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Progression of Structural Design Approaches by Dr. Naveed Anwar
- 1. Progression of Structural DesignApproaches Naveed Anwar, PhD
- 2. Naveed Anwar,AIT 2 How longdo we have before the building will collapse in this fire? - Asks the Fire Chief to the structural engineer / (Architect) 1974
- 3. Naveed Anwar,AIT Formalization ofbuilt environment 15th and 16th centuries: The visual appearance, sense of space and function became a distinct concern. 17th Century Onwards: Thinking about the load- bearing aspects of structures Late 17th and 18th centuries: Thinking separately about the role of individual materials following Galileo's work. Late 19th and 20th Century: Aesthetics should be given proper importance independent of load-bearing characteristics of the structure. Architects and Engineers
- 4. Naveed Anwar,AIT The beginningof Structural Engineering 4 recognition of the loads, and fact that loads have an effect on members and materials and that there is a resistance within materials to resist the loads effects and that there is some relationship between them
- 5.
- 6. Naveed Anwar,AIT Evolution ofour Understanding of Structures Limits on the allowable stresses to achieve in-direct FOS Explicit consideration of partial FOS. Formulation of limit state design principles. Formulation of ultimate strength. The recognition of the difference between brittle and ductile failure. The introduction of capacity based design approaches. Performance based design and more explicit linkage between demand and performance. Risk integrated based design, and a more and holistic approach towards consequence based engineering.
- 7. Naveed Anwar,AIT Structural Design 7 “StructuralDesign is the process of proportioning the structure to safely resist the applied forces in the most cost effective and friendly manner” A systematic investigation of the stability, strength and rigidity of structures Where Safety is a prime concern
- 8. Naveed Anwar,AIT Ensuring Safetythrough Factor of Safety! 8 Capacity > Demand 𝑪𝑪 𝑫𝑫 = 𝑭𝑭𝑭𝑭𝑭𝑭 𝐶𝐶 = 𝐷𝐷 𝑥𝑥 𝐹𝐹𝐹𝐹𝐹𝐹 𝐶𝐶 𝐹𝐹𝐹𝐹𝐹𝐹 = 𝐷𝐷 𝐶𝐶 𝐹𝐹𝐹𝐹𝐹𝐹𝐹 = 𝐷𝐷 𝑥𝑥 𝐹𝐹𝐹𝐹𝐹𝐹𝐹
- 9. Naveed Anwar,AIT Approach 1:WorkingStress Design 9 Strain Stress fy 0.5fy Unused Strength and Ductility 𝐶𝐶 𝐹𝐹𝐹𝐹𝐹𝐹 = 𝐷𝐷
- 10. Naveed Anwar,AIT The “RealBehavior” 10
- 11. Naveed Anwar,AIT Approach 2-Ultimate Strength Design ( ) ( ) ( ) += += += ∑∫∫ ∑∫∫ ∫∫ ∑ = = = ...),( 1 ...., 1 ...),( 1 ...., 1 ...),( 1 ..., 1 121 3 121 2 121 1 i n i ii x y y i n i ii x y x x y n i iiz xyxAxdydxyxM yyxAydydxyxM yxAdydxyxN σ γ σ γ φ σ γ σ γ φ σ γ σ γ φ y h c fc Strain Stresses for concrete and R/F Stresses for Steel f1 f2 fn fs NA CL Horizontal • The Strain becomes the primary concern, • Full “Strengths” of the materials at “ultimate” strains are utilized with appropriate “other” factors
- 12. Naveed Anwar,AIT The StressStrain Models 12
- 13. Naveed Anwar,AIT Considering Interactionof Actions 13 Nz Mx My • Shear-Torsion Interaction • Shear – Flexure interaction • Torsion-Flexure Interaction • Shear-Torsion-Flexure Interaction The realization that “Envelop Results” can not be used for design
- 14. Naveed Anwar,AIT The Strutand Tie Approaches – Post Crack Strength 14 An RC Beam and “Hidden”Truss A RealTruss
- 15. Naveed Anwar,AIT The Strutand Tie Approaches • Extensively used for • “D” regions of all members • Shear Design • Torsion Design • “Deep” Beams • Brackets • Corbels • Joints • Pipecaps • Shear walls • Transfer girders • …
- 16. Naveed Anwar,AIT Approach 3- Limit State Design • Limit State Design concept is an advancement over both Working Stress and Ultimate Strength design approaches. • Attempts to ensures safety at ultimate loads and serviceability at working loads. • The basic idea involves the identification of all potential modes of failure (i.e. identify significant limit states and determination of acceptable levels of safety against occurrence of each limit state. • This philosophy uses more than one safety factors attempting to provide adequate safety 16
- 17. Naveed Anwar,AIT Limit StateDesign 17 Types of Limit State Description Ultimate Limit states • Loss of equilibrium • Rupture • Progressive Collapse • Formation of plastic mechanism • Instability • Fatigue Serviceability limit states • Excessive deflections • Excessive crack width • Undesirable Vibration Special limit states Due to abnormal conditions and abnormal loading such as • Damage or collapse in extreme earthquakes • Structural effects of fire, explosion • Corrosion or deterioration
- 18. Naveed Anwar,AIT Partial Factorsof Safety 18 Characteristic value of material basic strength Design Strength Design member capacity Characteristic value of Load Design load Design member capacity Ym Yb Yf Ya Verification Yi Material safety Factor Member Factor Load Factor Structural Analysis Factor Structure Factor The value of Safety Factor tells how much confidence we have on our knowledge.
