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Demands and recommendations
for assessment and mitigation of risk
under exceptional earthquakes
Final Report of WG2 Topic 5
A. Plumier University of Liege, Belgium
R. Landolfo University of Naples “Federico II”, Italy
D. Dubina The Politehnica University Timisoara, Romania
European COoperation
in the field of Scientific and Technical research
Transport and Urban Development
COST Action C26:
“Urban Habitat Constructions Under Catastrophic Events”
COST C26 FINAL CONFERENCE
Naples, Italy
16-18 September 2010
Demands and recommendations for assessment and
mitigation of risk under exceptional earthquakes
STATE OF THE ARTIntroduction to the concept of exceptional earthquakes
Features of existing seismic codes contributing to a reduction of risk
Guidance for the assessment of existing structures.
Measures to reduce risk under earthquakes
CONTRIBUTIONS FROM COST MEMBERSAssessment of existing structures
Assessment of seismically strengthened structures
Innovative structural solutions
Improvement in design methods
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.Use only the most reliable global typologies and local details
Impose details for seismic robustness
Use typologies with q factor greater in reality than the q indicated by the code.
Do design following concepts associated with seismic motion typology
RECOMMENDATIONS FOR FURTHER DEVELOPMENTSImprovements in seismic design codes
Some specific aspects of research needs related to new design
Some specific aspects of research needs related to existing constructions
STATE OF THE ART
Introduction to the concept of exceptional earthquakesSeismic design reference earthquake
a given probability of being exceeded or a return period
=> greater values of accelerations can exist = “exceptional” earthquakes
abnormally large inelastic deformation demand to structures
Comments:
whatever the probability chosen, a certain risk of failure exists
● a level of earthquake > design EQ possible
● adequate choices in design => extra margins of safety => recommendations
● existing structures: “normal” intensity earthquake can be “exceptional”
inelastic deformation demand greater than the capacity
● uncertainties exists => uncertainty on exact level of probability of failure of a design
Base Shear V Target Displacement
1,5Target Displacement
Displacement Exceptional Earthquake
Roof
Displacement d
Structure 2
Structure 1
Pushover curves of 2 structures
valid for a given design earthquake.
Structure 2 has can survive
an exceptional earthquake.
STATE OF THE ART
Uncertainties affecting seismic design
● Uncertainty on the action. Every earthquake => modified seismic map
=>“exceptional” earthquake of to-day = design earthquake of to-morrow
● Ignored aspects of seismic motion
directivity effects in near-fault regions and soft soil conditions
ground motions with long period pulse-type form => large period TC
Structures with T< TC => accelerations greater than foreseen, q inappropriate
● Many codes : design earthquake only horizontal
recent earthquakes: damaging effects of vertical component
● For a given q, local ductility required by codes equal for all potential plastic zones
μΦ q RC structures θ q steel structures
some design, real distribution of strength of materials
=> some 1st formed plastic zones ductility request >> code
● Differential settlements in earthquakes add strains
STATE OF THE ART
Features of existing seismic codes contributing to a reduction of risk2 ways to design earthquake resistant structure:
structural elements large remain elastic = DCL
smaller elements deform plastically = DCM-DCH
Since the 80’s, design codes give rules for ductile design
Provides safety if:
● An intended global plastic failure mechanism is defined
no partial mechanisms like soft storey
numerous or large dimensions plastic zones
●“Dissipative zones” plastic deformation cycles small loss of resistance
● Other zones elastic “capacity designed”
Design criteria in codes=> global ductility of structures
“weak beam-strong column” rule for moment resisting frames
Eurocode 8 new : homogenization of overstrength over building heigth in CBF EBF
local ductility of components
Rules specific to material steel : classes of sections
reinforced concrete: ρ % longitudinal / transverse reinforcing steel
=> local ductility μ global ductility behaviour factor q
a margin of safety on local ductility real ductility may be 2 x >> strictly required
Conclusion: ductile design provide some safety for exceptional EQ
STATE OF THE ART
Guidance for the assessment of existing structures.
● A difficult issue needs to be addressed in prevision of catastrophic earthquakes
● Many progresses for engineered structures (steel, reinforced concrete)
to evaluate the limit state of “collapse”
extensive experimental basis & background studies are still needed
● Robust documents: FEMA 356
Eurocode 8 Part 3 (EN1998-3:2004)
● Evaluations of the seismic vulnerability of individual structures
Research work needed to improve regulations for assessment of collapse conditions
Especially masonry
● Evaluations of the seismic vulnerability of groups of structures
Work to do. Especially masonry
STATE OF THE ART
Measures to reduce risk under earthquakes Recent research work
LESSLOSS project.
