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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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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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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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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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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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1994 Northridge Earthquake
Widespread
failure ofwelded flange -bolted web
momentconnections
1994 Northridge Earthquake
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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
(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