Contributions of Seismic Isolation to Earthquake- Resilient Communities

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1 Contributions of Seismic Isolation to Earthquake- Resilient Communities Stephen Mahin Byron and Elvira Nishkian Professor of Structural Engineering Director, Pacific Earthquake Engineering Research Center University of California, Berkeley Open Seminar - JSSI 15th Anniversary International Symposium on Seismic Response Controlled Buildings for a Sustainable Society Seismic Performance Goals? Preserve Life Safety and Prevent Collapse

2 If collapse can be prevented, what level of damage is acceptable? Local Failure Buckled Bars Fractured Spiral Fractured Bar Fracture Yielded and buckled members Permanent Offset Moderate damage may mean a building looses its functionality Natural disasters cause widespread moderate damage Such damage can have substantial long-lasting social, economic and cultural impacts on a city.

3 Sustainable development UN Brundtland Commission meet needs of present generations without compromising the ability of future generations to meet their needs! Selection of materials;! Use of recycled materials;! Consideration of material re-use and disassembly for reuse;! Energy efficiency! Durability and longevity! Reparability! More efficient and lower impact construction;! More efficient design methods, and more efficient structural systems and layouts;! Integration of structural forms to help achieve the needs of other disciplines;! Reducing the impacts of abnormal events such earthquakes by minimizing the need for repair and disruption of service. Resilient Beyond Safety: Issues for Sustainable and Earthquake-Resilient Structures In Earthquake Engineering, our future challenge is to develop new or improved structures that:! protect public safety, and are! economical, but that! can be constructed quickly with minimal disruption to the public and to the environment, and! can withstand strong earthquake ground shaking (and other hazards) safely, with little disruption or cost associated with post-earthquake inspections and repairs. Such approaches are consistent with, and supportive of, emerging trends related to sustainable development and green design. Recycle

4 Resilient structures, networks and communities Are able to withstand a design level earthquake and be able to return to service in a reasonable amount of time. Type of Earthquake Frequent Rare (Design Level) Performance Very little damage Reparability Very rare Life safety based on collapse prevention 20 years ago Resilient structures, networks and communities Are able to withstand a design level earthquake and be able to return to service in a reasonable amount of time. Type of Earthquake Frequent Rare (Design Level) Very rare Performance Very little damage Basic safety Collapse prevention Emphasis Today

5 Resilient structures, networks and communities Are able to withstand a design level earthquake and be able to return to service in a reasonable amount of time. Type of Earthquake Frequent Rare (Design Level) Performance Very little damage Reparability Emphasis Very rare Tomorrow Life safety based on collapse prevention Resilient structures, networks and communities

6 Beyond Safety: Issues for Sustainable and Earthquake Resilient Structures " Safety " Reduce post-earthquake disruption and speed recovery of normal operations # Systems that: $Place damage known locations $Make it easy to inspect $Make it economical to repair # Systems that: $Minimize lateral displacements $Minimize accelerations $Minimize residual displacements Recycle Beyond Safety: Disaster Resilient Structures Numerous structural concepts possible " Self-centering structural concepts " Rocking Foundations " Next-generation braced and damped systems " Inertial damping systems " Seismic Isolation Recycle

7 Beyond Safety: Disaster Resilient Structures Numerous structural concepts possible " Self-centering structural concepts " Rocking Foundations " Next-generation braced and damped systems " Inertial damping systems " Seismic Isolation Recycle Modern approach to seismic design Structures respond inelastically " Reduces design forces $ Smaller members $ Smaller foundations $ Smaller accelerations " Special efforts needed to achieve desired inelastic deformation mechanism $ Capacity design " Displacement sensitive regions (plastic hinges) " Force controlled regions $ Special validated details are needed in regions allowed to yield. Displacement demand Plastic hinges Displacement demand Manufactured mechanical devices

8 Using seismic isolation to improve resilience of structures Manufactured mechanical devices with very high: " Inelastic displacement capacities " Assurance of achieving specified behavior during many cycles so that post-earthquake repair may be unnecessary High system ductilities µ > possible Improving performance of structures using seismic isolation For design level earthquake, normally designed so inelastic deformations occur mainly in isolators. " Inelastic action in only a few easy to inspect locations " Rest of structure remains essentially elastic Analysis models are expected to be very reliable Behavior expected to be dependable

9 Early research " Basic Development and Evaluation of FPS concept (Zayas, Low and Mahin, 1987) Early research " Basic Development and Evaluation of FPS concept (Zayas, Low and Mahin, 1987)

10 Improving performance of structures using seismic isolation Some continuing challenges: " Retrofitting very weak existing structures " Design for intense, near-fault ground motions results in: # Large lateral isolator displacement demands # High isolator strength and stiffness # High accelerations and forces in superstructure " Typical design criteria target enhanced performance (what to do for ordinary buildings?) " Typical design & analysis assumptions may result in inefficient and costly structures Rare earthquake Q d High strength & displacement needed Q d Low strength needed Frequent earthquake! Rare earthquake Frequent earthquake!