Guidelines on seismic vulnerability reduction in urban environment
● Screening of buildings at urban scale to identify retrofitting need; www.lessloss.org
● Conventional retrofitting methods;
● New retrofitting techniques Fibre Reinforced Polymers (FRP)
Design methods, user friendly tool, steel rebars + FRP
durability – fatigue - masonry infill transverse & in-plane urban scale;
● Dissipative devices INERD pin connections precast concrete portal , steel CBF
● Base isolation of historical buildings
● Mitigation of hammering between buildings a methodology
● Displacement based methodology of analysis for underground structures in soft soils
PROHITECH project
Exhaustive overview: issues in seismic protection existing/historical buildings
● Innovative technologies
damage in structural fuses practical implementation sometimes difficult
delicate: historical masonry constructions stiff & brittle
reduced efficiency of displacement-based hysteretic dissipation devices
better: viscous dampers
● Need of non-intrusive reversible techniques
COST 26 WG2 Topic 5 CONTRIBUTIONS FROM COST MEMBERS
Introduction
4 years of work on topics:
● Characterization and modeling of seismic action
● Evaluation of structural response under exceptional seismic actions
● Performance based evaluation and risk analysis
● Innovative protection technologies and study cases
● Demands and recommendations for damage prevention
under exceptional earthquakes
101 papers
In the following: a selective review of contributions
CONTRIBUTIONS FROM COST MEMBERS
Assessment of existing structures
Characterization & degree of accuracy of seismic input:
≠ levels type / importance of construction
Study of site seismicity: for important cases
seismic input type of analysis tool
Stratan and Dubina (2008)
discuss record selection for non-linear dynamic time-history analysis THA
from the viewpoint of current codified suggestions and requirements:
number & type of record: far or near fault, recorded, artificial, scaling procedure
Lungu et al. (2008)
study methods to assess soil conditions
to use information to define earthquake actions
Consider the specific Bucharest case
= example to develop EC8 & to harmonize National European seismic codes
Sickert et al. (2008)
use fuzzy stochastic analysis methodology to deal with uncertainties
of structural model & seismic input
important in modern performance-based evaluation methodology
Results: still research.
Long term: contribute to performance-based guidelines for a rational assessment
CONTRIBUTIONS FROM COST MEMBERS
Assessment of existing structures
Some structural types are not well covered in codes
Example: thin, lightly reinforced, structural RC walls
Fishinger et al. (2008)
Walls serve as:
● partitions between rooms
● lateral stiffness and strength
Tools for assessing
flexural-shear-axial interactions
Fishinger et al. (2008)
precast prestressed RC frames
Main source of risk: weak connections
Analysis of thin lightly reinforced RC shear walls
CONTRIBUTIONS FROM COST MEMBERS
Improvements of design rules
Present design codes: based on research over the past 20-30 years.
Several clarification / improvements needed
Steel structures: classification of beams and beam-columns available ductility
plastic overstrength
Eurocode 8 cross section classification = Eurocode 3
strength and stiffness degradation of plastic hinge not considered
Landolfo et al. (2008): a step in this direction
CONTRIBUTIONS FROM COST MEMBERS
Improvements of design rules
Not covered with detail by seismic codes: soil-foundation-structure interaction
Apostolska et al. (2007): behavior of typical RC wall structures
Pushover analysis - capacity spectrum method => target displacement
In soft soils: significant soil deformation
reduction of plastic deformation of structure may even remain elastic
a fixed-base model would indicate spread of plasticity
=> smaller q
Indication on the importance of soil-structure interaction for rigid structures on soft soil
Flexible soil-foundation system Fixed base model
CONTRIBUTIONS FROM COST MEMBERS
Innovative structural solutions for retrofitting investigated in COST C26
● Buckling-restrained braces BRB ● Steel shear panels
● Novel bracing types ● Composite fiber reinforced materials
Mazzolani et al. (2007) & D’Aniello et al. (2008)
Theoretical & experimental studies on retrofitting of under-designed RC buildings
● novel “all-steel” BRB, eccentric braces, composite fiber-reinforced materials
● investigation: collapse tests existing RC structures + retrofit systems
● Eccentric brace: increases stiffness & strength
limited global ductility because large plastic deformation exhaust shear link capacity
● FRP: limited improvements of stiffness & strength
increased ductility of existing members & overall structure
● Buckling-restrained braces: intermediate results
increase stiffness & strength & global structural ductility
Results used to improve knowledge & to develop guidelines
BRB’s
EBF’s
FRP wrapped
On columns
CONTRIBUTIONS FROM COST MEMBERS
Innovative structural solutions for retrofitting investigated in COST C26
Mazzolani et al. (2007)
● similar experiments, metal shear walls ,
steel and aluminum
● large increase of global stiffness,
strength, overall displacement-capacity
● significant local damage to RC members
Bordea et al. (2007):
Combination of “global” &“local” seismic retrofitting
Various combinations FRP- BRBs
Pushover analyses of case studies
Conclusion:
BRB’s alone: not able to meet code requirements
combination OK
laboratory testson-site testing
of prototypes
CONTRIBUTIONS FROM COST MEMBERS
Improvement in design methods
Dubina et al (2007 )
Concept of “Mixed Steel Building Technology”:
HSS used for high yield strength Grade up to 690 MPa
Conventional steel for low yield strength and ductility
Attractive application: dual frames with V braces
high seismic demand for strength in columns and beams
due to unbalanced tension and compression forces in braces
Michalopoulos et al. (2007).
