ick Foundation Analysis and Design

Transcription

1 ick Foundation Analysis and Design Work: ick Foundation Location: Description: Prop: Detail analysis and design of ick patented foundation for Wind Turbine Towers Gestamp Hybrid Towers Date: 31/10/2012 File: Author: Jorge Franco Rey Ingeniero de Caminos, Canales y Puertos Colegiado:

2 1. ABSTRACT 4 2. SOFTWARE USED 7 3. AIMS AND DIFFERENT LEVEL ANALYSIS LEVEL 1 ANALYSIS LEVEL 2 ANALYSIS LEVEL 3 ANALYSIS DIFFERENT ANALYSIS LEVELS RESULT VERIFICATION VERIFICATION A OF THE LEVEL 1 ANALYSIS MODEL COMPARISON CHART APPLICATION TO OCTAGONAL SHAPE FOUNDATIONS VERIFICATION B OF THE LEVEL 1 ANALYSIS MODEL COMPARISON CHART VERIFICATION C VERIFICATION D COMPARISON CHART WIND TURBINE TOWER CONSIDERED DESIGN CODE GEOTECHNICAL DESIGN SKIN FRICTION ADHESION SLOPE STABILITY DURING EXCAVATION STRUCTURAL DESIGN CONCRETE COVER INTERNAL FORCES BEAMS CENTRAL RING AND TOP SLAB REFERENCES 34 M-2

3 10. ANNEX 1: WIND TURBINE LOADS ANNEX 2: LEVEL 1 CALCULATIONS ANNEX 3: LEVEL 2 CALCULATIONS TOP SLAB DESIGN GENERAL MODEL DATA SLAB FLEXURAL DESIGN SLAB SHEAR DESIGN SLAB SUSPENSION REINFORCEMENT CENTRAL RING DESIGN OUTER HORIZONTAL REINFORCEMENT OUTER VERTICAL REINFORCEMENT INNER HORIZONTAL REINFORCEMENT INNER VERTICAL REINFORCEMENT ANNEX 4: LEVEL 3 CALCULATIONS INTRODUCTION MODEL 03AAB: EXTREME LOADS (NORMAL SITUATION) MATERIALS LOADS CONSTRUCTION STAGES FINITE ELEMENT MESH RESULTS MODEL 03AAB: OVERTURNING ANALYSIS MATERIALS LOADS CONSTRUCTION STAGES FINITE ELEMENT MESH RESULTS FIGURES INDEX 23 M-3

4 1. Abstract The ick foundation presents four principal elements: a) A top slab whose function is to achieve a more uniform distribution of pressure on the ground b) A central reinforced ring c) Several radial reinforced beams or stiffening ribs that forms a composite section with the top slab. d) Pedestal Fig. 1: Geometry of the ick foundation M-4

5 Fig. 2: Bottom view of the foundation Why this design? The design of the ick foundation is different from other existing foundations in which the ribs are arranged on the top of the slab. Our goal is developing a better structural performance with a T section that provides more efficiency and ductility. This is because the bending stresses to which it is subjected in the foundation act precisely the bottom-up direction, so that the traction is placed on the bottom and the compressions in the top. Fig. 3: Beam bending moment M-5

6 With the configuration of the ick foundation you have a T-section, highly efficient, which compressions are distributed across the top of the T head, getting a higher inner lever arm and a significant improvement in the ductility by reducing the relative depth of the neutral axis. In the pictures below compares the flexural strength between the ick foundation and a foundation with stiffening ribs placed in top position: Fig. 4: ick foundation. Ultimate Bending Moment = KNm, x/d=0,12 Fig. 5: Conventional ribbed foundation. Ultimate Bending Moment = KNm, x/d = 0,38 As can be seen, with the ick foundation design, at equal volume amount of concrete reinforcement, gives a flexural strength 10% higher and a depth relative neutral fiber considerably lower (0.12 versus 0.38), which allows a higher ductility, a higher resistant reserve and an improved fatigue behavior for the concrete. The performance of the ick foundation has been tested with an advanced FEM geothecnical calculation with Midas GTS software that takes into account the construction stages, the different properties of the soil and the interaction between the soil and the foundation. M-6

