RFS-CT HISTWIN High-Strength Steel Tower for Wind Turbine

Transcription

1 RFS-CT HISTWIN High-Strength Steel Tower for Wind Turbine WP3.1 EVALUATION OF SHELL THICKNESSES BACKGROUND DOCUMENT Contractors AUTH, FCTUC Authors C. Baniotopoulos, I. Lavasas, G. Nikolaides, P. Zervas Last modified 13/04/2009 Reviewed by Date dd/mm/yyyy

2 TABLE OF CONTENTS 1. WORK PACKAGE DESCRIPTION BACKGROUND DOCUMENT GENERAL ANCHORING SYSTEM CONFIGURATION F.E. STRUCTURAL MODEL VERIFICATIONS REFERENCES... 8 WP3.1, Background document

3 1. WORK PACKAGE DESCRIPTION WP leader: AUTH Contractors: FCTUC Task: Review of a current design procedure for preloaded anchors. Deliverables: Background document Starts: 01/07/2006 Ends: 30/09/2006 WP3.1, Background document 1/8

4 2. BACKGROUND DOCUMENT 2.1. General Anchoring of the wind turbine steel tower to the R.C. pedestal by means of preloaded anchors is one of the two most commonly configurations used, the other being the embedment of the lowest course of the shell to the foundation. Although no special benefit is gained concerning the plastic limit state (LS1), preloading to the anchors is mandatory for the fatigue limit state (LS4), the safety requirements of which otherwise are not met by conventional anchoring. Anchors are partially prestressed, at a level of about 50%, which is a good compromise, satisfying the verifications of the anchors against fatigue in one hand and keeping the R.C. foundation local strain within the acceptable margins, on the other. Preloaded anchors are not explicitly covered by the Eurocodes. The combination though of the relevant Standards, such as: [EC 3-1-8], [EC 3-1-9], [EC 2-1-1], [ETAG 001] etc. can provide an acceptable degree of computational competence. It is worth mentioning in more detail the specific clauses of [EC 3-1-8] determining the design of the anchoring components for the plastic limit state, which are: : Basis of design : Connections made with bolts, rivets or pins Bolts, nuts and washers : Connections made with bolts, rivets or pins Anchor bolts : Connections made with bolts, rivets or pins Categories of bolted connections : Connections made with bolts, rivets or pins Positioning of holes for bolts and rivets : Connections made with bolts, rivets or pins Design resistance of individual fasteners : Connections made with bolts, rivets or pins Slip-resistant connections using 8.8 or 10.9 bolts : Structural joints Design Resistance Shear forces : Structural joints Design Resistance Equivalent T-stub in compression : Structural joints Design Resistance Design Resistance of basic components ( ) : Structural joints Design Resistance Design resistance of column bases with base plates 2.2. Anchoring system configuration The anchoring system consists of the set of partially presressed anchors at a circumferential arrangement, connecting the base plate of the tower to the circular washer plate WP3.1, Background document 2/8

5 Figure 2.1-2: In site arrangement of anchors Figure 2.1-1: Detail to anchors embedded in the R.C. pedestal. The uplift forces are counterbalanced by the resistance of the embedded plate, while the shear force is delivered by the friction between the base plate and the (preferably nonshrink) grout, at the top of the pedestal. The PE covering of the anchor body establishes the absence of any cohesion between the anchors and the concrete, allowing thus the preloading, as soon as the R.C. elements develop their full capacity. Inevitably, the complicity of the configuration results in an equally complicated stress state and therefore a finite element model is required for the competent discretization of the foundation and the anchoring system. Furthermore, it is highly recommended that this model should be build together with the one of the tower, in order that the 2 nd order effects of the later and the contribution of the soil - structure interaction are properly taken into account F.E. structural model The proposed structural design is based on non linear analysis [GMNA], due to the presence of non-linear elements (unilateral contact and cable type elements), as described in the subsequent paragraphs. The foundation, including the non-shrink mortar to the top of pedestal, is modeled by means of brick elements. The footing is supported to the ground via unilateral contact elastic springs, having a constant per area unit equal to the soil subgrade reaction modulus. Corresponding horizontal springs are necessary for the global equilibrium of the structure. The base plate is rigidly attached to the shell of the tower and it is connected to the nonshrink mortar elements thru unilateral contacts with friction links. The embedded in the interior of the pedestal washer plate is modeled using plate elements. The washer plate is connected to the superjacent concrete elements thru unilateral contact conditions. WP3.1, Background document 3/8

