Download Vulnerable Buildings

Vulnerable Buildings
An Overview and History
David W. Cocke, S.E., F.SEI, F. ASCE
President, Structural Focus
Vulnerable
adj. susceptible to
physical or emotional
attack or harm.
Synonyms: helpless,
defenseless,
powerless, impotent,
weak, susceptible
From Lucile Jones, 2008 Shakeout Scenario
Building Vulnerabilities
Photos
• Configuration
• Irregularities
• Geotechnical Conditions
• Liquefaction
• High accelerations
• Fault rupture
Downtown Los Angeles
S. F. Marina District liquefaction, 1989
Building Vulnerabilities
• Construction Details
• No anchorage
• No bracing
• No ductility
• Construction Quality
• No quality control
• Weak or weakened materials due
to neglect
• Missed details
1995
Great Hanshin
EQ
1933
Riley Act
1933
Field Act
1927
UBC 1st
Published
1939
Garrison
Act
1949
L.A. Parapet
Ordinance
1973
Alquist Act
1973
UBC
Update
1935
UBC
Update
1984
L.A. URM
Updated
1986
CA URM
Law
1994
Senate Bill
1994
Tilt-Up
Ordinance
1996
L.A. Parapet
Updated
1996
L.A. Reinforced
Concrete &
Masonry Bldgs
Timeline: Legislation, Code Changes, Earthquakes
1998
L.A. Soft-Story
Ordinance
1991
UBC
Update
1997
UBC
Update
2015
z00
2014
South Napa
EQ
2010
2011
Christchurch
EQ
1994
Northridge
EQ
1990
1980
1970
1969
SF Parapet
Ordinance
1985
Mexico City
EQ
2000
1971
San Fernando
EQ
1960
1950
1964
Alaska
EQ
1940
1930
1933
Long Beach
EQ
1989
Loma Prieta
EQ
2013
S.F. Soft-Story
Ordinance
2015
L.A. Retrofit
Ordinances
1933 Riley Act
Local government must have a Building Department
1933 Field Act
New schools must be built earthquake-resistant – Department State Architect
1935 UBC Update
Identifies specific seismic issues – lateral bracing, material stress, overturning
movement, foundation ties, and additions to existing buildings
Established new code for schools
1939 Garrison Act
Existing schools must perform a seismic upgrade
1949 Los Angeles Parapet Ordinance
LABC Section 91.8114
Catalog and reinforce parapets
Mandatory, 100% compliance
1969 San Francisco Parapet Ordinance
LABC Section 91.8114
Catalog and reinforce parapets
Mandatory, 100% compliance
1973 Alquist Act
New hospitals must be built earthquake-resistant
1973 UBC Update
Identifies new issues – building separation and high-rise buildings
Adds analyses of individual floors, drift
New code for hospitals
1983 San Francisco Parapet Ordinance
Mandated lateral bracing of parapets within 5 years.
1984 Los Angeles URM Ordinance
LABC Sections 91.3403§4, 91.3404§4, 91.8502§1
Catalog and reinforce URM buildings and report
Mandatory, 100% compliance as of 2006
1986 California URM Law
Local governments adapt mandatory strengthening programs
Establish seismic retrofit standards
Reduce the number of occupants in URM buildings
1991 UBC Update
Earthquake regulations section splits into more
detailed sections.
Identifies new issues – soil types, ground movement,
and non-building structures
Requires mathematical models of force distribution
and lateral movement
1994 Senate Bill 1953
Amends 1973 Alquist Act to upgrade existing hospitals
1994 Tilt-Up Ordinance
Catalog and analyze tilt-up buildings and report, reinforce if needed
Mandatory, 100% compliance as of 2006
1996 Los Angeles Parapet Ordinance Updated
Catalog and reinforce parapets
Mandatory, 100% compliance
1996 Los Angeles Reinforced Concrete and Masonry Buildings
with Flexible Diaphragms
Catalog and analyze, Voluntary
1997 UBC Update
Defines near-fault zones and prohibits highly irregular structures
Adds factor to shear equations
Removes pre-qualified steel connection details
Adds redundancy factor to design forces and deformation compatibility requirements
1998 Los Angeles Soft-Story Ordinance (Division 93)
Catalog and analyze
Voluntary, 4% compliance as of 2006
And recently…
2013 San Francisco Soft-Story Ordinance
Catalog, analyze, and reinforce
Mandatory, 100% compliance by 2020
2015 Los Angeles Soft-First-Story Buildings Retrofit Ordinance, Division 93
Mandates retrofit of soft-first-story wood framed buildings to make the first floor
as strong as the second.
