Performance Based Design, Value Engineering and Peer Review

Transcription

1 Naveed Anwar, PhD Performance Based Design, Value Engineering and Peer Review

2 Excellence the quality of being outstanding or extremely good 2

3 To be Excellent, something must be above average, better than standard, and of higher performance How to achieve excellence through innovative, explicit, verifiable and demontratable process 3

4 Building Industry relies on Codes and Standards Codes Specify requirements Give acceptable solutions Prescribe (detailed) procedures, rules, limits (Mostly based on research and experience but not always rational) Spirit of the code is to help ensure Public Safety and provide formal/legal basis for design decisions Compliance to letter of the code is indented to meet the spirit 4

5 The First Code - Hammurabi's (1772 BC) Implicit Requirements Explicit Collapse Performance Consequence of non-performance Clause 229: If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death. 5

6 Public Safety and the Codes In case you build a new house, you must also make a parapet for your roof, that you may not place bloodguilt upon your house because someone falling might fall from it - Prescriptive Modern Codes, c2000 Ref: Teh Kem, Associate Prof. NUS Performance Oriented Law of Moses (1300 BC) The Bible, Book of Deuteronomy, Chapter 22, Verse 8 6

7 Railing height added by resident to feel safe and reduce risk (Only done by residents of higher floors) Public Safety and Codes Railing height deemed sufficient by the code (Acceptable to residents of lower floors) 7

8 The Responsibility Client/Owner Architect Structural Designer Geotech Consultants Peer Reviewer General Building Codes Structural Design Codes Law Makers Building Officials Legal and Justice System Builder/Contractor Public/ Users/ Occupants 8

9 Lack of Resources for Communities Population Natural or Man-made Phenomena Inappropriate Built Environment Urbanization and Unplanned development Disaster Hazard Vulnerability Exposure Risk To reduce risk of disaster and increase safety, we need tp estimate hazard properly, and Reduce Vulnerability 9

10 How modern codes intent to ensure Safety Define appropriate/estimated hazard or load levels Prescribe limits on structural systems, members, materials Define procedures for analysis and design Provide rules for detailing Provide specifications for construction and monitoring Hope that all of this will lead to reduced vulnerability and safer structures 10

11 The Modern Codes With intent to make buildings safe for public (ACI ) Extremely Detailed prescriptions and equations using seemingly arbitrary, rounded limits with implicit meaning (IS ) 11

12 A Move Towards Performance Based Prescriptive Codes restrict and discourage innovation Objective Requirements Prescribed Solution Performance Based approach encourages and liberates it Objective Requirements Alternate Solution 12

13 Ensuring Explicit Safety Performance (And increase Disaster Resilience)

14 Design Approaches - Intuitive Design Code Based Design Performance Based Design Wind Earthquake

15 Earthquakes as a Catylist for PBD Performance based design can be applied to any type of loads, but was initially developed and targeted for earthquake loads 15

16 Explicit Performance Objective in PBD Performance based design investigates at least two performance objectives explicitly Service-level Assessment Codes arbitrary implicit Design Level Collapse-level Assessment Ensure continuity of service for frequent hazards (Earthquake having a return period of about 50) Ensure Collapse prevention under extreme hazards (the largest earthquake with a return period of 2500 years) 16

17 Performance Level Definitions Owner Engineer Will the building be safe? Can I use the building after the hazard? How much will repair cost in case of damage? Free to choose solutions, but ensure amount of yielding, buckling, cracking, permanent deformation, acceleration, that structure, members and materials experiences How long will it take to repair? Need a third party to ensure public safety and realistic Performance Guidelines Peer Review 17

18 Performance Objectives for Seismic Design Level of Earthquake Seismic Performance Objective Frequent/Service (SLE): 50% probability of exceedance in 30 years (43-year return period) Serviceability: Structure to remain essentially elastic with minor damage to structural and non-structural elements Design Basis Earthquake (DBE): 10% probability of exceedance in 50 years (475-year return period) Code Level: Moderate structural damage; extensive repairs may be required Maximum Considered Earthquake (MCE): 2% probability of exceedance in 50 years (2475-year return period) Collapse Prevention: Extensive structural damage; repairs are required and may not be economically feasible 18