7 2. Software used - Propietary software developed under licensed Microsoft Excel v Midas Gen v 2012, by Midas Information Technology, license nº U Midas GTS v 2012 by Midas Information Technology, license nº U Fagus 6 by Cubus AG, license nº BN M-7

8 3. Aims and different level analysis The main objective in this document is to study and analyze the patented ick Foundation, presenting all the results of calculation and design, both from a geotechnical and structural point of view. Three levels of complexity are used in the study of the ick foundation: 3.1. Level 1 analysis Classic treatment. We have implemented an application to obtain the soil pressure distribution between a circular foundation and further obtain the overturning a sliding security factor and the effective uniform soil pressure in order to calculate the beam (ribs) internal forces. Fig. 6: Soil pressures on a circular foundation. See [1] Fig. 7: Graph to calculate circular foundations. See [1] The following implicit expression is resolved numerically through the Microsoft Excel Solver add-in. With the solution, we have the neutral axis depth and we can obtain the soil pressure distribution under the foundation. We know that the ick foundation is not a perfect circular slab with a smooth surface in contact with the soil, as other common foundations for wind turbines, but at this level study we think is sufficiently approximate. With the analysis of levels 2 and 3 we will check the veracity of this statement. M-8

9 With the preceding formulation we can obtain the maximum soil pressure, but for use in bearing capacity analysis and beams internal forces an effective foundation area is used. For a circular or octagonal foundation, an elliptical effective foundation area (Aeff) can be defined as [3]: With major axes Fig. 8: Effective foundation The effective foundation area Aeff can be represented by a rectangle with the following dimensions: Our Excel based application obtains the full graphical display of the elliptical and square effective area, as can be seen in the next figure: M-9

10 Fig. 9: Elliptic and rectangular effective areas M-10

11 3.2. Level 2 analysis We have developed a finite element model with Midas/Gen 2012, based on plate elements. This type of elements has a more simplex treatment and easy post processing instead of solid elements. Fig. 10: Plate-based FEM model The soil is represented by only-compression non-linear springs by a coefficient of subgrade reaction. The loads supplied by the wind turbine manufacturer are transferred to the foundation by a rigid link from a central node in which the loads are applied, as we can see in the next figure: Fig. 11: Detail of load transfer from the tower M-11

12 3.3. Level 3 analysis In this level, we have developed an advanced 3D geotechnical finite element model with the Midas GTS program. In this model we define the real 3D geometry of the soil and the different construction stages (excavation, concrete pouring, Fig. 12: Advanced finite element model M-12

13 4. Different analysis levels result verification 4.1. Verification A of the Level 1 analysis model This analysis model is based in the elastic theory of a circular foundation over an elastic bed. The full mathematical development can be found on reference [1]. To check the results of this model a plate-based FEM analysis model was performed. Here are the results 1 : Fig. 13: Results with the Excel application we have developed for Level 1 analysis 1 The loads applied are fictitious, only used to check the results of the two models. M-13

14 Fig. 14: Circular plate resting on elastic bed with the same modulus of subgrade reaction Fig. 15: Vertical displacements of the foundation M-14

15 Fig. 16: Detail of the vertical displacements Fig. 17: Neutral axis determination in the FEM model M-15

16 Fig. 18: Spring reactions that show us the soil pressure distribution Comparison chart Max. soil pressure (KN/m2) Max. Vertical displacement (mm) Neutral axis distance (m) Excel Level 1 model 352,98 35,30 5,80 Midas Gen FEM platebased model 358,32 35,83 6,00 % Dif. 1,51% 1,50% 3,45 % M-16

17 Application to octagonal shape foundations In order to simplify the rebar of the top slab, we used a circumscribed octagonal shape. We think the differences are small and the Level 1 application to determine soil pressures is sufficiently approximate. Fig. 19: Circumscribed octagonal shape top slab M-17

18 4.2. Verification B of the Level 1 analysis model In order to check the beam internal forces obtained with the level 1 analysis model, we have develop a simple model based on beams resting on an multilinear point spring support. Fig. 20: Simple bar finite element model M-18

