Patent Publication Number: US-2011061321-A1

Title: Fatigue reistant foundation system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for building fatigue resistant foundations for supporting columns, towers and structures under heavy cyclical loads such as wind turbines towers for onshore and offshore installations. 
     The proposed foundation system will be used specifically for the Multi-Megawatt class wind turbines. Several wind turbine manufacturers have successfully developed large wind turbines with rated power ranging from 1.5 to 10 MW. Several wind turbine manufacturers are planning mass production of large multi Megawatt turbines for onshore and offshore installations. The installation of the E-126 model turbine by Enercon with a 7 MW rated power required a xx meter diameter circular foundation with 1,400 cubic meters of concrete and xx tons of rebar. The installation of the 5M turbine by RePower with a 5 MW rated power required a 23 meter diameter circular foundation with 1,300 cubic meters of concrete and 180 tons of rebar. The task of building such large foundations is monumental and requires a great deal of construction planning and logistics. The proposed foundation designs and their associated construction methods provide cost-effective solutions for such challenging foundation projects. 
     Several wind turbine foundations, that have been constructed in the last 10 years in the US and Europe, have structural problems stemming from thermal cracking during construction or from fatigue cracking and required repairs. The present invention improves the geometry of the foundation in order to enhance dissipation conditions for the due to the typical temperature rise after casting and also provides a cost effective fatigue resistant design. 
     2. Description of the Related Art 
     Conventional gravity style foundations for large wind turbine usually comprise a large, thick, horizontal, heavily reinforced cast in situ concrete base; and a vertical cast in situ cylindrical pedestal that is installed over the base. There are several problems that are typically encountered during the construction of such foundations. 
     Fatigue resistant of such conventional footings is achieved by over sizing the structural concrete elements and the reinforcing elements such that the resulting stress amplitudes are small enough for the structural elements to bass fatigue design checks. 
     The main problem is the monumental task of managing large continuous concrete pours, which require sophisticated planning and coordination in order to pour more than four hundred cubic yards of concrete per footing, on average, in one continuous pour, without having any cold joints within the pour. 
     Another problem is logistics coordinating with multiple local batch plants the delivery plan of the large number of concrete trucks to the job site in a timely and organized manner. 
     A further problem is the complexity of installing the rebar assembly into the foundation which requires assembling two layers of steel reinforcing meshes that are two to six feet apart across the full area of the foundation, while maintaining strict geometric layout and specific spacing. This rebar assembly is made of extremely long and heavy rebar which requires the use of a crane in addition to multiple workers to install all components of the assembly. The rebar often exceeds forty feet in length, thus requiring special oversized shipments which are very expensive and usually require special permits. That labor intensive and time consuming task requires large number of well trained rebar placing workers. 
     Another important problem is the fact that majority of the construction process consist of field work which could be easily compromised by weather conditions and other site conditions. 
     Another problem is thermal cracking of concrete due to overheating of the concrete mass. When concrete is cast in massive sections for wind tower foundations, temperature can reach high levels and the risk of thermal cracking becomes very likely. Thermal cracking often compromises the structural integrity of the foundations as reported in many projects in Europe and North America. 
     Multi-cell caissons used in offshore installations always lack multi-axial post-tensioning elements and their fatigue resistant rely completely on heavily reinforced oversized concrete elements which involves expensive and labor intensive construction. 
     BRIEF SUMMARY OF THE INVENTION 
     It is desired to have cost-effective foundation system that can reduce construction materials and labor for large wind turbines. The wind turbine foundations can then be built to the standards of the Fatigue Resistant Foundation System which uses concrete rib stiffeners, with a cast in place slab on grade element and a central pedestal to build an integral foundation that will behave structurally as a monolithic foundation structure. Other concrete components can be included such as secondary and perimeter beams, diaphragms, or intermediate stiffeners and rib stiffened or flat slab sections. The foundation system relies on the use of many prefabricated components including rebar meshes and cages, pedestal cage assembly, pre cut post-tensioning strands, preassembled strand bundles, pre-cut post-tensioning duct sections and prefabricated concrete forms. 
     The present invention pertains to a fatigue resistant foundation for wind towers which comprises a plurality of components, namely a central vertical pedestal, a substantially horizontal continuous bottom support slab with stiffened perimeter, a plurality of radial reinforcing ribs extending radially outwardly from the pedestal and a three-dimensional network of vertical, horizontal, diagonal, radial and circumferential post-tensioning elements embedded in the footing that keeps all the structural elements under heavy multi-axial post compression, reduces stress amplitudes and deflections and allows the foundation to have a desirable combination of high stiffness and superior fatigue resistant while improving heat dissipation conditions during construction by having a small ratio of concrete mass to surface area thus eliminating the risk of thermal cracking due to heat of hydration. 
     Although the application is written for a wind turbine tower as the column being supported by the foundation, any tower or column can be used on the foundation including but not limited to, antennas, chimneys, stacks, distillation columns, water towers, electric power lines, bridges, buildings, or any other structure using a column. 