2015 Los Angeles Pre-1980 Concrete Buildings Retrofit Ordinance, Division 95
Mandates that concrete buildings designed prior to the 1976 Uniform Building Code
meet the Basic Safety Objective of ASCE 41.
Vulnerable Building Types in California
• Older Single Family Homes
• Unreinforced Masonry Bearing Wall Building – “URM’s”
• Tilt-up Concrete Walls Buildings – “Tilt-ups”
• Welded Steel Moment Resisting Frames – “Moment Frames”
• Soft Story Wood Frames – “Soft Stories”, “Ding-bats”, “Tuckunder”
• Concrete Buildings with Non-ductile Details – “Non-ductile
Concrete”
• Older Hillside Structures – built on slope
Older Single Family Homes
Negative Features
• Inadequate and non-existent anchor bolts
• Unbraced cripple walls
• No sheathing, only plaster/sheetrock walls
• Unreinforced and/or unbraced masonry
chimneys
Issues:
• Short cripple walls can collapse
• Walls can shift off of foundation
• Chimneys can collapse into building or into
yard
Older Single Family Homes
• Retrofit not mandated
• New codes address deficiencies
• Guidance from CEA and others
Photos from California Earthquake Authority
Older Single Family Homes
• Retrofit not mandated
• New codes address deficiencies
• Guidance from CEA and others
Photos from California Earthquake Authority
URM’s
• Constructed from 1800’s up to 1930’s
(Adobe buildings much earlier…)
• Significant fatalities and injuries from this
building type (Bluxome St., others)
• Roof and floors are supported by masonry
bearing walls
Negative Features:
• Heavy brittle walls are rarely adequately
anchored to floors & roof
• Walls have low in-plane strength, especially
with window openings
• Walls can have low out-out-plane resistance
Photos
URM’s
Issues
• If walls separate from roof and
floors, left unsupported against
vertical collapse
• If walls fail, roof and floors are
left unsupported against
vertical collapse
• Have little extra resistance after
initial failure – longer duration
EQ’s and aftershocks
Photos
Coalinga earthquake damage 1983
URM’s
Retrofit • 1986 State Bill mandated local
Ordinances for voluntary or mandatory
minimal retrofit level
• Add or supplement anchors between
walls and floors/roof
• Strengthen tall slender walls to resist outof-plane loading
• Strengthen in-plane wall shear strength
• Add “back-up” vertical support for floors
and roof
Retrofit photo
From ASCE 31‐03
Tilt-up’s
• Constructed from 1950’s to
present
• Significant property loss
Negative Features:
• Walls are rarely adequately
anchored to roof diaphragms
in older (pre 1990’s) buildings
• Large floor plan results in
significant lateral deflections in
flexible diaphragms
Northridge EQ damage, 1994
Tilt-ups
Issues:
• If walls separate from roof and floors, left
unsupported against vertical collapse
• If walls fail, roof and floors are left unsupported
against vertical collapse
• Have little extra resistance after initial failure –
longer duration EQ’s and aftershocks
Retrofit –
•
•
•
•
•
Mandated in 1994 (Division 91)
Add or supplement wall-to-roof anchors
Strengthen roof diaphragm
Strengthen walls
Division 96 - Voluntary
Northridge EQ damage, 1994
Welded Steel Moment Frames
•
•
•
•
Conceived in late 1950’s, became prevalent by 1970’s
Considered a “superior” system until Northridge EQ in 1994
No collapses have been reported
Lateral forces are resisted by “frame action” of the columns and
beams/girders
Negative Features:
• Welded connections in pre-Northridge (pre 1994) buildings
exhibited “brittle” failures
Issues:
• Failure of welded connections resulted in significant
deflections and some limited loss of vertical support
Welded Steel Moment Frames
• Recorded failures during Northridge EQ
(1994) resulted in significant and
immediate research and subsequent
guidelines:
• FEMA 267 – Interim Guidelines (8/95)
• FEMA 350 – Design Criteria for New (7/00)
• FEMA 351 – Seismic Evaluation & Upgrade
Criteria (6/00)
• FEMA 352 - Post-EQ Evaluation & Repairs (7/00)
