Seismic Rehabilitation of Buildings. Anindya Dutta, Ph.D., S.E. Systematic vs. Simplified

Transcription

1 Seismic Rehabilitation of Buildings Anindya Dutta, Ph.D., S.E. Systematic vs. Simplified Systematic Rehabilitation Any Performance for any EQ Any Building Any deficiency Simplified Rehabilitation Life Safety for 10%/50 year EQ Simple Buildings conforming to model building types Limitations number of stories deficiencies 1

2 Applicable Standards The Basic Steps Start Determine Existing Configuration Select Performance Objectives Determine Design Ground Motion(s) Evaluate Building Determine Deficiencies Develop Preliminary Design Perform Analysis Acceptable? No Done! 2

3 Building Configuration Defines overall behavior of building including: Lateral Force Resisting Systems Intended by designer Actual, including: structural elements non-structural elements Gravity Load Resisting System Building Configuration Hierarchy of parts that comprise a building: Elements Components Actions 3

4 Elements Horizontal or vertical subassemblies that comprise a structure: braced frame moment frame shear wall diaphragm isolation system Components Individual members that comprise an element: beam column joint brace pier footing damper 4

5 Actions Independent degrees of freedom associated with a component, each with an associated force and deformation: axial force - elongation moment - rotation torional moment - twist Building Configuration Primary Any element (component) {action} required to provide the building s basic lateral resistance similar to the concept of a participating element in the building code Secondary Any element (component) {action} that is not required to provide the building s basic lateral resistance may participate but is not required to do so 5

6 Primary & Secondary Permits engineer to utilize judgment in determining whether a building meets the intended performance levels Secondary elements are permitted to experience more damage than primary elements Acceptance criteria for secondary elements are more permissive than for primary elements Primary & Secondary Slabs (as diaphragms) (Primary) Slabs & interior columns (as frames) (Secondary) Plan Walls at elevator & stair (Secondary) Perimeter walls (Primary) Elevation 6

7 Determining Configuration Developing an understanding of the existing construction configuration is essential to: understanding its probable behavior developing reliable rehabilitation designs Sources of data: Original drawings, specs & calculations Renovation & remodel drawings Previous engineering reports Interviews with building maintenance office, owners, tenants Interviews with original designers On-site observation & inspection Building Configuration Economic conditions often lead to gathering less rather than more data The κ factor a reliability based coefficient used to adjust component strength values based on the quality of knowledge about component properties 7

8 How much do you know? Level of knowledge is: Comprehensive complete original documents or extensive surveys (destructive & non-destructive) extensive in-place testing performed material strength coefficients of variation < 20% foundation types & capcities well defined κ = 1.0 Minimum plan & elevation information based on documents &/or surveys visual confirmation of primary elements limited program of inplace materials testing performred or default values of Chapters 4 through 8 utilized foundation types known κ = 0.75 Minimum Levels of Knoweldge Foundations type, dimensions and materials of components Collapse Prevention & Life Safety Levels type, composition, consistency, relative density, and layering of soils through zone of foundation stress influence Enhanced levels unit weight γ, shear strength, soil friction angle φ, compressibility, shear modulus G and Poissons ratio ν 8

9 Minimum Levels of Knoweldge Structural steel Use of default values, or: material strength original documents + 3 coupons from every each group of components of a specified grade, or limited original documents + 3 coupons from every component type 6 coupons from each element type, every 4 stories weld metal 1 sample for laboratory testing Minimum Levels of Knoweldge Concrete 3 compressive tests for each element type minimum of 6 for entire structure minimum of 3 for each class of concrete minimum of 3 for each story, 200 cu yds, or 5,000 sq. ft. of surface area for lightweight concrete - similar requirements for splitting tensile strength 9

10 Minimum Levels of Knoweldge Masonry visual examination & default values, or: establish compressive strength and modulus of elasticity using test prisms flat jack tests in-place bed shear tests for URM 3 samples for each 3 stories, or 3,000 sq. ft. of masonry surface if original records specify strength 6 samples as above, if not Minimum Levels of Knoweldge Timber original documents defining grade + 2 visual verifications of grade for each component type for each 2 stories, or species and age is known, 3 visual verifications of grade for each component type for each 2 stories, or if no knowledge, 6 visual veritications of grade for each component type for each 2 stories absent documentation, at least 3 connectors of each type removed for grading and testing 10

