Resume

2003: circumferential cracks for DIBt foundations, vertical movements:

2005: vertical movement of embedment

2010: insufficient back anchoring of embedment

2010: weak circumferential reinforcement

2010: bad distribution of main reinforcement

The new Vestas design

The new Vestas design

Post tension connection between tower and foundation* no movements between steel and concrete* improved life time for steel part* full integration of steel parts (anchor cage)

Circular / Octogonal Shape & Radial / Concentric reinforcement* optimal shape* optimized reinforcement, no overlap between orthogonal and radial/concentric mesh

Base flange below main reinforcement* no additional reinforcement required* no risk of forming initial cracking at post tension

Grouted joint between tower and foundation* grouting in one work sequence* optimized installation of grouting using hydrostatic pressure in the wet grout* optimized multiaxial stress situation at flange edges* optimized contact pressure between flange and grout having no stress peaks

Cabel guidance below foundation and through inner ø 1 meter* site specific cable layout, PVC guidance NOT included in the Vestas delivery* PVC guidance below foundation plate, no short anchors with lower elasticity* ø 1 m free space at foundation centre for PVC guidance

Restricted inclination of upper suface* material saving versus concrete quality and shear reinforcement

Overall shear reinforcement* stable and robust foundation not sensitive to poor concrete quality

Soil distribution, bending in plates.

Examples based on V112 119m IEC IIa foundation design approved by DNV

1: Plastic distribution DNV

For the Abnormal Load Case:Total normal force Vd = 20827 KNBending at foundation base Md = 125049 KNmEccentricity of normal force e = 6.00 mFoundation radius R = 9.55 m

Effective length L' = 12.39 mEffective width B' = 5.92 mSoil pressure σ = 284 KN/m²

Bending moment, 1 m strip along centre axis M = 5683 KNm

Load Concentration Factor LCF:

Based on geometrical considerations the variation of the plate stiffness is calculated by dividingthe compressed area in a number of strips:

Load Concentration Factor LCF: LCF = 1.21

Bending moment peak M * LCF = 6876 KNm

2: Plastic distribution elliptical compressed area

A simple comparison using the elliptical shape from the DNV model

Elliptical main axis Le = 12.39 mElliptical main axis Be = 5.92 mSoil pressure σ = 284 KN/m²

Bending moment, 1 m strip along centre axis M = 6810 KNm

3: Elastic distribution

Foundation radius R = 9.55 mEffective width R = 8.58 mSoil pressure at edge σ = 398 KN/m²Soil pressure at pedestal σ = 77 KN/m²Bending moment, 1 m strip along centre axis M = 6988 KNm

Model M Index1 Plastic rectangular incl LCF 6876 1002 Plastic elliptical 6810 993 Elastic 6988 102

KNm

Punching

From the V112 3MW 119m IEC IIa:

Normal force at tower base VEd = 5595 KN

Bending at tower base MEd = 122040 KNm

Eccentricity of normal force e = 21.81 mTower mean diameter D = 3.90 mInclination of punching cone 1:1.5 θ = 33.69 deg.Perimeter Du = 12.85 m

1: EN 1992-1-1, chapter 6.4

The section in valid for "…solid slabs, waffle slabs with solid areas over columns, and foundations"

For normal force and bending the max shear force per meter is calculated as:

vEd = β * VEd / (ui * d)

β is a factor to cover for eccentric loading β = 1 + 0.6 * π * e / Du

For the example above β = 4.20

Inclination of punching cone 1:2 θ = 26.60 deg.Perimeter Du = 15.39 mFor the example above β = 3.67

2: Plastic distribution

Line load from normal force at Du: pN = VEd / (π * Du)

Line load from bending at Du: pM = MEd / Du²

Equalizing these formula with EN 1992-1-1:VEd / (π * Du) + MEd / Du² = β * VEd / (π * Du)

β-factor β = 1 + π * e / Du

For the example above β = 6.33

3: Elastic distribution

Line load from normal force at Du: pN = VEd / (π * Du)

Line load from bending at Du: pM = 4 * MEd / (π Du²)

Equalizing these formula with EN 1992-1-1:VEd / (π * Du) + 4 * MEd / (π * Du²) = β * VEd / (π * Du)

β-factor β = 1 + 4 * e / Du

For the example above β = 7.79

4: Prof Nölting suggestion

Prof Nölting checked a large number of model test and compared the β-factor against the ratiobetween load eccentricity and load diameter - in our case e/D.

Nölting's suggestion β = 1 + e / D

For the example above β = 6.59

5: Vestas method

Due to the extreme eccentricities one sees for wind foundations Vestas is using an alternative method based on the strut model.

If the foundation is loaded by a normal force alone the foundation part inside the anchor cageplays an insignificant role - it is assumed that all forces is taken by outer parts:

At section 1 - at the tower diameter: pN = VEd / (π * D)

At section 2 - at the perimeter: pN = VEd / (π * D) * (D / Du) = VEd / (π * Du)

If the foundation is loaded by bending alone the foundation part inside the anchor cageplays an important role as a diagonal compression strut is formed - it is assumed that half the forces are taken by outer parts:

At section 1 - at the tower diameter: pM = 4 * MEd / (π * D²)

At section 2 - at the perimeter:pM = 0.5 * 4 * MEd / (π * D²) * (D / Du) = 2 * MEd / (π * D * Du)

Equalizing these formula with EN 1992-1-1:VEd / (π * Du) + 2 * MEd / (π * Du²) = β * VEd / (π * Du)

Vestas' suggestion β = 1 + 2 * e / D

For the example above β = 12.19

Model β Index1 EN 1992-1-1, chapter 6.4 4.20 342 Plastic distribution 6.33 523 Elastic distribution 7.79 644 Nölting's method 6.59 543 Vestas' method 12.19 100

Shear

At pedestal:

EN 1992-1-1 (6.9) VRd,max = αcw * bw * z * v1 * fcd / (cot(θ) + tg(θ))

As all. shear, trust model vRd,max = v1 * fcd * sin(θ) * cos(θ)

Approximate value: vRd,max = fcd / 4

Away from the pedestal:

EN 1992-1-1 (6.8) VRd,s = (Asw / s) * z * fwyd * cot(θ)

As all. shear, trust model vRd,s = 1.2 * ρw * fwyd

Tower - Grout - Concrete connection

Identified behaveour:* stress distribution differs from uniform* combination of anchor forces and tower reaction downward most significan* flange thickness to width plays significant role

Traditionel design: σ = ΣP / A < σall

Stress concentration σ = SCF * ΣP / A < σall

Confinement σ = SCF * ΣP / A < Cf * σall

Concrete Grout

Low cycles: Cf = 1.73 Cf = 1.3 (75% of concrete value)

High cycles: Cf = 1.15 Cf = 1.15