By

By BAI Xue

Supervisor Prof. HE Minjuan

Design of Foundation for Wind Turbine Towers

For centuries mankind has used wind resources for sailing.Today we use wind turbines to produce electricity.

CONTENTS

1. Foundation Design Overview

2. Tower-foundation Connections

3. Prestressed Beam-slab Foundation

4. Partially Prestressed Concrete Foundation

5. Case Study

6. Further Work

Types of foundations for offshore wind turbines

1. Foundation Design Overview

a) Gravity footing

b) Monopile foundation

c) Monopod bucket foundation

d) Jacket foundation

Types of foundations for onshore wind turbines

Spreading footing foundation

a) Slab foundation

b) Beam-slab foundation

a b

Requirements

Wind

Soil Resistance

Weight of Soil

and Concrete to

Resist Tilting

Foundations rely upon soil and concrete to resist

overturning force at the extreme wind loads.

Tower-foundation Connection

Insert Ring Anchor Bolt

1. Foundation Design Overview

Forces should be transmitted effectively from tower to

foundation via connections.

Failure examples

Large Windmill collapses in NH

due to loss of equilibrium.Turbine falls at Fenner wind

farm due to connection failure.

1. Foundation Design Overview

Insert rings

Insert ring

Easy to connect to tower

Concrete is under the repeated variable

amplitude tensile-compressive load.

Concrete around the flange is prone to

crack and fatigue damage.

M

F

N

Tensile

stressCompressive

stress

2. Tower-foundation Connections

Movement

Ft

Damaged

Concrete

Repeated

Load

Fatigue damage

Damaged concrete above

the flangeDamaged concrete in the surface Damaged concrete at

reinforcement holes level

2. Tower-foundation Connections

Damage propagates to the

surface.

Damage propagates and

increases upwards.

Rongcheng wind farm in Shandong Province

Damage originates from

the flange of anchor ring.

MF

N

Tensile

stressCompressive

stress

Solution

2. Tower-foundation Connections

MF

N

Compressive

stress

Compressive

stress

Insert Ring

Prestressed Anchor Bolt

Concrete foundation is always in a state of compression.

Therefore, the stress change in the concrete under the application of repetitive wind loads is not critical.

Replaced by

Prestressed

Anchor Bolt

Concrete under tensile-compressive load.

Insert Ring

3. Prestressed Beam-slab Foundation

Foundation systemBeam-slab foundation Prestressed anchor bolts

Reduces the weight and

volume of materials

used and reduce cost.

Achieve continuous stiffness

between tower and foundation.

Foundation Plan A-A profile

Allow foundation to have a desirable combination of high stiffness and superior fatigue resistance.

Practical application

2009 2010 2011 2012 SUM

Number 164 652 1733 614 3163

Prestressed Beam-slab Foundation Applied in Wind Farm

This foundation system has been adopted by more than ten Chinese

turbine manufacturers with rated power ranging from 1.5 to 6MW.

3. Prestressed Beam-slab Foundation

Regional distribution

4. Partially Prestressed Concrete Foundation

Concrete under the flange of the anchor

ring is exposed to plastic deformation

under long-term initial prestress load.

Initial Prestress Load

High prestress load also requires higher

standards for concrete, steel plates and

flanges.

The minimum required value of prestress should be defined.

Problems arise

Comparison

Fully Prestressed Foundation

Tower and foundation should always keep contact even under the extreme wind

load.

Partially Prestressed Foundation

Tower and foundation can separate at extreme wind load, while keep contact under

operating load.

Prestress loss under long-term loads should be considered sufficiently.

4. Partially Prestressed Concrete Foundation

Degree of prestressing

0.9 0.9 0.9

1.2u eP f A

① The pretension of a bolt is defined in accordance with Code for design of steel structures (GB50017) as follows:

= nominal lowest ultimate tensile strength;uf

= effective diameter of ordinary anchor bolt at threaded section.eA

②In addition, a 15% extra-pretention is applied during the construction stage.

Design pretension load of bolts

design pretension load of bolts;

0P

P

The prestressing degree of a bolt is defined as follows:

P 0P partial pretension load of bolts.

0twk cgk pc

① The criterion that tower and foundation should keep contact under operating load is ensured by:

= concrete stress on the windward side under operating load based on SLS;twk

pc = concrete compressive stress induced by prestress of bolts;

cgk = concrete compressive stress induced by gravity of wind turbine;

= percentage of concrete compression force over exterior load.

② In addition to prestress loss during construction stage, shrinkage and creep of concrete and relaxation of bolts in long-term should also be taken into consideration.

Criteria for the partial prestressing

③ The design tension of partially prestressed bolts is defined as:

0 /1.15preloadP P

Where, 1.15 is considered as the prestress loss during construction stage.

④ Partially prestressed concrete foundation should also ensure adequate response with regard to the serviceability and ultimate limit states, as well as the additional requirements related to fatigue and minimum construction reinforcement.