19 Define Performance Levels Based on FEMA 451 B 19

20 Link the Hazard to Performance Levels Loading Severity Hazard Resta urant Resta urant Consequences Resta urant Vulnerability Structural Displacement 20

21 Performance-based design More explicit evaluation of the safety and reliability of structures. Provides opportunity to clearly define the levels of hazards to be designed against, with the corresponding performance to be achieved. Code provisions are intended to provide a minimum level of safety. Shortcoming of traditional building codes (for seismic design) is that the performance objectives are considered implicitly. Code provisions contain requirements that are not specifically applicable to tall buildings which may results in designs that are less than optimal, both from a cost and safety perspective. Verify that code-intended seismic performance objectives are met. 21

22 How to Apply PBD

23 The Building Structural System - Conceptual The Gravity Load Resisting System The structural system (beams, slab, girders, columns, etc.) that acts primarily to support the gravity or vertical loads The Lateral Load Resisting System The structural system (columns, shear walls, bracing, etc.) that primarily acts to resist the lateral loads The Floor Diaphragm The structural system that transfers lateral loads to the lateral load resisting system and provides in-plane floor stiffness 23

24 Structural System Source: NEHRP Seismic Design Technical Brief No. 3 24

25 PBD Guidelines PEER 2010/05, Tall Building Initiative, Guidelines for Performance Based Seismic Design of Tall Buildings PEER/ATC 72-1, Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings ASCE/SEI 41-13, Seismic Evaluation and Retrofit of Existing Buildings LATBSDC 2014, An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located in the Los Angeles Region 25

26 Required Information Basis of design Geotechnical investigation report Site-specific probabilistic seismic hazard assessment report Wind tunnel test report 26

27 Basis of Design Description of building Structural system Codes, standards, and references Loading criteria Gravity load, seismic load, wind load Materials Modeling, analysis, and design procedures Acceptance criteria 27

28 Geotechnical Investigation Report SPT values Soil stratification and properties Soil type for seismic loading Ground water level Allowable bearing capacity (Factors to increase in capacity for transient loads and stress peaks) Sub-grade modulus (Vertical and lateral) Liquefaction potential Pile foundation Ultimate end bearing pressure vs. pile length Ultimate skin friction pressure vs. pile length Allowable bearing capacity Allowable pullout capacity Basement wall pressure 28

29 Site-specific Probabilistic Seismic Hazard Assessment Report Recommend response spectra (SLE, DBE, MCE) Ground motions scaled for MCE spectra If piles are modeled in nonlinear model, Depth-varying ground motions along the pile length Springs and dashpots If vertical members are restrained at pile cap level, Amplified ground motions at surface level 29

30 Depth-varying Ground Motions along Pile Length 30

31 SPECTRAL ACCELERATION Response Spectra 2.5 Response Spectra 2.0 Service Level Earthquake (SLE) 50% of probability of exceedance in 30 years (43-year return period) Design Basis Earthquake (DBE) 10% of probability of exceedance in 50 years (475-year return period) SLE (g) DBE (g) MCE (g) Maximum Considered Earthquake (MCE) 2% of probability of exceedance in 50 years (2475-year return period) NATURAL PERIOD (SEC) 31

32 Wind Tunnel Test Report Wind-induced structural loads and building motion study 10-year return period wind load 50-year or 700-year return period wind load Comparison of wind tunnel test results with various wind codes Floor accelerations (1-year, 5-year return periods) Rotational velocity (1-year return period) Natural frequency sensitivity study 32

33 Performance-based Design Procedure

34 Overall PBD Process Initial Investigations Preliminary Design Wind Tunnel Test Detailed Code Based Design Service Level Evaluation Collapse Level Evaluation Peer Review Final Design 34

