Performance Based Seismic Design
Course 2
- Design procedure in modern seismic design codes
- Performance based design (1)
Course notes are available for download athttps://www.ct.upt.ro/studenti/cursuri/stratan/pbsd.htm
Design procedure in modern seismic design codes
Modern seismic design codes
Design and construction of structures controlled through codes
Design codes provide a set of minimum requirements, which, if enforced, should provide a minimum accepted level of "safety, serviceability and durability".
These objectives are generally implemented through a set of prescriptive criteria which impose:– Acceptable construction materials (r.c., steel, timber, etc.)
– Types of structures and non-structural systems (moment resisting frames, concentrically or eccentrically braced frames, structural walls, etc.)
– Minimum levels of strength, stiffness [and ductility]
– Detailing rules (minimum cross-section size, minimum rebar diameter, minimum and maximum spacing of bolts, etc.)
Modern seismic design codes
"Modern codes": P100-1/2013, EN 1998-1: 2004, etc.
Fundamental requirements:– Life safety: The structure shall be designed and constructed to withstand
the design seismic action without local or global collapse, thus retaining its structural integrity and a residual load bearing capacity after the seismic events. P100-1/2013: the seismic action with a probability of exceedance of 20% in 50 years or a return period of 225 yearsEN 1998-1: probability of exceedance of 10% in 50 years or a return period of 475 years
– Damage limitation. The structure shall be designed and constructed to withstand the seismic action without the occurrence of damage and the associated limitations of use, the costs of which would be disproportionately high in comparison with the costs of the structure itself. P100-1/2013: a probability of exceedance of 20% in 10 years or a return period of 40 yearsEN 1998-1: probability of exceedance of 10% in 10 years or a return period of 95 years
Modern seismic design codes
Fundamental requirements (life safety and damage limitation) are verified by checking the structure for two limit states:
Ultimate Limit State (ULS)– associated to collapse and other forms of structural degradation
that may endanger human lives
– verification of ULS implies a balance between strength and ductility
Serviceability Limit State (SLS)– associated to degradations, that lead to limitation of use
– limitation of structural and non-structural damage
– generally, check for SLS involves limitation of interstorey drifts, in order to protect non-structural elements, equipment, etc.
Modern seismic design codes
Design at the ULS– Selection of design concept (dissipative / low-dissipative) and
ductility class (H, M, L) of the structure
– Seismic action defined through elastic response spectra
– Design seismic action (reduced using the behaviour factor q) depending on the type of structure and ductility class. The value of the behaviour factor q affected by the regularity of the structure. Values of behaviour factor q – empirical, based on observations in past earthquakes and engineering judgement.
– Structural response estimated using an elastic analysis (LFM or spectral analysis)
– Dissipative components: strength checks and requirements aimed at providing the necessary degree of ductility (materials, cross-section class, slenderness, transversal reinforcement, etc.)
– Non-dissipative components: overstrength with respect to dissipative components promoting a favourable plastic mechanism (global). Capacity based design.
Modern seismic design codes
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Modern seismic design codes
Design at the SLS
Displacement analysis at SLS:
– ds lateral displacement at SLS
– de lateral displacements determined from design earthquake action
– reduction factor to account for a lower mean return period of SLS earthquake (=0.4-0.5)
SLS checks
– brittle non-structural elements
– ductile non-structural elements
– without non-structural elements
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inelastic response
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Modern seismic design codes
P100-1/2013: limitation of lateral drifts at ULS
Prevention of loss of life due to total failure of non-structural elements
Displacement analysis
Check of interstoreydrifts at ULS:
es dqcd
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inelastic response
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Modern seismic design codes
Design codes Reality
Recommends regular and simple structures.
Most structures do not fulfil code regularity requirements.
Structural response modelled using anelastic static analysis. Ductility of the structure quantified through the behaviour factor q.
Most structures will respond in the inelastic range during a design earthquake, the response being affected by the dynamic nature of the seismic action.
Check at the SLS by limiting lateral drifts.
- Only non-structural components are targeted.
- Other response quantities may be more relevant (acc.)
… …
Modern seismic design codes
Modern seismic design codes are prescriptive and try to be simple.
Though the objective of the seismic design codes is to provide certain performance levels, the ability of the structure to attain a certain performance level is NOT evaluated as a part of a traditional design process.
The real performance of code-designed buildings can be better than the minimum requirements, or worse.
Building owners and tenants have generally the perception that structures designed to modern codes have a high safety level, and their damage is not probable.
Performance based design
Need for performance based seismic design
Earthquakes produced at the end of the 20th century (e.g. 1994 Northridge earthquake and 1995 Kobe earthquake):– Has forced recognition that damage, sometimes severe, can
occur in buildings designed in accordance with the code.