     In one embodiment, the present invention pertains to a wind turbine foundation having a plurality of components, namely a central vertical pedestal, a substantially horizontal bottom support slab, and a plurality of radial reinforcing ribs extending radially outwardly from the pedestal. The ribs are prefabricated and transported to job site, but the pedestal and support slab are poured in situ at the site out of concrete. The prefabricated ribs are equipped with load transfer mechanisms, for shear force and bending moment, along the conjunctions with the cast in situ support slab. The prefabricated ribs are also equipped at their inner ends with load transfer mechanisms, for shear force and bending moment, along the conjunctions with the cast in situ pedestal. The ribs are arranged in a circumferentially spaced manner around the outer diameter of the pedestal cage assembly before or after slab reinforcing steel is installed. Forms are then arranged for the pedestal and support slab. The support slab is cast in situ by pouring concrete into the forms and then pedestal concrete is poured over the slab into the pedestal form. When the concrete cures the support slab is united to the prefabricated ribs and the ribs are also united to the pedestal. The final result is continuous monolithic polygonal or circular foundation wherein loads are carried across the structure vertically and laterally through the continuous structure by the doweled and spliced reinforcing steel bars which are integrally cast into the pedestal, ribs and support slab. The combination of the high stiffness of the ribs, solid pedestal and continuous slab construction across the pedestal, and through or under ribs, allows the slab to behave structurally as a continuous slab over multiple rigid supports resulting in small bending and shear stresses in the slab, reducing deflections and increasing the stiffness of the foundation, improving fatigue conditions as well as allowing for the benefits of an economical design. Support slab reinforcing steel covers the entire footprint of the foundation and extends, without interruption, across the slab area and into the pedestal to improve the structural performance of the foundation under different loading conditions. Perimeter beams or thickened slab edges around the perimeter add stiffness and strength to the foundation and provide the benefits of a two-way slab system. Circumferential post-tensioning of slab edge is used to increase the structural capacity of the ribs and the pedestal by creating eccentric post compression in the ribs and by reducing stress amplitudes in the slab, ribs and pedestal. 
     The foundation of the present invention substantially reduces the amount of concrete used in wind turbine foundation of spread footing style, simplifies the placement of rebar and concrete in the foundation, allows for labor and time savings and shortens foundation construction schedule when compared to conventional designs. 
     This invention provides the wind energy industry with a foundation system suitable for utility scale wind turbines including 1.5 MW through 10 MW and even larger, wherein the amount of cast in situ concrete work is limited, and the number of concrete trucks required for the foundation is small and manageable level and the amount of rebar used in the foundation is 60% less than conventional footings. 
     In one embodiment, the present invention relies on using prefabricated components that meet size and weight limits for standard ground freight shipping through typical roads and highways, without resorting to special permitting for oversize or overweight shipments, keeping in mind that the foundation width for large turbines can easily exceed sixty feet in width. 
     In another embodiment, the present invention uses specific combinations of precast components with cast in situ components designed to speed up construction without compromising the rigidity and structural continuity and optimization of the foundation. The combination of high strength, high stiffness prefabricated ribs, solid pedestal construction and continuous slab construction across the pedestal, and through or under ribs, allows the slab to behave structurally as a continuous slab over multiple rigid supports resulting in small bending and shear stresses in the slab, reducing deflections and increasing the stiffness of the foundation, substantially reducing fatigue as well as allowing for the benefits of rapid construction and economical design. 
     The present invention improves the geometry of the foundation in order to enhance dissipation conditions for the heat of hydration due to the typical temperature rise after casting. This design feature is achieved by reducing the thickness of the support slab and the ratio of concrete mass to surface area, thus reducing the risk of thermal cracking and protecting the structural integrity of the foundations. 
     The present invention optimizes the design support slab by configuring slab reinforcing to span between supporting ribs and allowing it to continue under or across the ribs. Each slab panel is triangular or pie-shaped and is prestressed along all three sides such that a multi-axial prestress is generated in slab panel. Slab panels with radial and perimeter post tensioning elements form a robust horizontal trussed diaphragm and as a result, the required slab thickness is optimized and the amount of cast in situ concrete is reduced. 
     The present invention reduces the maximum rebar length for field installation to roughly 7.6 meters (twenty five feet), which is significantly shorter when compared to conventional footing that may requires 15.2 to 18.3 meters (fifty to sixty foot) long reinforcing bars. 
     The present invention allows rib dowels, or post tensioning tendons, extending inwardly into the pedestal at one end, to continue without interruption between distal ends of the foundation. As a result each pair of ribs on opposite ends of the pedestal will behave structurally as one continuous beam across the width of the foundation. 
     The present invention reduces fatigue for concrete and rebar in the foundation by minimizing stress concentrations through appropriately configured connections and component geometry. The solid and deep construction of the pedestal allows for great reduction of stresses across the pedestal and at the conjunctions between the pedestal and surrounding. Dowels into the pedestal are relatively deep to reduce stresses near the surface zone of the pedestal. The solid pedestal offers generous bearing conditions for the tower base plate and improves geometry as needed to minimize fatigue. 
     The present invention employs prestressing and/or post tensioning techniques in order to maximize the performance of the foundation, or to extend its life span. Besides the vertical tensioning of anchor bolts, tensioning of horizontal and diagonal tendons along the length concrete ribs and across the pedestal besides circumferential  112  and radial  111  post tensioning strands imbedded in the slab are employed. Post-tensioning of the ribs is done in an eccentric manner to counter balance and reduce the stresses from the dead loads on the foundation. This can be accomplished by setting an eccentric post tensioning load pattern in the ribs with higher axial force at the bottom than at the top of the rib. The circumferential post tensioning load in the slab provides additional desirable eccentric prestressing of the ribs and the pedestal and helps increase rib load capacity and rib fatigue resistance. 