35 Preliminary design Structural system development Finite element modeling Check overall response Preliminary member sizing Bearing wall system Dual system Special moment resisting frame Intermediate moment resisting frame Linear analysis models Different stiffness assumptions for seismic and wind loadings Modal analysis Natural period, mode shapes, modal participating mass ratios Gravity load response Building weight per floor area Deflections Structural density ratios Slab thickness Shear wall thickness Coupling beam sizes Column sizes Lateral load response (DBE, Wind) Base shear, story drift, displacement 35

36 Detailed Code-based Design Modeling Nominal material properties are used. Different cracked section properties for wind and seismic models Springs representing the effects of soil on the foundation system and basement walls Gravity load design Slab Secondary beams Wind design Apply wind loads from wind tunnel test in mathematical model Ultimate strength design 50-year return period wind load x Load factor 700-year return period wind load Serviceability check Story drift 0.4%, Lateral displacement H/400 (10-year return period wind load) Floor acceleration (1-year and 5-year return period wind load) 36

37 Detailed code-based design Seismic design (DBE) Use recommended design spectra of DBE from PSHA Apply seismic load in principal directions of the building Scaling of base shear from response spectrum analysis Consider accidental torsion, directional and orthogonal effects 5% of critical damping is used for un-modeled energy dissipation Define load combinations with load factors Design and detail reinforcement 37

38 Scaling of Response Spectrum Analysis Results Source: FEMA P695 June

39 SLE Evaluation Linear model is used. Site-specific service level response spectrum is used without reduction by scale factors. 2.5% of critical damping is used for un-modeled energy dissipation. 1.0D L ± 1.0 E SLE Seismic orthogonal effects are considered. Accidental eccentricities are not considered in serviceability evaluation. Response modification coefficient, overstrength factor, redundancy factor and deflection amplification factor are not used in serviceability evaluation. 39

40 Acceptance Criteria (SLE) Demand to capacity ratios 1.5 for deformation-controlled actions 0.7 for force-controlled actions Capacity is computed based on nominal material properties with the strength reduction factor of 1. Story drift shall not exceed 0.5% of story height in any story with the intention of providing some protection of nonstructural components and also to assure that permanent lateral displacement of the structure will be negligible. 40

41 MCE Evaluation Nonlinear model is used. Nonlinear response history analysis is conducted. Seven (or more) pairs of site-specific ground motions are used. 2.5% of constant modal damping is used with small fraction of Rayleigh damping for un-modeled energy dissipation. Average of demands from seven ground motions approach is used. Capacities are calculated using expected material properties and strength reduction factor of

42 Expected Material Strengths Source: LATBSDC

43 Deformationcontrolled Actions Behavior is ductile and reliable inelastic deformations can be reached with no substantial strength loss. Results are checked for mean value of demand from seven sets of ground motion records. Force-deformation relationship for deformation-controlled actions Source: ASCE/SEI

44 Force-controlled Actions Behavior is more brittle and reliable inelastic deformations cannot be reached. Critical actions Actions in which failure mode poses severe consequences to structural stability under gravity and/or lateral loads. 1.5 times the mean value of demand from seven sets of ground motions is used. Non-critical actions Actions in which failure does not result structural instability or potentially life-threatening damage. Mean value of demand from seven sets of ground motions is used with a factor of 1. Force-deformation relationship for force-controlled actions Source: ASCE/SEI

