FEMA P-807-1:
Guidance and Recommendations for the
Seismic Evaluation and Retrofit of
Multi-Unit Wood-Frame
Buildings with Weak First Stories
David Mar, Mar Structural Design
Garrett Hagen, Degenkolb Engineers
Project Team
Project Technical Committee
FEMA/ATC
David Mar (Project Tech. Dir.)
Christina Aronson
Kelly Cobeen
Michael Mahoney
Garrett Hagen
Bob Hanson
Daniel Zepeda
Justin Moresco
Project Review Panel
Project Working Group
Jonathan Buckalew
Kamiar Kalbasi Anaraki
Kristijan Kolozvari
Sina Basereh
Jay Kumar
Kaat Ceder
John Wallace
Christopher Neumann
Cynthia Zabala
Carmen O’Rourke
Weichiang Pang
Justin Tan
Note: All images are referenced in the report, u.o.n.
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Background
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Soft, Weak, or Open-Front (SWOF) Buildings
Highly variable in configuration and materials
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Soft, Weak, or Open-Front (SWOF) Buildings
Prevalent in California: Often housing people with limited resources
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Soft, Weak, or Open-Front (SWOF) Buildings
Typically:
 Non-Engineered
 No Plans
 Archaic Materials
 Archaic Construction Practices
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Soft, Weak, or Open-Front (SWOF) Buildings
Vulnerable and Dangerous
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FEMA P-807 (2012) Recap
 Goal: to create safe, cost-effective, ground floor retrofits
 Utilizes existing finishes for structural strength
 Uses the Weak-Story Tool (WST) for assessment
 Intended to be more economical and reliable than IEBC
Appendix Chapter A4
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Topics that Drove the Initiation of FEMA P-807-1
 Poor design and detailing observed in SWOF retrofits
 Interest in Line Retrofits, prevalent in Southern California
 Interest in effect of diaphragm load-deflection behavior
 Expansion to address broader national audience
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What FEMA P-807-1 Does Do
 Evaluates performance of various retrofits with high-fidelity models
 Provides recommendations based on building behavior, including
related to ordinance scope and engineering design
 Includes retrofit design and detailing examples
What FEMA P-807-1 Does Not Do
 Does not revise FEMA P-807 or the Weak-Story Tool (WST)
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Archetype Buildings
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Building Inventory / Survey
Collected Information from Southern California (Los Angeles Area) and
Northern California (San Francisco Bay Area) on:
 Building Configuration (Using West Hollywood Screening Form)
 Age of Construction
 Number of Stories
 Materials of Construction
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Building Configuration Data
Building Types in Southern California
[Figure A-4]
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Age of Construction
Decade of construction for buildings in
Southern California [Figure A-5]
Decade of construction for buildings in Northern
California [Figure A-10]
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Number of Stories
4 Stories; 1%
3 Stories;
33%
2 Stories; 66%
Number of stories for buildings in
Southern California [Figure A-6]
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Number of stories for buildings in
Northern California [Figure A-8]
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Materials of Construction
Floor finishes for Berkeley
buildings ordered by date of
construction. Tall bars indicate
carpet. Short bars indicate
hardwood. [Figure A-12]
Wall and ceiling finishes for
Berkeley buildings ordered by
date of construction. Tall bars
indicate gypsum wallboard.
Short bars indicate plaster.
[Figure A-13]
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Selected: SWOF Long Side Open (LO)
Includes Two- and Three-Story Versions: LO2, LO3
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Selected: SWOF Short Side Open (SO)
Includes Two- and Three-Story
Versions: SO2, SO3
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Selected: Materials of Construction
1950s
Plaster
Straight Lumber Sheathed Diaphragm
Carpet
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1960s
Gypboard
Diagonal Lumber Sheathed Diaphragm
Carpet
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Analytical Studies
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OpenSees Models
Mass and Spring Assemblage
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Walls They are brittle and weak.
