformance Demonstration


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Demonstrate conformance with requirements of Sect. 9.2a by one of

the following methods:

I. Conduct qualifying cyclic tests in accordance with Appendix S.

Tests conducted specifically for the project, with test specimens thatare representative of project conditions.

or

Tests reported in the literature (research literature or other

documented test programs), where the test specimens arerepresentative of project conditions.

9.2b Conformance Demonstration

Demonstrate conformance with requirements of Sect 9 2a by one of


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Demonstrate conformance with requirements of Sect. 9.2a by one of the following methods (cont):

II. Use connections prequalified for SMF in accordance with Appendix P

Use connections prequalified by the AISC ConnectionPrequalification Review Panel (CPRP) and documented in StandardANSI/AISC 358 -"Prequalified Connections for Special and IntermediateSteel Moment Frames for Seismic Applications"

or

Use connection prequalified by an alternative review panel that isapproved by the Authority Having Jurisdiction.

9.2b Conformance Demonstration - by Testing


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Test connectionin accordancewith Appendix S

Appendix SQualifying Cyclic Tests of Beam-to-Column

and Link-to-Column Connections


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Testing Requirements:

Test specimens should be representative of prototype(Prototype = actual building)

Beams and columns in test specimens must be nearly full-scalerepresentation of prototype members:

- depth of test beam 0.90 depth of prototype beam- wt. per ft. of test beam 0.75 wt. per ft. of prototype beam

- depth of test column 0.90 depth of prototype column

Sources of inelastic deformation (beam, panel zone, connectionplates, etc) in the test specimen must similar to prototype.

Appendix S

Testing Requirements (cont):


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Lateral bracing in test specimen should be similar to prototype. Connection configuration used for test specimen must match

prototype.

Welding processes, procedures, electrodes, etc. used for test

specimen must be representative of prototype.

See Ap pendix S for o th er requirements .

Typical Test Subassemblages


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Exterior Subassemblage Interior Subassemblage

Typical Exterior Subassemblage


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Lbeam

Interstory Drift Angle = Lbeam

Typical Exterior Subassemblage


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Typical Interior Subassemblage


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Hcolumn

Interstory Drift Angle =

Hcolumn

Typical Interior Subassemblage


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Appendix S

Testing Requirements - Loading History


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Apply the following loading history:6 cycles at = 0.00375 rad.

6 cycles at = 0.005 rad.





2 cycles at = 0.03 rad.2 cycles at = 0.04 rad.

continue at increments of 0.01 rad, with twocycles of loading at each step

Appendix S Testing Requirements - Loading History


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-0.05

-0.04

-0.03

-0.02

-0.010

0.01

0.02

0.03

0.040.05

0.06

I n t e

r s t o

r y D

r i f t A

n g

l e

Acceptance Criteria for SMF Beam-to-Column Connections: After completing at least one loading cycle at 0.04 radian, the measured flexuralresistance of the connection, measured at the face of the column, must be at least0.80 M p of the connected beam

Example of Successful Conformance Demonstration Testper Appendix S:


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-30000

-20000

-10000

0

10000

20000

30000

40000

-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Interstory Drift Angle (rad)

B e a m

M

o m e n

t a

t F a c e o f

C o

l u m n

( i n - k

i p s )

0.8 M p

- 0.8 M p

M0.04 0.8 M p

M0.04 0.8 M p

9.2b Conformance Demonstration........by use of Prequalified Connection


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A Prequalified connection is one that has undergone sufficient

testing (per Appendix S)

analysis

evaluation and review

so that a high level of confidence exists that the connection canfulfill the performance requirements specified in Section 9.2a for

Special Moment Frame Connections

9.2b Conformance Demonstration .....by use of Prequalified Connection


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Requirements for Prequalification of Connections:Appendix P - Prequalification of Beam-to-Column

and Link-to-Column Connections

Authority to Prequalify of Connections:AISCConnection Prequalification Review Panel (CPRP)

Information on Prequalified Connections:Standard ANSI/AISC 358 - "Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications "


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ANSI/AISC 358 - "Prequalified Connections for Special and I di S l M F f S i i A li i "


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Intermediate Steel Moment Frames for Seismic Applications "

Connections Prequalified in ANSI/AISC 358 (1st Ed - 2005)

Reduced Beam Section (RBS) Connection

Bolted Unstiffened and Stiffened Extended End-Plate Connection

Reduced Beam Section (RBS) Moment Connection


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RBS Concept:

Trim Beam Flanges Near Connection Reduce Moment at

Connection

Force Plastic Hinge Awayfrom Connection

Example of laboratory performance of an RBS connection:


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Whitewashed connection prior to testing:


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Whitewashed connection prior to testing:


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Connection at 0.02 radian......


