ACTS Conference, June 2014 Dbayyeh, Lebanon Seismic Evaluation and Retrofit of Existing Reinforced Concrete Structures

Transcription

1 ACTS Conference, June 2014 Dbayyeh, Lebanon Seismic Evaluation and Retrofit of Existing Reinforced Concrete Structures Wassim M. Ghannoum University of Texas at Austin

2 Outline 1. Seismic Evaluation and Retrofit Hazard Evolution of standards Process in the U.S. 2. The Future of Seismic Evaluations and Retrofit in the U.S. Memorandum of understanding between ASCE and ACI Updates to concrete provisions

3 Hazard 1. Seismic design codes introduced in the 80 s 2. Most construction prior to that period did not have adequate strength or detailing 3. Just after that period, design forces were relatively low and system-level vulnerability not adequately addressed 4. A large body of nonductile building stock is present around the world

4 Hazard 1. Nonductile concrete buildings are the single largest threat to life in urban areas 2. In California alone nonductile concrete buildings number over 20,000

5 Hazard 2011 Christchurch, NZ Mw = % of fatalities caused by collapse of concrete buildings 80% of the downtown had to be demolished and rebuilt

6 Hazard Wenchuan, China (2008) Mw = ,000 fatalities Main source of deaths: concrete and masonry construction

7 Hazard Kocaeli, Turkey (1999) Mw = 7.4 Deaths: 17,225, Injuries: 44,000 77,300 homes and 245,000 businesses destroyed

8 Seismic Evaluation and Retrofit in the U.S. Tools for Existing Buildings 0.75*1976 UBC ATC 3-06 (1978) ATC 14 (1987) FEMA ASCE Rapid evaluation of existing buildings FEMA ASCE Advanced evaluation and rehabilitation of existing buildings

9 Seismic Evaluation and Retrofit in the U.S. ASCE 31 is a Standard for rapid seismic evaluation of buildings ASCE 41 is a Standard for detailed seismic evaluation and retrofit of existing buildings Widely adopted in the U.S. and worldwide

10 Seismic Evaluation and Retrofit in the U.S. ASCE ASCE ASC E Seismic Evaluation and Retrofit of Existing Buildings

11 ASCE Chapter 1 General Requirements Chapter 2 Seismic Performance Objectives and Ground Motions Chapter 3 Evaluation and Retrofit Requirements Chapter 4 Tier 1 Screening Chapter 5 Tier 2 Deficiency-Based Evaluation and Retrofit Chapter 6 Tier 3 Systematic Evaluation and Retrofit Chapter 7 Analysis Procedures and Acceptance Criteria Chapter 8 Foundations and Geologic Site Hazards Chapter 9 Steel Chapter 10 Concrete Chapter 11 Masonry Chapter 12 Wood and Cold-Formed Steel Chapter 13 Architectural, Mechanical, and Electrical Components Chapter 14 Seismic Isolation and Energy Dissipation Chapter 15 System-Specific Performance Procedures Chapter 16 Tier 1 Checklists ASCE 31 ASCE 31

12 ASCE/SEI Tier 3 Process 1. Evaluate seismic hazard A small break is given for existing buildings 2. Determine desired performance objectives 3. Analyze structure under hazard using Modeling Parameters linear/nonlinear, dynamic/static 4. Ensure member and structural demands do not exceed Acceptance Criteria

13 Earthquake Hazard Level Seismic Evaluation and Retrofit in the U.S. Building Performance Level Operational Immediate Occupancy Life Safety Collapse Prevention 50% / 50 yr (72 year) 20% / 50 yr (225 year) BSE-1 (~475 year) BSE-2 (~2475 yr)

14 ASCE/SEI Generic Backbone Curve Q/Q y a b a, b & c: Modeling Parameters 1.0 B IO C IO, LS & CP: Acceptance Criteria IO: Immediate Occupancy LS: Life Safety CP: Collapse Prevention D LS c CP E (Peak deformation) A θ y θ or D MP or AC

15 Future of Seismic Evaluations and Retrofit in the U.S. ASCE 41 is in need of updating Document is over 1000 pages long Covers most material types Committee has fewer than 30 voting members Agreement between ACI and ASCE signed in 2014

16 Future of Seismic Evaluations and Retrofit in the U.S. ACI committee 369, Seismic Repair and Rehabilitation starting a new Standard ACI 369 Standard will be the source document for concrete provisions of ASCE 41 Agreement is a foundation for material specialty organizations and ASCE to work together

17 ACI 369 Standard Seismic Evaluation and Retrofit of Existing Concrete Buildings Chapter 1 Scope Chapter 2 Material Properties and Condition Assessment Chapter 3 General Assumptions and Requirements Chapter 4 Concrete Moment Frames Chapter 5 Precast Concrete Frames Chapter 6 Concrete Frames with Masonry Infills Chapter 7 Concrete Shear Walls Chapter 8 Precast Concrete Shear Walls Chapter 9 Concrete Braced Frames Chapter 10 Cast-in-Place Concrete Diaphragms Chapter 11 Precast Concrete Diaphragms