Stucco plus Gypsum Wallboard
1200
800
800
400
400
0
0
-400
-400
-800
-800
-1200
-10
-7.5
-5
-2.5
0
2.5
5
Gypsum Wallboard
1200
7.5
10
-1200
-10
Weak Walls
(WW)
-7.5
-5
-2.5
Drift (%)
1200
Figures from Section 2.3.1
Stucco with Plaster on Wood Lath
800
400
400
0
0
-400
-400
-800
-800
-7.5
-5
-2.5
0
2.5
5
7.5
10
2.5
5
7.5
Gypsum Plaster on Wood Lath
1200
800
-1200
-10
0
Drift (%)
10
-1200
-10
Strong Walls
(SW)
-7.5
-5
-2.5
Drift (%)
0
2.5
5
7.5
10
Drift (%)
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Diaphragms
Strong
SD
Diagonal Wood Sheathing
Force (lb/ft)
1200
Weak
WD
Brittle
BD
Brittle Diaphragm
1200
Horizontal Wood Sheathing
1200
Lower Bound
LBD
Very Weak
VWD
Very Weak Diaphragm
1200
800
800
800
800
800
400
400
400
400
400
0
0
0
0
0
-400
-400
-400
-400
-400
-800
-800
-800
-800
-800
-1200
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
-1200
-10
-7.5
Drift (%)
-5
-2.5
0
Drift (%)
2.5
5
7.5
10
-1200
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
Drift (%)
-1200
-10
-7.5
-5
-2.5
0
Drift (%)
2.5
Lower Bound Diaphragm
1200
5
7.5
10
-1200
-10
-7.5
-5
-2.5
0
2.5
Drift (%)
Figures from Section 2.3.1
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5
7.5
10
Primary Diaphragms
Strong
SD
Diagonal Wood Sheathing
1200
Force (lb/ft)
Weak
WD
800
800
400
400
0
0
-400
-400
-800
-800
-1200
-10
-7.5
-5
-2.5
0
2.5
5
Horizontal Wood Sheathing
1200
7.5
10
-1200
-10
-7.5
-5
Drift (%)
-2.5
0
2.5
5
7.5
10
Drift (%)
Figures from Section 2.3.1
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Concerns for Diaphragms
Brittle
BD
Brittle Diaphragm
1200
Lower Bound
LBD
Very Weak
VWD
Very Weak Diaphragm
1200
800
800
800
400
400
400
0
0
0
-400
-400
-400
-800
-800
-800
-1200
-10
-7.5
-5
-2.5
0
Drift (%)
2.5
5
7.5
10
-1200
-10
-7.5
-5
-2.5
0
2.5
Lower Bound Diaphragm
1200
5
7.5
10
Drift (%)
-1200
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
Drift (%)
Figures from Section 2.3.1
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Pushover Strengths
Similar in both directions & similar for both wall types
1
X
Y
0.8
0.6
0.4
LO3‐SW‐WD
Strong Walls
0.2
0
0
5
Vx = 156 k
Vy = 158 k
W = 504 k
10
15
X
Y
0.8
V/W
V/W
1
0.6
0.4
LO3‐WW‐SD
Weak Walls
0.2
20
0
0
Story Drift (%)
5
Vx = 90 k
Vy = 93 k
W = 303 k
10
15
20
Story Drift (%)
[Figure 2-18]
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Incremental Dynamic Analyses
Capacity is usually exhausted
after 5% drift
Collapse is modeled explicitly:
This is indicated by the flattening of IDA
due to side-sway collapse from P-Delta
[Figure 2-20]
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Unretrofitted Building Results
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Multiple Failure Modes
There is not a singular (obvious) vulnerability (i.e., open front).
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SWOF Buildings Are Vulnerable to Multiple Failure Modes
52% Collapses in the perpendicular to open-front direction (blue, Y)
37% Collapses in the parallel to open-front direction (red, X)
11% Collapses initiated in the diaphragm
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Contrary to Conventional Expectations
The Open Front (or Line) is Not Exceptionally Vulnerable.
14% Collapses in the
open line
14% Collapses in the
X translation
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9% Collapses in the
“non-vulnerable” side
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Failure Mode Examples: SO3-WW-BD
Y: 48% of failures
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Failure Mode Examples: SO3-WW-BD
X (open line): 14% of failures
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Failure Mode Examples: SO3-WW-BD
X (non-vulnerable front): 9% of failures
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Failure Mode Examples: SO3-WW-BD
D (diaphragm-2nd story failures): 11% of failures
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Retrofit Elements and Criteria
Three Types: FEMA P-807, Line, Optimized Line
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Retrofit Elements and Modeling
[Figure 2-23]
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FEMA P-807 Retrofit Criteria
 Performance-Based Criteria
 Design Force: 0.5SMS = 1.0g at selected site near Los Angeles City
Hall
 20% probability of exceedance of drift levels associated with onset of
strength loss (as surrogate for collapse)
 This is consistent with City of Los Angeles retrofit ordinance
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Line Retrofit Criteria
 Design Force = 0.75*ASCE 7 Determined Force
= 0.75(2/3SMS) = 1.0g at selected site near Los Angeles City Hall
Common Systems for Line Retrofits
R
Ω0
Cd
Light-frame (wood) walls sheathed with rated
wood structural panels (plywood, OSB)
6 1/2
3
4
Light-frame walls sheathed with shear panels
of all other materials
2
2 1/2
2
2 1/2
1 1/4
2 1/2
Steel special cantilever column systems
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Optimized Line Retrofit Criteria
 Design Force = 0.75*ASCE 7 Determined Force
= 0.75(2/3SMS) = 1.0g at selected site near Los Angeles City Hall
Common Systems for Line Retrofits
R
Ω0
Cd
Steel special cantilever column systems
2 1/2
1 1/4
2 1/2
Use capacity design
Remove drift limit
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Summary of Archetypes Studied
Primary Set of Archetypes
Material Types
Long Side Open
Short Side Open
WW-SD
SW-WD