     OBJECTS OF THE INVENTION 
     An object of this invention is to provide the wind energy industry with a fast, reliable, yet cost-effective foundation system that is suitable for most wind energy projects, including projects using the largest utility scale turbines and tallest towers, while providing a foundation lifespan that is longer than conventional foundation systems. 
     Another object of this invention is to reduce the cost of wind energy projects by realizing savings in the areas of rebar quantity, form work, concrete trucking service, concrete pouring and finishing, logistics, man-hours and crane operations. 
     It is the object of this invention is to provide foundation system suitable for large wind turbines including utility scale turbines ranging from 1.5 MW to 10 MW and larger, wherein the amount of cast in situ concrete work is limited and the number of concrete trucks and the amount of rebar required for the foundation are reduced to a manageable level when compared to conventional gravity style foundations. 
     Another object of this invention is to improve dissipation conditions for the heat of hydration and the typical temperature rise after casting. That goal is achieved by reducing the ratio of concrete mass to surface area. When concrete is cast in massive sections for wind tower foundations, temperature can reach high levels and the risk of thermal cracking becomes very high unless cooling techniques or special admixtures are applied. Thermal cracking often compromises the structural integrity of the foundations. 
     A further object of this invention is to improve foundation structural properties due to fabrication of some structural components in a fully controlled environment of a precast concrete plant or a suitable facility at or near project site and to utilize benefit from advancement in concrete construction in areas such as concrete admixtures, special cements and fiber reinforcement. 
     Still another object of this invention is to utilize desirable features and benefits associated with mass production of precast concrete such as high reliability and uniform consistency and high compressive strength. 
     Another important object of this invention is to minimize chances for errors in bar placement, spacing and layout by providing pre-marked spacing for splicing slab rebar with existing dowels extending from ribs. 
     A further object of this invention is to use light weight, small diameter, short and easy to handle rebar for the cast in situ concrete. 
     A further important object of this invention is to provide the wind energy industry with a solution for all weather foundation construction. 
     Still another important object of this invention is to improve safety and accessibility around foundations under construction, and reduce hazardous conditions for construction crew. 
     A further significant object of this invention is to increase productivity and increase the number of footing that can be built in a given time frame using the same number of workers, when compared to conventional foundation designs built under similar conditions. 
     Another object of this invention is to employ prestressing and/or post tensioning techniques in order to maximize the performance of the foundation, improve its fatigue resistant and extend its life span. 
     Another object of this invention is to provide the wind energy industry with reliable and readily available designs, and prefabricated components, for every wind energy project wherein foundation designs are pre-approved by and coordinated with turbine manufactures and certification agencies. 
     A further object of this invention is to use standard designs to reduce engineering work and simplify the permitting process, as well as improve project construction schedule. 
     Still another object of this invention is to speed-up construction by using many prefabricated components including rebar meshes and cages, bolt cage assembly, pre cut post-tensioning strands, preassembled post-tensioning bundles, pre-cut post-tensioning duct sections and prefabricated concrete forms. 
     It is also the object of this invention is to provide wind energy developers with the ability to select pre-approved complete foundation designs for wind turbine foundation based on project and site variables including turbine model and tower height; site geotechnical characteristics; and desired foundation style such as gravity, anchored or piling. 
     Another object of this invention is to provide foundation contractors with the convenience and economy of using commercially available prefabricated components with complete assembly and detail drawings that can be delivered to any project site with short lead time. 
     A further object of this invention is to improve the quality and productivity of foundation construction due to experience gained from practicing standard construction techniques with repetitive production steps. 
     Still another object of his invention is to produce foundation designs suitable for shallow and deep offshore installations. 
     Another object of this invention is to use the modular foundation system for other tower structures such as chimneys, stacks, distillation columns and telecommunication towers. 
     Yet another object of the foundation is to improve tower base bearing resistant in concrete pedestals supporting wind towers such that it become possible to build the pedestal and the foundation with concrete having the same compressive strength without increasing the diameter of the pedestal. 
     Another object of the invention is to build wind tower foundation in one continuous concrete pour. 
     The final object of the invention is to independently produce prefabricated components for offshore foundations that can be assembled on a barge without having the critical path of completing to a first component before a second component can be constructed. 
     Other objects, advantages and novel features of the present invention will become apparent from the following description of the preferred embodiments when considered in conjunction with the accompanying drawings. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the foundation showing the rebar before pouring the concrete. 
         FIG. 2A  is a perspective view of a pedestal and ribs in a second embodiment with a pier for off shore applications. 
         FIG. 2B  is a perspective view of a pedestal and ribs. 
         FIG. 3A  is an inner perspective view of a rib showing connections to the pedestal and the slab. 
         FIG. 3B  is an outer perspective view of a rib showing connections to the pedestal and the slab. 
         FIG. 4  is a perspective view of a rib and forms for forming the pedestal and slab. 
         FIG. 5  is a perspective view of the bolt assembly and alignment apparatus. 
         FIG. 6  is a top view of the foundation prior to pouring the concrete showing the rebar and template for the anchor bolts and post tensioning elements. 
         FIG. 7  is a perspective view of a raised rib having means for raising the rib above the slab. 
         FIG. 8  is a perspective view of the foundation showing the alignment apparatus and a pedestal forming section. 
         FIG. 9  is a perspective view of the foundation showing the rebar and rebar cage. 
         FIG. 10  is a perspective view pedestal cage assembly with anchor bolt and reinforcing. 
         FIG. 11  is a perspective view of the foundation. 
