Moment Resisting Frames - Part1

7/27/2019 Moment Resisting Frames - Part1

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Moment Resisting Frames

Definition and Basic Behavior of Moment

Resisting Frames

Beam-to-Column Connections: Before and After

Northridge

Panel-Zone Behavior

AISC Seismic Provisions for Moment Resisting

Frames: Special, Intermediate and Ordinary


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Resisting Frames


Northridge

Panel-Zone Behavior




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MOMENT RESISTING FRAME (MRF)

Advantages

Architectural Versatility High Ductility and Safety

Disadvantages

Low Elastic Stiffness

Beams and columns with moment resisting

connections; 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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Achieving Ductile Behavior:

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

hinge locations.

Detail plastic hinge regions to sustainlarge inelastic rotations prior to the onset

of fracture or instability.

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

Understand and Contro l Inelast ic Behavior:


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Behavior of an MRF Under Lateral Load:

Internal Forces and Possible Plastic Hinge Locations


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M VNote high shear (V) in Panel Zones


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Possible Plastic Hinge Locations

Beam(Flexural Yielding)

Panel Zone

(Shear Yielding)

Column(Flexural & Axial

Yielding)


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Plastic

Hinges

In Beams


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Plastic Hinges

In Column

PanelZones


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Plastic Hinges

In Columns:

Potential for

Soft Story

Collapse


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Critical Detailing Area for Moment Resisting Frames:

Beam-to-Column Connections

Design

Requirement:

Frame must develop

large ductility

without failure of

beam-to-column

connection.


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Resisting Frames


Northridge

Panel-Zone Behavior




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Moment Connection Design Practice Prior to

1994 Northridge Earthquake:

Welded flange-boltedweb moment connection

widely used from early

1970s to 1994


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Moment Connection Design Practice Prior to

1994 Northridge Earthquake:

Considered less desirable because of

slip of bolts (pinched hysteresis loops)

and net section rupture

All-bolted

connection?


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Pre-Northridge

Welded Flange Bolted Web Moment Connection

Backup Bar

Beam Flange

Column FlangeStiffener

Weld Access Hole


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Welded Flange

Bolted Web

Moment

Connection

Weld tabs in

place


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Stages of

construction ofwelded flange

bolted web

moment

connection.

Beam web bolted

to shear tab.


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Typical: 3/8 root

30-degree bevel on beam

flange

Bottom flange

back-up bar tack

welded into place.


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Weld tabs tack

welded into place,

extending groovegeometry beyond

flange edges.


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First weld pass

has been placed

with flux-cored arcwelding (FCAW)

process.


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Bottom groove

continues to be

filled.

Note interruption at

middle portion of

flange.


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Completed bottom

flange groove

weld.

Pre-Northridge

practice: back-upbar and weld tabs

left in place.


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Beam top flangewith back-up bar

and weld tabs in

place.


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Completed top

flange groove

weld.


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Ultrasonic Testing

(UT) on a moment

connection with a

cover plate.

UT used to detect

defects.


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Experimental Data on Pre-Northridge

Moment Connection

Typical Experimental

Setup


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Initial Tests on Large Scale Specimens:

Tests conducted atUC Berkeley ~1970

Tests on W18x50 and

W24x76 beams Tests compared all-

welded connections

with welded flange-

bolted web

connections

Note on initial test specimens


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Note on initial test specimens

Loss of redundancy, deeper beams, thicker

column flanges!

Relatively modest beam and column sizes

At the time, nearly all beam-to-columnconnections in buildings designed to transfer

moment

Over the years, cost premium for full momentconnections led engineers to limit number of

bays of framing designed as ductile moment-

resisting frames


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All-Welded Detail


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Welded Flange Bolted Web Detail


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Observations from Initial UC Berkeley Tests


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Observations from Initial UC Berkeley Tests:

Large ductility developed by all-welded

connections.

Welded flange-bolted web connectionsdeveloped less ductility, but were

viewed as still acceptable.

At that time (early 1970s), little

information available on level of

ductility needed to survive astrong earthquake.

Welded flangebolted web connection beam the de facto

standard, used in a large number of moment frames

Less costly to

fabricate!

Subsequent Test Programs:


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Subsequent Test Programs:

Welded flange-bolted web connections

showed highly variable performance.

Typical failure modes: fracture at ornear beam flange groove welds.

A large number of laboratory testedconnections did not develop adequateductility in the beam prior to connectionfailure.