11 Building Configuration Adjacent Hazards obtain sufficient information on adjacent structures to assess the potential effects of: Shared Elements (Party Walls) Pounding Debris & Other Hazards Rehabilitation Objectives Specification of Desired performance level Hazard level at which performance is achieved Rehabilitation Objective = + Ground Motion x% - 50 years Performance Level 11

12 Building Performance Structural Performance S-1 Immediate Occupancy Level S-2 Damage Control Range S-3 Life Safety Level S-4 Limited Safety Range S-5 Collapse Prevention Level S-6 Not Considered Non-structural Performance N-A Operational Level N-B Immediate Occupancy Level N-C Life Safety Level N-D Hazards Reduced Level N-E Not Considered Structural Performance Levels Immediate Occupancy - S1 original strength maintained original strength essentially maintained minor damage - repairable at convenience Typical damage: steel frames: minor yielding, no fractures concrete frames: minor hairline cracking URM walls: cracks < 1/8 width timber: minor hairline cracks in gypsum& plaster sheathing 12

13 Structural Performance Levels Life Safety - S3 significant damage to secondary elements limited damage to primary elements stiffness and strength degradedbut significant residual strength remains Typical damage: Braced frames - braces buckle & yield Concrete walls - boundary elements spall coupling beams shear Wood walls - nails partially withdrawn, edge members split Structural Performance Levels Collapse Prevention - S5 significant damage to primary & secondary elements little remaining stiffness or strength Typical damage: steel frames: extensive distortions, many fractures concrete frames: extensive hinge formation, cracking & spalling URM walls: extensive cracking, veneer sheds 13

14 Non-structural Performance Operational - NA nonstructural components are capable of supporting building s normal function lighting, plumbing, HVAC, computer systems, operate utilities available (from normal or emergency supply) this level includes consideration of the ruggedness of mechanical, electrical, and architectural components Non-structural Performance Immediate Occupancy - NB basic access & life safety systems remain operable, provided power is available other non-structural components are secured most but not all would be expected to operate (rigorous equipment qualification, such as is required for level NA is not included) 14

15 Non-structural Performance Life Safety - NC extensive damage to non-structural components no falling hazards egress routes may be impaired, but not blocked Non-structural Performance Hazards Reduced - ND extensive damage to nonstructural components no major falling hazards parapets, large plaster ceilings, exterior wall panels 15

16 Building Performance Levels Structural Performance Level Immediate Occup. -S1 Damage Contol - S2 Life Safety - S3 Limited Safety - S4 Collapse Prev. - S5 Non-structural Performance Level Not Cons. - S6 Operational - NA Immediate Occupancy - NB Life Safety - NC Hazards Reduced - ND Not Considered - NE 1-A 1-B 1-C 2-A 2-B 3-B 2-C 3-C 4-C 5-C 6-C 2-D 3-D 4-D 5-D 6-D 4-E 5-E 6-E Performance Levels Lateral Shear Demand Joe s Beer! Food! Operational Joe s Life Safety Beer! Food! Collapse Prevention Lateral Deformation 16

17 Performance Levels & Acceptance Criteria Acceptance of performance is judged on a component action level Each component action is defined as: deformation controlled, or force controlled Permissible levels of inelastic displacement or strength demand are defined for each performance level Acceptance Criteria F D 17

18 Acceptance Criteria Ductile Component (Action) Q CE F 2 1 Collapse Prevention Primary Secondary Immediate Occupancy a b 3 4 g e d Life Safety Primay Secondary D Acceptance Criteria Typical Semi-Ductile Component (Action) Q CE Q CL F Immediate Occupancy g 1 a Life Safety Primay Secondary e 2 Collapse Prevention Primary & Secondary D Deformation controlled behavior unless a < g 18

19 Acceptance Criteria Typical Non-Ductile Component (Action) F 1 Collapse Prevention Primary & Secondary Q CL Force Controlled Behavior Immediate Occupancy & Life Safety Primay Secondary g D Earthquake Hazards Ground Motion Liquefaction Lateral Spreading Land Sliding Ground Fault Rupture 19

20 Ground Shaking Hazards Performance Objectives may be based on any hazard level probabilisitic or deterministic Site Specific vs. General Two specific reference hazard levels defined Basic Safety Earthquake - 2 Hazard level with a 2% chance of exceedance in 50 years when 2% / 50 year hazard exceeds a 0.4g spectrum hazard need not be taken greater than 150% of the median hazard resulting from a characteristic earthquake on any known active fault 20