45 Classification of Actions Component Action Classification Criticality Shear walls Coupling beams (Conventional) Flexure Deformation-controlled N/A Shear Force-controlled Critical Flexure Deformation-controlled N/A Shear Force-controlled Non-critical Coupling beams (Diagonal) Shear Deformation-controlled N/A Girders Columns Diaphragms Basement walls Mat foundation Piles Flexure Deformation-controlled N/A Shear Force-controlled Non-critical Axial-Flexure Deformation-controlled N/A Shear Force-controlled Critical Flexure Force-controlled Non-critical Shear (at podium and basements) Force-controlled Critical Shear (tower) Force-controlled Non-critical Flexure Force-controlled Non-critical Shear Force-controlled Critical Flexure Force-controlled Non-critical Shear Force-controlled Critical Axial-Flexure Force-controlled Non-critical Shear Force-controlled Critical 45

46 Concrete Element SLE/Wind DBE MCE Core walls/shear walls Stiffness Assumptions in Analysis Models Flexural 0.75 I g Shear 1.0 A g Flexural 0.6 I g Flexural ** Shear 1.0 A g Shear 0.2 A g Basement walls Flexural 1.0 I g Shear 1.0 A g Flexural 0.8 I g Shear 0.8 A g Flexural 0.8 I g Shear 0.5 A g Coupling beams (Diagonal-reinforced) Flexural 0.3 I g Shear 1.0 A g Flexural 0.2 I g Shear 1.0 A g Flexural 0.2 I g Shear 1.0 A g Coupling beams (Conventional-reinforced) Flexural 0.7 I g Shear 1.0 A g Flexural 0.35 I g Shear 1.0 A g Flexural 0.35 I g Shear 1.0 A g Ground level diaphragm (In-plane only) Flexural 0.5 I g Shear 0.8 A g Flexural 0.25 I g Shear 0.5 A g Flexural 0.25 I g Shear 0.25 A g Podium diaphragms Flexural 0.5 I g Shear 0.8 A g Flexural 0.25 I g Shear 0.5 A g Flexural 0.25 I g Shear 0.25 A g Tower diaphragms Flexural 1.0 I g Shear 1.0 A g Flexural 0.5 I g Shear 0.5 A g Flexural 0.5 I g Shear 0.5 A g Girders Flexural 0.7 I g Shear 1.0 A g Flexural 0.35 I g Shear 1.0 A g Flexural 0.35 I g Shear 1.0 A g Columns Flexural 0.9 I g Shear 1.0 A g Flexural 0.7 I g Shear 1.0 A g Flexural 0.7 I g Shear 1.0 A g 46

47 Evaluation of Results

48 Evaluation of Results Results extraction, processing and converting them into presentable form takes additional time. Results interpretation i.e. converting numbers we have already crunched into meaningful outcome for decision-making. Since each of these performance levels are associated with a physical description of damage, obtained results are compared and evaluated based on this criterion to get performance insight. 48

49 Overall Response Base shear Ratio between inelastic base shear and elastic base shear Story drift (Transient drift, residual drift) Lateral displacement Floor acceleration Energy dissipation of each component type Energy error 49

50 Base shear (kn) Base shear (%) Base Shear 300, , , , , , , , ,000 81, ,000 30,878 57,826 39, X Along direction Wind (50-yr) x 1.6 Elastic MCE Inelastic MCE-NLTHA Elastic SLE Y 0.0 X Y Along direction Wind (50-yr) x 1.6 Elastic MCE Inelastic MCE-NLTHA Elastic SLE 50

51 Story level Transient Drift 70 GM GM GM-CHY006 GM-JOS 40 GM-LINC GM-STL GM-UNIO Average 10 Avg. Drift Limit Max. Drift Limit Drift ratio 51

52 Story level Residual Drift 70 GM GM GM-CHY006 GM-JOS 40 GM-LINC 30 GM-STL GM-UNIO 20 Average 10 Avg. Drift Limit Max Drift Limit Drift ratio 52

53 Story level 70 Lateral Displacement GM GM GM-CHY GM-JOS GM-LINC 30 GM-STL 20 GM-UNIO 10 Average Lateral displacement (m) 53