4. Partially Prestressed Concrete Foundation

5. Case Study

Turbine: WD88-1500 -70 (1500kW Turbine)Location: Delingehaer County in Qinghai Province Height of Tower: 68mTower-foundation Connection:

Prestressed anchor bolt M36

General

Elevation of foundation

Results

Cases Design pretention force(kN)

Full prestressing 410

Partial prestressing 295

Comparison between full and partial prestressing

The partial prestressing degree of bolts is given as follows:

0 0.72P

P 80% is chosen as prestressing degree conservatively.

,

1

2r eqS

meanM

5. Case Study

Fatigue load

Method: Fatigue equivalent load cycle method

,

1

2mean r eqM M S

Cases My (kN m) Fx(kN) Fz(kN)

Maximum Case1

24818.25 301.45 3024.45

MinimumCase2

608.75 17.35 2928.25

Maximum and minimum of fatigue loads

1,

, 0( )

mn r i m

r eq ieq

SS

N

Equivalent range of load cycle;,r eqS

Equivalent number of allowed cycles;eqN

m Exponent that defines the slope of the S-N cycle;

rS Range of load cycles;

n Number of cycles.

For reinforced concrete fatigue analysis, m=7.

Fatigue loads are calculated in means of rain flow count algorithm, which transforms the cyclic load into an equivalent simpler set of loads.

5. Case Study

Concrete fatigue analysis

The fatigue life should be verified for compressive concrete under the flange of the anchor ring.

min maxminS maxSIn this project the fatigue analysis is performed separately for concrete and

reinforcement.

Preload values Checking

points

Partial

prestress

P=330kN

Points on the

windward side 2.72 4.73 0.14 0.25

Points on the

leeward side 4.83 6.85 0.25 0.36

Full prestress

P=410kN

Points on the

windward side 3.85 5.86 0.2 0.31

Points on the

leeward side 5.96 7.99 0.31 0.42

Checking points stress of concrete under fatigue analysis

min maxminS maxS

max max / cS fmin min / cS f

5. Case Study

Concrete fatigue verification methods

Code for design of concrete structures (GB50010-2010) only provides fatigue analysis of concrete at 2X106 cycles. Therefore, it’s not suitable to verify fatigue resistance of wind turbine foundations.

The fatigue life of concrete is determined on CEB-FIP Model code 2010 .The fatigue verification for checking points is satisfied.

Wind turbine foundation is subject to high-cyclic load. The number of cycles can be up to 107.

Fatigue life of concrete in accordance with CEB-FIP Model code 2010

Preload value Checking points CEB-FIB MODEL

CODE2010 (lgN)

Partial prestress

P=330kN

Points on the windward side 42.06

Points on the leeward side 31.09

Full prestress

P=410kN

Points on the windward side 36.61

Points on the leeward side 21.94

5. Case Study

Reinforcement fatigue analysis

JL2

JL1Bottom Slab

Stress amplitude Checking points JL1

(N/mm2)

JL2

(N/mm2)

Bottom slab

(N/mm2)

Bending

reinforcement top

Points on the windward side 3.77 9.80 27.96

Points on the leeward side 0 67.94 59.76

Bending

reinforcement

bottom

Points on the windward side 19.08 0 11.23

Points on the leeward side 63.39 68.77 45.06

Checking points stress amplitude of reinforcement under fatigue analysis

Reinforcement fatigue verification methods:

Europe Code 1992-1-1-2004 and CEB-FIP Model

code 2010 both provide damage equivalent stress

range at 107 cycles .

The result shows the reinforcement has sufficient

fatigue resistance.

5. Case Study

Utilization ratiosHighest utilization ratios of the design

Part Strength analysis

Ultimate limit state

Fatigue analysis

Europe code 1992

Bending reinforcement top 0.59 0.64

Bending reinforcement bottom 0.68 0.64

Concrete compression 0.82 0.68

The utilization ratios are calculated by dividing required capacity by

provided capacity.

Limitation

The method ‘Fatigue equivalent load cycle’ is intended for fatigue calculation of components of the wind power plant such as the steel connections. It’s not clear whether it’s reasonable for fatigue analysis of reinforcement concrete.

However, it’s more straightforward because fatigue verification of concrete in Europe code requires one equivalent stress range.

6. Further Work

1. The uncertainties regarding whether ‘Fatigue equivalent load cycle’ can be used for design of reinforced concrete foundation needs to be verified.Fatigue analysis would be more simple through the use of equivalent load.

2.Due to the lack of reasonable S-N curves for concrete, cumulative damage of concrete is hard to evaluate. Fatigue verification methods for turbine foundation require further investigations.

3. Guidelines of partial prestressing and preload loss require further evaluation through on site survey.

4. Dynamic response monitoring of critical elements of turbine foundation needs to be carried out to analyze its fatigue performance afterwards.

Thank you for your attention

Department of Building Engineering, Tongji University, Shanghai, China