Demands and recommendations for assessment and mitigation of risk under exceptional earthquakes Final Report of WG2 Topic 5 A. Plumier University of Liege, Belgium University of Naples “Federico II”, Italy R. Landolfo D. Dubina The Politehnica University Timisoara, Romania COST C26 FINAL CONFERENCE Naples, Italy 16-18 September 2010 European COoperation in the field of Scientific and Technical research Transport and Urban Development COST Action C26: “Urban Habitat Constructions Under Catastrophic Events”
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
STATE OF THE ART Introduction to the concept of exceptional earthquakes Seismic 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 Structure 2 Pushover curves of 2 structures valid for a given design earthquake. Structure 1 Structure 2 has can survive an exceptional earthquake. Roof Displacement d
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 T C Structures with T< T C => 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 1 st 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 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 ● “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 ρ % longitudinal / transverse reinforcing steel reinforced concrete: => 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
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
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 Analysis of thin lightly reinforced RC shear walls Fishinger et al. (2008) precast prestressed RC frames Main source of risk: weak connections
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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