21 Basic Safety Earthquake - 2 Spectral acceleration - g s 2.0 Probabilistic hazard estimate Current code (zone 4 - UBC) 1.0 Deterministic hazard estimate 0 Distance to fault in kilometers Basic Safety Earthquake - 1 Hazard level with a 10% chance of exceedance in 50 years Need not be greater than 2/3 of the Basic Safety Earthquake - 2 level 21

22 Ground Motion Maps Two series of 2 maps are provided BSE-2 10% - 50 years S s contours for 5% damped response acceleration for short period structures on class B sites S 1 contours for 5% damped spectral response acceleration at 1 second period on class B sites General Procedure for Determining Ground Shaking Hazards Determine hazard level of interest (x% - 50 years) Obtain S 1, S S from maps Convert to appropriate return period (probability of exceedance) S = S x%/ 50 10% / 50 PR 475 Modify for site class effects S = F S S = F S DS a s ; D1 v 1 Determine general design spectrum n 22

23 General Response Spectrum Spectral acceleration - S a g s S a =S DS S a =S D1 / T T s Period - T seconds Rehabilitation Objectives Building Performance Levels Basic Safety Objectives Earthquake Hazard Levels 50% / 50 years 20% / 50 years 10% / 50 years - BSE-1 2% / 50 years - BSE-2 Operational 1-A Immed. Occ. 1-B Life Safety 3-C Collapse Prev. 5-E 23

24 Other Objectives Enhanced objectives Basic Safety + better performance Limited objectives Reduced rehabilitation Design for lower performance than Basic Safety Partial Rehabilitation Only certain deficiencies are addressed Rehabilitation measures may be designed for any performance Structural Evaluation / Analysis Four alternative analysis procedures 2 Linear static (LSP) dynamic (LDP) Nonlinear static (NSP) dynamic (NDP) 24

25 Linear Procedures Build mathematical model only primary elements included Determine primary component action demands apply equivalent lateral forces (LSP) perform response sepctrum or time history analysis (LDP) no R factors used in analysis response (forces) scaled to level that produces real response deformations Linear Procedures Determine demands on secondary component actions 2 methods available build second model consisting only of secondary elements & impose deflections from primary model include secondary elements in primary model at greatly reduced stiffness check acceptability of all primary & secondary component actions 25

26 Acceptance - Linear Methods Deformation controlled actions: mκ QCE QUD = QG ± QE force controlled QE κ QCL QUF = QG + CCC J Nonlinear Procedures Build mathematical model include primary & secondary elements Evaluate nonlinear response of structure static pushover response history Check acceptability deformation demands for deformation controlled actions strength demands for force controlled actions 26

27 Selection of Analysis Procedures Linear Procedures may generally be used for any structure unless: Energy dissipation elements are utilized Base isolation is used, with certain types of isolation devices Building is highly irregular irregularity defined in terms of distribution of computed demand - capacity ratios throughout structure Selection of Analysis Procedures Linear Static Procedure can be used for any structure that qualifies for linear analysis, unless: height exceeds 100 feet large setbacks exist (except penthouses) a severe mass or stiffness irregularity exists determined based on pattern of interstory drifts predicted by linear analysis building has a non-orthogonal lateral force resisting system Linear Dynamic Procedure may be used if linear procedures are applicable 27

28 Selection of Analysis Procedures Nonlinear Procedures may be used for any structure, but not recommended for timber structures if used, must have comprehensive knowledge of structure Selection of Analysis Procedures Nonlinear Static Procedure may be used for any structure must be used in paralell with LDP if higher mode effects are significant determined based on mass participation of higher modes more liberal acceptance criteria provided for LDP when run in paralell with NSP 28

29 Selection of Analysis Procedures Nonlinear Dynamic Procedure may be used for any structure independent peer review required Nonstructural Evaluation Classify importance of elements critical safety systems other systems 29

30 Nonstructural Evaluation Force sensitive elements provide adequate bracing for lateral forces Deformation sensitive elements isolate from structural drift, or limit structural drift Shaking sensitive elements operability may be effected by internal damage Nonstructural Evaluation Procedure for Force sensitive elements suggested acceptance criteria 30

31 Nonstructural Evaluation Procedure for deformation sensitive elements suggested acceptance criteria Quality Assurance Engineer s responsibility Building Official s responsibility Owner s responsibility Contractor s responsibility 31

32 Quality Assurance the importance of field observation 32