6 The partially prestessed anchors are linear elements of cable type, active in tension only. Each anchor is represented by a single element, attached solely to the two steel flanges, excluding any connection to the concrete and the non-shrink mortar. Figure 2.3-1: Foundation F.E. model Figure 2.3-2: Cross section to foundation F.E. model The loads are applied to the model as follows: At the 1 st iteration step, only the self weight of the tower is considered. The nominal preload forces are applied to the anchors at next iterations, step by step. Finally, the loads of the combination are implemented gradually to the model. WP3.1, Background document 4/8

7 It is noted though that the procedure presented above can not be performed to the seismic combinations, since spectral response analysis is incompatible with non-linearities. Therefore, all unilateral contact links have to be replaced with rigid ones, preventing in this way the investigation of the anchor forces. A proposal to overcome this problem is demonstrated as follows: Solve the spectral response case, the tower the rigidly connected to the foundation. Determine the forces acting on the pedestal, using the SRSS combination. Construct a new model of the foundation only, incorporating all non-linearities. Apply the reactions of the tower to the foundation model as static loading and execute the iterative analysis As an alternative, in the rare case of the earthquake being crucial for the design of the tower, the additional representation of the seismic action in terms of acceleration timehistories may be adopted and the spectral response can be carried out. Regarding the direct estimation of the anchor forces by hand calculation, such a task is practically not feasible, since the position of the neutral line of the anchoring system can be determined only with the introduction of the elasticity conditions, which requires an iterative calculation procedure. A less accurate assesment of the anchor forces can be achieved with the aid of a simpler Finite Element model, including only the bottom flanges (with unilateral elastic support) and the anchors (see Figure [F-2.3-3]). Figure 2.3-3: Alternative F.E. model for the estimation of anchor forces 2.4. Verifications The design of the foundation anchoring system covers the verification of the following components (see Figure [F-2.4-1]): WP3.1, Background document 5/8

8 Washer and base plates (Von Misses stresses) Prestressed anchors (tensile forces) Non-shrink mortar (compressive and shear stresses) Concrete (compressive, shear and punching shear stresses) Reinforcement bars Rigid body equilibrium for the structure. Figure 2.4-1: Stress diagram The required reinforcement is calculated by the integration of the tensile stresses of the brick elements at the check points as marked in Figure [F-2.4-2]: [1], [2] : Footing, bottom mesh [3], [4] : Footing, top mesh Figure 2.4-2: Check points WP3.1, Background document 6/8

9 [5] [6] [7] : Top of the pedestal mesh : Pedestal vertical rebar, for the transfer of the tensile anchor forces : Circumferential reinforcement, controlling the split-up forces due to the anchor preloading stress due to Poisson ratio.. WP3.1, Background document 7/8

10 2.5. References [1] EC 2-1-1: Design of concrete structures General rules and rules for buildings, 2004 [2] EN : Design of steel structures General rules and rules for buildings, 2005 [3] EN : 2006: Design of steel structures Towers, masts and chimneys Towers and masts, 2006 [4] EC 3-1-8: Design of steel structures Design of joints, 2005 [5] EC 3-1-9: Design of steel structures Fatigue, 2005 [6] EN : Design of structures for earthquake resistance General rules, seismic actions and rules for buildings, 2004 [7] EN : Design of structures for earthquake resistance Towers, masts and chimneys, 2005 [8] ETAG 001: Metal anchors for use in concrete, 1997 [9] GL Wind 2003 IV Part1: Guideline for the Certification of Wind Turbines, 2004 [10] ACI : Code Requirements for Nuclear Related Concrete Structures Appendix B: Anchoring to Concrete, 2001 WP3.1, Background document 8/8