         FIG. 12  is a perspective view of the rib for supporting a lattice style tower. 
         FIG. 13  is a perspective view of the foundation for offshore installation. 
         FIG. 14  is a perspective view of the foundation for offshore installation. 
         FIG. 15  is a perspective view of the foundation for offshore installation. 
         FIG. 16  is an elevation view of the foundation for offshore installation. 
         FIG. 17  is a perspective view of the foundation for offshore installation. 
         FIG. 18  is a perspective view of the foundation for offshore installation. 
         FIG. 19  is a perspective view of the foundation for offshore installation. 
         FIG. 20  is a perspective view of the foundation with rock anchors. 
         FIG. 21  is a perspective view of the foundation with rock anchors. 
         FIG. 22  is a perspective view of the foundation with rock anchors. 
         FIG. 23  is an elevation view of the foundation with rock anchors. 
         FIG. 24  is a perspective view of an offshore foundation with micro-piles. 
         FIG. 25  is an elevation view of an offshore foundation with micro-piles. 
         FIG. 26  is a perspective view of the foundation. 
         FIG. 27  is an elevation view of the foundation. 
         FIG. 28  is a perspective view of rock anchored foundation. 
         FIG. 29  is a perspective view of rock anchored foundation. 
         FIG. 30   a - 30   d  show circumferential post tensioning view of the foundation. 
         FIG. 31   a  is a plan view of the foundation. 
         FIG. 31   b  is a section view of the foundation. 
         FIG. 32   a  is a section view of the foundation. 
         FIG. 32   b  is a section view of the rib  16 . 
         FIG. 33   a  is an elevation of rib reinforcing details. 
         FIG. 33   b  is a plan view of rib reinforcing details. 
         FIG. 34   a  and  FIG. 34   b  are section views of the rib  16 . 
         FIG. 34   c  and  FIG. 34   d  are pedestal reinforcing details. 
         FIG. 35   a  and  FIG. 35   b  are section views of the pedestal. 
         FIGS. 36   a  and  36   b  are slab  20  reinforcing plans. 
         FIG. 37   a  and  FIG. 37   b  are elevation and plan views of a rib with unbounded post tensioning elements. 
         FIG. 38  FIG. is a plan view showing circumferential post tensioning in the foundation. 
         FIG. 39   a - FIG. 41   b  show details of the a foundation with prefabricated ribs. 
         FIG. 42   a - FIG. 42   d  show tendon duct arrangements in a pedestal  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention pertains to a wind turbine foundation for wind turbines. The foundation comprises a plurality of components, namely a central vertical pedestal, a substantially horizontal bottom support slab, and a plurality of radial reinforcing ribs extending radially outwardly from the pedestal. The ribs may be prefabricated and transported to job site, but the pedestal and support slab are poured in situ at the site out of concrete. Alternatively the ribs may be cast in situ. 
     The present invention pertains to a fatigue resistant foundation  100  for wind towers which comprises a plurality of components, namely a central vertical pedestal, a substantially horizontal continuous bottom support slab with a stiffened perimeter, a plurality of radial reinforcing ribs extending radially outwardly from the pedestal and a three-dimensional network  500  of vertical, horizontal, diagonal, radial and circumferential post-tensioning elements embedded in the footing that keeps all the structural elements under heavy multi-axial post compression, reduces stress amplitudes and deflections and allows the foundation  100  to have a desirable combination of high stiffness and superior fatigue resistance while improving heat dissipation conditions during construction by having a small ratio of concrete mass to surface area thus eliminating the risk of thermal cracking due to the heat of hydration. 
     A construction site is prepared by excavation and flattening and preparation of soil for the foundation  100 . The foundation  100  may be set on pilings, on piers, or have anchors (soil anchors or rock anchors  404  or micro-piles  401  or other types) in a conventional manner. The present invention ensures good contact between foundation  100  and soil, or sub-base, by casting. 
     The foundation  100  is cast against prepared soil, or crushed stone sub-base, or a mud slab or a membrane sheet in case of offshore foundations assembled on a barge or in dry docks. Known grouting and leveling techniques under precast elements can be employed for ensuring plumb installation and good soil contact. 
     In one embodiment of the invention the foundation  100  may be set on a mud slab  14  or on compacted granular fill. The mud slab  14  is often a thin plain concrete layer intended to provide a clean and level base for foundation installation. After the foundation site has been prepared, a plurality of three or more precast stiffener ribs  16  are placed on the mud slab  14  or compacted granular fill inside of the excavation pit  12 . The precast concrete stiffener ribs  16  may have means for leveling or other leveling techniques can be employed for level and plumb installation. If desired, grouting techniques can be used to ensure complete rib base contact with the mud slab or sub-base. The precast concrete stiffener ribs  16  may have bases  21  with left shear key  38  and/or shear connectors and right shear key  36  and/or shear connectors. The precast concrete stiffener ribs  16  may also have a vertical shear key  34 . The shear keys  34 ,  36  and  38  and associated dowels  40 ,  42  and  46  are to ensure continuous connections, with complete transfer of shear and bending loads, between the precast concrete rib stiffener  16  and the cast in place concrete which is to be poured into the foundation  100 . The precast concrete stiffener ribs  16  have upper dowels  40  and lower dowels  42  extending on the right and left sides of the base  21  which interconnect with and spliced to upper mesh rebar  22  and lower mesh rebar  24  installed between the ribs  16  and connected to dowels  40 ,  42  to form reinforcement for the slabs of foundation  100  when the concrete is poured. The base  21  of rib  16  and the top of rib  16  also have dowels  46  radially entering the pedestal  10  in the center of the foundation. 
