11.10.2015

1

SL-Rasch GmbH

Uncertainties in Wind Load Estimations on Membrane Structures

Alexander Michalski, SL-Rasch GmbH, Stuttgart

Novel Structural Skins, COST Action

EUROMEM Nantes, September 2015

SL-Rasch GmbH

Dr. A. R. Dykes, British Institution of Structural Engineers, 1976

"Engineering is the art of modeling materials we do not wholly

understand, into shapes we cannot precisely analyze so as to

withstand forces we cannot properly assess, in such a way that the

public has no reason to suspect the extent of our ignorance."

�Accurately captures the essence of the challenge of structural

engineering

�Uncertainties in

• Material

• Structural Model

• Forces (external, internal)

11.10.2015

2

SL-Rasch GmbH

Wind Climate

Atmoshperic Boundary Layer

Structure

SL-Rasch GmbH

3 fundamental aspects of wind loaded structures

Atmospheric

Boundary LayerStructureWind climate

11.10.2015

3

SL-Rasch GmbH

Van-der-Hoven Spectrum

Atmospheric

Boundary LayerWind climate Structure

Structural

frequencies

SL-Rasch GmbH

Wind climate

11.10.2015

4

SL-Rasch GmbH

Flow characteristics on earth

SL-Rasch GmbH

Flow characteristics on earth

Meteorology: Mathematical/Numerical model

11.10.2015

5

SL-Rasch GmbH

Statistical wind climate model: Reference wind speed

SL-Rasch GmbH

Code-based reference wind speed• Represents the wind climate of each location

• Expected value of the wind speed which occurs with a defined probability of occurence

• Valid for predefined meteorologic boundary conditions

• 10 m height, 10-Minutes mean wind speed >independent of wind direction

• Expected maximum wind speed in designed lifetime (extreme value of wind speed, as

determined by extreme value theory)

11.10.2015

6

SL-Rasch GmbH

Atmospheric Boundary Layer

SL-Rasch GmbH

Atmospheric Boundary Layer

• Lowest part of the atmosphere

• behavior is directly influenced by friction with surface

• Wind speed increases with increasing height above the ground

11.10.2015

7

SL-Rasch GmbH

Mathematical description

Wind speed

Fluctuations

Mean wind speed

Fluctuating

components

ui(z,t)

Mean

wind

speed

SL-Rasch GmbHGust wind pressure including measurement error and model error

11.10.2015

8

SL-Rasch GmbH

Structure

SL-Rasch GmbH

What causes loads to flow-exposed buildings?

11.10.2015

9

SL-Rasch GmbH

Wind Loaded Membrane Structures

Wind tunnel, Institute of Hydromechanics University of Karlsruhe

Load carrying behavior

− Activation of tangential membrane stresses

− Large deflections, membrane adapts to external loads until equilibrium condition is achieved

Static structural response

− Double curved surfaces � Determination of wind loads

Dynamic structural response

− Low self-weight, Inertial forces could be neglected, the structure reacts directly to load changes

− Added mass, added damping effects: Influence of surrounding mass of air increases

Convertible membrane structures ���� Behaviour in the retraction process

SL-Rasch GmbH

Wind loading based on wind tunnel tests

11.10.2015

10

SL-Rasch GmbH

Wind tunnel setup

Wind tunnel, Institue of Hydromechanics

University of Karlsruhe

Wind tunnel, Chair of Fluidmechanics

TU München

SL-Rasch GmbHReynolds number similarity

Dynamic similarity of the moving fluid particls

� Movement of fluid particals must be dynamically similar

uref…Reference velocityLref…Reference length scale, nnnn… dynamic viscosity of air

Example:

u=20m/s, L=60m, n=1.53 e-5m²/s

Re = approx 7 x e7

� Reynolds-number similarity could not be

achieved in small scale wind tunneltests

11.10.2015

11

SL-Rasch GmbH

Wind tunnel tests

SL-Rasch GmbH

Measurement results

11.10.2015

12

SL-Rasch GmbH

Werodynamic pressure coefficients: Rigid models

Windkanal Modell 1:200Windkanal Modell 1:200

SL-Rasch GmbH

Computational Wind Engineering

11.10.2015

13

SL-Rasch GmbH

Fluid Structure Interaction Simulation

CFD

Flow analysis

PAM Flow

CSD

Structural

analysis

PAM Crash

Pressure/ friction

Deflection

Wind

generation

Form-

finding

LISA

Wind load simulation Structural response simulation

Michalski, A., P. D. Kermel, et al. (2011). Validation of the computational fluid-structure interaction simulation at real-scale tests of a

flexible 29 m umbrella in natural wind flow. Journal of Wind Engineering and Industrial Aerodynamics 99(4): 400-413

SL-Rasch GmbH

Strain measurements at the umbrella arms and mast

11.10.2015

14

SL-Rasch GmbH

Photogrammetric measurement of membrane deformation

SL-Rasch GmbH

Photogrammetric measurement of membrane deformation

11.10.2015

15

SL-Rasch GmbH

Requirements for measurement

• Winddirection 262°

• Windspeed > 12 m/s

• Stationary datasets

• Measurement period 10 Minutes

• Sampling rate: 10 Hz

• Windfeld 10.02.09

WR 262°

Measuring site

SL-Rasch GmbH

Reference positionWind measurement mastsWind

inflow

Windinflow

Simulation 1 – 7 Mio elements Simulation 2 – 12 Mio elements

Computational Wind Simulation

11.10.2015

16

SL-Rasch GmbH

SL-Rasch GmbH

11.10.2015

17

SL-Rasch GmbH

Mast foot moment M(t)

Mmast

Mmean=250

Mmean= 220

RMS=128

RMS=102

Comparison of numerical and experimental results

Michalski, A., Haug E., Wüchner R. Bletzinger K.-U. (2011) Validierung eines numerischen Simulationskonzeptes zur Strukturanalyse

windbelasteter Membrantragwerke, Der Bauingenieur, Band 86, März 2011, S. 129 - 141

SL-Rasch GmbH

Influence of the mesh density on the results

7 Mio elements12 Mio elements

Normalized root mean squares of umbrella response (fine model)

Normalized root mean squares of umbrella response (coarse model)

11.10.2015

18

SL-Rasch GmbH

Computational Fluid Dynamics Simulation

Michalski et al., “Computational Wind Engineering of Large Umbrella Structures”, Journal of Wind Engineering and Industrial

Aerodynamics, Publication 2015, under review

SL-Rasch GmbH

Fluid-Structure-Interaction Simulation

Umbrella response

AFTER structural modification

Wind

Umbrella response

BEFORE structural modification

11.10.2015

19

SL-Rasch GmbH

SL-Rasch GmbH

FSI Simulation of partially folded umbrellas

11.10.2015

20

SL-Rasch GmbH

FSI simulation of partially folded umbrellas

SL-Rasch GmbH

Conclusion

George Edward Pelham Box (born 18 October 1919)

"Essentially, all models are wrong, but some are useful“

� Model uncertainties

�How wrong do they have to be to not be useful

� Try to predict nature as best as possible

Apply several methods

Perform cross checks

Sensitivity analysis

Parametric study

Validation, Verification

Industry

Public, private Organisations

Universities

11.10.2015

21

SL-Rasch GmbH

Real scale experiments: A challange for building structures

Studio MSB