同济大学：《建筑结构抗震》课程教学资源（课件讲稿）Chapter 9 Seismic Isolation.pdf_大学文库

12/6/2012 1 College of Civil Engineering, Tongji University Seismic Design of Building Structures Seismic Isolation and Energy Dissipation Gilberto Mosqueda Associate Professor University of California, San Diego Department of Structural Engineering Course Seminar, Tongji University, December 12, 2012 Outline of Presentation • Damage to Structures in Recent Earthquakes • Damage to structural and nonstructural systems • Disruption of critical services • Seismic Isolation • Principles of seismic isolation structures • Hardware – elastomeric and friction bearings • Dynamic properties of isolated structures • Code requirements • Applications to Buildings • Energy Dissipation • Damping devices • Example applications • Current Research in Protective Systems Course Seminar, Tongji University, December 12, 2012 Investment in Building Construction • Only a small portion of construction investments in buildings is related to the cost of the structural skeleton system • For earthquake resistant design, focus is mainly on structural system Typical investment of building construction (from Miranda, 2003) Course Seminar, Tongji University, December 12, 2012 Recent Earthquakes • Feb 27th, 2010 (M 8.8) Maule, Chile earthquake • Collapse of few modern buildings and bridges, several RC buildings damaged beyond repair • Massive amount of nonstructural damage that lead to large impact and significant disruption to Chilean society • Sept 4, 2010 (M 7.1) and Feb 22nd, 2011 (6.3) NZ earthquakes • Structural collapse mainly of URM structures • Significant liquefaction • Large amount of nonstructural damage • March 11th, 2011 Great Eastern Japan earthquake • Not considering tsunami inundation zone, there was very little structural damage due to shaking • Majority of damage in large cities including Tokyo and Sendai was mainly nonstructural Course Seminar, Tongji University, December 12, 2012 Photo Associated Press Structural Damage to Buildings in Chile Course Seminar, Tongji University, December 12, 2012 Photos E. Miranda Structural Damage to Buildings in Chile

12/6/2012 2 Course Seminar, Tongji University, December 12, 2012 Photo by E. Miranda Ceiling-sprinkler head dynamic interaction Nonstructural Damage to Hospitals in Chile • 130 hospitals (71%) in Chile were affected, 18% of beds were lost • Damage to structural and nonstructural systems Photo by G.Mosqueda Course Seminar, Tongji University, December 12, 2012 Closeup view of some sprinkler heads sheared off Photo by E. Miranda Ceiling-sprinkler head dynamic interaction Nonstructural Damage to Airports in Chile • Damage to many types of nonstructural systems and equipment at Santiago airport led to closure of facility Photo by G. Mosqueda Course Seminar, Tongji University, December 12, 2012 Overall view of wood ceiling Photos by E. Miranda Ceiling-sprinkler head dynamic interaction Nonstructural Damage to Airports in Chile • Fire sprinkler damage/leakage in Conception airport led to flooding and closure of facility Course Seminar, Tongji University, December 12, 2012 Damage in Recent Earthquakes • A key objective of seismic design codes is life-safety or collapse prevention • There have been a few cases of structural collapse of modern buildings in recent earthquakes • In most cases, these structures have been subjected to fairly strong shaking • In performance-based design, objective is to limit overall losses • Large story drifts result in damage to structural system • Low to moderate story drifts can result in damage to nonstructural components such as partition • Large accelerations in structures result in damage to contents, piping and other mechanical equipment • Functionality of a structure following earthquake requires acceptable performance of both structural and nonstructural systems Course Seminar, Tongji University, December 12, 2012 Seismic Isolation • One of the most effective techniques to protect structures from EQ • Provides a flexible ‘isolation’ layer with lateral stiffness well below the structure lateral stiffness • Reduces natural vibration period of structural system • Limits energy transfer to structure reducing both accelerations and story drifts • Induces large displacement demands across the isolation interface • Requires specialized hardware that can provide large lateral displacement with vertical load capacity and re-centering capabilities Fixed - base structure Isolated Structure Image DIS-Inc Course Seminar, Tongji University, December 12, 2012 Principles of Seismic Isolation • Effect of period shift in seismically isolated structures • Reduction in seismic forces • Increase in lateral displacement demand of isolation system • Increased damping tends to improve seismic performance (Christopoulos and Filiatrault 2006)