- 19. Naveed Anwar,AIT Improving Factorsof Safety 19 Factors Example Structure Level Importance Factors (1.0 to 1.5) Structure type “R” factors Member Level Over strength Factors Action Level Different “fi” factors for flexure, shear, … Material Level Different gamma factors for concrete and steel Load Factors Different for different loads, based on probability of variance Load Combination Factors Deferent for various probability of simultaneous occurrence
- 20. The Development and Roleof Building Codes
- 21. Naveed Anwar,AIT The FirstBuilding Code: Code of Hammurabi 21 • The earliest known written building code was the Babylonian law of ancient Mesopotamia • Also known as the code of King Hammurabi (who ruled Babylon from 1792 BC to 1750 BC). • Found in 1901 in Khuzestan, Iran • Contains detailed accounts of laws pertaining to builders as well as construction conflicts.
- 22. Naveed Anwar,AIT Code ofHammurabi 22 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.
- 23. Naveed Anwar,AIT Ancient BuildingCode: Laws of Moses 23 “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”. -The Bible, Book of Deuteronomy, Chapter 22,Verse 8
- 24. Naveed Anwar,AIT Development ofBuildings Codes 24 “Rebuilding of London Act” after the “Great Fire of London” in 1666 AD. In 1680 AD, “The Laws of the Indies” Spani sh Crown London Building Act of 1844. In USA, the City of Baltimore first building code in 1859. In 1904, a Handbook of the Baltimore City In 1908 , a formal building code was drafted and adopted. The Internat ional Building Code (IBC) by (ICC). European Union, the Eurocodes.
- 25. Naveed Anwar,AIT The ModernCodes 25 (ACI 318 – 11) Extremely Detailed prescriptions and equations using seemingly arbitrary, rounded limits with implicit meaning (IS 456-2000)
- 26. Naveed Anwar,AIT The GeneralCode Families UBC, IBC ACI, PCI, CRSI, ASCE, AISI, AASHTO British, CP and BS Euro- codes China, USSR, Japan 26
- 27. Naveed Anwar,AIT Are AllBuildings Codes Correct ? • If they differ, can all of them be correct ? • Did we inform the structures to follow which code when earthquake strikes ? • Codes change every 3 or years, should be upgrade our structures every 3 or 5 years to conform ? 27
- 28. Naveed Anwar,AIT 2 Ismy Structure safe? What level of Richter earthquake my structure sustain?
- 29. Naveed Anwar,AIT Prescriptive Codes– A Shelter • Public: • Is my structure safe ? • Structural Engineer: • Not sure, but I did follow the “Code” As long as engineers follow the code, they can be sheltered by its provisions 29
- 30. Newer Design Approaches Primarily gearedtowards Earthquakes and Extreme Events
- 31. Naveed Anwar,AIT Design forSeismic Resistance and Extreme Events • Force/stress based design • Assume reduced forces, limit the stresses • Displacement based design • Allow force D/C to exceed, as long as displacements can be limited • Capacity based design • Put “fuses” in the structure limit the force capacity, hence the demand • Energy based design • Total energy input is collectively resisted by kinetic energy, the elastic strain energy and energy dissipated through plastic deformations and damping 31
- 32. Naveed Anwar,AIT Progression ofSeismic Resistance Design 32 Historical Approach: Earthquake forces proportional to building mass (Vdes = 5 - 10% ofWt), TraditionalCodes: Elastic earthquake forces reduced for linear design (Vdes =Vmax /R) Lack of Knowledge on Earthquake Demand and Building Capacity Linear Elastic Building Response V Vdes Yield Max V Vdes Elastic Forces reduced for Design by R Inelastic Response
- 33. Naveed Anwar,AIT Current SeismicDesign Approach 33 CurrentTrend: a) Inelastic earthquake demand based on, inelastic capacity of building b) Resolution of demand vs. capacity generates Performance Point c) Design based on displacement, ∆des des Sd Performance Point Demand Reduced Based on Inelastic Capacity of Building
- 34. Naveed Anwar,AIT Ductile linkAnalogy for Capacity based design 34 C. V. R. Murty, 2002
- 35. Naveed Anwar,AIT Performance BasedDesign (PBD) • Explicitly link the performance with earthquake hazard • Why it was needed? • Traditional codes not suitable/adequate • Explicit verification not specified or required in most codes • Public does not care about the code, or theories or procedures, they care about “safety” and ‘performance” 35
- 36. Naveed Anwar,AIT Prescriptive VsPerformance 36 Approach Procedure Outcome Prescriptive (emphasis on procedures) Specify “what, and how to do” Make Concrete: 1:2:4 Implicit Expectation (a strength of 21 MPA is expected) Performance Based Approach (emphasis on KPI) What ever it takes (within certain bounds) Explicit Performance Concrete less than 21 MPA is rejected
