2/15/2023
Welcome to the Webinar on FEMA P-749
Earthquake-Resistant Design Concepts (Part B)!
Instructor. Ronald O. Hamburger, P.E., S.E., is a
Consulting Principal at Simpson Gumpertz & Heger and
author of the FEMA P-749 report.
Handouts. Webinar handouts will be sent in the chat and are available at this link: tinyurl.com/2p8tpy3c
PDH Certificates. Participants who are both registered and in attendance today will receive a PDH
certificate by email within 4 weeks.
Q&A. Use the Q&A window at the bottom of your screen to pose questions. Some questions will be
answered live; others will be distributed by email within 4 weeks.
Recording. A link to the recording will be sent by email after the event.
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B
Technical Criteria
FEMA P-749: Training Part B
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FEMA P-749
 An introduction to the U.S. building regulation
process as it relates to seismic safety and
sustainability
 Audience
A. The General Public – including regulators,
emergency planners, lenders, developers and
others with an interest in earthquake risk and
safety
B. Design Professionals – including engineers,
architects, students and others who want a
detailed introduction into the design
requirements in current building codes
FEMA P-749: Training Part B
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Poll Question
 Which best describes your technical background
 Architecture or engineering student
 Practicing Architect
 Practicing Civil or Structural Engineer
 Other type of Engineer
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Poll Question
 Rate your experience level with seismic design
 I have not designed a structure for seismic
resistance
 I have some experience in seismic design
 I have extensive seismic design experience
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 6
Seismic Design Process
FEMA P-749: Training Part B
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Linear Elastic Response
F
2F
x
m
2x
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Period of Vibration - T
F
x
T
𝑇
FEMA P-749: Training Part B
2𝜋
𝑚
𝑘
2𝜋
𝑊
𝑘𝑔
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Linear SDOF Dynamic Response
F
x
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Damped Response
•
Damping is a function of
•
•
•
•
•
Materials of construction
Level of response
It is common to express the amount of damping present as a % of critical damping
Critical damping is the amount of damping that will bring a displaced SDOF structure to rest
in one cycle (time T)
Building codes use 5% damping as a reasonable average value for most structures
responding to design earthquakes
FEMA P-749: Training Part B
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Linear SDOF Response to Ground Shaking
m(ẍ(t) + ü(t)) + cẋ(t) + k(x)x(t) = 0
FEMA P-749: Training Part B
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Linear Response to Ground Shaking
1‐Second Structure
2‐Second Structure
FEMA P-749: Training Part B
4‐Second Structure
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Acceleration Response Spectrum
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Acceleration Response Spectrum
F
x
 Structure has a period of T = 2 seconds
 Entering the response spectrum plot find at T= 2 seconds, the spectral
acceleration (Sa) has a value of 0.2g
 Find the maximum force in the structure as 𝐹 𝑚𝑎
𝑊 ⁄𝑔 𝑆
0.2𝑊
 Find the maximum displacement as x=F/k, or
𝑆 𝑔
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Nonlinear Response
Linear Response
Nonlinear Response
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Seismic Design Process
1. Determine the structure’s use (occupancy) and Risk Category
2. Based on the building site, determine the design ground shaking,
3. Determine the building’s Seismic Design Category, based on
ground shaking and Risk Category
4. Select an appropriate seismic force-resisting system
5. Design the seismic force resisting system, considering the effects
of structural irregularities, if present
6. Design the nonstructural components in the building
This presentation follows the design process as it is embedded in the
ASCE 7-22 standard, which will be referenced by IBC-2024
FEMA P-749: Training Part B
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 7
Determine Seismic Design Criteria
FEMA P-749: Training Part B
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1- Determine Building’s Use - Risk Categories
 ASCE 7-22 Table 1.5-1 defines
risk categories in a general
manner
 The building code provides
more specific definition of the
types of structures assigned to
each category.
 Here we focus primarily on the
ASCE 7 criteria, but also talk to
the IBC requirements
 Note that where adopted, the
IBC takes precedence over
ASCE 7
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Risk Category
Performance Intent
I.
Structures not usually used for human occupancy
(barns, greenhouses)
II.
Ordinary occupancy structures
(stores, offices, homes, warehouses)
III.
<10% chance of collapse in MCER shaking
Structures with:
 Very large occupancies (high rise office buildings)
 Occupants with limited mobility (schools, nursing homes, prisons)
 Contain potentially hazardous materials
IV.
Structures:
 Essential to post earthquake response and recovery
 Contain highly hazardous materials
FEMA P-749: Training Part B
<5% chance of collapse in MCER shaking
limited potential for release of materials
<2.5% chance of collapse in MCER shaking
remain functional following most earthquakes
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2- Determine Design Ground Shaking
Smoothed, design spectra, specified by the building code are intended to
represent the response expected of many possible earthquakes, each of
which will have somewhat different motion
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Design Spectra
 The design response spectrum will be different for each site.
 The design spectrum is affected by:
 Regional seismicity
 Earthquake magnitudes
 Frequency of occurrence