12/6/2012 3 Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware • Reliable hardware for seismic isolation has only matured in last 30 years • Seismic isolation bearings require the following properties • Lateral flexibility relative to the structural system • Vertically stiff with ability to carry the gravity load under large lateral deformations • Sufficient initial stiffness to prevent relative motion under wind and other non-seismic loads • Two main types of seismic isolation bearings • Elastomeric bearings • Sliding friction bearings • Energy dissipation may be inherent in bearings, supplemental energy dissipation devices can be provided if not sufficient Mainly used in China Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware - Elastomeric • Elastomeric Bearings consist of alternating layers of steel and rubber • Lead Rubber bearings (shown below) include a lead core for enhanced energy dissipation • High damping rubber bearings Image DIS-Inc Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware - Elastomeric • Thin layers of rubber improve vertical stability of bearing Image T. Saito Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware - Elastomeric • Force-Displacement relationship for various types of elastomeric bearings • Shear strain defined as lateral displacement/total height of rubber (From Thompson et al. 2000) High Damping Rubber Lead Rubber Low Damping Scragging Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware - Sliding • Sliders on smooth flat surface that dissipates energy by friction with parallel linear springs to provide re-centering capabilities • Typically PTFE on polished stainless steel surface • Slider on concave surface to provide re-centering capabilities through gravity – Friction Pendulum bearing shown below Course Seminar, Tongji University, December 12, 2012 Seismic Isolation Hardware - Sliding • With Friction Pendulum Bearings, the structure displaces laterally similar to a pendulum, providing a re-centering force by gravity = Pendulum K2=W/R Friction QD=W

12/6/2012 5 Course Seminar, Tongji University, December 12, 2012 Theory of Seismic Isolation • Determine frequencies and mode shapes via modal analysis • Solve eigenvalue problem Similar to isolated rigid structure Increases compared to s Course Seminar, Tongji University, December 12, 2012 Theory of Seismic Isolation • Examination of modal response provides further insight into effectiveness of seismic isolation The response of each mode is governed by the following equations (Christopoulos and Filiatrault 2006) Course Seminar, Tongji University, December 12, 2012 Theory of Seismic Isolation • Modal participation factors for earthquake loading (Christopoulos and Filiatrault 2006) Course Seminar, Tongji University, December 12, 2012 Theory of Seismic Isolation • Modal participation factor for second mode is small if s and b are well separated (typically by factor of 3 or more) • Earthquake excitation vector is similar to the first mode and therefore almost orthogonal to second mode • Earthquake excitation at second mode frequency will not be transmitted to structure • NOTE: Damping has been neglected to allow for the application of modal analysis Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Specific design provisions for seismically isolated buildings introduced in many countries • China: Code for Seismic Design of Buildings GB 50011-2010 • U.S.: ASCE 7-10, Chapter 17 • Provisions as discussed here mainly relate to ASCE 7-10 Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • In U.S., seismic isolation is mainly applied to important facilities having high performance requirements • Hospitals or emergency response centers • Retrofit of historical buildings • In China and Japan, it is applied to important facilities but also has much more widespread applications to regular buildings • Seismically isolated structures typically designed to achieve higher performance standards than fixed-based structures • No nonstructural damage for moderate earthquakes • No major structural damage for strong earthquakes (fixed-based buildings are designed for life-safety) • Design usually always requires dynamic time-history analysis • Static analysis design methods first applied for preliminary design and to develop minimum design criteria • Special criteria for peer review of design and prototype testing of bearings

12/6/2012 6 Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Equivalent linear bearing properties for static design method • Stiffness and damping used to determine structure period and seismic demands assuming rigid structure of weight W • Note that properties are based on design displacement DD, method is iterative Equivalent viscous damping ratio Isolated Period (Christopoulos and Filiatrault 2006) Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Design displacement for Design Based Earthquake (DBE) • SD1 = Design spectral acceleration at 1 sec period • BD = Damping reduction factor based on damping ratio • To account for torsion • Prototype isolators constructed and tested based on preliminary design, then actual properties are used to refine analysis • Similarly, displacement DM for Maximum Expected Earthquake (MCE) also computed based on spectral acceleration SM1 with probability of exceedence of 2% in 50 years Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Structure designed for DBE demands • Structure designed to be essentially elastic, strength reduction factor R<2 (much less than for fixed based design) • If structure yields, structure period increases and effectiveness of isolation is reduced • Could attract larger deformations from structure (higher mode participation) • Seismic isolation system designed for MCE demands • Bearings must be able to support gravity loads at DM • Failure of isolation system considered catastrophic Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Design based on dynamic analysis • Incentives to use dynamic analysis • Minimum design based on static analysis as a sanity check • (Christopoulos and Filiatrault 2006) Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Design Based on Dynamic Analysis • Pairs of horizontal components from at least three recorded seismic events necessary for time-history analysis • Events must be representative of site, soil, and source characteristics and have durations consistent with DBE or MCE. • Time- histories site within 15 km from major active fault requires nearfault characteristics • Ground motions scaling: • For each ground motion pair, SRSS of 5% damped spectrum computed • Motions scaled so that average SRSS spectrum not below target spectrum for the MCE in the period range 0.5TD to 1.25TD • Calculation of design values: • If three time-histories used: Design based on maximum response quantities from three time-history analyses • If seven time-histories used:Design based on mean response quantities from seven time-history analyses Course Seminar, Tongji University, December 12, 2012 Design of Seismic Isolated Structures • Design Review: • To be conducted by independent team of design professionals familiar with base isolation technology (see ASCE 7-10, S. 17.7) • Prototype Testing: • To verify deformation and damping characteristics of base isolation system • Conducted on two full-size specimens of each type of isolator used including the wind restraint system. • Prototype specimens tested not used in actual construction • See ASCE 7-10, S. 17.8 for details of testing program and acceptance criteria

