2.2 Peter Delphin

8/10/2019 2.2 Peter Delphin

1/32

Considerations for the structuraldesign of wind turbine generatorfootings in accordance with

Australian Standards

Peter Delphin

March 19, 2014


2/32

Introduction

This presentation will give an overview of the use of thefoundation load document for the structural design of windturbine generator (WTG) footings and how it may be adaptedfor use with Australian Standards.

We will look at the following: Building Code of Australia

WTG Foundation Load document

AS1170 Loading Standards

Structural design requirements

WIF Page 2March 28, 2014


3/32


4/32


5/32

Building Code of Australia (BCA)

Annual probabilities of exceedance:

The BCA defines annual probabilities of exceedance forDesign Events for Safety based on the Importance level forthe structure.

For an importance level of 2, the following values areobtained:

Wind 1:500 annual probability of exceedance

Earthquake 1:500 annual probability of exceedance



6/32

WTG Foundation Load Document


7/32


8/32


Wind turbines are classified in accordance with a referencewind speed measured at hub height of the tower.

Vref is a wind speed averaged over 10 minutes

Ve50

is an extreme 3 second gust wind speed with a return period of 50 years

The wind turbulence category A, B or C is also appended to the class where Arepresents high turbulence, B medium and C low.


Wind turbine class I II III

Vref [m/s] 50 42.5 37.5Ve50 [m/s] 70 59.5 52.5


9/32


Design load cases (IEC 61400-1)


Design Situation DLC Wind Conditions

1. Power production 1.1 to 1.5 Vin< Vhub< Vout , Vr

2. Power production plus

occurrence of fault

2.1 to 2.4 Vin< Vhub< Vout , Vr

3. Start up 3.1 to 3.3 Vin< Vhub< Vout , Vr

4. Normal shutdown 4.1 to 4.2 Vin< Vhub< Vout , Vr

5. Emergency shutdown 5.1 Vr

6. Parked (standing still or idling) 6.1 to 6.4 50-year return, 1-year

return

7. Parked and fault conditions 7.1 1-year return

8.Transport, assembly,

maintenance and repair

8.1 to 8.2 Vmaint


10/32


The design loads given in the foundation load document aremultiplied by partial safety factors depending on the limitstate being analysed and the operating condition. The partialsafety factors are defined in IEC 61500-1


Design situation Partial safety factor

Ultimate limit state, Normal and extreme, N 1.35

Ultimate limit state, Abnormal, A 1.1

Ultimate limit state, Transport, erection,

installation and maintenance, T

1.5

Serviceability limit state 1.0

Fatigue limit state 1.0


11/32


Typical presentation of loads at tower base and coordinate system

March 28, 2014WIF Page 11


12/32

AS1170 Loading Standards


13/32

AS1170 Loading

AS1170 specifies general procedures and criteria for thestructural design of buildings or structures in limit stateformat.

The AS1170 standard is made up of 5 parts:

Part 0: General principles Part 1: Permanent, imposed and other actions

Part 2: Wind actions

Part 3: Snow and ice actions

Part 4: Earthquake actions



14/32

AS1170 Loading

Limit states

States beyond which the structure no longer satisfies the designcriteria

Ultimate limit state

States associated with collapse, or other similar forms of structuralfailure

Serviceability limit state

States that correspond to conditions beyond which specifiedservice criteria for a structure are no longer met.



15/32

AS1170 Loading

AS1170.0 specifies combinations of actions for the ultimatelimit states for stability and strength.

Stability:

Stabilising effects Ed,stb= [0.9G]

Destabilising effects Ed,dst= [1.2G, Wu, psicQ]

Strength:

Ed= [1.2G, Wu, psicQ] permanent, wind and imposed action

Ed= [0.9G,Wu] permanent and wind action

Ed= [G, Eu, psieQ] permanent, earthquake and imposed action



16/32

AS1170 Loading

The 50-year return period wind speed at hub height can be calculatedfrom AS1170.2 and compared with the Ve50wind speed based on thewind turbine classification.