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3000

4000

5000

Mp


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-3000

-2000

-1000

0

1000

2000

3000

-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05

Drift Angle (radian)

B e n

d i n g

M o m e n

t ( k N - m

)

RBS Connection

Mp

ANSI/AISC 358:

Prequalification Requirements for RBS in SMF


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Beam depth: up to W36 Beam weight: up to 300 lb/ft

Column depth: up to W36 for wide-flange

up to 24-inches for box columns Beam connected to column flange

(connections to column web not prequalified)

RBS shape: circular

RBS dimensions: per specified design procedure

ANSI/AISC 358:Prequalification Requirements for RBS in SMF

cont......


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Beam flange welds: - CJP groove welds- Treat welds asDemand Critical - Remove bottom flange backing and provide

reinforcing fillet weld

- Leave top flange backing in-place; fillet weldbacking to column flange

- Remove weld tabs at top and bottom flanges

Beam web to column connection:

- Use fully welded web connection (CJP weldbetween beam web and column flange)

See ANSI/AISC 358 for additional requirements (continuity plates, beamlateral bracing, RBS cut finish req'ts., etc.)

RBS with welded web


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RBS with welded web

connection:


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Examples of RBS Connections.....


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AISC Seismic Provisions - SMF - Panel Zone Requirements9.3a Shear Strength


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The minimum required shear strength,R u , of the panel zone shall betaken as the shear generated in the panel zone when plastic hinges formin the beams.

To compute panel zone shear.....Determine moment at beam plastic hinge locations

(1.1 R y M p or as specified in ANSI/AISC 358)

Project moment at plastic hinge locations to the faceof the column (based on beam moment gradient)

Compute panel zone shear force.

Panel Zone Shear Strength (cont)


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M pr-2 M pr1

V beam-2

V beam-1

Beam 1 Beam 2

Plastic Hinge Location


s h s h

M f1 M f2

M pr = expected moment at plastic hinge = 1.1R y M p or as specified in ANSI/AISC 358

V beam = beam shear (see Section 9.2a - beam required shear strength)

sh = distance from face of column to beam plastic hinge location (specified in ANSI/AISC 358)


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c

f b

f u V

t d

M R Panel Zone Required Shear Strength =


Panel Zone Design Requirement:


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Panel Zone Design Requirement:

R u v R v wh ere v = 1.0

R v = nominal shear strength, basedon a limit state of shear yielding, ascomputed per Section J10.6 of the

AISC Specification


To compute nominal shear strength, R v , of panel zone:


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When P u 0.75 P y in column:

pc b

2 cf cf

pc y v

t d d

t b31t d F 6 .0 R

(AISC Spec EQ J10-11)

Where: d c = column depth

d b = beam depth

bcf = column flange width

t cf = column flange thickness

F y = minimum specified yield stress of column web

t p = thickness of column web including doubler plate


To compute nominal shear strength, R v , of panel zone:


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When P u > 0.75 P y in column (not recommended) :

y

u

pc b

2 cf cf pc y v P

P 2 .19.1t d d t b31t d F 6 .0 R (AISC Spec EQ J10-12)

If shear strength of panel zone is inadequate:

- Choose column section with larger web area


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- Weld doubler plates to column

Options for Web Doubler Plates

AISC Seismic Provisions - SMF 9.4 Beam and Column Limitations

Beam and column sections must satisfy the width-


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y

thickness limitations given in Table I-8-1

y

s

f

f

F E 30 .0

t 2 b

Beam Flanges

Beam Web

y

s

w F E

45 .2 t h

b f

t f

h

t w

Column Flanges sf E 30 .0 b



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y f F t 2

Column Web

125 .0 P

P

y

u

y

u

y

s

w P

P 54.11

F

E 14.3

t

h

125 .0

P

P

y

u

y

s

y

u

y

s

w F E

49.1P

P 33.2

F E

12 .1t h

Note: Column flange and web slenderness limits can be taken as p in AISCSpecificationTable B4.1, if the ratio for Eq. 9-3 is greater than 2.0

AISC Seismic Provisions - SMF 9.5 Continuity Plates


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Continuity Plates

9.5 Continuity Plates


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Continuity Plates


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AISC Seismic Provisions - SMF 9.5 Continuity Plates


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Continuity plates shall be consistent with therequirements of a prequalified connection as specified inANSI/AISC 358 (Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications)

or

As determined in a program of qualification testing inaccordance with Appendix S

ANSI/AISC 358 -Continuity Plate Requirements

Continuity PlatesFor Wide-Flange Columns:


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Continuity plates are required, unless:

yc yc

ybybbf bf cf F R

F R t b8 .14.0 t

6 b

t bf cf

and

t cf = column flange thickness

bbf = beam flange width

t bf = beam flange thickness


For Box Columns:


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Continuity Plates

For Box Columns:

Continuity plates must be provided.