     Doweling of rebar between ribs and foundation components can be achieved rebar dowels extending from the prefabricated elements or by using rebar couplers, bar extenders or any mechanical rebar splicing system. 
     Arrays of grout or epoxy filled sleeves arranged in the slab  20  could receive corresponding arrays of vertical dowels extending from the bottom of prefabricated ribs or perimeter beams  190  or other prefabricated components. 
     Shear keys can be replaced with, or combined with, corbels or shear studs, or other shear connectors such as angled rebar or embedded steel shapes. 
     In another embodiment an array of steel beams, are encased into the web of the rib and extend inwardly into the pedestal cavity at the inner most end of ribs, and serve as a suitable shear force transfer mechanism between the rib and the pedestal. 
     In another embodiment the foundation  100  comprises a steel frame fully is encased in concrete and has a central tower receiving metal cylinder fixed to an array of radially extending steel girders encased in concrete beams and rigidly connected at their outer ends to an array of perimeter beams  190  encased in the concrete foundation and a reinforced concrete slab-on-grade  20  covering the foot print of the foundation  100  and connected to the said steel frame. 
     In one embodiment the ribs are treated with concrete bonding agent along surfaces where cast in place concrete is received. 
     In another embodiment the foundation  100  is provided with drains around the perimeter and the top surface of the slab  20  is slightly sloped towards the said drains such that water is drained away from foundation  100 . 
     In another embodiment the ribs or other foundation elements are covered or coated with protective material for extending the life span of the footing. 
     In one embodiment the ribs  16  are placed on the mud slab  14  first and then the pedestal cage  50  made of an array of rebar, preferably Z or C shaped rebar and circumferential rebar is assembled around anchor bolt assembly. Alternatively the pedestal cage  50  is assembled first or a preassembled pedestal cage  50  dropped into place first and then the ribs  16  with dowels  46  are slid into place so that dowels  46  and shear connectors fit between the elements of pedestal cage  50  rebar assembly. 
     As best seen in  FIG. 3   b , the precast concrete stiffener rib  16  has lifting lugs  32  to help place the stiffener rib  16  into the excavated construction area. The base  21  has a flat bottom surface such that the ribs may stand on their own on the mud slab  14  or compacted granular fill or during transportation from precast plant to foundation site. The precast concrete stiffener ribs  16  have prestressing elements  58  running through the ribs  16  radially from the outside of the ribs  16  and through pedestal  10 . The radial prestressing elements  58  (or post tensioning elements) may be anchored to the opposite side of the pedestal or optionally run through the opposing precast concrete stiffener  16  on the other side of the pedestal  16  and anchored at the end of the opposite rib  16 . Once the ribs  16  and the pedestal cage  50  are in place, the dowels  46  extending radially inward from ribs  16  may be connected to, or spliced with, corresponding dowels arranged in the pedestal cage. Inside of a cage  50  are additional rebar dowels  48  which will facilitate the continuity of the structural components through the pedestal  10  as well as resist bearing, shear and bending loads. 
     Also inside of pedestal reinforcement cage  50  is a bolt assembly  60  comprising a bolt template  52  an embedment ring  54  and anchor bolts  56  protected by a PVC sleeve  57  or wrapped with a material to prevent bonding between the anchor bolts  56  and concrete to be poured. The anchor bolts  56  have a top portion which is used to attach the base flange  301  of a tower or column to the pedestal  10 . A grout trough template  52  at the bottom of the bolt template  52  may be used to create a grout trough  90  to ensure a good connection of the tower or column to the pedestal  10 . The grout trough  90  will be formed by removing the bolt template  52  from the anchor bolts  56  after the concrete has been poured. Radial dowels, prestressing elements or shear connectors at the inner end of ribs  16  should be spaced to clear anchor bolts # and other reinforcement arranged in pedestal cage  50 . 
     In a preferred embodiment, for fully cast in place foundations, slab forms may sit directly on the mud slab and rib forms  16   b  are supported and kept elevated above slab  20  elevation by means of adjustable and reusable support legs arranged in the rib forms  16   b . Small footings or thickened mud slab areas could be used under rib form support legs. Pedestal forms  102  can be supported by rib forms  16   b  or by separate support legs. 
     When ribs  16  are prefabricated, the bolt assembly  60  is held in place and the anchor bolts  56  are properly oriented by an alignment apparatus  130  can be utilized. The alignment apparatus  130  has a central post  132  with arms  134  attached perpendicularly to the center post and having legs  136  for attachment to the top of the ribs  16  to provide added stability, and bolt circle proper alignment during construction. The legs  136  have an adjustable height relative to the arms  134 . 
     The arms  134  may have braces  138  attached to the central post  132  for holding the arms  134  straight. The central post  132  may also have rod supports  135  for holding reinforcement rebar such as reinforcement rebar  80  which are spliced to dowels  46 . The alignment apparatus  130  also has adjustable support members  140  for attachment between the arms  134  and the bolt template  52  to align the anchor bolts  56  so they are upright. The alignment apparatus  130  can support the bolt assembly  60  without the central post  132  by relying on the legs  136  supported by ribs  16 , which allows the lower portion of the central post  132  to be removed if desired. Alignment apparatus  130  can be used as a template to ensure proper location, elevation and orientation of ribs  16 . 