54 Story level 70 Floor Acceleration GM GM GM-CHY GM-JOS GM-LINC GM-STL GM-UNIO 10 Average Absolute acceleration (g) 54

55 Energy dissipation (%) Energy dissipation (%) Energy dissipation (%) Energy Dissipation Total dissipated energy Total dissipated energy Time (sec) From shear walls Time (sec) Conventional reinforced coupling beams Total dissipated energy Diagonal reinforced coupling beams Time (sec) 55

56 Component Responses Component Response Pile foundation Mat foundation Shear wall Column Beams Conventional reinforced coupling beam Diagonal reinforced coupling beam Flat slab Basement wall Diaphragm Bearing capacity, pullout capacity, PMM, shear Bearing capacity, flexure, shear Flexure (axial strain), shear PMM or flexural rotation, axial, shear Flexural rotation, shear Flexural rotation, shear Shear rotation, shear Flexural rotation, punching shear In-plane shear, out-of-plane flexure and shear Shear, shear friction, tension and compression 56

57 How to Work with PBD 57

58 Integrated PBD for Earthquake and Wind 58

59 Earthquake and Wind PBD are Compatible! Site specific Seismic Hazard Study Site specific Climate Analysis Various Earthquake levels SLE, DBE, MCE etc Various Wind Return period and Velocities Hazard Response Spectrum Wind Force in Frequency Domain Earthquake Ground Motion Time History Wind Tunnel Pressure in Time Domain Wi nd 59

60 Consider winds of higher intensity and longer return periods Determine static and dynamic impacts through wind tunnel studies Incorporate wind tunnel dynamic measurements into dynamic analysis of structural models Set appropriate performance criteria for motion, deformation, strength, ductility, energy decimation etc. Make the Wind PPD consistent with Earthquake PBD Possible Way forward 60

61 Wind Pressure Variation and Dynamic effects 61

62 Wind Return Period Wind Performance Level Structural System Response Overall Damage Wind Performance Objective Design Criteria 1 year Perception Threshold No Permanent Interstory Undamage None Perception of movement Bldg. Acceleration <5 milli -g Suggested Structural Performance Criteria for Wind 10 years Motion Comfort 50 years Operational 100 years Limited Interruption 475 years Life Safety No Permanent Interstory No Permanent Interstory No Permanent Interstory Permanent Interstory Undamage Undamage Minor Damages Major Damages Controlled Comfort Non-Structural Damage Structural Damage No Collapse Bldg. Acceleration <15 milli -g Story drift is limited to 0.2% Story drift is limited to 0.3% Story drift is limited to 0.5% Residual Drift < h/ years Collapse Prevention Permanent Interstory Extensive Damages No Collapse Story drift is limited to 1% Residual Drift < h/500 62

63 Wind Earthquake Time Varying Loading Wind Tunnel Testing Site Specific Investigation Compare Loading Mean + Fluctuating + Resonant Fluctuating + Resonant PBD Wind and PBD Earthquake Overall Structural Damage ASCE ASCE Structural System Response ASCE ASCE (Using ASCE 41 as a sample) Members Deformation Control Limits ASCE ASCE Material Behavior Uncrack to Crack under yield to Crack beyond yield point Crack under yield to Crack beyond yield point Structural members controlled Some members are Force and Deformation Controlled Some Members are Force 63 and Deformation Controlled

64 Performance Based Design Explicit confirmation of higher or expected performance level using innovative solutions Value Engineering Get the best value for resources Peer Review Provide an independent view and confirmation 64

65 Value Engineering Balancing Cost and Performance

66 Cost and Performance C P General Belief Easy to do! CC P Cost Effective Design Can be done C P High Performance Design Can be done C P Highly Innovative Design Hard to do! 66

67 What is the Cost of a Project? Cost may include Financial Cost (loan, interest, etc) Planning and Design Cost Direct Construction Cost Maintenance Cost Incidental Cost Liquidated Cost (lost profit etc) Opportunistic Cost Environmental Cost Emotional Cost Non-determinist Resources Cost may be: Consumption of Particular Resources, at Particular Time Sustainability may be: <Consumption of all resources, and their impacts through throughout the life cycle> 67

