WIND ISSUES IN THE DESIGN OF
TALL BUILDINGS



Peter A. Irwin
RWDI


Los Angeles Tall Building Structural Design Council
M...
Wind Issues for Structural Design

Structural integrity under ultimate loads
Deflections under service loads
Building moti...
Relationship between importance of
wind and height



  Importance
     of wind
       loads




               Building h...
Vortex shedding
                 Shedding frequency N is
                 given by
                            U
         ...
Peak response due to vortex excitation




Magnitude of
peak response
            1
 ∝
   density × damping
Vortex excitation on a tapered spire
Mode 1
Vortex excitation on a tapered spire
Mode 2
Shape strategies to reduce excitation


Softened corners

Tapering and setbacks

Varying cross-section shape

Spoilers

Po...
Taper effect - Petronas towers
Taipei 101 – corner softening



Original




                      25%
                REDUCTION
           IN BASE MOMEN...
Shape effect, Taipei
101 wind tunnel tests
Burj Khalifa – 828 m
        Set backs, changing cross-section, orientation

Completed Building
                          ...
Shanghai Center – 632 m
      Twisting and tapering
151 storey tower in Korea
     Creation of bleed slots at edges to suppress vortex shedding

Full scale rendering
        ...
Use of corner slots to bleed air through
                   corners on a tall building

             Plan view            ...
With vortex
      excitation                           700 yr
  10 yr                                50 yr

              ...
where
                                                     where

Reliability considerations for flexible buildings

   Ex...
First order, second moment reliability
   analysis for rigid buildings
   Code analysis method

  Vw = (Vqi + Van )1 / 2 =...
Reliability of flexible buildings with
monotonic response

Wind load varies as power law

                  W ∝U      n


...
Required load factors for flexible
       buildings with monotonic response
      Rigid building
  Vw = (Vqi + Van )1 / 2 ...
Wind loads determined directly at
      ultimate return period by wind
      tunnel method
Building                Require...
Consequence of vortex shedding on
           reliability assessment – Nakheel Tower
                            Low ratio ...
Reynolds number effects


Originate from viscosity of air
Can cause changes in flow patterns on a small
scale model relati...
Effect of Reynolds number on pressure
               coefficient around a circular cylinder
                        1.0


...
1:50 Scale Model of Upper Portion of Burj Khalifa
High Reynolds Number Tests, Re ~ 2x106


                               ...
Comparison of Mean Pressure Coefficients
at Low and High Reynolds Number - Burj
Dubai
                             Pressur...
High Reynolds number tests on
             Shanghai Center



   Example of Mean Pressure Coefficients




Pressure tap co...
Damping and dynamic response
                    beyond the elastic limit

                                     F2        ...
Effect of stiffness reductions and inelastic
                deflections on effective viscous damping ratio


            ...
Damping and inelastic response
      research

Non-linear time domain analysis of structures under
realistic wind loading ...
Building motions and criteria


Problem is complex due to variability amongst
people
What return period should be used?
Wh...
Table 1:
 Table 1:         Building Motion Criteria. Historical review of 19
                  buildings wind tunnel teste...
Summary of computed frequencies
of 19 buildings

 0.40
 0.35

 0.30
 0.25

 0.20
 0.15

 0.10
 0.05
        50   100     1...
Summary of Predicted and
Improved Acceleration Responses

                             30


                             2...
Buildings with Supplemental Damping System



              Building Name and
                   Location                 ...
Park Tower, Random House and Wall Center with
Damping Systems

Chicago                    New York




                   ...
Trump world Tower and Bloomberg Center with
Damping Systems


New York




                            New York
Chicago Spire – Very Long period – Higher
Mode Effects
Higher modes can affect response
                   ~ questions re frequency dependent motion criteria
                   ...
Chicago spire - motion and deflection control through use of damping system.
Assessing building motions.
                                                               25

               ISO Criteria...
Meso-scale Modelling of June 1988 Event
Thank you
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WIND ISSUES IN THE DESIGN OF TALL BUILDINGS