Research on more economical Base Isolation systems
Iurorio et al (2007)
Establish design data & method
for buildings stabilised by cold formed steel walls
Design based on a parametric study performed with
an analytical method which predicts
the nonlinear shear - top wall displacement relationship
based on screw connections test response.
CONTRIBUTIONS FROM COST MEMBERS
Improvement in design methods
Bordea et al (2010)
Potential design value of q of a reinforced concrete building
designed for gravity loads retrofitted with BRB
Performance Based Evaluation of the RC frame before and after retrofitting
Nonlinear static and Incremental Dynamic Analysis
To validate IDA results: 2 full scale tests of a portal frame of the structure,
1 with BRB, one without BRBs monotonic and cyclic loadingMRF vs. MRF+BRB experimental test
-200
-150
-100
-50
0
50
100
150
200
-160 -120 -80 -40 0 40 80 120 160
RC Top Displacement [mm]
Forc
e [K
N]
Retrofitted RC frame (MRF+BRB) Initial RC frame (MRF)
Experimental q
with BRB’s ≈ 4 >> to the original q= 1,5
Experimental test set up
Cyclic pushover curves
initial RC frame (MRF)
retrofitted frame (MRF+BRB)
CONTRIBUTIONS FROM COST MEMBERS
Improvement in design methods
Dinu et al (2010)
Experimental work: 2 storey frames with dissipative shear walls
Calibration of behaviour factor q
Different beam-to-column joints
Observations
q ≈ 5 => Steel Plate Shear Walls SPSW q ≈ q of MRF’s or EBF’s
Numerical parametrical investigations
on multi-storey frames under way
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.
► Use only the most reliable global typologies and local details
► Impose details for seismic robustness
► Use typologies with q factor greater in reality than the q indicated by the code
► Design following concepts associated with seismic motion typology
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.
► Use only the most reliable global typologies and local details
● Conceptual design, regularity, etc
● Else
Eurocode 0: ≠ reliability coefficient KFI can characterise ≠ typologies of structure
More prone to defects => KFI > 1 KFI multiplier of design action
A very unreliable typology => KFI=q
Example: Algerian code RPA2003
● RC MRF’s more than 3 storeys high: forbidden in zones IIb and III
● RC MRF any height forbidden if infills at upper floors no infills at ground floor
Meaning: RC MRF’s not reliable ( Boumerdes 2003) due to uneven concrete quality
=> give up MRF’s and their 100’s critical zones
=> favour wall structures: one big plastic hinge very dissipative by its dimensions
To put the idea into practice: ranking KFI typologies, details
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.
► Impose details for seismic robustness
Details for seismic robustness: additional = construction measures
independent of analysis and design,
applied to improve reliability of structures designed to the code
Robustness is required by Eurocode 1: “the ability of a structure to resist events…
without effects disproportionate to the cause…
in particular the ability to avoid progressive failure, a chain in which a local failure
generates a global failure, effect out of proportion to the original local problem”.
Zanon et al, 2010
INERD concept
Mitigation of soft storey problems
of RC MRF’s:
a) b) c)
The INERD concept
a column locally composite
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.
► Use typologies with q factor greater in reality than the q indicated by the code
q in design codes: lower bound of many values (tenth of thousands)
In reality, a great scatter
q depends on strength of materials, spans, seismicity level, etc...
Postulate: the energy dissipation is greater
if many potential dissipative zones start yielding simultaneously
=> EC8 “homogenisation” rule: overstrength ratio Ω within 25% over the building height.
But fy, real & fc,real ≠ fyd or fcd
Design in favour of an early formation of a global plastic mechanism.