12/6/2012 7 Course Seminar, Tongji University, December 12, 2012 Application of Seismic Isolation – U.S. • ~300 isolated buildings • Specialized applications, limited to essential buildings and historic retrofits in high seismic zones • Designed for higher performance standards • 5-10% increased construction cost impediment to ordinary structures SF City Hall Stanford hospital Course Seminar, Tongji University, December 12, 2012 Application of Seismic Isolation – U.S. Berry Street Project in San Francisco • 2-story isolated vertical addition to existing 3-story building • Objective: maximize additional space but minimize disruption • 1200 mm LRB and 600 mm elastomeric sliders to accommodate large displacements under light vertical loads Course Seminar, Tongji University, December 12, 2012 Application of Seismic Isolation - Japan • ~5000 isolated buildings in Japan • Applications spurred by 1995 Kobe EQ • Earthquake awareness based on historical seismicity • Commercial and residential use • High rises (50 stories), small homes • Public demand and market for seismic protection Application of seismic isolation in Japan (clark et al. 2000) Course Seminar, Tongji University, December 12, 2012 Application of Seismic Isolation – China • ~600 isolated buildings • Applications slowly started in 1993 and increase in 1997 • Introduced into code in 2001, recently revised in 2010 • Potential for significant increase in use following 2008 Wenchuan Earthquake • Highest isolated structure is 20 stories SOURCE: Pan et al. 2012 Seismic Isolation of building group in Beijing Course Seminar, Tongji University, December 12, 2012 Performance in Earthquakes • USC University Hospital in Los Angeles during 1994 Northridge EQ • 8-story steel superstructure supported by 149 elasotimeric isolators • Reduced accelerations by 66% at the base and 40% at the roof. (USGS, http://pubs.usgs.gov/fs/2003/fs068-03/fs068-03.pdf) Course Seminar, Tongji University, December 12, 2012 Performance in Earthquakes • Building in Ojiya during the 2004 Niigata, Japan EQ • Lateral acceleration is reduced by 25% • Vertical acceleration increased by 150% (how to isolate vertically?) SOURCE: L. Masanori, BRI

12/6/2012 8 Course Seminar, Tongji University, December 12, 2012 Research in Seismic Isolation • Concept of seismic isolation by rocking or sliding dates back to 300 BC • Research into modern isolation systems initiated with Lead Rubber Bearings in New Zealand in 1970’s. • Numerous experimental and numerical studies have been done on seismic isolation including shake table studies • Current research on hardware • Modeling and component testing of seismic isolation devices • Behavior of elastomeric bearings at large displacements, including stability under large axial loads • Verification of bearing behavior at full-scale • Current research on seismic isolation systems • Verification of system level behavior at full scale • Life-cycle cost analysis to demonstrate cost benefit • Behavior of seismic isolation systems under extreme earthquakes • Applications to nuclear power plants Course Seminar, Tongji University, December 12, 2012 Shake table tests of a full‐scale 5‐story steel moment frame building (PI: K. Ryan, Reno; S. Mahin, Berkeley; G. Mosqueda, San Diego)  triple friction pendulum isolators  lead rubber bearing/cross linear slider  Fixed base o Simulations designed to impose large displacement demands in isolation systems o Simulations both with and without vertical component of ground motion o 4th and 5th floor included nonstructural systems E-Defense Experiments: 5 story steel moment frame Course Seminar, Tongji University, December 12, 2012 E-Defense Experiments: 5 story steel moment frame Course Seminar, Tongji University, December 12, 2012 E-Defense Experiments: 5 story steel moment frame Course Seminar, Tongji University, December 12, 2012 E-Defense Experiments: 5 story steel moment frame Course Seminar, Tongji University, December 12, 2012 E-Defense Experiments: 5 story steel moment frame Level Peak Acceleration Profile Peak Acc. (g) • Measured response 50 60 70 80 90 100 -0.5 0 0.5 -0.48446 -0.11864 0.14598 Fixed Base TPB Isolated LRB Isolated 50 60 70 80 90 100 -0.5 0 0.5 0.5844 0.14362 -0.23067 Base Shear Coefficient X‐direction Y‐direction Time (sec)