     The ribs  16  can be of any shape or size depending on the specifications of the tower and loads thereon. For example the ribs  16  may be trapezoidal, rectangular, tee shaped or I beam shaped. The ribs  16  may have intermediate stiffener plates or diaphragms for improved structural performance. The ribs  16  or rib forms  16   b  may receive ramps or catwalks thereon for easy access to the forms during construction. 
     Ribs  16 , or rib forms  16   b , may have means for receiving and supporting perimeter forms  18 , such as bolts or threaded inserts for receiving and supporting the pedestal forms  102 . The ribs  16 , or rib forms  16   b , may also have attachment means  15  for holding base forms  17 . The pedestal forms  102  may be equipped with platform sections for allowing access around the pedestal and the rest of the footing. 
     With all the rebar, ribs  16 , pedestal  100 , bolt assembly frame  80  and optional alignment apparatus  130  in place concrete forms may be attached such that concrete can be poured to form the pedestal and base of the foundation. Pedestal forms  102  may attach to the ribs  16 , or rib forms  16   b , by bolts  18  or by any other means. Similarly the base perimeter forms  17  may be attached to the ribs  16 , or rib forms  16   b , by bolts  15  or by any other means. Alternatively the base perimeter forms may be supported to the ground or the mud slab. 
     With all the parts assembled all the rebar in place and the conduit for the prestressing tendons or rods of the foundation in place, concrete is ready to be poured into the pedestal  10  and between the ribs  16 . The pouring of the concrete can be accomplished quickly and slab areas between the ribs  16  can be finished as the pedestal  10  concrete is still being poured. The concrete may be used to build the pedestal  10  and the slab  20  in one pour. Alternatively the base for the entire foot print of the footing can be poured in a first pour then the pedestal  10  can be formed in a second pour. 
     When bonded multi-strand post tensioning system is used in the foundation  100 , the prefabricated components will be fitted with ducts and anchor hardware according to design specifications. The cast in place components will be fitted with matching ducts to facilitate the continuity of tendons across the foundation  100 . After the jacking of tendons, duct grouting is carried out as required. If the un-bonded, bundled mono-strand system is employed, no duct or grouting is required. 
     The structural load capacity of the foundation  100  is increased significantly by the combination or radial (or diametric) and circumferential post tensioning  59 . Circumferential post tensioning  59  creates a desirable symmetric bi-axial post compression in the slab  20 . Circumferential post tensioning  59  is applied at an elevation well below the neutral axes of the ribs  16  thus creating eccentric post compression in the ribs  16  and the pedestal  10  and resulting in increased nominal moment and shear capacity of the ribs  16  as well as improvement in multi-axial fatigue resistant of the pedestal  10 , ribs  16  and the slab  20 . Radial or diametric post tensioning elements  58  extend from rib to opposite rib across the pedestal  10 . Radial post-tensioning is applied with an eccentric load pattern, with higher post compression below the neutral axis of the rib. When all the prestressing elements are jacked, the foundation  100  is kept under heavy multi-axial eccentric post compression stress, thus increasing rib structural capacity to resist soil support reaction and providing low deflections, high stiffness and low stress amplitudes resulting in high fatigue resistant and high durability. Backfill is added over the foundation  100  for increased stability and stiffness of the foundation  100 . 
     After the concrete sets, post tensioning is carried out and the foundation  100  is backfilled with compacted granular fill to stabilize the foundation  100  against overturning. 
     Alternately the bolt assembly can be replaced by a tower section  56   b  embedded in pedestal  10  concrete and the embedded section  56   b  having means  56   c  for receiving a tower base by means of a bolted connection arranged at the top of the section. The embedded metal cylindrical tower section  56   b  encased in pedestal  10  concrete is provided with holes for rebar and post tensioning tendons  58  to extend through the metal cylinder. Post tensioning  58  tendons can extend through holes arranged in the cylinder and across the pedestal  10 , through the ribs  16  to be anchored on distal ends of the foundation. 
     Pedestal  10  can be any size or shape, round, triangular, square, polygon or other shape depending on the specifications of the tower and loads thereon. The ribs  16  can be in any pattern around the pedestal  10 . In one embodiment shown in  FIG. 2  the foundation  100  may have a square pedestal  10  and ribs  16  at the corners parallel to the faces of the pedestal. The pedestal  10  may have a stepped construction with an enlarged lower cross section to reduce the length of the cantilevered ribs  16 . 
     Pre-assembled reinforcement sections (meshes) of the slab  20  components can be lowered into place in the slab  20  to speedup construction. All rebar dowel or metal shear connectors extending through a construction joints may be galvanized or Epoxy coated to prevent corrosion. The use of mechanical couplers in the foundation  100  shall be limited or avoided. Specified mechanical couplers must be tested and certified for the number of load cycles in the life span of the foundation  100 . 
     In another preferred embodiment, the ribs are cast in place in reusable rib forms  16   b . The ribs  16  are cast in place jointly with the pedestal  10  in one continuous pour over the slab. Optionally, the ribs  16 , the pedestal  10  and the slab  20  are all jointly cast in one pour. All rib internal components including rebar assembly with dowels and prestressing elements are placed inside the forms then cast in place concrete is poured into the rib forms  16   b  as well as into pedestal  10  and slab  20  forms. 
     Rib reinforcing cages can be assembled above grade and lowered into the foundation in one or more sections. 