● Select typologies which activate simultaneously all potential plastic zones
Examples: “zipper” EBF
stronger “weak beams-strong columns” condition ΣMRc≥ 2,0ΣMRb
● Use industrialised dissipative zones for which Ω ≈ 1 over the building height
Examples: BRB’s INERD pin connections => q increase from 3,3 to 6,4
A « zipper » EBF
enforces
a global plastic mechanism
RECOMMENDATIONS FOR THE DESIGN OF STRUCTURES
SUBMITTED TO EXCEPTIONAL EARTHQUAKES.
► Do design following concepts associated with seismic motion typology
Stratan & Dubina in (Mistikadis et al, 2007)
To resist severe earthquakes:
● Balanced stiffness and strength between members, connections and supports
● Overall conception and detailing=> enhanced redundancy
● Conceptual design considers features of possible ground motion.
RECOMMENDATIONS FOR FURTHER DEVELOPMENTS
Improvements in seismic design codes
Performance: at present life protection
Other parameters of interest exist: life cycle, maintenance and repair costs
performance of non-structural components
=> performance based approach to design and construction.
Buildings can be designed to perform at different levels of hazards with different risk
Further developments & more wide use of PBD needed before integration in codes
Significant US steps in direction of
Performance-Based Seismic Design and Assessment of Buildings
FEMA 283 (1996) FEMA 349 (2000) FEMA 356
FEMA 445 (2006) Objectives:
► revise the discrete performance levels of 1st generation procedures
create new performance measures: repair costs, occupancy interruption time, losses
more meaningful to stakeholders;
► create procedures for estimating repair costs, occupancy interruption;
► develop a framework for assessment that communicates limitations
in ability to accurately predict response, uncertainty of earthquake hazard.
FEMA461 Testing Protocols for Determining the Seismic Performance
Characteristics of Structural and Non-structural Components, 2000
Similar documents should be drafted for seismic design practice in Europe
RECOMMENDATIONS FOR FURTHER DEVELOPMENTS
Improvements in seismic design codes
Joint Research Centre of EU Commission publishes in 2007 EUR 22858 EN-2007
Pre-normative research needs to achieve improved Design Guidelines
for seismic protection in the EU. Technical Support to the implementation,
harmonization and further development of the Eurocode 8
EUR 22858 EN-2007- General Requests
● a common methodology to evaluate earthquake hazard in Europe
● assessment and strengthening methodology for more economical and safe solutions
● low intrusive strengthening techniques for monuments & historical buildings
● design and upgrading of mechanical & electrical equipments of lifelines and industry
EUR 22858 EN-2007- More specific requests
● Primary vs. secondary seismic elements: further evaluation of the concept
● Flat slab systems ● Prestressed concrete ● Masonry buildings
● Interaction structure-foundation-soil ● Protection of equipments ● Irregular buildings
COST C26 requests
● differentiated design criteria for low/moderate and moderate/high seismic risk regions;
● specific criteria for low dissipative structures, in particular for low/ moderate seismicity
● design provisions for new structural systems, materials and protection technologies,
COST C26 has addressed most of those aspects
Similar research priorities in EUR 22858 EN & COST C 26
RECOMMENDATIONS FOR FURTHER DEVELOPMENTS
Some specific aspects of research needs related to new design
Acceptable risk of collapse behaviour factor q assigned to structural typologies
Historically, q mainly based on experience during past earthquakes
on engineering judgment
Recently, numerical validation
But unavailability of adequate hysteresis models
=> system response studied only in stable range of behaviour
Future research need to consider refined hysteresis models
to correctly capture collapse conditions of structures
Examples of interest●Tall buildings are sensitive to P-Delta effects
strength deterioration=> important P-Delta effects
● Masonry constructions are at risk of collapse even for earthquakes of small intensity
Numerical analyses with refined hysteresis models
could improve the design rules for new constructions.
RECOMMENDATIONS FOR FURTHER DEVELOPMENTS
Some specific aspects of research needs related to existing constructions
Non-engineered masonry
Inherent difficulties of the problem, probabilistic approach required
The fragility of a structure, i.e. the probability of exceedance of a given damage state
for a given earthquake intensity,
must be combined with the rate of exceedance of that earthquake intensity,
in order to calculate the probability of that damage state.
Consideration of both epistemic and random uncertainties
earthquake intensity has a random component
structure behavior and assessment affected by both uncertainties.
More complex for grouped constructions
=> a smaller degree of confidence in the results
larger difficulties in damage assessment process
Research efforts needed to develop scientifically sound methods
to evaluate monetary & life losses.