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PBD.pptx

  1. 1. Special Considerations and Challenges in Seismic Design of Tall Buildings Asian Institute of Technology | Thailand 1-2 June 2018
  2. 2. Buildingsand Structuresare expected tobe 2 • Safe • Secure • Serviceable • Reliable • The contents of the structures are often much more valuable than structure itself • The loss of service/operations/business is a often larger than repair costs • Protective • Friendly • Sustainable • Affordable
  3. 3. Howdoes CTBUHlook at Tall 3 Relatively Tall. Both for public and the professions who design and construct Proportion Slenderness, in plan and in elevations Systemsand Technologies Uses something “different” than ordinary buildings 3
  4. 4. Key Challenges in(Tall) Buildings 4 • Taller • Slender • Twisting • Unusual forms • Multi Use • Changing Plans • Larger column freespaces • Smaller Cores • Minimizing Floor Height • Minimize floor depth • Minimize column size • Minimize structural cost • Inclined columns • Free form • Unusual requests
  5. 5. Win d Gravity Earthquake 5 5 emaze.com Main Challenges ! Optimizing for one, may de-optimize for others!
  6. 6. 6 Focus oftheTalk–“Tallness Range” Low Rise >500m >300m >200m >100m <50m >150m Source:CTBUReport,2018
  7. 7. Main Structural Concerns 01 02 03 04 05 06 Stabilityand integrity Strengthand Servivbility Deformation Drift Ductility Energy Dissipatio n 7 07 Motion Perception
  8. 8. • Direct LoadTransfer Systems • Flat Slaband Flat Plate • Beam-Slab • Waffle Slab • Wall Joist • Indirect LoadTransferSystem • Beam, Slab • Girder, Beam, Slab • Girder, Joist • Materials • Steel/ Composite Deck • Reinforced Concrete • Post-tensioned slabsystems 8 Choosing the “Right” Gravity Load Resisting System Least weight Fast Construction cycle Leaststructuraldepth
  9. 9. 4Commandments forLateralLoad Systems 1 Resistoverturning forces due to lateral loads by using vertical elements placed asfar apart as possible 2 Channel gravity loadsto those vertical elements resisting overturning forces 3 Link these vertical elements together with shear-resisting structural elements with minimum shear lag to activate entire perimeter of the building 4 Axial loaded membersin compression to resist overturning forces 9
  10. 10. 10 10 SeismicLoad WindLoad Depend on •focus of earthquake •Shaking intesity •ground conditions •Mass and stiffness distribution Depend on • Wind speed • terrain • topography ofthe location • Force increases withheight • Geometry andexposed area v A üg m  Excitation isan applied displacement at the base  force will be distributed along interior and exterior lateral load resisting elements  Excitation isan applied pressure or force on thefacade  force will act mainly on exterior frames then transferred to floor diaphragms
  11. 11. Basic Physicsof Dynamics 1 1 • Newton’s View, for rigidbodies F= ma
  12. 12. Structuralengineer’s View 𝑀 𝑢 + 𝐶 𝑢+ 𝐾 𝑢=𝐹 for linear elastic, deformablebodies 12
  13. 13. Typical Linear Dynamic Response of Tall Building 13 Animation
  14. 14. Dynamic Equilibrium Damping- Velocity Mass-Acceleration Stiffness- Displacement External Force The basic variable is displacement and its derivatives 14 𝑀 𝑢 + 𝐶 𝑢+ 𝐾 𝑢+ 𝐹 𝑁 𝐿=𝐹 Nonlinearity 𝑀 𝑢 + 𝐶 𝑢+𝐾 𝑢
  15. 15. Nonlinear and Analysis for PBD 15
  16. 16. BuildingIndustryrelieson Codes and Standards • CodesSpecify requirements • Giveacceptable solutions • Prescribe (detailed) procedures, rules, limits • (Mostly based on research and experience but not always rational) Spirit of the codeis to help ensure Public Safety and provide formal/legal basis for design decisions 16 Compliance to letter of thecode is indented to meet thespirit
  17. 17. Seismic Response Linear Time History Analysis Ku  FNL  FEQ Pushov er Analysi s Ku  FEQ Equivalent Static Analysis Ku  FEQ Response Spectrums Response Spectrum Analysis Acceleration Records Nonlinear Time History Analysis 17 𝑀 𝑢 + 𝐶 𝑢+ 𝐾 𝑢+ 𝐹 𝑁 𝐿=𝐹 𝑀 𝑢 + 𝐶 𝑢+ 𝐾 𝑢=𝑀 𝑢 𝑔 Free Vibration 𝑀 𝑢 + 𝐾 𝑢=0
  18. 18. The“ArbitraryFactors”inCodes 18
  19. 19. 19  For most buildings, dynamic wind response may be neglected  Gust factor approach  predict dynamic response of buildings with reasonable accuracy  Structures are designed to respond elastically under factored loads  Structures are designed to respond inelastically under factored loads  it is not economically feasible to design structures to respond elastically to earthquake ground motion Design for SeismicEffects Design for WindLoad
  20. 20. 20  Structures are designed to respond inelastically under factored loads  it is not economically feasible to design structures to respond elastically to earthquake ground motion Design for SeismicEffects
  21. 21. 21 0 5 10 15 20 25 30 35 40 45 0 TheProblemwithRFactor The elastic forces obtained from the standard RSAprocedure The RSA elastic forces reduced by 𝑅 The inelastic forces obtained from the NLRHAprocedure The actual reduction in RSA elastic forces. The “reward” of making a nonlinear model The underestimation causing a “false sense of safety” due to directly reducing the RSA elastic forces by 𝑅factor 10 20 30 40 50 60 Story Shear (x106 N) Story Level • The R factor may vary from 2 to 8 depending on definition of structure type • R factor could “off” by a factor of 2 to 4 • Other names for R factor are Response Factor, BehaviorFactor (q), Structure Type factor (K)etc., Fawad Najam,2017
  22. 22. Effectof Modes on Story Moment 22 22
  23. 23. Effectof Modes on Story Shear 23 23
  24. 24. Are All Buildings Codes Correct ? 24 • All codes have different values of R and other factors • If they differ, can all of them be correct ? • Did we inform the structures to follow which code when earthquake or hurricane strikes ? • Codes change every 3 or years, should we upgrade our structures every 3 or 5 years to conform ?