  1. 1. WIND ISSUES IN THE DESIGN OF TALL BUILDINGS Peter A. Irwin RWDI Los Angeles Tall Building Structural Design Council May 7, 2010
  2. 2. Wind Issues for Structural Design Structural integrity under ultimate loads Deflections under service loads Building motions and occupant comfort Uncertainties in building structural properties (stiffness, damping) Uncertainties in wind loading Uncertainties in wind climate Codes and standards Computational Fluid Dynamics
  3. 3. Relationship between importance of wind and height Importance of wind loads Building height
  4. 4. Vortex shedding Shedding frequency N is given by U N =S b S = Strouhal number U = wind speed b = building width Directions of fluctuating force wind
  5. 5. Peak response due to vortex excitation Magnitude of peak response 1 ∝ density × damping
  6. 6. Vortex excitation on a tapered spire Mode 1
  7. 7. Vortex excitation on a tapered spire Mode 2
  8. 8. Shape strategies to reduce excitation Softened corners Tapering and setbacks Varying cross-section shape Spoilers Porosity or openings
  9. 9. Taper effect - Petronas towers
  10. 10. Taipei 101 – corner softening Original 25% REDUCTION IN BASE MOMENT Modified
  11. 11. Shape effect, Taipei 101 wind tunnel tests
  12. 12. Burj Khalifa – 828 m Set backs, changing cross-section, orientation Completed Building Early 1:500 scale wind tunnel tests
  13. 13. Shanghai Center – 632 m Twisting and tapering
  14. 14. 151 storey tower in Korea Creation of bleed slots at edges to suppress vortex shedding Full scale rendering 1:500 wind tunnel model
  15. 15. Use of corner slots to bleed air through corners on a tall building Plan view Plan view with without slots slots Crosswind Base moments motion reduced by 60% Wind Wind
  16. 16. With vortex excitation 700 yr 10 yr 50 yr Without vortex excitation (a) (b) With vortex 50 yr 700 yr excitation Without vortex excitation (c) (d)
  17. 17. where where Reliability considerations for flexible buildings Expression for load factor λw 1 α 2 β Vw λW = e Kw Vw = Coefficient of variation of wind load β= Reliability index Kw = Bias factor α= Combination factor
  18. 18. First order, second moment reliability analysis for rigid buildings Code analysis method Vw = (Vqi + Van )1 / 2 = (0.2 2 + 0.32 )1 / 2 = 0.36 2 2 1 α 2 β Vw 1 ( 0.752 ×3.5×0.36 ) λW = e = e = 1.56 Kw 1 .3 Wind tunnel method Vw = (0.22 + 0.12 )1 / 2 = 0.22 1 ( 0.752 ×3.5×0.22 ) λW = e = 1.54 1 .0 Both the coefficient of variation and bias factor are smaller in the wind tunnel method. Load factor ends up at about the same.
  19. 19. Reliability of flexible buildings with monotonic response Wind load varies as power law W ∝U n Rigid building Vw = (Vqi + Van )1 / 2 2 2 Flexible building Vw = (Vqi + Van + Vdyn )1 / 2 2 2 2 Vdyn ≈ ndampVς2 + ( n − 2) 2V f2 + ( n − 2) 2VV2 2 Damping term Frequency term Velocity sensitivity term
  20. 20. Required load factors for flexible buildings with monotonic response Rigid building Vw = (Vqi + Van )1 / 2 = (0.2 2 + .12 )1 / 2 = 0.22 2 2 ( 0.752 ×3.5×0.22 ) λW = e = 1.54 Typical flexible building Vw = (Vqi + Van + Vdyn )1 / 2 = (0.2 2 + 0.12 + 0.112 )1 / 2 = 0.249 2 2 2 ( 0.752 ×3.5×0.249 ) λW = e = 1.63 Highly flexible building Vw = (Vqi + Van + Vdyn )1 / 2 = (0.2 2 + 0.12 + 0.22 2 )1 / 2 = 0.31 2 2 2 ( 0.752 ×3.5×0.31) λW = e = 1.84 Note:- Wind tunnel method is assumed in all cases.
  21. 21. Wind loads determined directly at ultimate return period by wind tunnel method Building Required Actual type n ndamp Load Factor Load Factor Ratio 1.264n Rigid 2 0 1.54 1.60 1.04 Flexible 2.5 0.25 1.63 1.80 1.10 Very Flexible 3 0.5 1.84 2.02 1.10 Slightly conservative
  22. 22. Consequence of vortex shedding on reliability assessment – Nakheel Tower Low ratio of 1000 year to 50 year loads Uncertainty in loads is mostly dictated by uncertainty in this peak Need to carefully assess uncertainty in peak vortex shedding response since the normal assumption that uncertainty in wind speed governs is no longer valid. Damping and frequency uncertainties become important.
  23. 23. Reynolds number effects Originate from viscosity of air Can cause changes in flow patterns on a small scale model relative to full scale Not a concern on sharp edged structures Can be a concern on curved shape buildings Is lessened by high turbulence and surface roughness
  24. 24. Effect of Reynolds number on pressure coefficient around a circular cylinder 1.0 U 0.5 θ b θ degrees 0 20 60 100 140 Cp -0.5 Reynolds number -1.0 Ub Re = -1.5 ν -2.0 Sub-critical Re < 2×105 -2.5 Kinematic viscosity of air Transcritical Re > 3×106
