wind turbine foundationsstructural design principles & practices

MNSEA tradeshow & seminarMay 8, 2018

Christopher Kopchynski, PE,Barr Engineering Co.

Introduction to Barr

Engineering

Barr’s presenter – Mr. Christopher A. Kopchynski

• cak@barr.com• 30 years of structural engineering

experience• 20 years of wind turbine foundation

experience• licensed professional engineer in 7

Canadian provinces and 12 U.S. states

Barr Engineering

Co.

• www.barr.com• corporate headquarters:

Minneapolis, Minnesota• employee owned, 700 people• engineering wind turbine

foundations since 1994• 370 facilities• 21,700 wind turbines• 38,500 MW of nameplate capacity• 4,000 MW in construction• 2,000 MW in design

integrated process

Mechanical &ElectricalFactors

The PhysicalWorld

StructuralFactors

Materials

GeotechnicalFactors

Foundation

Construction

5

overturning moment is (vi)king

Aerodynamic Forces

Overturning Moment

Vertical Load

Horizontal Load

• large lever arm

• high aerodynamic thrust

• vertical load and horizontal load relatively small50

m to

120

m

300kN to 1100 kN

15MN-m to 150 MN-m

dynamic structural behavior

TowerStrain_shortened_Barr.wmv

strain readings

moment and power vs. time

POWER

MOMENT

Foundation Rotation vs. Momentrotation verses moment

fatigue loads

11

24220

21553

24606

21285

22850

21537

22786

20914

22424

23771

21261

24044

22330

23063

21178

24724

20924

24391

21669

20000

20500

21000

21500

22000

22500

23000

23500

24000

24500

25000

10 15 20 25 30 35 40

Mom

ent  (kN‐m

)

Elapsed Time (seconds)

Moment verses Time

design fatigue spectrum

12

represented by sets of corresponding:

• ranges, Sr,I

• means, Sm,I

• cycles, i=1 to n

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design input - tower bottom flange dimensions

• provided by original equipment manufacturer (OEM)

• flange outside and inside diameter• bolt circle inside and outside diameter• number of bolts• bolt hole diameter• flange thickness• tower shell outside diameter• tower shell thickness

design input – foundation loads

• provided by OEM• normal extreme• abnormal extreme• operating 1% exceedance• operating 0.1% exceedance• markov matrix fatigue loads• seismic - tower & turbinecomponent weights

design input – stiffness requirements

• provided by OEM• foundation rotational and lateral stiffness

design input – geotechnical soil properties

• provided by project• soil chemistry• groundwater level• ground improvement

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design codes• International Building Code• American Concrete Institute ACI-318• American Institute of Steel Construction

AISC 360• American Society of Civil Engineers

ASCE 7• state and local building codes

industry standards

• International Electro Technical Commission IEC 61400-1

• Germanisher Lloyd GL Wind Guidelines 1.1

• Det Norske Offshore Standard DNV-OS-C502

• American Wind Energy Association, ASCE/AWEA RFP2011

technical references

• DNV, Guideline for Design of Wind Turbines

• Arya, O’Neill and Pincas, Design of Structures and Foundations for Vibrating Machines

• Fahey, Soil Stiffness

new standards

• DNV GL ST-0126, “Support Structures for Wind Turbines”

• IEC 61400-6, “Wind Turbines – Part 6: Tower and Foundation Design Requirements”.

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materials - concrete

• minimum 4500 psi (31 MPa), typical 5000 psi (35 MPa)

• rare: extremely large loads 6000 psi (41 MPa)

• ¾” (19m) to 1 ½”(38mm) coarse aggregate size• fly ash common• non-alkali silicareactive aggregates

materials – reinforcing steel

• ASTM A615 grade 60 ksi (414 MPa) or grade 75 (517 MPa)

• plain deformed bars• #4 (12mm) to#11 size (36mm)• rarely:large loads #14 size (43mm)

materials – anchor bolts

• cold formed threads• ASTM A615, grade 75 (517MPa)• ASTM A722, type 1 (827 Mpa)• custom grade 90 (620 Mpa)• common size 1 3/8” (35mm)• rare: large loads 1 ¾” (46mm)• pvc sleeves

materials – template ring and embedment plate

• ASTM A36 (248 MPa)• 1 in (25mm) to 2 in (50mm) thick• dimensions matching tower flange

materials – bottom flange grout

• modified cementitious or epoxy• 8000 psi (55MPa) to 15000 psi

(103 MPa)• 1 ½ in (38mm) to 3 in (75mm)

thick• formed with shoulders or placed

in grout pockets• rare: for large loads deep grout

pockets

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check a1 –global

overturning stability

• use unfactored extreme loads• use characteristic dead load resistance• per AWEA/ASCE FS>1.5• per IBC 0.6W>1.0

− W=stabilizing weight of the foundation

check a2 –foundation

contact extreme load

• GL wind guidelines• greater than 50% contact• use unfactored extreme loads• use characteristic dead load resistance

check a3 –foundation contact 1%

exceedance load

• GL wind guidelines• 99.5% to 100% contact• use unfactored normal loads• use characteristic dead load resistance

check a4 –bearing

capacity

• allowable stress design (ASD) check• allowable bearing capacity per

geotechnical report• assume DNV RISO soil stress

distribution• applied normal operating, normal

extreme, abnormal extreme, and seismic less than allowable

riso soil distribution

stiffness

• use formulas from Arya• assume rigid foundation on elastic medium• reduce small strain modulus considering soil strain using

Fahey

shear modulus ratio:

• stiffness must be greater than minimum specified

rotational stiffness:

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check b1 –anchor bolt

• use AISC 360• ultimate limit state (ULS) design check• factored loads and material resistance

factors• select pre-tension based on maximum

fatigue load and increase 20 to 30% to account for losses

check b2-grout

• select grout thickness and strength• use ACI 318• ULS check

check b3 –embedment

plate

• select embedment plate thickness• use AISC 360• ULS check• nut to nut bending• nut to edge bending• rupture of washer through plate

check b4 –foundation connection

• use ACI 318• select embedment ring

height above foundation

• ULS checks• tower flange pedestal

bearing• vertical reinforcing

moment strength

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check d –footing

bottom and top

reinforcement

• use ACI 318• ULS check• one-way footing bending• top reinforcement upwind bending• bottom reinforcement downwind

bending• two way moment/shear transfer

between the pedestal and footing

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check c –footing

concrete shear

strength

• use ACI 318• ULS design check• one-way shear• select coarse aggregate size• above code: use Concrete International

technical reference to account for size effects

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check e –fatigue limit

state (FLS)

• use DNV OS C502• footing concrete compression in

bending• footing concrete one-way shear• pedestal concrete bearing• footing top and bottom reinforcing in

tension• pedestal/footing vertical reinforcing

typical constructiom

sequence

site preparation

48

excavation

49

mud mat

50

lay down bearing bars

51

place remaining

bottom bars

52

set anchor bolt cage

53

place top mat rebar

54

set forms

55

place footing

concrete

56

finish top of footing

57

set conduit and

grounding

58

set pedestal form & place

concrete

59

remove pedestal

forms

60

backfill and compact soil

over foundation

61

finish grade site

62

extend conduit into transformer

pad

63

set bottom tower and

grout

64

tension anchor bolts

65

erect remaining

turbine parts

66

commission wind farm

67

questions?