Soil type and depth
Distance to the earthquake source (fault)
Depth of source
Mechanism of fault rupture
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Determining Design Response Spectrum
Chapter 11
1. Determine Site Class
 Detailed procedures of Chapter 20
 Default Site Class
2. Determine spectral response ordinates
 USGS Values (obtained from ASCE Hazards tool)
 Site-specific Study (Chapter 21)
3. Determine design spectral response
parameters (Chapter 11)
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Site Classification
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Example 𝒗𝒔 Calculation
20’
40’
ASCE 7 20.4-1
Stiff Clay
vs=600 ft/sec
𝑣̅
⁄
,
,
=1082 ft/sec
Dense Sand
vs=1600 ft/sec
Rock
vs=3000 ft/sec
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Default Site Class
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Determine Spectral Response Ordinates
 Design spectral ordinates are
obtained from an online USGS
database.
 Access to this database is
available through the free
ASCE 7 Hazard Tool at
https://asce7hazardtool.online/
 Enter building address (or
coordinates), Site Class, Risk
Category
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Full Report Format
Summary Input Data
Results
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Full Report Format
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Alternate - Seismic Maps – Chapter 22
FEMA P-749: Training Part B
𝑆
2
𝑆
3
𝑆
2
𝑆
3
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2-Period Response Spectrum
TL
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Actual v Default Site Class
Site Class C
Default Site Class
0.5g
0.3g
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3- Determine Seismic Design Category
 ASCE 7 categorizes structures according to their seismic risk, that is
the consequences of their failure.
 There are 6 Seismic Design Categories ranging from A to F, with
SDC A being the least seismic risk (low occupancy, low hazard).
 The Seismic Design Category affects many aspects of the design
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Determining Seismic Design Category
ASCE 7 Section 11.6
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 8
Design the Structure
FEMA P-749: Training Part B
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 8
Design The Structure
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4 - Select Seismic Force Resisting System
 The available seismic force-resisting systems depend on:
 Seismic Design Category
 Structure Type
 Building Structure
 Non-building Structure
 Structure Height
FEMA P-749: Training Part B
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Structure Types
Buildings
 Typically enclosed
 Intended for human occupancy
 Common structural systems




Load bearing walls
Braced frames
Moment frames
Dual systems
Design of buildings is governed by ASCE 7 Chapter 12
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Structure Type
Non-building structures with
structural systems like buildings
 May be enclosed, partially
enclosed or open
 Human occupancy is incidental
to its intended use
 Common structural systems