  25. 25. 1:50 Scale Model of Upper Portion of Burj Khalifa High Reynolds Number Tests, Re ~ 2x106 Note: Full scale Re~7x107
  26. 26. Comparison of Mean Pressure Coefficients at Low and High Reynolds Number - Burj Dubai Pressure Taps
  27. 27. High Reynolds number tests on Shanghai Center Example of Mean Pressure Coefficients Pressure tap count
  28. 28. Damping and dynamic response beyond the elastic limit F2 k2 F1 k1 1 δE1/ 2 ζ eff = 2π E x 1 x2 η = ( x2 − x1 ) / x1  k 2  1 k2 2  1 − η +   η  1 k1  2 k1   ζ eff = π k 1 + 2η + 2 η 2 k1
  29. 29. Effect of stiffness reductions and inelastic deflections on effective viscous damping ratio 1 δE1/ 2 ζ eff = k2/k1=0.5 2π E Example: For 20% k2/k1=0.75 deflection beyond elastic limit effective increment in damping ratio = 0.024. This would be additive to the damping ratio below the elastic limit which is typically ( x2 − x1 ) / x1 assumed to be 0.01 to 0.02.
  30. 30. Damping and inelastic response research Non-linear time domain analysis of structures under realistic wind loading will allow us to evaluate whether more efficient structures can be developed. How feasible is it to load the structure beyond its elastic limit and how far can one go with this? Can we learn how to keep the structure stable and robust after going plastic. What can be learnt from earthquake engineering? More full scale monitoring needed at representative deflections.
  31. 31. Building motions and criteria Problem is complex due to variability amongst people What return period should be used? What quantity best encapsulates comfort: acceleration; jerk; something in between; angular velocity; noises combined with motion, etc? Designers have to make decisions and move on. What is the actual experience using traditional approaches?
  32. 32. Table 1: Table 1: Building Motion Criteria. Historical review of 19 buildings wind tunnel tested by RWDI in the 1980s and 1990s. First Order Modes Frequencies Assumed Building (Hz) Building Height Damping Ratio Number (m) fx fy ftor (% of critical) 1* 249.4 0.193 0.199 0.295 2.0 2 163.4 0.176 0.224 0.250 2.0 3 198.1 0.189 0.184 0.300 1.5 4 137.2 0.185 0.135 0.323 2.0 5 236.2 0.154 0.169 0.400 2.0 6 178.0 0.244 0.250 0.400 2.0 7 215.0 0.177 0.149 0.331 1.5 8 110.9 0.192 0.164 0.250 2.0 9 163.0 0.195 0.208 0.224 1.5 10 124.4 0.170 0.224 0.204 2.0 11* 207.8 0.147 0.171 0.250 1.5 12* 145.2 0.411 0.213 0.440 2.0 13 94.0 0.278 0.243 0.139 2.0 14 141.2 0.370 0.216 0.356 2.0 15 143.3 0.135 0.180 0.333 2.0 16 259.4 0.131 0.125 0.263 1.25 17 175.4 0.201 0.157 0.200 1.5 18* 247.8 0.131 0.154 0.211 2.0 19* 259.4 0.179 0.159 0.236 2.0 * For these buildings, wind tunnel studies predicted peak resultant accelerations above the 15 –18 milli-g range.
  33. 33. Summary of computed frequencies of 19 buildings 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 50 100 150 200 250 300 Buil di ng Hei ght , H ( m ) fx fy f = 33/H Fitting Curve
  34. 34. Summary of Predicted and Improved Acceleration Responses 30 25 Acceleration ( milli - g ) 20 15 10 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Building # 15-18 milli-g range accelerations Above 15-18 milli-g range accelerations Reduced acceleration responses using SDS
  35. 35. Buildings with Supplemental Damping System Building Name and Location 10-Year Peak Resultant Acceleration Building ( milli – g ) Number SDS Type Installed Without SDS With SDS 1 Park Tower, Chicago, IL TMD 24.0 16.6 11 Random House, New TLCD 25.3 15.4 York, NY 12 Wall Centre, Vancouver, TLCD 28.0 15.2 BC 18 Bloomberg Tower, New TMD 22.5 15.5 York, NY 19 Trump World Tower, New TMD 27.4 17.3 York, NY
  36. 36. Park Tower, Random House and Wall Center with Damping Systems Chicago New York Vancouver
  37. 37. Trump world Tower and Bloomberg Center with Damping Systems New York New York
  38. 38. Chicago Spire – Very Long period – Higher Mode Effects
  39. 39. Higher modes can affect response ~ questions re frequency dependent motion criteria such as ISO comfort criteria. 700 600 500 2 nd harmonic 1st harmonic 400 Height, m 300 200 100 0 -1.5 -1 -0.5 0 0.5 1 1.5 Mode deflection shape
  40. 40. Chicago spire - motion and deflection control through use of damping system.
  41. 41. Assessing building motions. 25 ISO Criteria for 20 Peak acceleration, milli-g single frequency 15 Resid Co entia mmercial 10 l 5 Moving room 0 simulations of multiple 0.1 Frequency, Hz 1 frequencies
  42. 42. Meso-scale Modelling of June 1988 Event
  43. 43. Thank you
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