     In a preferred embodiment rib forms  16   b  with internal rib reinforcing cages are preassembled and lowered into the foundation by cranes to mesh with slab reinforcing sections already placed in the foundation. The radial reinforcing panel of the slab  20  enables the meshing rib dowels between slab reinforcing without geometric interference. 
     Ribs  16  can also be made in segments and eventually united by means doweling or by using segmented post-tensioned construction techniques. Rib anchor zone with anchor trumpets and hardware can be prefabricated separately of higher strength concrete than the rest of the rib. 
     Prefabricated perimeter beams  190  with post tension ducts could serve as perimeter forms become part of the structure. An array of precast, rectangular or L-shaped beams with means for connecting to the slab  20  and the ribs  16  can be used. The perimeter (edge) beams can rest directly on the mud slab and connect to the slab  20  using horizontal dowels and shear key arranged at its inner side. Optionally the perimeter beam is elevated and connects to the top of the slab  20  using dowels extending from its bottom. The precast perimeter beams  190  may have dowels and shear keys extending from their sides ends for connecting to the ribs  16 . In this case the ribs  16  will have corresponding dowels and shear keys for receiving and supporting perimeter beams  190 . The connection between ribs  16  and perimeter beams  190  is established using closure pours in small cavity at the conjunctions. 
     In another embodiment the foundation  100  pertains to hybrid gravity based and rock anchored foundation system. Ribs  16  can be made with arrangement, mechanisms and connecters for receiving piles  400  or micro-piles  401  or anchors  404  in different configurations. Vertical through holes in the ribs  16  can provide means for receiving a pile or an anchor. Bearing elements and grouting are arranged on top of each rib to establish the required structural connection. An array of bearing plates  404   b  with tensioning nuts  404   c  on each soil/rock anchor is used to compress the foundation  100  against supporting soil. Vertical through holes with corrugations for the anchor extend through the foundation. Bearing plates  404   b  with tensioning nuts  404   c  can be placed on top of the pedestal  10  or in the foundation  100 . If desired ribs  16  may have piers extending vertically from the ribs  16  and the top of pier elevation is raised above grade to make anchor bolts accessible for tensioning and testing. Typical rock or soil anchor construction and grouting methods can be utilized. Another option is to house rock anchor bolts and bearing plates  404   b  and tensioning nuts  404   c  in accessible corrosion protection compartments above the foundation  100 . 
     In another embodiment the invention pertains to a foundation  100  that comprises the following elements:
         1. A vertically extending pedestal  10  that is cast in situ, out of concrete, the pedestal  10  serving to receive and support the tower structure;   2. A substantially horizontal support slab  20  that is cast in situ out of concrete, the support slab  20  covering an area of ground larger than that covered by the pedestal  10 ;   3. A plurality of radial ribs  16  extending radially outwardly from the pedestal  10  and spaced around the pedestal  10 , each rib is being joined along the base thereof to the support slab  20  being joined along an inner side thereof to the pedestal  10 , each rib has means for receiving a rock or soil anchor;   4. An optional plurality of perimeter beams  190 , or stiffened slab edge  190   a , spanning continuously, near the perimeter of the foundation, between ribs  16  and supporting the slab  20  may be employed;   5. An array of soil or rock anchors  404  extending through the foundation  100 , preferably through the ribs  16 , may extend down into the ground below the foundation, each anchor having a bearing element in or above the foundation  100  and compressing the foundation against support soil when the anchors are tensioned.       

     The prefabricated components can be molded at a facility under controlled conditions for good quality concrete setting and controlled rebar spacing which is superior to what can be obtained on a job site and at a lower cost. The ribs  16 , acting as deep stiff horizontal cantilever support, allow the base of the foundation slabs to have a relatively small thickness using less cast in place concrete and rebar thus lowering the cost for each foundation. 
     Alternatively ribs  16  can have reusable temporary supports  170 , or other means, arranged at the ribs  16  to hold the ribs  16  in place, maintain them plumb during construction and elevate them at a predetermined height over slab reinforcing. This style of ribs  16  is intended to be raised above the ground or mud slab  14  so that the foundation support slab  20  can be poured in place continuously under ribs. Dowels and shear connectors for this style may be arranged at the bottom of the rib for connecting with base slab  20  which extends under the raised rib. When the concrete cures the continuous support slab  20 , extending under the ribs, is united to the prefabricated ribs  16  and the ribs  16  are also united to the pedestal  10 . The rib inner ends will be partially encased in the pedestal  10  to increase rib torsional end resistance. The final result is continuous monolithic foundation wherein loads are carried across the structure vertically and laterally through the continuous structure by the doweled and spliced reinforcing steel bars which are integrally cast into the pedestal, ribs  16  and support slab  20 . The combination of the high stiffness of the ribs  16 , solid pedestal  10  and continuous slab  20  construction across the pedestal  10 , and under ribs  16 , allows the slab  20  to behave structurally as a continuous slab  20  over multiple rigid supports resulting in small flexural and shear stresses in the slab  20 , reducing deflections, improving fatigue conditions and increasing the stiffness of the foundation as well as allowing for the benefits of an economical design. 
     Cast in situ concrete can be shielded from extreme weather, including heat, cold, rain and snow, by simply extending blankets, covers or shields between ribs  16  during construction, and then using heaters or fans as required to regulate temperature, humidity of concrete to allow for proper setting and curing conditions. 