2 Story
LO2-WW-SD
LO2-SW-WD
3 Story
LO3-WW-SD
LO3-SW-WD
2 Story
SO2-WW-SD
SO2-SW-WD
3 Story
SO3-WW-SD
SO3-SW-WD
Variants studied:
 Addition of wing walls
 Varying diaphragm properties
 No open-front vulnerability
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Retrofitted Building Results
FEMA P-807, Line, Optimized Line
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Failure Mode Examples: SO3-WW-BD-L
Perpendicular to Open-Front (Y): 64% of failures
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Failure Mode Examples: SO3-WW-BD-L
Parallel to Open-Front (X, back wall): 27% of failures
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Seismic Retrofit Effectiveness
3-Story Type LO Weak
Wall Strong Diaphragm
3-Story Type SO Weak
Wall Strong Diaphragm
[Figure 2-25]
[Figure 2-24]
Long side open
FEMA P-807 works
Line retrofits are
Moderately Effective
Short side open
FEMA P-807 works
Line retrofits are
Ineffective
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Performance Summary – Primary Set
POC at
Sa = 1.0g
[Figure 2-26]
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Wing Wall Archetype Study
similar POC
SOW3-WW-SD
1
For the Wing Wall Condition:
Optimized Line and FEMA P‐807
retrofits have a similar POC
X
Y
0.8
0.6
V/W
Wing walls make Y direction strong –
thus not needing retrofit
0.4
0.2
0
0
5
10
15
20
Roof Drift (%)
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Performance Summary – Weak Diaphragm Effects
3-Story Type SO Weak Wall
POC at Sa = 1.0g
[Figure 2-28a]
[Figure 2-29]
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Performance Summary – Hazard Variation
[Figure 3-6]
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Recommendations
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Assumptions and Limitations
 Drawn from the analytical studies
 Influenced by archetypes used
 With living space at ground story, most representative of Southern
California
 Used best available tools to estimate anticipated performance, but
results believed to be somewhat biased to overpredict collapse
 Emphasis should be on general trends
 Use caution in extrapolating to other configurations
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Introduction to Data Plots
Example fragility function
results from analytical
studies [Figure 3-3]
0.5SMS
1.0SMS
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Introduction to Data Plots
Primary Study
POC Data at
Sa = 1.0g
[Figure 3-11]
Archetype
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Introduction to Data Plots
Archetype
Archetype
Primary study POC at 0.5SMS= 1.0g [Figure 3-5]
Primary study POC at 1.0SMS= 2.0g [Figure 3-5]
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Importance of Retrofit (Rec A)
In high-seismic-hazard
regions, consider
including SWOF
buildings in seismic
retrofit programs
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Type of Retrofit (Rec B, Part 1)
Require full-story
retrofits, where
practicable
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Type of Retrofit (Rec B, Part 2)
Where it is not possible to
require a FEMA P-807 or
full-story screening,
perform screening for
open-front wall lines on all
exterior walls of the
building, including those
perpendicular to the
evident open-front wall
SCREEN
LEFT SIDE
ALSO
EVIDENT
OPENFRONT
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SCREEN
RIGHT
SIDE ALSO
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Type of Retrofit (Rec B, Part 3)
 Recognize lower
performance with the
lower-bound diaphragms
(LBDs) in three-story
SWOF buildings in retrofit
ordinance documents
 Provide suggestions for
increasing the retrofit
performance
Diaphragm POC Data at Sa = 1.0g with Varying Diaphragm
Model Properties [Figure 3-9]
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Building Prioritization (Rec C)
Where prioritizing
SWOF building retrofits,
give priority to SWOF
buildings three stories
or more
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Local Seismic Hazard (Rec D, Part 1)
When considering
adoption of a seismic
retrofit ordinance, take
into consideration local
seismic hazard and
resulting variation in
POC for unretrofitted
buildings
Primary Study POC Data with Varying Sa [Figure 3-6]
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Scope of Retrofit (Rec D, Part 2)
Consider whether twostory, long-side-open
SWOF buildings of the
type studied need to be
included in SWOF
building retrofit
ordinances
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Level of Retrofit (Rec E)
 When implementing
FEMA P-807 retrofits, it
is recommended that
the criterion meet or
exceed 20% POE at
0.5SMS
 For moderate and high
seismic, a higher target
may be achievable
 For very high seismic,
second story strength
may limit performance
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Other Vulnerable Conditions (Rec F)
Consider addressing all SWOF building
configurations, including:
 Partial open-front wall lines on several sides of
the building with first-story residential units (West
Hollywood Building Type C),
 Tuck-under parking configurations on hillsides
(West Hollywood Building Types D and E),
 Tuck-under parking configurations with no firststory residential units (West Hollywood Building
Types F and G),
 Residential units over commercial space, and
 Multi-family dwellings over crawlspaces on flat or
hillside sites.