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Bottom flange groove weld fracture


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Top flange fracture, initiated at left edge

at weld-runoff region.


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Summary of Testing Prior to

Northridge Earthquake Welded flange bolted web connection

showed highly variable performance

Identical specimens (different welder),welds inspected vast difference indemonstrated ductility or lack thereof(Engelhardt and Hussain, 1993)

Many connections failed in laboratory withlittle or no ductility

Reasons not well understood.

1994 Northridge Earthquake


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Widespread

failure ofwelded flange -bolted web

momentconnections



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January 17, 1994

Magnitude = 6.8

Epicenter at Northridge - San Fernando

Valley(Los Angeles area)

Fatalities: 58

Estimated Damage Cost: $20 Billion

(structural and non-structural)

Northridge - Ground Accelerations


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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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Collapse of first

story of a wood-

framedapartment

building.


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Nonductile reinforced

concrete frame building

(collapse of entire story)


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Modern precast

parking garage

Damage to Steel Buildings in the


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Damage to Steel Buildings in the

Northridge Earthquake

Initially not identified (not readily visible); foundaccidentally later during repairs to nonstructuralelements, observations of elevator problems, etc.

Large number (more than 100 of approx. 500 inregion)of modern steel buildings sustained severedamage at beam-to-column connections.

Primary Damage: Fracture in and around beam flange

groove welds

Damage was largely unexpected by engineeringprofession

Pre-Northridge


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Backup Bar

Beam Flange

Column FlangeStiffener

Weld Access Hole

Pre-Northridge

Welded Flange Bolted Web Moment Connection


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Weld Tab


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Divot failure


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Divot type fracture

(laboratory test

specimen)


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Damage Observations


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g

A large number of steel moment framebuildings suffered 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

Observations from Studies of


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Fractured Connections

Many connections failed by brittle fracture with littleor no ductility

Brittle fractures typically initiated in beam flangegroove welds

Response to Northridge Moment Connection Damage


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(S) Structural Engineers Association of California

(A) Applied Technology Council

(C) California Universities for Research in Earthquake Engineering

Nearly immediate elimination of weldedflange - bolted web connection from US

building codes and design practice

Intensive research and testing efforts tounderstand causes of damage and to develop

improved connectionsAISC, NIST, NSF, etc.

SAC Program (FEMA)

Causes of Moment Connection Damage in


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Northridge

Welding

Connection Design

Materials

Causes of Northridge Moment


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Connection Damage:

Welding Factors Low Fracture Toughness of Weld

Metal Poor Quality

Effect of Backing Bars and Weld Tabs

Weld Metal Toughness


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g

Most common Pre-

Northridge welding

electrode (E70T-4)

had very low

fracture

toughness.

Typical Charpy V-

Notch: < 5 ft.-lbs at

700F

(7 J at 210C)

Welding Quality


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Many failed connections showed evidence of

poor weld quality

Many fractures initiated at root defects in

bottom flange weld, in vicinity of weld accesshole


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Lack of penetration defect


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Weld Backing Bars and Weld Tabs


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Backing Bars:

Can create notch effect Increases difficulty of inspection

Weld Tabs: Weld runoff regions at weld tabs contain numerous

discontinuities that can potentially initiate fracture


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Note: shows

evidence of

lamellar

tearing


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Causes of Northridge Moment Connection

D


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Design Factors:Stress/Strain Too High at Beam Flange Groove Weld

Inadequate Participation of Beam Web Connection in

Transferring Moment and Shear Effect of Weld Access Hole

Effect of Column Flange Bending

Other Factors

Damage:

Including presence of composite floor slab

Panel Zone (more later)

ss

F


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Mp

Increase in Flange Stress Due to

Inadequate Moment Transfer Through Web Connection

Flang

e

Stres

Fy

Fu


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Stress

Concentrations:

Weld accesshole

Shear in flange

Inadequate

flexural

participation of

web connection


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Base Metal

Tri-Axial Stress Condition


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Causes of Moment Connection Damage in


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Northridge:

Material Factors (Structural Steel)

Actual yield stress of A36 beams oftensignificantly higher than minimumspecified

FEMA 267, August 1995


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Interim Guidelines: Evaluation,

Repair, Modification and

Design of Welded SteelMoment Frame Structures

Advisory No. 1 1997

Advisory No.2 - 1999

Moment Resisting Frames - Part1

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