  25. 25. Code Comparisonfor Seismic Performance 25 • Compare Performance of buildings designed todifferent codes • ACI 318-14 +ASCE 7-10 • BS 8110-1997 +EURO-8 • EURO-2-2004 +EURO-8 • Forlow-seismic and high seismiczone • Manila >VeryHigh • Bangkok >Low tomedium • All produce different level orperformance in differentcomponents !! Two MS Thesis, 2016 at AIT
  26. 26. Shift From Prescriptive to Performance Based Approach
  27. 27. A Move TowardsPerformance-based Approach • Prescriptive Codes restrict and discourage innovation Objective Requirements Prescribed Solution Objective Requirements Alternate Solution • Performance Based approach encourages and liberates it 27
  28. 28. Design Approaches Intuitive Design Prescriptive Code Based Design Performance Based Design >> >>> 28
  29. 29. 29 Looking at some Design Challenges
  30. 30. Providing (Hiding)the Outriggers 30
  31. 31. Outrigger Effects 1 + 1 K:1 +1=8 31 K:1+1=2 2
  32. 32. 32 Effectivenessof Outriggers Reduce Reducethe natural period – Good forwent response Reduce Reducetop displacement Reduce Reduce drift Reduce Reducemoment in shearwalls Follow Follow the All 4Commandments Donot reduce Donot reduce shear in shearwalls Need Need spaceto implement
  33. 33. 33 Real VsVirtual Outriggers • Virtual Outriggers are more acceptable” from architectural planning and circulation viewpoint • Theyare nearly aseffective as“real” outriggers Direct or “real”Outriggers In-direct or “viryual”Outriggers
  34. 34. 3 Adding Belts • More evendistribution of axial loads on perimeter columns • Reduces possibility of tension in columns or foundatons • Provides virtual outrigger effect inboth directon 4
  35. 35. Buckling RestraintBraces, BRB 35
  36. 36. BRB–An efficient Outriggerand Damper 36
  37. 37. 37 N1-S1 CoreOnly N1-S2 N1-S3 N2-S3 N3-S3 Flag Walls–an Alternative to Outriggers
  38. 38. 38 CoreOnly Config1 Config2 Config3 Config4 Config5 Flag Walls–an Alternative to Outriggers
  39. 39. Staggered Wallsas Outriggers 39
  40. 40. TheDiaphragm DesignChallenges 40
  41. 41. PodiumFloor Diaphragm Behavior 41
  42. 42. Diaphragm TransferForces Large diaphragm transfer forces should be anticipated at offsets or discontinuities of the vertical elements of the seismic-force- resisting system. (a) Setback in the building profile (b) Podium level at grade. 42
  43. 43. Podium and Backstay Effects Backstay Effects Title: Effects of podium interference on shear force distributions in tower walls supporting tall buildings Author: Mehair Yacoubian, Nelson Lam, Elisa Lumantarna, John. L. Wilson, 2017 43
  44. 44. Typical Diaphragm Components Chord (Diaphragm) Chord (Diaphragm) Shear (Diaphragm) Shear Wall Diaphragm 1 2 4 Shear Friction (Support) 3 Collector (Support) 44
  45. 45. Realistic Model - FiniteElement Model • Finite element modeling of a diaphragm can be useful for assessing the force transfer among vertical elements, force transfer around large openings or other irregularities. Shear Walls 45 Shear Walls Shear Walls
  46. 46. EFFECTOF COMMON PODIUMON THESEISMICPERFORMANCE TOWERS 46
  47. 47. Modeling Options • Individual design of tower and podium separately in practice • Restraint of resources such as software, processing time, understanding and references Single tower without podium 48 Single tower with half podium Single tower with whole podium Twin tower with whole podium
  48. 48. • Restraint of resources such as software, processing time, understanding and references • To study the effect of various options on seismic response estimation Problem Statement Single tower with whole podium Twin towers with whole podium Actual Building Single-tower less than Multi-tower Single-tower greater than 49 Multi-tower Design Results UNECONOMICAL Design UNSAFE Design
  49. 49. 50 TowersandPodiums
  50. 50. 51 Effectof Soil-Structure Interactionon Seismic Responsesof Tall Buildings
  51. 51. 52 A B C Siteeffects Soil-structure interaction 52
  52. 52. 53 MODELS Without SSI With SSI Referencemodel Equivalent Linear Nonlinear Model 3B Model 3A FE(DirectApproach) Code-based (SubstructureApproach) Model inpractice Model 2A Model 2B Equivalent Linear Model 1 Fixed-base 53
  53. 53. 54 Continuous improvements in our understanding, research, learning and practice Way Forward
  54. 54. T h a n ky o u

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