Required thickness of continuity plates


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a) For one-sided (exterior) connections, continuity plate thickness shall beat least one-half of the thickness of the beam flange.

b) For two-sided (interior) connections, continuity plate thickness shall be atleast equal to the thicker of the two beam flanges on either side of the

column

For other design, detailing and welding requirements for continuity plates - See ANSI/AISC 358

t



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t cp

t bf t cp 1/2 t bf

t cp



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p

t bf-2 t bf-1

t cp larger of ( t bf-1 and t bf-2 )


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AISC Seismic Provisions - SMF 9.6 Column-Beam Moment Ratio

Section 9.6 requires strong column - weak girder design for SMF (with a few exceptions)


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design for SMF (with a few exceptions)

Purpose of strong column -weak girder requirement:

Prevent Soft Story Collapse

AISC Seismic Provisions - SMF 9.6 Column-Beam Moment Ratio

The following relationship shall be satisfied at beam-to-l ti


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column connections:

0 .1M

M * pb

* pc Eqn. (9-3)

9.6 Column-Beam Moment Ratio

0 .1M

M * pb

* pc


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pb

* pc M the sum of the moments in the column above and below the joint at

the intersection of the beam and column centerlines.M * pc is determined by summing the projections of thenominal

flexural strengths of the columnsabove and below the joint to thebeam centerline with a reduction for the axial force in the column.

It is permitted to take M * pc = Z c ( F yc - P uc /Ag )

* pbM the sum of the moments in the beams at the intersection of the beamand column centerlines.

M * pb is determined by summing the projections of theexpectedflexural strengths of the beamsat the plastic hinge locations to thecolumn centerline.

C ColumnL0 .1M M

* pb

* pc


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C BeamL

M* pc-top

M* pc-bottom M* pb-left

M* pb-right

Note:M* pc is based on minimum specified yieldstress of column

M* pb is based on expected yield stress of beamand includes allowance for strain hardening

V beam-right

Left Beam Right Beam


Computing M* pb


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M pr-right M pr-left

beam right

V beam-left Plastic Hinge Location

s h+d col /2

M pr = expected moment at plastic hinge = 1.1R y M p or as specified in ANSI/AISC 358

V beam = beam shear (see Section 9.2a - beam required shear strength)sh = distance from face of column to beam plastic hinge location (specified in

ANSI/AISC 358)

M* pb-left M* pb-r ight

s h+d col /2

M* pb = M pr + V beam (s h + d col /2 )

Top Column

Computing M* pc M pc- top

M* pc- top

V col-top


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Bottom Column

M pc = nominal plastic moment capacity of column, reduced for presence of axial force; cantake M pc = Z c (F yc - P uc / Ag ) [or use more exact moment-axial force interaction

equations for a fully plastic cross-section]V col = column shear - compute from statics, based on assumed location of column inflectio

points (usually midheight of column)

M* pc-bot tom

M* pc = M pc + V col (d beam /2 )

M pc-bot tom

d beam

V col-bottom

AISC Seismic Provisions - SMF 9.8 Lateral Bracing of Beams

Must provide adequate lateral bracing of beams in SMF


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Must provide adequate lateral bracing of beams in SMFso that severe strength degradation due to lateraltorsional buckling is delayed until sufficient ductility isachieved

(Sufficient ductility = interstory drift angle of at least 0.04rad is achieved under Appendix S loading protocol)

Lateral Torsional BucklingLateral torsionalbuckling controlled by: b

r

L


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y r

L b = dis tanc e betw een beam lateral br aces

r y = w eak axis radius o f gyrat ion

L b L b

Beam lateral braces (top & bottom flanges)


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Effect of Lateral Torsional Buckling on Flexural Strength and Ductility:

M


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M

M p

Increasing L b / r y


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AISC Seismic Provisions - SMF 9.8 Lateral Bracing of Beams

In addition to lateral braces provided as a maximum spacing


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of Lb = 0.086r y E / F y :

Lateral braces shall be placed near concentrated forces, changes in cross-section and other locations where analysis indicates that a plastic hinge

will form.

The placement of lateral braces shall be consistent with that specified inANSI/AISC 358 for aPrequalified Connection , or as otherwise determined

by qualification testing.


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ANSI/AISC 358 -Lateral Bracing Requirements for the RBS

For beams with an RBS connection:


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When a composite concrete floor slab is present, no additionallateral bracing is required at the RBS.

When a composite concrete floor slab is not present, provide anadditional lateral brace at the RBS. Attach brace just outside of the RBS cut, at the end farthest from the column face.


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Section 9Special Moment Frames (SMF)

9.1 Scope


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9.2 Beam-to-Column Joints and Connections9.3 Panel Zone of Beam-to-Column Connections


9.5 Continuity Plates9.6 Column-Beam Moment Ratio

9.7 Lateral Bracing of at Beam-to-Column

Presentaion-AISC Seismic Design-Module2-Moment Resisting Frames

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