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Screening (Rec G, Part 1)
 When performing screening,
consider the overall
configuration of the first-story
walls relative to upper stories
 Methods could include
summed wall lengths in each
story in each direction, a
FEMA P-807 check, or an
ASCE/SEI 41 quick strength
check
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Screening (Rec G, Part 2)
Where it is not possible
to require a
FEMA P-807 or fullstory screening,
perform screening for
open-front wall lines on
all exterior walls of the
building, including
those perpendicular to
the evident open-front
wall
SCREEN
LEFT SIDE
ALSO
EVIDENT
OPENFRONT
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SCREEN
RIGHT
SIDE ALSO
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Type of Retrofit (Rec B, Part 2)
Where it is not possible to
require a FEMA P-807 or
full-story screening,
perform screening for
open-front wall lines on all
exterior walls of the
building, including those
perpendicular to the
evident open-front wall
SCREEN
LEFT SIDE
ALSO
EVIDENT
OPENFRONT
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SCREEN
RIGHT
SIDE ALSO
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Evaluation (Rec H)
 Provide a full-story
evaluation
 The FEMA P-807
methodology and
FEMA P-807 WeakStory Tool (WST) are
believed to be the
best available tools
for evaluation
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Line vs Optimized Line (Rec I)
 Design new vertical
steel elements based
on strength only
 Consistent with the
optimized line retrofits
included in the studied
archetypes, drift limits
need not be
considered
Primary Study POC Data at Sa = 1.0g [Figure 3-11]
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Selected Retrofit Design
Recommendations
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Chapter 4 – Retrofit Design Recommendations
 4.2 Seismic-Force-Resisting System Elements in Retrofit Design
 4.3 Protection of Existing Structural Systems
 4.4 Load Path to New Retrofit Elements
 4.5 Collectors, Moment Frame Beams, and Columns
 4.6 Foundations
 4.7 Quality Assurance Requirements
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4.3.1 Protecting the Existing SFRS
LOAD MUST NOW
GO THROUGH
STUD FROM STUD
ATTACHMENT,
THEN TO SILL AND
THEN TO
COLLECTOR
ISSUE: DISRUPTED
LOAD PATH
ALTERNATIVE DETAIL
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4.3.2 Protecting the Existing Gravity System
ISSUE: WEAK-AXIS LOAD
ON (E) BEAM
ALTERNATIVE DETAIL
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4.4.1 Load Path for Lumber Sheathed Diaphragm
ISSUE: COLLECTOR LENGTH IS
FUNCTION OF (E) DIAPHRAGM TYPE
ALTERNATIVE DETAIL
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4.5.1 Collector Length
 Determine collector length
based on the capacity of
the new vertical element.
Extending full diaphragm
length is preferred when
practicable
 Distribute vertical elements
to reduce collector lengths
and resulting forces
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4.5.2 Vertical Elements Outside Building Footprint
ISSUE: CANTILEVERED COLUMN
OUTSIDE BUILDING FOOTPRINT
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EXAMPLE DETAIL
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4.5.3 Bracing Requirements for New Steel Systems
 Special Cantilevered Column
Stability Requirements: Per
AISC 341-10 and 341-16, Section
E6.4a and Section D1.2a (stability
of moderate ductility beams)
 Recommended Practice: Where
the distance between required
bracing is not less than twice the
column height, lateral torsional
bracing is only required at the
base of the column
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4.6.1 Foundation Sliding, Uplift, Overturning
 Often difficult to make
sliding calculations work.
Recommended alternative
is extending new foundation
to and doweling into
existing foundation
 Same can be true for uplift
and overturning
 For capacity design of
foundation, need to
evaluate soil bearing to
ultimate level
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4.6.2 Detailing of Cantilevered Columns
2024 IBC will clarify that
ductile detailing
requirements are not
required for foundation
grade beam, provided
designed for column
capacity or for
overstrength forces
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4.6.3 Detailing of Cantilevered Columns
PLAN VIEW OF BUILDING
DRIFT
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Thank you!
Questions?
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