19 Fig. 21: Loads applied to the model We have limited the maximum soil pressure on the beam end using a multi-linear spring with this hysteresis curve: Fig. 22: Multi-linear point spring support Fig. 23: Multi-linear point spring support definition The limitation of 350 KN respond to a maxium soil pressure equal to 350 KN / (1,20m 0,588m) = 496 KN/m2 0,588 m is the separation between nodes of the model and 1,20 m is the beam width. M-19

20 We set the self weight of the beams to obtain the full weight of beams, central ring and top slab. Fig. 24: Self weight point spring reaction. The Fz value includes the self-weight of the foundation and backfill. Fig. 25: Point spring reactions. Load case 1 M-20

21 Fig. 26: Bending moments load case Comparison chart Factored bottom bending moment Load Case 1 (KNm) Factored top bending moment Load Case 1 (KNm) Excel Level 1 model Midas Gen bar model ,50 = ,5 1,50 = % Dif. 6,8% 9,7% The bar model is a simplification in which all the forces are taken by the bars. In reality, the top slab takes a part of the forces, downloading the beams. M-21

22 4.3. Verification C In this point we compare the results of internal forces obtained with the level 2 model. Fig. 27: Level 2 analysis FEM model Fig. 28: Beam and top slab internal forces obtained with Level 2 analysis model The total maximum beam bending moment (web + top slab) is: M = 4145,54 (beam web) ,34 (top slab) = 5951,88 KNm And the design moment will be 5951,88 1,50 = 8927 KNm The bending moment is considerably lower than the obtained with Level 1 manual calculations. M-22

23 4.4. Verification D In this point we compare the results of internal forces and maximum soil pressure obtained with the level 3 model, made with Midas GTS software. Fig. 29: Level 3 FEM model Fig. 30: Soil pressure distribution, Load Case 1 (Resulting Extreme Loads, Normal situation). The maximum compression is 270 KN/m2. M-23

24 Fig. 31: Fig. 32: Vertical soil pressure under beam M-24

25 Fig. 33: Vertical soil pressure under top slab. Longitudinal cut M-25

26 Fig. 34: Vertical soil pressure under top slab. Transversal cut Fig. 35: Beam forces integration. The unfactored bending moment is My = 3610 KNm 2 = 7220 KNm M-26

27 Comparison chart Factored bottom bending moment Load Case 1 (KNm) Maximum soil pressure Load Case 1 (KN/m2) Excel Level 1 model ,30 Midas Gen bar model ,50 = Not obtained Midas Gen plate model (Level 2) Midas GTS 3D FEM model (Level 3) 5951,88 1,50 = 8927,82 162, ,50 = Peak beam: 262 KN/m2 (*1) Peak slab: 160 KN/m2 (*2) (*1) Peak soil pressure under beam. In the Fig. 48 can be seen that this is a peak stress and that rapidly decreases to a value of aprox. 160 KN/m2 (0,16 MPa). Under the slab (*2) the peak stress is aprox. 160 KN/m2. We can see that the results are similar. The forces obtained with the model of level 1 are the largest of comparison and will be used for beam design in this document. Therefore, the design of the ick foundation with the downward ribs is functioning properly. Compared to a conventional foundation with the flat base it can be seen that there is a little stress concentration under the beam end that, in any case, has an acceptable value. Can also be seen the collaboration of the top slab in the distribution of loads over the soil. M-27

28 5. Wind Turbine Tower considered All the calculations have been developed with the following Wind Turbine: VESTAS V MW Mk7.5 HH 80m IECS T-flange The foundation loads can be found in Annex 1. The foundation is designed with a steel tower, but is valid for a full concrete or a hybrid tower. Both full concrete and hybrid tower has the advantage of a major self-weight, which is a favourable effect for the foundation. 6. Design code In this document the Eurocodes are used. own national appendix to the Eurocodes wich specifies values that should be used to fulfill the demands. In this document the recommended values are used. Also, in concordance with DIN 1054, Limit states of failure in the ground and soil pressure distribution are verified using Design Approach 2 in which the entire calculation is performed using characteristic values and the partial factors are introduced at the end, when the ultimate limit state condition is checked. The global ULS factor used to obtain the factored loads for the structural concrete design is 1,50. M-28

