Moment Resisting Frames - Part2

7/27/2019 Moment Resisting Frames - Part2

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Strategies for Improved Performance of Moment

Connections:

WELDING

Required minimum toughness for weld metal:

Required CVN forall welds in SFRS:20 ft.-lbs at 00 F

Required CVN forDemand Criticalwelds:40 ft.-lbs at 700 F

ANSI/AISC 341-10 Section A3.4


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WELDING

Improved practices for backing bars and weld tabs

Typical improved practice:

Remove bottom flange backing bar

Seal weld top flange backing bar

Remove weld tabs at top and bottom flange welds

Greater emphasis on quality and quality control (AISCSeismic Provisions Chapter J)


Connections:


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Typical Pre-Northridge

Bottom Flange Weld


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Improved POST-Northridge Bottom Flange Weld

Weld tabs andrunoff regions

removed; ground

smooth

Back-up bar removed; root visually

inspected, defects removed; smallreinforcing fillet weld placed at

bottom of groove weld


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Typical Pre-Northridge

Top Flange Weld


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Improved POST-

Northridge Top

Flange Weld

Weld tabs and

runoff regions

removed;

ground

smooth

Back-up bar left in place; smallfillet weld placed between bar

and column face


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Connections:Materials (Structural Steel)

Introduction of expected yield stress into designcodes

Fy = minimum specified yield strength

Ry = 1.5 for ASTM A36 (hot-rolled shapes and bars)= 1.1 for A572 Gr. 50 and A992

Expected Yield Stress = Ry Fy

ANSI/AISC 341-10 Table A3.1


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Connections:

Materials (Structural Steel)

Introduction of ASTM A992 steel for wide flangeshapes

ASTM A992

Minimum Fy = 50 ksi

Maximum Fy = 65 ksi

Minimum Fu = 65 ksi

Maximum Fy / Fu = 0.85


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Pre-Northridge

Improved


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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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Reinforced Connection

Cover-Plated

Connection

Cover plates fillet welded

to beam flanges, thencombinedbeam flange

and cover plate groove

welded to face of column


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Cover-Plated Connection

Improvedperformance in

general, but

costly to

construct


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Cover-Plated Connection About 2/3 of specimensdeveloped total plasticrotation of 0.03 radwithout brittle fracture

(strong panel zone) Others, panel zone

yielding dominatedresponse

2 specimens with boltedwebs failed in brittlemanner (< 0.02 radplastic rotation)

Brittle fracture inspecimen for whichwelding procedure notenforced

Failure in specimen withLONG cover plate

Not sufficiently

reliable?

Susceptible to sameproblems of weld

quality and through-

thickness behavior of

column flange?


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Flange Rib

Connection

Like cover-plated connection,

connection is stronger than

beam, plastic hinge formation

forced away from face of

column


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Flange RibConnection


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Lee, C.H. et al. (2005) Experimental Study of Cyclic Seismic Behavior of

Steel Moment Connections Reinforced with Ribs, Journal of Structural

Engineering, Vol. 131, No. 1, January 1, 2005


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Haunched

Connection

May be at bottom flange or

both top and bottom flanges

Initially tested as four pre-Northridge connections

repaired with bottom

triangular T-shaped haunches

H h d C ti


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Haunched Connections

Generally good

performance inlaboratory

Costly to construct

Not included asprequalified for newbuildings in FEMA350

Recommended SeismicDesign Criteria for NewSteel Moment-FrameBuildings(2000) More on FEMA 350 later


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Reduced Beam

Section (RBS)

Also called Dogbone

connection; less costly, simplerthan reinforced connections

Forces hinge formation to

occur within reduced section


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RBS has become one of the most common moment

connection details used in current practice.

(More details later )


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Proprietary Connection

Side Plate

Connection

Beam flanges NOT directly

welded to column flanges;

forces transferred through sideplates.

http://www.sideplate.com/


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Connections Investigated Through

SAC-FEMA Research Program


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Reduced Beam

Section (RBS)


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WeldedUnreinforced

Flange - Bolted

Web (WUF-B)


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Welded

Unreinforced

Flange - Welded

Web (WUF-W)


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Free Flange

Connection


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Bolted Unstiffened

End Plate


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Bolted Stiffened

End Plate


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Bolted Flange

Plate


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Double Split Tee

Typically treated

as a partially-

restrainedconnection

(effects of

connection

flexibility must

be included inoverall frame

analysis).