• FEMA 353 Specifications & Quality Assurance
(7/00)
Welded Steel Moment Frames
• Inspections and repairs mandated in areas of
a > 0.2g
• New Design Practice changed:
• New welding procedures and documentation
• New inspection procedures
• Changes in pre-approved welded
connections
• No two-ways connections
• Different design configurations to move plastic
hinging away from the welded joint are
available
Soft-Story Wood Framed
• Common apartment style in San
Francisco Marina District (early
1900’s)
• Common “ding-bats” in post-war
era in Southern California
San Francisco, Loma Prieta Earthquake, 1989
• Significant damage, injuries and
fatalities 1994 Northridge EQ
(16 fatalities at Northridge
Meadows Apartments)
Northridge Earthquake, 1994
Soft-Story Wood Framed
Negative Features:
• “Rigid” box on top of soft or weak
stories – all energy and movement is
concentrated in lower story
Issues:
• Drift in lower story is significant leading
to catastrophic collapse, sometime
sudden, of lower story
• Very little resistance to long duration
shaking and aftershocks
Northridge Earthquake, 1994
Non-ductile Concrete
• Constructed since early 1900’s
• First significant damage noted in San Fernando
EQ in 1970
• Code changes after San Fernando EQ
• Significant damage, some injuries and fatalities
(Cypress Freeway, Oakland 1989)
• LA City Division 95 - voluntary retrofit ordinance
Photos
Negative features:
• Concrete joints and columns had little ability to resist loading in longer
durations and significant drift – little ductility
• Buildings can be very large and multiple stories
• Buildings are difficult & expensive to evaluate because steel rebars are
not visible
Non-ductile Concrete
Issues:
• Column failure can lead to
collapse of floors and roof
• Buildings are very heavy,
collapse can be catastrophic
Photos
Limon earthquake, Costa Rica, 1992
Olive View Hospital, 1971
Current Focus
Resilience by Design
This report presents the recommendations of a joint
Los Angeles-US Geological Survey task force
for thoughtful, attainable, and comprehensive
policies the City of Los Angeles can adopt in order to

protect the lives of our citizens during earthquakes

improve the capacity of the City’ to respond to the
earthquake

prepare the City to recover quickly after the
earthquake and protect the economy of the City
and all of southern California
…recommendations addressing four areas of seismic
vulnerability…
1. Pre-1980 buildings that present an unacceptable risk to the lives of our citizens, including
“non-ductile reinforced concrete,” and “soft-first-story wood framed” buildings
2. Water system infrastructure (including impact on firefighting capability)
3. Communications infrastructure
Strengthen Our Buildings
Task Force Recommendations:
1. Mandatory Retrofit of Soft-First-Story
2. Mandatory Retrofit of Non-Ductile
buildings
3. Mandatory Retrofit of Buildings that
are Excessively Damaged in Earthquakes
4. Adoption of a Back to Business Program
5. Voluntary Building Rating System
Buildings
Reinforced Concrete
Mandatory EQ Hazard Reduction in Existing
Wood Frame Buildings with Soft, Weak or
Open-Front Walls
Northridge Apt. Buildings, 1994
Mandatory EQ Hazard
Reduction in Existing Wood
Frame Buildings with Soft,
Weak or Open-Front Walls
• Approximately 13,500 in City of
LA jurisdiction
• Cost about $5k/unit or 3-6
months rent
Mandatory EQ Hazard Reduction in
Existing Wood Frame Buildings with
Soft, Weak or Open-Front Walls
Ordinance 183893,
Article 1, Division 93
The purpose of this division is to
promote public welfare and safety by
reducing the risk of death or injury
that may result from the effects of
earthquakes on existing wood-frame
multi-story buildings with soft, weak or
open front walls.”
Mandatory EQ Hazard Reduction in
Existing Wood Frame Buildings with
Soft, Weak or Open-Front Walls
• Retrofitting will address the first floor structural deficiency.