29 7. Geotechnical design The geotechnical design has to consider several possible types of failure. The ones that are verified in this memory are: Settlements calculation : soil pressure verification. : overturning and sliding security factor. The ick foundation allows selectively taking into consideration the skin friction resistance and adhesion between the soil and the radial beams and the soil. This has been implemented as described below Skin friction Fig. 36: Soil pressure over the radial beams and central ring The unit frictional resistance is: Koh is the at rest soil pressure coefficient: o is the effective medium soil pressure over the lateral surface of beams and central ring: M-29

30 And Fig. 37: Geometrical definition of the ick foundation 7.2. Adhesion The unit adhesion between soil and concrete is: cu is the undrained shear strength. M-30

31 7.3. Slope stability during excavation An important aspect is the stability of the slopes during excavation. In the advanced model 3D finite element the excavation has been considered as a construction stage, so the computation show us when it is necessary to excavate with a certain slope and / or arrange a shoring or formwork. Moreover, the mounting procedure of the ick foundation is based in pre-assembling the reinforcement of beams and central ring for later introduction into previously excavated trenches. This procedure has the advantage of allowing the reinforcement assembling of several foundations before starting the excavation. Where required by ground conditions, on the preassembled reinforcement should be attached a formwork (like a geomesh, a wire mesh or pecafil system) that will prevent spillage during concreting. The geomesh or wire mesh has the advantage of allowing the passage of concrete so that it is in contact with the ground and can be counted on the frictional resistance and adhesion aforementioned. Fig. 38: Example of pecafil formwork Fig. 39: Example of geomesh It is normal that the slopes are not perfectly vertical. In this situation the concrete volume will be greater and hence the weight of the foundation as well, which represents a favorable effect for the foundation. Also, in this case, the concrete surface will be greater and therefore the soil pressures will be smaller. M-31

32 8. Structural design The reinforced concrete analysis is made with the Eurocode Concrete cover Exposure class: XC2 Structural class: S4 Cover of concrete poured directly over the soil: 75 mm Cover of concrete poured over a regularization concrete bed: 40 mm 8.2. Internal forces The internal forces for the design of the ick foundation are obtained as described below: Beams We use the Level 1 analysis. With the effective soil pressure distribution we load the bottom of each beam in her influence trapezoidal area: Fig. 40 The beams forces are calculated as a T-section cantilever fixed to the central ring, subjected to a variable linear load, depending on the bandwidth that corresponds to each point of its length: M-32

33 Fig. 41: Loading scheme of a radial beam The design of the beams are made with Midas/Gen design capabilities Central ring and top slab We use the Level 2 analysis. With the plate finite element model we obtain the internal forces of both components. The design of the central ring and top slab are made with Midas/Gen design capabilities. En Madrid (Spain), 09 de Noviembre de 2012 Fdo.: Jorge Franco Rey Ingeniero de Caminos, Canales y Puertos, Col Director Técnico / Technical Manager M-33

34 9. References [1] Cálculo de cimentaciones circulares. Antonio Angulo, Ingeniero de Caminos, Canales y Puertos. Revista de Obras Públicas, Junio de [2] Germanischer Lloyd (GL), Guideline for the Certification of Wind Turbines, Edition 2010 [3] Det Norske Veritas (DNV), Design of offshore wind turbine structures, Offshore standard DNV-OS-J101, September 2011 [4] Det Norske Veritas (DNV), Offshore concrete structures, Offshore standard DNV-OS-C502, October 2010 [5] H.Svensson, LUND University, Design of foundations for wind turbin [6] Guía de cimentaciones en obras de carretera. Gobierno de España, Ministerio de Fomento, Dirección Gral. de carreteras, 2009 [7] DIN-1054: [8] European Committee for Standardization (ECS), Eurocode 7 Geotechnical design [9] European Committee for Standardization (ECS), Eurocode 2 Design of Concrete Structures Part 1.1: General rules and rules for buildings M-34