Demands and recommendations for assessment and mitigation ... · PDF fileDemands and recommendations for assessment and mitigation of risk under exceptional earthquakes Final Report

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

http://www.lessloss.org/

PROHITECH project Exhaustive overview

Advanced mixed reversible technologies for seismic protection

PROHITECH project Exhaustive overview

Advanced mixed reversible technologies for seismic protection

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


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


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


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


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


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



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.



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)



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


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



► 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



► 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



► 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



► 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


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


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.


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.

Demands and recommendations for assessment and mitigation ... · PDF fileDemands and recommendations for assessment and mitigation of risk under exceptional earthquakes Final Report

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