68 Cost and Performance Enhancement of Performance Dynamic response parameters Lateral load response Vertical load response Demand and capacity ratios Response irregularity, discontinuity Explicit Performance Evaluation at Service, DBE and MCE Cost Effectiveness Capacity utilization ratio Reinforcement ratios Reinforcement volume ratios Concrete strength and quantity Rebar quantity Constructability, time and accommodation of other constraints 68 68

69 Local Vs Global Optimization Simple Example of a Column Stack What and how can we optimize? Concrete Strength Steel Strength Column Size Rebar Amount Composite Section Material Cost, Labor Cost, Formwork Cost, Management and operations Cost, Time?? 69

70 Cost and Performance P (Increased Performance, Same Cost) (Base Cost and Performance) M P (Reduced Cost for Same Performance) (Base Cost and Performance) M 70

71 Demand Capacity (DC Ratio) Definition of D/C: It is an index that gives an overall relationship between affects of load and ability of member to resists those affects. This is a normalized factor that means D/C ratio value of 1 indicates that the capacity (strength, deformation etc) member is just enough to fulfill the load demand. Two types of D/C ratio Members with brittle behavior D/C is checked by Strength (Elastic) Members with ductile behavior D/C is checked by deformation (Inelastic) Total D/C ratio of the member is combined of these two. 71

72 Cost Effectiveness > Utilization Ratio Utilization Ratio Compare, What is Needed against What is Required One measure The Demand/ Capacity Ratio (D/C) Not Cost Effective Ideal Not Safe Columns Demand/ Capacity No. % D/C< % 0.5<D/C< % 0.7<D/C< % 1<D/C< % 1.5<D/C< % D/C> % Total % 72

73 Focus should be Maximum Value for Resources Cost effective, not Low Cost 73

74 Peer Review To ensure Basic Design the Performance Evaluation and Value Enginering are done right

75 The Responsibility Client/Owner Architect Structural Designer Geotech Consultants Peer Reviewer General Building Codes Structural Design Codes Law Makers Building Officials Legal and Justice System Builder/Contractor Public/ Users/ Occupants 75

76 Peer Review What exactly is design peer review? It is a process whereby a design project (or aspect of) is reviewed and evaluated by a person, or team, not directly involved with the project, but appropriately qualified to provide input that will either reinforce a design solution, or provide a route to an improved alternative. Why is it so important? Very few can claim to be all-encompassing experts. The invaluable input from broad base and independent experience at each stage of a design project will often result in technical improvements, lower costs, avoidance of sourcing issues, and improved performance. 76

77 When is Peer Review needed New York Building Code, adopted by many cities Structural Peer Review is required for: Buildings included in Structural Occupancy Category IV as defined in the Building Code. Buildings with aspect ratios of seven or greater. Buildings greater than 500 feet (160 m) in height or more than 1,000,000 square feet (100,000 Sqm) in gross floor area. Buildings taller than seven stories where any element supports in aggregate more than 15 percent of the building area. Buildings designed using nonlinear time history analysis, pushover analysis or progressive loading techniques. Important Slender Tall or large Critical Use NLA 77

78 Responsibility Structural Engineer of Record (SER). The structural engineer of record shall retain sole responsibility for the structural design. The activities and reports of the Reviewing Engineer shall not relieve the structural engineer of record of this responsibility. Reviewing Engineer. The Reviewing Engineer s report states his or her opinion regarding the design by the engineer of record. The standard of care to which the Reviewing Engineer shall be consistent with Structural Peer Review services performed by professional engineers licensed/approved Retains Responsibility Evaluates, and gives opinion that may or may not be accepted by Client or SER 78

79 Excellence is Structures Design Codes and Guidelines High performance, Higher safety higher value, cost effective Sustainable Peer Review PBD Basic Design Value Engineering Client Public Officials 79

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