11 Improving performance of structures using seismic isolation Some continuing challenges: " Retrofitting very weak existing structures " Design for intense, near-fault ground motions results in: # Large lateral isolator displacement demands # High isolator strength and stiffness # High accelerations and forces in superstructure " Typical design criteria target enhanced performance (what to do for ordinary buildings?) " Typical design & analysis assumptions may result in inefficient and costly structures Q d Low strength needed Frequent earthquake! Bilinear Hysteresis Loop Shape Not Optimal Rare earthquake Rare earthquake High strength & displacement needed Q Hybrid LRB d with slider LB with viscous fluid damper Frequent Triple Pendulum Bearing earthquake! Hysteretic characteristics to improve performance for range of excitations Double Concave Friction Pendulum Bearing Triple Pendulum Friction Bearing Greater displacement capacity with smaller bearings Can vary radii and friction coefficients on each surface to modify bearing properties EPS

12 Details: Triple Pendulum Friction Isolation Bearing Manufactured by Earthquake Protection Systems Test set up Becker & Mahin, 2008

13 TPS bearings tested 2D non-rotational behavior of bearings with friction set as 0.04, 0.08, 0.12 Becker & Mahin, 2008 Test Results Becker & Mahin, 2008

14 Earthquake shaking Becker & Mahin, 2008 Test results Becker & Mahin, 2008

15 Highly durable behavior Significant wearing on slider liners following more than large earthquakes did not significantly effect properties or response of structure Comparison of Behavior after 28 Repetitions of Earthquake Motions Slider Set 1 Unrestrained Sine Wave T=1s Amp=5in Test number 30 vs Test number 58 Test #30 Test #58 Becker & Mahin, 2008

16 Early research " Basic Development and Evaluation of FPS concept (Zayas, Low and Mahin, 1987) Mass and stiffness eccentricities produce very limited torsional response Current research " Mass eccentricities in isolated structures (Stojadinovic, Mieler & Keldrak, 2009) Center of mass and center of resistance coincide Landers, x only

17 NEES tips Current Research Tools for Isolation and Protective Systems (Keri Ryan, Steve Mahin, Gilberto Mosqueda, Lucy Arendt) "Typical design criteria target enhanced performance (what to do for ordinary buildings?) "Typical design & analysis assumptions may result in inefficient and costly structures $Performance-based evaluation and design procedures for seismic isolation systems! Utilizing PEER and ATC 58 approaches! Examine full range of acceptable behavior and identify component properties needed to realize performance goals Various configurations increase possible applications of isolation Moat Podium From:EERC-89/09:Zayas, Low, Bozzo and Mahin

18 Reducing Drift- and Acceleration-related Damage Exploring bearing characteristics and system dynamics Data for proof of concept evaluation and model calibration Braced Frame System (Inelastic) DCFP and TFP bearings Yielding superstructure Uplift on bearings Displacement restraint effects Morgan & Mahin, 2007

19 Comparison of Model and Experiment Stage I TP Cyclic Model TP Cyclic Model with velocitydependent friction Morgan & Mahin, 2007 Comparison of Model and Experiment Stage II Hysteresis Cumulative Energy Dissipation Morgan & Mahin, 2007

20 Comparison of Model and Experiment Stage III Hysteresis Cumulative Energy Dissipation Morgan & Mahin, 2007 Comparison of Isolator Displacements 3 story braced frame - 60 LA ground motions 35 Isolator Displacement (in) Can be designed to reduce isolator displacements For conventional designs, similar isolator displacements Triple Pendulum Single Bilinear Pendulum Isolator Spectral Acceleration (T = 2 sec) Morgan & Mahin, 2007

21 Peak Structural Drift Ratios 2.00% 72 - yr yr yr 1.80% 1.60% 1.40% Maximum PIDR 1.20% 1.00% 0.80% 0.60% 0.40% 0.20% 0.00% 50% reduction Ground Motion Scale Factor Targeted to be same for MCE Single Bilinear Pendulum Isolator Triple Pendulum Triple Pendulum Isolator Power (Single Pendulum) Expon. (Triple Pendulum) Morgan & Mahin, % 3.00% 2.50% Peak Structural Story Drift Ratios 72 - yr yr yr Fixed Base BRBF Maximum PIDR 2.00% 1.50% 1.00% 0.50% 0.00% Ground Motion Scale Factor Single Pendulum Triple Pendulum BRBF (I = 1.5) Power (Single Pendulum) Expon. (Triple Pendulum) Morgan & Mahin, 2007

