Performance Based Seismic Design of Structure IEER CUET, Bangladesh

1. EQE 6110
PERFORMANCE-BASED SEISMIC
DESIGN OF STRUCTURES
Mohammad Raihan Mukhlis
Assistant Professor (Research)
Institute of Earthquake Engineering Research (IEER)
Chittagong University of Engineering & Technology (CUET)
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©Mohammad Raihan Mukhlis

2. COURSE OUTLINE
1. Introduction to Performance-Based Seismic Design of Structures
2. Fundamental considerations for Performance-Based Seismic Design (PBSD)
and Seismic Inputs
3. Analysis Methods: Nonlinear Static Analysis, Nonlinear Dynamic Analysis
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3. Analysis Methods: Nonlinear Static Analysis, Nonlinear Dynamic Analysis
4. Modeling Approaches for Key Structural Members
5. ASCE-41, FEMA-356, ATC-40, JRA 2002 Guidelines for Performance-
Based Design of Structures.

3. LECTURE 1
Introduction to Performance-Based Seismic
Design of Structures
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4. PERFORMANCE-BASED SEISMIC DESIGN (PBSD)
 The process of designing the structure for seismic resistance has been
undergoing a critical reappraisal in recent years, with the emphasis
changing from strength to performance.
 This lead to an approach towards a new design concept called
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Performance-Based Seismic Design (PBSD).
 Parameters to evaluate structural performance:
 story drift
 damage indices
 structural failure mechanism

5. SEISMIC DESIGN APPROACH:
2 Approaches
1. Force based seismic design (FBD) – Traditional Design Approach (Codes)
2. Displacement based seismic design (DBD) – New Design Approach
FORCE BASED SEISMIC DESIGN (FBD):
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 FBD Focus on the seismic force over the structure.
 Forces are induced by the earthquake resulting displacement in the
structures.
 During elastic stage, these forces are related to the elastic stiffness
of the system, but in inelastic condition the relationship becomes
more complex, depending on the displacement history throughout
the excitation.

6.  Therefore force considerations are important in FBD.
 Structural strength should exceeds the design applied loads to avoid
structural collapse.
 Linear elastic analysis of the structure is performed for the lateral forces
calculated from the procedure by Equivalent static load method.
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 In FBD the acceptability of a structure is judged on the basis of force
based quantities.
 For example the design base shear should not be less than the base
shear calculated assuming linear elastic structural response for the
seismic loading representation.

7.  FBD is Suitable only within elastic range of structural behavior.
 FBD uses building displacement as the final check to determine the
structural performance.
 If the final displacement in FBD larger than the standard value then the
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design procedure should be repeated from the beginning.
 That’s why FBD needs extra effort to improve the structural
performance.
FBD cannot provide
FBD cannot provide the appropriate means for implementing concepts
the appropriate means for implementing concepts
of Performance
of Performance-
-based Earthquake Engineering (
based Earthquake Engineering (Bertero
Bertero and
and Bertero
Bertero,
,
2002)
2002)

8. DISPLACEMENT BASED SEISMIC DESIGN (DBD):
 DBD is known broadly as any seismic design methods that use
displacement related quantities directly to judge performance acceptability.
 For example the drift under specified seismic loading should not
exceed the specified drift limit corresponding to a defined damage
level.
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level.
 DBD uses building displacement as the designed target performance.
 The aim of DBD is to design a structure which would achieve, rather than
be bounded by a given performance limit state under a given seismic
intensity .