• Specifically did not want to require work on the interior and at
the same time did not want to drive damage into upper stories
• The design base shear in a given direction shall be 75% of
design base shear as specified in the seismic provisions of
ASCE 7.
• Using 75% of code has been the traditional approach used in
previous voluntary seismic retrofit ordinances adopt by LA City.
• Lateral Vertical Systems. Strengthening systems with
concrete walls or masonry walls, or steel braced frame
shall not be permitted.
• Specifically did not want to drive damage into the upper stories
Mandatory EQ Hazard Reduction in
Existing Wood Frame Buildings with
Soft, Weak or Open-Front Walls
• Special provisions for Horizontal Structural Irregularities in buildings
with 3 or more stories.
• Special issues, has additional requirements
• The Building Department may approve alternate design
methodologies that improve the whole first story seismic
performance that are equivalent to the life safety objectives in this
ordinance.
• Acknowledgement that new methodologies are in development and will
be available in the near future
• Special provisions for additional anchorage for buildings on hillsides.
• Hillside structures have significant additional configuration issues to be
addressed.
Mandatory EQ Hazard Reduction in
Existing Wood Frame Buildings with
Soft, Weak or Open-Front Walls
Before Retrofit
Retrofit Construction
Mandatory EQ Hazard Reduction in Existing
Non-Ductile Concrete Buildings
Cypress Viaduct, 1989
Kaiser MOB, 1994
Mandatory EQ Hazard
Reduction in Existing
Non-Ductile Concrete
Buildings
• Building list is to be
confirmed
• Approximately 1,500 in
City of LA jurisdiction
• How many others in
Southern California?
Mandatory EQ Hazard Reduction in Existing
Non-Ductile Concrete Buildings
Ordinance 183893,
Article 1, Division 95
The purpose of this division is to promote
public welfare and safety by reducing the
risk of death or injury that may result from
the effects of earthquakes on existing
concrete buildings…The poor performance
of these older buildings is typically due to
deficiencies in the lateral forces resisting
system that render the building incapable
of sustaining gravity loads when the
building is subjected to earthquakeinduced lateral displacements.
Photos
Mandatory EQ Hazard Reduction in Existing
Non-Ductile Concrete Buildings
• Mandatory retrofitting for all concrete buildings designed to a
building code prior to the 1976 Los Angeles City Building Code.
• The Task Force felt that the 1976 LABC made significant changes
to improve ductility
• Elements not designated to be part of the lateral force resisting
system shall be retrofitted to be adequate for gravity load effects
and seismic displacement due to the full (100%) of the design story
drift specified in the current Los Angeles Building Code seismic
provisions.
• An effort to address deformation compatibility and prevent
collapse mechanisms
Mandatory EQ Hazard Reduction in Existing
Non-Ductile Concrete Buildings
The building shall meet one of the following criteria:
1. The minimum standard for retrofitting is the Basic Safety Objective
in ASCE 41.
• At this time, is the best standard for defining Performance
Objective
2. Strength of the code conforming lateral resisting system shall meet
or exceed seventy five percent (75%) of the base shear specified
in the current Los Angeles Building Code seismic provisions.
• Roughly equivalent to ASCE 41 Basic Objective
3. Other methods approved by the Department deemed to be
equivalent
Mandatory EQ Hazard Reduction in Existing
Non-Ductile Concrete Buildings
• Historical buildings: Qualified historical
buildings shall comply with requirements of
the California Historical Building Code (CHBC)
established under Part 8, Title 24 of the
California Code Regulations.
• Stays consistent with the treatment of
existing qualified Historical buildings. The
CHBC’s provisions are already in place to
address seismic retrofitting.
Address Vulnerabilities
Increase Resilience
Even in small steps….
Example:
1986 URM State Bill and Subsequent Ordinances
• No one died in a URM in Northridge EQ in 1994
• Only 19% of inspected URM’s needed repairs after
Northridge EQ compared to 33% of buildings overall
• Statewide - jurisdictions have retrofitted or demolished 88%
of URMs with mandatory programs but only 22% with
voluntary programs
Lucy Jones, EERI presentation 2015
Questions?