R lt f SAC FEMA R h P


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Results of SAC-FEMA Research Program

Recommended Seismic Design Criteria

for 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 Criteria

for Welded Steel Moment-Frame Buildings

FEMA 353Recommended Specifications and Quality Assurance

Guidelines for Steel Moment-Frame Construction for Seismic

Applications


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FEMA 350

Nine momentconnection details just

shownprequalifiedconnections

Recommended designprocedures, limits of

usage (e.g., OMF only,W36 beams andshallower, flange

thickness limits, webconnection, etc.)

Not a standard; butstill a valuable

reference


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New standardReplaced FEMA 350

Prequalifiedrigorousprogram of testing, analytical

evaluation and review by theconnection prequalificationreview panel (CPRP).

Reduced Beam Section(RBS)

Bolted Stiffened andUnstiffened Extended EndPlate

ANSI/AISC 358-05

Connection Prequalification


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Connection Prequalification

AISC Connection Prequalification

Review Panel ANSI/AISC 341-10 Chapter K

Connections shall be prequalified based on

test data satisfying Section K1.3, supportedby analytical studies and design models.The combined body of evidence must besufficient to assure that the connection can

supply the required story drift angle forSMF and IMF systems . on a consistentand reliable basis within the specified limitsof prequalification.


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ANSI/AISC 358-10

Proprietary Connections

Reduced Beam Section (RBS)


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Reduced Beam Section (RBS)

Bolted Unstiffened Extended End Plate (BUEEP)


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Bolted Unstiffened Extended End Plate (BUEEP)

Bolted Stiffened Extended End Plate (BSEEP)


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Welded Unreinforced Flange Welded


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Welded Unreinforced FlangeWelded

Web (WUF-W)

Kaiser Bolted Bracket (KBB)


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Kaiser Bolted Bracket (KBB)


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Steel Connections: Proprietary or Public Domain? by P. Cordova & R.

Hamburger, Modern Steel Construction, October 2011


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Experimental Evaluation of Kaiser

Bolted Bracket Steel Moment-Resisting Connections

Scott M. Adan and William Gibb

AISC Engineering Journal 2009

http://www.steelcastconnections.com/

ConXtech ConXL moment connection


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ConXtech ConXL moment connection


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http://dcm-designs.com/steel-prefabricated-moment-frame/

This innovative

connection system

enables beams to besimply lowered and locked

onto square columns in

the field, resulting in a

dimensionally accuratestructural chassis. The

system is often referred to

as a full-scale erector set.

http://www.conxtech.com

/conx-system/

VIDEO CLIP

Connections in process of prequalification
http://www.conxtech.com/conx-videos/introduction-to-conxtech/http://www.conxtech.com/conx-videos/introduction-to-conxtech/


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Connections in process of prequalification

Double Tee

Simpson Strong Frame SENSE TSC

Side Plate

SOM Pin Fuse Joint

Double Tee


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Double Tee

http://www.aisc.org/uploadedcontent/2012NASCCSessions/N11/

Simpson Strong Frame (Yield Link)


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Simpson Strong Frame (Yield Link)



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SidePlate


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SidePlate


SOM Pin Fuse Joint


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SOM Pin Fuse Joint

Steel Connections: Proprietary or Public Domain? by P. Cordova & R.

Hamburger, Modern Steel Construction, October 2011

SOM Pin Fuse Joint


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SOM Pin Fuse Joint


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http://www.som.com/content.cfm/pin_fuse_joint


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A number of

experiments havebeen conducted

with specimens

with weak panel

zones


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Very weak panel

zone; localizedkinks cause

strain

concentrations,

ultimately leadingto fracture in

vicinity of beam

flange groove

welds.


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Same specimen

as previous slide.

Connection failed

at moment well

below Mp


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"kink" at corners

of panel zone


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-1200

-800

-400

0

400

800

1200

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

Panel Zone g (rad)

PanelZoneShe

arForce(kips)

Composite RBS Specimen with

Weak Panel Zone

g

Observations on Panel Zone Behavior


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Very high ductility is possible.

Localized deformations (kinking) at cornersof panel zone may increase likelihood offracture in vicinity of beam flange groovewelds.

Current AISC Seismic Provisions permitslimited yielding in panel zone (SpecificationJ10.6 for available strength)

Further research needed to better defineacceptable level of panel zone yielding

Moment Resisting Frames - Part2

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