SLIDE 1 Demands and recommendations for assessment and mitigation of risk under exceptional earthquakes
Final Report of WG2 Topic 5
University of Liege, Belgium
University of Naples “Federico II”, Italy
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
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Demands and recommendations for assessment and mitigation of risk under exceptional earthquakes
STATE OF THE ART
Introduction 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 MEMBERS
Assessment 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 DEVELOPMENTS
Improvements in seismic design codes Some specific aspects of research needs related to new design Some specific aspects of research needs related to existing constructions
SLIDE 3 STATE OF THE ART Introduction to the concept of exceptional earthquakes
Seismic design reference earthquake a given probability of being exceeded
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.
SLIDE 4 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
SLIDE 5 STATE OF THE ART Features of existing seismic codes contributing to a reduction of risk
2 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
plastic deformation cycles small loss of resistance
“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
SLIDE 6 STATE OF THE ART Guidance for the assessment of existing structures.
needs to be addressed in prevision of catastrophic earthquakes
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
SLIDE 7 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
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
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PROHITECH project Exhaustive overview Advanced mixed reversible technologies for seismic protection
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PROHITECH project Exhaustive overview Advanced mixed reversible technologies for seismic protection
SLIDE 10 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
SLIDE 11 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
- f 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
SLIDE 12 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
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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
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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
SLIDE 15 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
SLIDE 16 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 tests
- n-site testing
- f prototypes
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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.
SLIDE 18 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 loading
MRF vs. MRF+BRB experimental test
50 100 150 200
40 80 120 160
RC Top Displacement [mm] Force [KN] 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)
SLIDE 19 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
- n multi-storey frames under way
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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
SLIDE 21 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
SLIDE 22 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
a)
The INERD concept a column locally composite
SLIDE 23 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
SLIDE 24 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.
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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
SLIDE 26 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
SLIDE 27 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
- n experience during past earthquakes
- n 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.
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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.