Need for performance based seismic design
– Property and insured losses as a result of the Northridge Earthquake, recognized as the most costly earthquake in U.S. history, led to an awareness that the level of structural and nonstructural damage that could occur in code-compliant buildings may not be consistent with public notions of acceptable performance.
Need for performance based seismic design
Increased losses are due to several factors:– A larger density of the building stock in seismically active areas
– An increasingly older building stock
– Larger costs of business interruption
– Larger share of losses due to failure of nonstructural components and contents (in case of high-tech industries and medical facilities).
However, there is a major misperception on the part of many owners, insurers, lending institutions and government agencies about the expected performance of a code conforming building.
Current codes clearly serve an essential and effective role in protecting building occupants. The design basis of the code is intended to provide a basic level of safety and a relatively economical means by which to construct buildings.
Stakeholders are demanding that practical and cost-effective means be developed to address the issues of damage control and loss reduction.
Need for performance based seismic design
Developments in the field of earthquake engineering
+
Need for seismic performance assessment methods able to provide stakeholders informations on expected level of building damage
+
Acknowledgement of the fact that prescriptive design approach used for new buildings are NOT suitable for evaluation of existing ones (which do not satisfy a series of requirements for regularity, detailing, etc.)
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Development of Performance Based Seismic Design methods (PBSD)
Performance based seismic design
Performance based seismic design (PBSD) offers engineers the possibility to design in a reliable way structures meeting a given performance level. At the same time, PBSD lets stakeholders to quantify financially (or otherwise) the risk to which their properties are prone, and to choose a performance level according to their needs, keeping a minimum level of safety.
In PBSD identification and evaluation of seismic performance of the building is part of the design process.
PBSD is an iterative process.
Performance based seismic design
Select performance objectives
Preliminary design
Assess performance
Revise design
Does performance meet objectives
DoneYESNO
Performance based seismic design
Performance-based design begins with the selection of design criteria stated in the form of one or more performance objectives.
Each performance objective is a statement of the acceptable risk of incurring specific levels of damage, and the consequential losses that occur as a result of this damage, at a specified level of seismic hazard.
Losses can be associated with – structural damage,
– nonstructural damage, or
– both.
They can be expressed in the form of – casualties,
– direct economic costs,
– and downtime (time out of service)
Performance based seismic design
Generally, a team of decision makers, including the – building owner,
– design professionals, and
– building officials,
will participate in the selection of performance objectives for a building.
This team may consider the needs and desires of a wider group of stakeholders including prospective tenants, lenders, insurers and others who have impact on the value or use of a building, but may not directly participate in the design process.
The basic questions that should be asked are: – What events are anticipated?
– What level of loss/damage/casualties is acceptable?
– How often might this happen?
Performance based seismic design
The notion of acceptable performance follows a trend generally corresponding to: – Little or no damage for small, frequently occurring events
– Moderate damage for medium-size, less frequent events
– Significant damage for very large, very rare events
Once the performance objectives are set, a series of simulations (analyses of building response to loading) are performed to estimate the probable performance of the building under various design scenario events.
In the case of extreme loading, as would be imparted by a severe earthquake, simulations may be performed using nonlinear analysis techniques. DISPLACEMENT
FO
RC
EIO LS
CP IO - Immediate Occupancy
LS - Life Safe
CP - Collapse Prevention
CO
CO - Completely Operational
Performance based seismic design
PBSD is an iterative process
In some cases it may not be possible to meet the stated objective at reasonable cost, in which case, some relaxation of the original objectives may be appropriate.
PBSD provides a systematic methodology for assessing the performance capability of a building, system or component.
It can be used to:– verify the equivalent performance of alternatives,
– deliver standard performance at a reduced cost,
– or confirm higher performance needed for critical facilities.
PBSD establishes a vocabulary that facilitates meaningful discussion between stakeholders and design professionals on the development and selection of design options.
Performance based seismic design
PBSD provides a framework for determining what level of safety and what level of property protection, at what cost, are acceptable to stakeholders based upon the specific needs of a project.
Performance-based seismic design can be used to:
– Design individual buildings with a higher level of confidence that the performance intended by present building codes will be achieved.
– Design individual buildings that are capable of meeting the performance intended by present building codes, but with lower construction costs.
– Design individual buildings to achieve higher performance (and lower potential losses) than intended by present building codes.
– Design individual buildings that fall outside of code-prescribed limits with regard to configuration, materials, and systems to meet the performance intended by present building codes.
– Assess the potential seismic performance of existing structures and estimate potential losses in the event of a seismic event.
– Assess the potential performance of current prescriptive code requirements for new buildings, and serve as the basis for improvements to code-based seismic design criteria so that future buildings can perform more consistently and reliably.