Load bearing walls
Braced frames
Moment frames
Dual systems
Design of building‐like structures is governed by ASCE 7 Chapters 12 and 15
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Structure Type
Non-building structure not like a
building
 May be enclosed, partially
enclosed or open
 Human occupancy, if it occurs,
is incidental to its intended use
 Does not have a building type
structural system
Design of non‐building not like buildings is governed by ASCE 7 Chapter 15
FEMA P-749: Training Part B
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Structural System Selection
 After the designer determines:
 Design ground motion for the site
 Type of structure
the designer chooses a structural system to use for the Seismic
Force Resisting System (SFRS).
 Structural systems are categorized by material and behavior.
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Structural System Selection: Buildings
 Buildings and structures classified as SDC A can use any structural
system.
 Buildings classified as SDC B-F must use one of the specific SFRS
or combinations of systems provided in Table 12.2-1 of the ASCE 7
standard.
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Structural System Selection: Buildings
 ASCE 7 Table 12.2-1 restricts
structural systems that have
proven to exhibit poor behavior in
past earthquakes to SDC A and
possibly SDC B.
 In other SDCs the structural
system may be restricted based on
maximum heights or weights.
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Structural System Selection Nonbuilding Structures
Table 12.2‐1
Table 15.4‐1
 All three ASCE/SEI 7 tables
provide:
 System SDC and height limits
 Values for three design parameters:
 Response Modification Factor – R
 Overstrength factor - o
 Deflection amplification factor - Cd
Table 15.4‐2
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5- Design the seismic force-resisting system
considering irregularities, if present
 ASCE 7 requires that structures be provided with sufficient strength
to resist specified earthquake forces in combination with other loads.
 The specific combinations of seismic load with other loads, including
dead and live loads, that members of the structural system must be
proportioned to resist are specified in the ASCE 7, Chapter 2.
 The structure must be analyzed independently in each of two orthogonal
directions.
 For each direction, the fundamental period (T), seismic base shear force (V),
and individual story forces (Fi) may be different.
 In some cases, the structure is designed for response in both directions
simultaneously
 Irregular structures are designed for larger forces
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Required Strength - SDC A
 Structures assigned to Seismic Design Category A are required to
have adequate strength to resist three different types of specified
forces:
 Global system seismic forces
 Continuity forces, and
 Wall anchorage forces
FEMA P-749: Training Part B
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Required Strength - SDC A
 The SFRS is designed
by applying a static
lateral force, equal to 1
percent (0.01) of the
structure’s weight
at each level.
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Required Strength – SDC A
 Continuity forces apply
to those elements that
“tie” or interconnect a
small piece of a
structure.
 Continuity forces equal
to 5 percent (0.05) of the
weight of the smaller
portion of the structure.
FEMA P-749: Training Part B
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Determining the Lateral Seismic Design Force
SDC B, C, D, E and F
Four methods:
 Equivalent Lateral Force (ELF) method ASCE 7-22 Section 12.8
 Simplified Method also available for some structures Section 12.14
 Modal Response Spectrum Analysis (RSA) ASCE 7-27 Section 12.9.1
 Linear Response History Analysis (LRHA) ASCE 7-27 Section 12.9.2
 Non-linear Response History Analysis (NLRHA) ASCE 7-27 Chapter 16
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Required Strength
 The specified earthquake forces are typically lower than the forces
that the design level earthquake will cause in these structures.
 The magnitude of the specified forces and how they are determined
depends on:
 Seismic Design Category (Seismic Hazard)
 Type of Structure
 Type of Element within the Structure
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Seismic Design Categories B through F
 The procedures used to calculate seismic lateral forces in SDC B
through F are similar, although for SDC B and C the ELF analytical
procedure is commonly used. IN SDC D, E and F it is more common
to use modal response spectrum or response history analysis.
 Structures must be designed for the effects of lateral and vertical
seismic forces
 The magnitude of the lateral seismic forces is determined accounting
for the structure’s inelastic response.
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Vertical Response
 The forces due to vertical earthquake shaking are taken as a fraction
of the demands on individual elements due to dead load, D.
Ev  0.2S DS D
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Structural Design Coefficients for Lateral Forces
Sa(T)
Linear elastic
structure
F=Sa(T)W
Nonlinear
structure
Ω 𝐹 𝑅
𝐹
𝑅
𝛿
𝑅

𝛿
𝐶
𝑅

 R, Cd and Ω0 are intended to
approximate the beneficial effects of
nonlinear behavior and ductility
 R – Response Modification factor
reduces the theoretical elastic
earthquake force to consider yielding
 Ω0 is an overstrength factor that
recognizes that peak strength is
larger than yield strength
 Cd is a factor used to determine drift,
considering nonlinear behavior
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Configuration and Regularity
 ASCE 7 attempts to encourage the design of structures with regular
configurations:





Uniform distribution of mass
Uniform distribution of strength
Uniform distribution of stiffness
Continuous structural systems
Continuous load path
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Configuration and Regularity
 Some irregularities result in requirements to perform
a more detailed analysis to better account for the
effects on the distribution of forces and associated
deformations.
 Some irregularities result in portions of the structure
being required to have higher strength to counter the
irregularity effects.
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Configuration and Regularity
 Two basic categories of irregularity are identified in ASCE 7:
 Horizontal, associated with irregular distribution of seismic force resistance in
plan
 Vertical, which cause a significant change in performance from one story to
another
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Horizontal Irregularities
Torsional & Extreme Torsional
Re‐entrant Corner
Out‐of‐plane Offset
Diaphragm Discontinuity
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Vertical Irregularities
Weak or soft story
In‐plane Offset
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Determining Lateral Forces
 Force magnitudes depend on:
 Deflected shape of the structure
 Weight of structure
 Location within structure
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Determining Lateral Forces
 The total lateral force or base shear, V, is:
V  C sW
where,
V 
Cs 
W 
the total base shear
seismic base shear coeff.
Weight of Building
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Determining Lateral Forces
 The value of Cs depends on:
 The fundamental period of the structure
 The Risk Category
 The structural system used
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Determining Lateral Forces
 Cs equals the lesser of
Cs 
 or
Cs 
S DS
R I 