9. 2 Approaches :
♦ Method A – For Existing structures – (Displacement-Based Design - DBD)
• Check of an already pre-designed structure and make
improvements (increase dimensions of cross section) only to
members that have problems.
♦ Method B - For New structures - (Direct Displacement-Based Design - DDBD)-
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♦ Method B - For New structures - (Direct Displacement-Based Design - DDBD)-
• First proposed by Priestley (1993)
• Design from the beginning the structure for a certain displacement.
The design displacement is usually determined by serviceability or
ultimate capacity considerations.
DBD/DDBD can provide
DBD/DDBD can provide the appropriate means for implementing
the appropriate means for implementing
concepts of Performance
concepts of Performance-
-based Earthquake Engineering
based Earthquake Engineering

10. WHY DO WE NEED A NEW PROCEDURE?
 Priestley et. al. [2007] founds at least three main weakness of FBD:
 Firstly, FBD relies on assumption of initial stiffness to determine the
structural period and the distribution of design forces among different
structural elements. Since the stiffness is dependent on the strength of
elements, this cannot be known until the design process is completed.
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elements, this cannot be known until the design process is completed.
 Secondly, allocation seismic force among elements based on initial
stiffness is illogical for many structures because it incorrectly assumes that
the different elements can be forced to yield simultaneously.

11. Performance levels, indeed, are described in terms of displacements, as
Performance levels, indeed, are described in terms of displacements, as
damage is better correlated to displacements rather than forces. As a
damage is better correlated to displacements rather than forces. As a
 Thirdly, there is no unique force-reduction factors (based on ductility
capacity) for a given structural type and material.
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damage is better correlated to displacements rather than forces. As a
damage is better correlated to displacements rather than forces. As a
consequence, new design approaches, based on displacements, have been
consequence, new design approaches, based on displacements, have been
recently implemented.
recently implemented.

12. WHAT IS INHERENTLY WRONG OR INADEQUATE IN THE EXISTING PROVISIONS FOR
DESIGN THAT WARRANTS A NEW LOOK AT THE ENTIRE PROCESS?
 Since the early development of seismic design codes, global response
modification factors (or R-factors) have remained at the core of seismic force
formulas.
 The main purpose of the force reduction factors used in seismic design is to
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 The main purpose of the force reduction factors used in seismic design is to
simplify the analysis process so that elastic methods can be used to
approximately predict the expected inelastic demands in a structure subjected
to the design loads.
 They account for reductions in seismic force values due to a variety of factors
including system inherent ductility, over strength, and redundancy.

13.  Of these, only the ductility component of the R-factor is generally implied in
the design provisions because systems with larger expected ductility have the
lowest reduction factors.
 Current codes also specify a displacement amplification factor Cd that
quantifies the expected inelastic displacement of the system.
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quantifies the expected inelastic displacement of the system.
 Both R and Cd factors are global response measures that do not provide an
assessment of structural performance at the component level.

14.  There is growing awareness that force-based design using R and Cd factors has
serious shortcomings. For instance, these factors are independent of the
building period and ground motion characteristics.
 Additionally, the same R-factor is used for moment-resisting reinforced
concrete (RC), steel, and braced frames. [see Table 6.2.24, BNBC 2006]
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concrete (RC), steel, and braced frames. [see Table 6.2.24, BNBC 2006]
It is clear that a single global response modifier cannot capture the
It is clear that a single global response modifier cannot capture the

progressive distribution of nonlinearities between various structural elements
progressive distribution of nonlinearities between various structural elements

the resulting redistribution of seismic demands inside the structure
the resulting redistribution of seismic demands inside the structure

the changes that occur during the course of the seismic motion
the changes that occur during the course of the seismic motion

15. MAIN COMPONENTS OF R FACTOR
 Figure shows the base-shear
versus roof displacement response
of a typical building structure.
 The vertical axis in Figure 1 shows
the base shear coefficient, which is
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the base shear coefficient, which is
the total shear normalized by the
seismic weight of the building
(V/W).
 The design base shear coefficient
of the building is Cs, while the
corresponding elastic strength is Ce.