Performance objectives
A performance objective is an acceptable risk of experiencing a specific damage level (with associated losses) for a specific level of seismic hazard
The concept of performance objective is equivalent to the one of limit state
For a building to fulfil a performance objective, it is necessary that it fulfils a given seismic performance levelassociated to a certain seismic hazard level
Generally design or evaluation of a building implies fulfilling of more performance objectives (limit states), depending on importance class and the requirements of the stakeholder
Choosing performance objectives
OPERATIONALFULLY
OPERATIONALLIFESAFE
NEARCOLLAPSE
EARTHQUAKE PERFORMANCE LEVELE
AR
TH
QU
AK
E D
ES
IGN
LE
VE
L
VERY RARE(2475 years)
(475 years)RARE
(225 years)OCASIONAL
(72 years)FREQUENT
PEROFRMANCE(FOR NEW CONSTRUCTION)
UNACCEPTABLEBASIC OBJECTIVES
ESSENTIAL/HAZARDOUS OBJECTIVES
CRITICAL OBJECTIVES
25
Performance based seismic design
Evolution of performance based seismic design
Three series of documents tried to develop procedures that can be used as seismic design criteria and which form the basis for performance based seismic design:– SEAOC Vision 2000 (1995)
– ATC 40 (1996)
– FEMA 273 and 274 (1996), followed by FEMA 356, ASCE/SEI 41-06 and ASCE/SEI 41-13
Evolution of PBD: SEAOC Vision 2000
The object of SEAOC Vision 2000 is to develop a background for some procedures that would permit the design of structures with predictable seismic performance adjusted to multiple performance objectives.
The document presents the concepts of performance levels for structural and nonstructural elements.
There are described five performance levels, and defined the limits of the corresponding transient and permanent drift. It is suggested the use of the concepts offered by capacity design for control of the inelastic behaviour of the structure and also to designate the ductile components of the structural system to lateral forces.
The design methods include different procedures such as conventional force-based methods, displacement-based methods and energy methods.
Evolution of PBD: ATC 40
The provisions of ATC 40 refer to a methodology that expresses the structural criteria in terms of achieving a performance objective.
The document is limited to reinforced concrete structures and it uses the capacity spectrum method to evaluate the behaviour of the structure. This procedure implies the determination of the capacity and demand spectra.
In order to build the capacity spectrum, a non-linear static (pushover) analysis is used, which leads to a force-displacement relation for the structure. The forces and displacements are then converted to spectral accelerations and spectral displacements using an equivalent single-degree-of-freedom system.
The system requirements are represented by highly damped elastic response spectra.
Evolution of PBD: FEMA 273/274 and FEMA 356
FEMA 273 and its successor, FEMA 356 (2000) and ASCE/SEI 41-06, ASCE/SEI 41-13 represent a variety of performance objectives with associated probabilistic ground motions.
Analysis and design methods for the multi-level performance range from linear static to inelastic time history analysis.
The documents defines performance levels for non-structural elements and systems and suggests drift limits for various lateral-load-resisting structural systems at different performance levels.
Are probably the most comprehensive documents at the present time providing a practical approach for performing a performance-based seismic evaluation of a structure
European codes
EN 1998-1: 2004 "Eurocode 8: Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings".
– Two fundamental (performance ) requirements : no-collapse (ULS) and damage limitation (SLS).
– Allows advanced structural analysis methods (nonlinear static and nonlinear dynamic methods).
– Basis for implementation of PBSD but a complete methodology is NOT present.
EN 1998-3: 2004 "Eurocode 8 - Design of structures for earthquake resistance Part 3: Assessment and retrofitting of buildings".
– Three limit states (performance objectives):– Near Collapse - NC,
– Significant Damage - SD, and
– Damage Limitation - DL.
– Application methodology present , but non-technical vocabulary is missing
Romanian codes
P100-1: 2013 "Cod de proiectare seismică - Partea I -Prevederi de proiectare pentru clădiri"– Two fundamental (performance ) requirements : life safety (ULS)
and damage limitation (SLS).
– Allows advanced structural analysis methods (nonlinear static and nonlinear dynamic methods).
– Basis for implementation of PBSD but a complete methodology is NOT present.
P100-3: 2008 "Cod de proiectare seismică – Prevederi pentru evaluarea seismică a clădirilor existente"– Requires checking of at least two limit states (performance
objectives), the same as the ones in P100-1:2006
– Application methodology present , but non-technical vocabulary is missing
Conclusions
PBSD has the objective of designing structures with controlled response for different levels of seismic hazard, depending on the concrete needs of stakeholders.
The concept itself is not new, representing an extension, generalisation, formalisation and quantification of the limit state method (Fajfar, 1998).
The new developments consist in procedures and methodologies for practical implementation of PBSD in future seismic design codes.