S D1
R I T

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Determining Lateral Forces
 For structures with a fundamental period of vibration greater than TL
Cs 

S D1T L
R I T2

 However, for any structure
C s  0.44S DS I
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Determining Required Strength – Near Fault Sites
 In SDC E and F, where S1 ≥ 0.6g, the base shear coefficient has an
additional lower bound imposed (i.e., the minimum seismic loads are
increased).
C s  0.5
min

S1
R I

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Vertical Distribution of Loads
 The base shear is then distributed vertically to each level of the
structure using
Fi 
w i hik
n
w
j 1
j
h
V
k
j
k  1.0 for T  0.5 sec.
k  2.0 for T  2.5 sec.
Interpolate for 0.5  T  2.5sec.
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Stiffness and Stability
 Lateral deflections (drift) must be checked if the simplified analysis
procedure is not used.
 The drift checks are used to:
 Protect nonstructural components
 Assure stability
 Avoid pounding with neighboring structures
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Stiffness and Stability
 Story drift, that is the ratio the
lateral movement in a single
story to the story height is used
to evaluate stiffness
 The drift for the 3rd story is
δ3/h3
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Stiffness and Stability
 The allowable story drift depends on the structural system and Risk
Category.
 The check is a comparison of estimated inelastic deflections to
allowable deflections:

Cd  i
  a hi
I
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Stiffness and Stability
 A calculation is required to evaluate stability

Px 
V x hx C d
 If this exceeds 0.1, then a P-Δ analysis must be included
 max 
0.5
 0.25
C d
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Strength Considerations – SDC D, E and F
 Strength considerations in these SDCs must include a check on
redundancy.
 Redundancy is considered sufficient if removal of any one element in
the SFRS does not:
 Reduce the lateral strength by more than 1/3
 Create an extreme torsional irregularity
 If the structure does not meet these redundancy requirements, its
strength must be increased by 30 percent.
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Structural System Selection: Buildings
 Once member sizes are confirmed adequate for strength and drift,
the structure must be configured and detailed to meet the specific
requirements of the associated material design standard




ACI 318 - concrete
AISC 341 - steel
TMS 402 - masonry
NDS - wood
 Chapter 14 of ASCE 7 adds some requirements to these standards,
however, the IBC does not adopt chapter 14
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Final Step – Detail the Structure
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 9
Anchor and Brace the Nonstructural
Components
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FEMA P-749
An Introduction to Seismic-Resistant
Building Practices in the U.S.
Part B Chapter 9
Anchor and Brace the Nonstructural
Components
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Nonstructural Components and Systems
 In SDCs C, D, E and F, the attachment of nonstructural components must
be designed for the seismic forces per ASCE 7 Chapter 13.
 Exceptions:
 Mechanical and electrical components in SDC C (except those assigned Ip = 1.5)
 Mechanical and electrical components in SDCs D, E or F mounted at floor level, Ip =
1.0, weigh < 400 lbs., and connected with flexible connections.
 Mechanical and electrical components in SDCs D, E or F, Ip = 1.0, mounted > 4 feet
above the floor, weigh <20 pounds, and connected with flexible connections
 Component Importance Factor (Ip) must be determined:
 Ip =1 except:
 Ip=1.5 when:
 Required for Life Safety (e.g., Fire Suppression)
 Containment of Hazardous Materials
 Essential to Risk Category IV function
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Nonstructural Components and Systems
 Components not exempt must be anchored to resist the seismic
forces and withstand displacements without failing and endangering
life-safety.
 Strength of attachments is determined as
0.3𝑆 𝐼 𝑊
0.4𝑆 𝐼 𝑊
𝐻
𝑅
𝐶
𝑅
1.6𝑆 𝐼 𝑊
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Lateral Design Force
𝐻
0.4𝑆 𝐼 𝑊
𝑅
𝐶
𝑅
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Nonstructural Components and Systems
 Designated seismic systems (Components with I=1.5) must be
certified by their manufacturers to have been tested or have survived
previous earthquakes with acceptable performance
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FEMA P-749, Chapter 10: Special Topics
 Performance-based Design
 Design for Tsunami
 Soil-Structure Interaction
 Protective Systems
 Seismic Isolation
 Energy Dissipation
 Nonbuilding Structures
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Poll Question
 How do you feel about seismic design criteria
now?
 It looks really complex and I am not sure if I can do
it
 I understand what this is about now and think I can
do it better
 I still have lots of questions
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