The IEC 61400-1 partial safety factor of 1.35 when applied to the 50-yearreturn period characteristic loads from the foundation load document is

equivalent to a wind with a recurrence period of approximately 580 years.This is comparable to the 1:500 BCA requirement

Therefore the factored extreme loads can be substituted for the ultimateloads, Wu, in the AS1170.0 load combinations.



17/32

AS1170 Loading

WTGs are typically located on

hills, ridges and escarpmentstherefore a topographic surveyplan is required to determine thecoefficient, Mtto calculate windspeed at hub height usingAS1170.2



18/32

AS1170 Loading

Wind speed comparison at hub-height:

AS1170.2 V50wind less than or equal to Ve50

Use factored extreme loads from load document

AS1170.2 V50wind greater than Ve50

Factor the DLC cases that use extreme wind by

Do notfactor the production load cases DLC 1 to 5



19/32

AS1170 Loading

Earthquake loadingAS1170.4

May or may not be included in the foundation load document

If not included a separate assessment will need to be carried out by thestructural engineer. The earthquake load should be added to anoperating wind load case since the earthquake will likely trigger sensorsthat shut down the WTG.

A simple dynamic analysis can be carried out to determine earthquake

forces. Knowledge of the tower construction and mass distribution of theWTG is required.



20/32

Structural design


21/32

Structural design

The structural design of the WTG footing requiresconsideration of the following:

- Ultimate limit state

Stability (overturning, sliding)

Ultimate bearing capacity of the foundation Bending and shear capacity of the footing

Transfer of load from tower to footing

- Serviceability limit state

In-service bearing pressures

Footing rotation/settlement

Crack control in concrete (reinforcement stress)



22/32

Structural design

-Fatigue limit state Concrete stress

Steel reinforcement stress

- Other

Dynamic rotational stiffness Durability



23/32

Structural design


- overturning

- sliding

AS1170.0 requirement

- Ed,std >= Ed,dst

i.e. stabilising >= destabilising



24/32

Structural design


- Soil bearing (triangular)

- Recommend at least 50% ofbase of the footing is in contact

with the ground as soilpressure increasesexponentially with increasingoverturning

- Equivalent uniform pressure

using DNV/Riso approach



25/32

Structural design

Ultimate limit state Bending Shear

Punching

Strength calculationscarried out in accordancewith AS3600 ConcreteStructures using loadcombinations:

1.2G+Wu 0.9G+Wu



26/32

Structural design

Serviceability limit state

- no-gapping requirement

- Footing in full contact with theground

- Expected settlement / rotation(geotechnical advice)

- Cracking in concrete reviewedby limiting reinforcement stress

in accordance with AS3600.

- Crack widths can also bedetermined using Eurocode



27/32

Structural design

Fatigue limit state

- AS3600 Concrete Structuresdoes not have any rules forfatigue assessment

-AS5100 Bridge Structures orEurocode EC2 can be used toassess fatigue in concrete andreinforcement

- Damage equivalent loading

simpler to check fatigue.

- Miners method can be used tosum individual stress rangesand damage ratios



28/32

Structural design

Other

- Rotational stiffness

The rotational stiffness of the footingsneeds to be checked to ensure that itis greater than the minimum value

specified in the foundation loaddocument.

This is necessary to avoid thepossibility that resonance could occurwhen the WTG is operating leading to

increased deflections and rocking.

Geotechnical advice required for thedynamic shear modulus of thefounding material



29/32

Structural design

Durability

Concrete cover to reinforcement and concrete compressivestrength are determined to comply with durabilityrequirements of AS 3600 Concrete Structures.

Detailing of the reinforcement should also comply withAS3600 requirements to control cracking due to shrinkageand temperature effects.



30/32

Summary


31/32

Summary

The WTG foundation load document and the IEC partialsafety factors can be used as ultimate loads for footingdesign.

Wind speed at hub height should be checked using

AS1170.2 and compared with IEC turbine classification.Load cases in the foundation load document using theextreme wind may need to be scaled accordingly.

AS3600 in combination with AS5100 or Eurocode EC2 for

fatigue assessment can be used to design reinforcedconcrete WTG footing.



32/32

Thank You

[email protected]

2.2 Peter Delphin

Documents

Transcript of 2.2 Peter Delphin