18 ACI 369 Standard MP and AC for RC columns 1. New MP for concrete columns Current MP defined inconsistently across document 2. Use new extended database with close to 500 column tests 3. Treat circular columns 4. Adjust AC based on new MP Conservatism should be incorporated in AC not MP Select AC at percentiles of MP to achieve consistent probabilities of exceedance across all members

19 MP for RC columns - Dataset Extended database with close to 500 column tests Ghannoum, W. M., and Sivaramakrishnan, B. (2012). "ACI 369 Rectangular Column Database. DOI: / Ghannoum, W. M., and Sivaramakrishnan, B. (2012). "ACI 369 Circular Column Database. DOI: /D39Z ;

20 Number of Columns Number of Columns Number of Columns Number of Columns MP for RC columns - Dataset Extended database with close to 500 column tests Rectangular Circular Rectangular Circular Axial Load Ratio Rectangular Circular s/d Rectangular Circular V V y <V o V y V y >V o Drift Transverse Reinforcement Ratio V y /V o

21 MP for RC columns - Dataset Data in form of lateral force-deformation plots Buenrostro, E. (2013). Deformations in Non-Seismically Detailed Concrete Columns." MS Report, University of Texas at Austin

22 MP for RC columns Parameters Most influential parameters for a were found to be Axial load ratio Transverse reinforcement ratio V y /V o

23 transverse Reinforcement Ratio Parameters Table bounds need updating Circular columns Table Modeling Parameters and Numerical Acceptance Criteria for Nonlinear Procedures Reinforced Concrete Columns Modeling Parameters a Acceptance Criteria a Conditions c P A = v A g f c b w s Plastic Rotations Angle (radians) Residual Strength Ratio Plastic Rotations Angle (radians) Performance Level a b c IO LS CP Condition i. b Condition ii. b c P A = v A g f0.035 c b w s V b w d f c d Rectangular columns (0.25) (0.5) (0.25) (0.5) (0.25) (0.5) (0.25) (0.5) Condition iii. b c P A g f c Condition iv. Columns controlled by inadequate development or splicing along the clear height b P A g f c c A = v b w s A v = b 0 w s Axial Load Ratio

24 MP for RC columns a Regression analysis results Rectangular columns P Vy ar (rad) ' t A f V g c Circular columns (spirals or hoops) P Vy ac ' t (rad) A f V g c o o ρ t V y /V o 0.2 Values of a at parameter boundaries P/(A g f c ) ρ t V y /V o a R (rad) a C (rad) * 0.071*

25 Cumulative Distribution Cumulative Distribution MP for RC columns a Analysis of fit Equation estimates ASCE estimates Equation estimates ASCE estimates Number of Tests = Standard Deviation Equation = Standard Deviation ASCE41-06 = Error in a R = Experiment - Estimate (rad) 0.3 Number of Tests = Standard Deviation Equation = Standard Deviation ASCE41-06 = Error in a C = Experiment - Estimate (rad) Median estimate achieved Improved fit for circular columns

26 Error in a R = Experiment - Estimate (rad) Error in a C = Experiment - Estimate (rad) MP for RC columns a Analysis of fit o hooks 90 o hooks Unknown spirals lapped Unknown Rectangular column s/d Circular column s/d Little bias with respect to s/d Little bias with respect to tie hooks

27 MP for RC columns b Behavioral model Due to the scarcity of columns tested to axial collapse a behavioral-model is selected instead of a regressionbased model Simplified Elwood-Moehle axial failure model L = P A v f yt s d c Drift ratio at axial failure

28 MP for RC columns b Analysis of fit Including a loading protocol factor of 0.5 on the drift at axial failure for columns tested under 6 load cycle per drift level or bi-axially Model Average Measured/ Calculated drift ratio at axial failure Standard Deviation Coefficient of Variation ASCE Suppl Elwood-Moehle Simplified Elwood Moehle with loading factor

29 Proposed behavioral model b (rad) Proposed behavioral model b (rad) MP for RC columns b Analysis of fit Using full database Tests not to axial collapse Tests to axial collapse Tests not to axial collapse Tests to axial collapse Experimental b 2R (rad) Experimental b 2C (rad) Model produces median estimates for rect. and circular columns tested to collapse

30 Acceptance Criteria AC defined as percentiles of MP a and b Fixed probability of exceeding a threshold behavior defined by MP a and b IO LS Primary CP Primary LS Secondary CP Secondary Criteria 10% of MP a 20 th percentile of MP a 35 th percentile of MP a 10 th percentile of MP b 25 th percentile of MP b Rectangula r 0.1 a R Circular a R 0.75 a R 0.60 b R 0.75 b R 0.30 a C 0.65 a C 0.60 b C 0.75 b C a C

31 Summary 1. Seismic hazard in the Middle-East is high and mitigation is needed 2. ASCE is the standard used in the U.S. for seismic evaluation and retrofit of buildings 3. Document used in many other applications as it contains MP and AC for a wide range of members and materials 4. ACI 369 standard will be the source document the concrete provisions of ASCE Wide ranging updates to concrete provisions are under way 6. Discussions on altering procedures in ASCE 41 also