- 37. Naveed Anwar,AIT Define PerformanceLevels 37 Restaurant Restaurant Restaurant Operational (O) Immediate Occupancy (IO) Life Safety (LS) Collapse Prevention (CP) Ref: FEMA 451 B
- 38. Naveed Anwar,AIT Link theDamage to Performance Levels 38 Structural Displacement LoadingSeverity Resta urant Resta urant Resta urant Hazard Vulnerability Consequences
- 39. Naveed Anwar,AIT Link Performanceother Indicators 39 Restaurant Restaurant Restaurant Operational (O) ImmediateOccupancy (IO) Life Safety (LS) Collapse Prevention (CP) 0 % Damage or Loss 99 % Ref: FEMA 451 B CasualtiesLowest Highest Rehab Cost to Restore after eventLowest Highest Retrofit Cost to MinimizeConsequencesHighest Lowest Downtime for RehabLowest Highest
- 40. Naveed Anwar,AIT How toWork with PBD 40 Requires: • Detailed modeling • Nonlinear-dynamic analysis • Appropriate computing tools, knowledge, skills and lots of patience
- 41. Naveed Anwar,AIT AIT’s Focuson PBD • Considerable research, development focus on PBD • Specially targeted to Tall Buildings and retrofitting of existing structures 41
- 42. Naveed Anwar,AIT Demand toCapacity Ratio – An Important indicator • This could apply to • Forces, actions (Moment, shear, etc) • Ductility • Deformations (drift, deflection) • Indicates the “safety”, “acceptability” as well as “efficiency” of design D/C > 1 <Not acceptable, in many cases D<C < 1 <Acceptable in most cases D/C =1 <Ideal design, not practical> 0.5 < D/C < 0.9 <Efficient design> 42
- 43. Naveed Anwar,AIT Beyond PBD •For public, the performance criteria still does reduce the effects of the events • Insurance companies want to have greater reliability of assessment of risk and damages 43
- 44. Naveed Anwar,AIT Risk BasedDesign • . 44
- 45. Naveed Anwar,AIT Consequence BasedEngineering • It is not enough to say “Cracking and non-structural damge is acceptable, as long as structure does not collapse” • A natural extension of the performance-based design approach • Structural consequences can be defined in terms of repair costs, casualties and loss of use duration (dollars, deaths and downtime) (Porter, 2003). • Other types of consequences which result from the inherent function of a structure, are addressed using importance factors for various occupancy categories in design codes (Yuxian 2013). 45
- 46. Naveed Anwar,AIT Consequence BasedEngineering • “Structural consequence and non-structural effects” determined entirely from the analysis of structural member as well as overall system behavior. • The consequence-based structural design approach proceeds through the analysis of expected system consequences, irrespective of the event triggering these consequences. • This philosophy requires the structural members to be designed for variable reliability levels, depending upon their contribution in causing adverse system consequences. 46
- 47. Naveed Anwar,AIT Special PurposesGuidelines from USA Applied Technology Council (ATC) Federal Emergency Manageme nt Agency (FEMA) and National Earthquake Hazards Reduction Program (NEHRP) PEER Guidelines forTall Buildings Tall Buildings Initiatives (TBI) CTBUH Guidelines 47
- 48.
- 49. Naveed Anwar,AIT A tusslebetween Heuristic and the Rational Should design be based on “Engineering Judgment” and intuition, Or controlled by explicit computations and, restrictive limits and rational approaches 49
- 50. Naveed Anwar,AIT 50 It isby logic we prove, but by intuition we discover
- 51. Naveed Anwar,AIT Intuitive design,verification, application 51 Félix Candela
- 52. Naveed Anwar,AIT Intuitive formsand designs 52 Pier Luigi Nervi
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- 54. Naveed Anwar,AIT The Roleof Computers and Software • Initially, computers were used to program the procedure we had • Now, we develop procedures that are suited for computing 54
- 55. Naveed Anwar,AIT Design Approachesevolved to match computing revolution 55
- 56. Naveed Anwar,AIT Some Toolsof the Trade 56 Integrated 3D Bridge Design Software Integrated Software for Structural Analysis and Design Integrated Analysis, Design and Drafting of Building Systems Integrated Design of Flat Slabs, Foundation Mats and Spread Footings Nonlinear Analysis and Performance Assessment for 3D Structures Design of Simple and Complex Reinforced Concrete Columns By Computers and Structures Inc. USA
- 57. Naveed Anwar,AIT A SwingTowards the AI • Rich Pictures • Analytical Hierarchy Process (AHP) • Artificial Neural Networks (ANN) • Genetic Algorithms (GA) • Expert Systems (ES) • Fuzzy Logic • Deep Thinking • Big Data and Data Mining 57
- 58. Naveed Anwar,AIT Mobile computingmight change how we design 58
- 59. Naveed Anwar,AIT Can wemake it safe ? 59
- 60.