22 2.5 Comparison of Peak Floor Spectral Accelerations Peak Floor Spectral Accel (g) Spectral Acceleration (T = 2 sec) Triple Pendulum Single Bilinear Pendulum isolator Morgan & Mahin, 2007 Comparison of Peak Floor Spectral Accelerations BRBF Peak Floor Spectral Accel (g) Isolated structures Triple Pendulum Single Bilinear Pendulum Isolator BRBF Spectral Acceleration (T = 2 sec) Morgan & Mahin, 2007

23 Performance-Based Evaluation!"#$%&%'(!)*+%*,-#&!#./+#&&#0!%(!1#+2&!'3!45678!0'9(:2#8!#1$; Enscoe & Mahin, 2009 Parameter Study Conditions Considered " 60 ground motions for Los Angeles (SAC) " Peak responses from shear building BRBF models (Troy Morgan s) with! y =.0033, S DS =1.2g, S D1 =0.68g " 2 isolated buildings $ T isolation =4 sec., R µ =1 $ Isolator yield displacements of 0.01 and 1 " 1 fixed base building $ R µ =3 (value added design) $ T 1 =0.53s Enscoe & Mahin, 2009

24 Median Response Values Building Hazard Level Median PGA (g) Median Story Drift Median Floor Acc. (g) Isolated (u y =.01") 50%/ % %/ % %/ % 0.25 Isolated (u y =1") 50%/ % %/ % %/ % 0.23 Fixed Base 50%/ % %/ % %/ % 0.76 Enscoe & Mahin, 2009 Median Response Values Building Hazard Level Median PGA (g) Median Story Drift Median Floor Acc. (g) Isolated (u y =.01") 50%/ % %/ % %/ % 0.25 Isolated (u y =1") 50%/ % %/ % %/ % 0.23 Fixed Base 50%/ % %/ % %/ % 0.76 Enscoe & Mahin, 2009

25 Median Response Values Building Hazard Level Median PGA (g) Median Story Drift Median Floor Acc. (g) Isolated (u y =.01") 50%/ % %/ % %/ % 0.25 Isolated (u y =1") 50%/ % %/ % %/ % 0.23 Fixed Base 50%/ % %/ % %/ % 0.76 Enscoe & Mahin, 2009 Estimated Cost of Repair Building Type Hazard Level Confidence Level Isolated (u y =0.01") Isolated (u y =1") Fixed Base 72 year EQ Median $41,000 $24,000 $423,000 90% $51,000 $35,000 $944, year EQ Median $74,000 $70,000 $735,000 90% $96,000 $107,000 $1,251, year EQ Median $126,000 $125,000 $2,034,000 90% $211,000 $223,000 $3,161,000 Enscoe & Mahin, 2009

26 Damage cost for isolated building only to partitions Type of Damage Damage to fixed base building is costly, widespread & disrupts occupants during repair Earthquake Resilience of Tall Buildings " The large number of occupants and capital investment in tall buildings suggests that traditional ductile design/collapse prevention approaches may be insufficient " Tall residential structures in US moving towards non-redundant systems Single RC core wall provides lateral resistance

27 Preliminary Results (Envelopes) " Accelerations 2400 Rinaldi228 Acceleration vs. Height 2400 Takatori090 Acceleration vs. Height Height (in) Standard Isolated Height (in) Standard Isolated " Story Drift Acceleration (g) Rinaldi228 ISD vs. Height 2400 Height (in) Standard Isolated Acceleration (g) Takatori090 ISD vs. Height 2400 Height (in) Standard Isolated Ogorzalek & Mahin, % 5% 10% 15% ISD (%) 0 0% 5% 10% 15% ISD (%) Concluding Remarks! Structural engineers can proactively achieve designs that are not only safe, but also resilient and economical in terms of initial construction cost and potential disruptions that might occur in the event of a damaging earthquake through: # improved understanding and exploitation of inelastic behavior of structural systems, and # use of innovative deformation control strategies, such as % seismic isolation, Most dependable % uplifting foundations, % self-centering systems, % other innovative systems.

28 Concluding Remarks Triple pendulum isolators, and some other new devices, offer: # Large lateral displacement capacity in compact form, # Ability to carry large and small vertical loads, # Reduces floor accelerations, # Dependable behavior under wide range of loads, rates, temperatures, and so on, # Durability, and # Simplicity in understanding/specifying behavior.