16.  As is evident from the response of
the structure, the yield strength of
the building is Cy (assuming a
bilinear idealization as shown).
 First yielding in a member in the
system should typically commence
at Cs, though material over strength
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at Cs, though material over strength
and member sizing may delay initial
yielding beyond this value.
 Hence, the actual force reduction
factor or response modification
factor as defined in IBC (2000) can
be defined as follows:

17.  The ratio Ce/Cy can be viewed as the response modification factor related to
ductility (R𝜇).
 While the ratio Cy/Cs contains two components: response modification
factor related to over strength (RΩ) and redundancy (RR).
 Hence, the R-factor can be broken down into three main components R𝜇 ,
RΩ , RR as indicated in following Equation:
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𝜇
RΩ , RR as indicated in following Equation:
R = Ce/Cs = (Ce/Cy) Cy/Cs)
R = R𝜇 Ω R

18.  Numerous factors influence each of the modifiers that appear in Equation.
 For example, the post-yield strain (or stiffness) can affect both R𝜇 and RΩ .
 The use of higher material strengths than those specified in the design,
satisfying minimum code requirements for detailing, the presence of
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nonstructural components, and the over sizing of members have an impact
primarily on RΩ and RR.
The product RΩ × RR is likely to have greater variability for RC than for steel
structures.

19. It is clearly difficult to isolate the different components of the reduction factor and
It is clearly difficult to isolate the different components of the reduction factor and
thereby provide engineers with an understanding of the demands imposed not only
thereby provide engineers with an understanding of the demands imposed not only
on the overall system but also on individual components in the system and the margin
on the overall system but also on individual components in the system and the margin
of safety against failure.
of safety against failure.
Since the total lateral force or base shear is the primary design parameter, the current
Since the total lateral force or base shear is the primary design parameter, the current
code format is regarded as a ‘‘strength
code format is regarded as a ‘‘strength-
-based’’ design procedure.
based’’ design procedure.
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code format is regarded as a ‘‘strength
code format is regarded as a ‘‘strength-
-based’’ design procedure.
based’’ design procedure.
The idea of distributing strength throughout the structure rather than relying on a
The idea of distributing strength throughout the structure rather than relying on a
single base
single base-
-shear parameter is recognized in the concept of capacity design (Park and
shear parameter is recognized in the concept of capacity design (Park and
Paulay
Paulay 1976),
1976),
The realization that displacements are more critical than forces initiated the move
The realization that displacements are more critical than forces initiated the move
toward displacement
toward displacement-
-based design (
based design (Moehle
Moehle 1992, 1996; Priestley and
1992, 1996; Priestley and Calvi
Calvi 1997;
1997;
Chopra and
Chopra and Goel
Goel 2001).
2001).

20. DEVELOPMENT OF PERFORMANCE-BASED SEISMIC DESIGN METHODS
 Performance-based design (PBD) has emerged as the new paradigm in seismic
engineering.
 There are several completed and ongoing efforts to develop performance-based
seismic design methodologies, such as –
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 ATC-40 (1996)
 FEMA-350 (2000)
 FEMA-356 (2000)
However, they were each developed with limited objectives.

21. ֍ ATC-40
 initiated the concept of performance‐based seismic evaluation to address the
vulnerability of existing non‐ductile concrete buildings in California.
 contain guidelines for the evaluation and rehabilitation of existing buildings.
 limited to the evaluation of existing reinforced concrete buildings.
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֍ FEMA-350
 applies to new steel moment frames only.

22. ֍ FEMA-356
 contain guidelines for the evaluation and rehabilitation of existing buildings.
 covers all building types.
 contains a blueprint for performance-based design even though it is a
guideline for seismic rehabilitation of structures.
 This has now been replaced with ASCE‐41.
֍ JRA (2002)
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֍ JRA (2002)
contain guidelines for the evaluation of existing bridges [Design Specifications
for Highway Bridges, Part V: Seismic Design].
 contain verification of limit state for three performance level.
Therefore FEMA
Therefore FEMA-
-356 referred to as a ‘‘pre
356 referred to as a ‘‘pre-
-standard’’ suggesting that it
standard’’ suggesting that it
may resemble the model of a Performance Based Design (PBD) code
may resemble the model of a Performance Based Design (PBD) code

23. Any Question?
Any Question?
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Any Question?
Any Question?
©Mohammad Raihan Mukhlis