     Another embodiment of the present invention pertains to a leveling technique that simplifies the tower base leveling process and shortens the number of steps required for grouting under a tower base. The bolt template is provided at the very top of the bolt assembly with at least three sets of additional bolts and corresponding threaded bolt inserts suitable for embedment in the concrete. Such leveling bolts  53  and inserts  53   b  will be located outside or inside the bolt circle of tower base, but directly under tower base flange. This allows for continuity of grout bed construction and provides an easy access to leveling bolts  53 . Small cutouts at leveling bolt locations may be used. Another benefit of this leveling technique is having the ability to apply continuous grout bed that is free of cold joints, under tower base flange in one session as well as having the ability to tension all anchor bolts in one work session. 
     The foundation design can be reconfigured to support lattice towers comprising multiple columns connections to foundations in a spaced array. The ribs  16  will be provided with column receiving components including embedded anchor bolts (or grouting around embedded element) and an integrated pier design into the rib. The rib geometry may be widened and enlarged at the integral pier. The array of said integrated piers ribs  16  are fitted with means for receiving and supporting the legs or the columns of the lattice tower  200 . The integrated piers can extend above final grade elevation, while the top of pedestal  10  may stay below final grade elevation. For this foundation style, pedestal elevation may be depressed and tower receiving components may not be required in the pedestal  10 . This configuration may also be used in offshore applications wherein a prefabricated gravity foundation  100  is connected to lattice tower structure  200  that is fitted with a wind tower receiving component at its top. The foundation  100  will be installed over prepared seabed and filled with a suitable backfilling material  13 , and surrounded with scour protection  13   b.    
     In permafrost conditions, the foundation  100  may be supported on an array of concrete piers deeply embedded and frozen into the ground. Anchors can be used to secure the ribs  16  to their supporting piers around the perimeter of the foundation. If a slab  20  is incorporated in the design, the slab  20  bottom elevation may be set above grade elevation. 
     This invention pertains to a fatigue resistant gravity based spread footing for use under heavy multi-axial cyclical loading of a wind tower  300  which comprises a plurality of components, namely a central vertical pedestal  10 , a substantially horizontal continuous bottom support slab  20  with stiffened perimeter, a plurality of radial reinforcing ribs  16  extending radially outwardly from the pedestal  10  and a three-dimensional network of vertical, horizontal, diagonal, radial (or diametric) and circumferential post-tensioning elements that keep the structural elements under heavy multi-axial post compression with specific eccentricities and orientations that are intended to reduces stress amplitudes and deflections and allows the foundation  100  to have a desirable combination of high stiffness and superior fatigue resistance while improving heat dissipation conditions during construction by having a small ratio of concrete mass to surface area thus eliminating the risk of thermal cracking due to heat of hydration. 
     Vertical prestressing of the pedestal  10  can be carried out independently of tower receiving elements. A pedestal  10  may have an array of vertical post tensioning elements  56  that does not connect to a tower  300 , and an embedded tower section  56   b  bolted to a tower structure  300 . 
     Radial post-tensioning  58 , extending across the foundation  100 , in pairs of ribs  16 , allows for the desirable structural continuity and the direct transfer of loads from a downwind ribs  16  into the pedestal  10  and then into the opposing upwind ribs  16 . Radial and circumferential post compression stresses in the slab  20  and/or perimeter beams  190  allows for a desirable reduction in stress amplitudes the structural continuity between slab  20  spans and/or perimeter beam  190  spans, across the ribs  16 , thus creating a desirable load sharing mechanism between adjacent ribs  16  by forcing more ribs  16  to be engaged in resisting tower loads. 
     The invention pertains to a durable, high-stiffness, fatigue-resistant foundation structure  100  for onshore wind tower installations which comprises:
         1. a central pedestal  10  that is made of cast-in-place concrete with concentric vertical prestressing elements and eccentric multi-axial horizontal and/or radial post-tensioning elements;   2. an array of cast-in-place eccentrically post-tensioned radial ribs  16 ;   3. a cast-in-place slab  20  with heavily post-tensioned thickened slab edge  190   a.  
 
All components are made of high strength reinforced concrete and are rigidly connected to each other to behave as a monolithic spread foundation structure. The structural components are rigidly connected with arrays of rebar dowels (passive reinforcing) or post-tensioning elements extending through the conjunctions. The slab  20  functions as a two-way slab system that is free of construction joints across the footprint of the foundation and spans continuously over multiple ribs  16 . Perimeter post tensioning  59   a  or circumferential post tensioning  59  of the slab  20  is applied at an elevation well below the neutral axes of the ribs  16  to cause eccentric loading of the ribs  16  and the pedestal  10 . Radial post-tensioning elements with an eccentric load pattern, with higher post compression at the bottom of the rib, extend from rib end to opposite rib across the pedestal  10 , or to the opposite end of the pedestal  10 . When all the prestressing elements are jacked, the foundation  100  is kept under heavy multi-axial eccentric post compression stress, thus increasing rib structural capacity to resist soil support reaction and providing low deflections, high stiffness and low stress amplitudes resulting in high fatigue resistant and high durability. Backfill  13  is added over the foundation  100  for increased stability and stiffness of the foundation  100 .
       

     Soil support reaction under the slab  20  is transferred from the slab  20  to the ribs  16  and thickened slab edge  190   a  (or perimeter beams  190 ) as in two-way slab systems with more load distribution going to the ribs  16  in the primary span. Perimeter or circumferential post-tensioning are applied, perpendicular to the ribs  16 , in the orientation of the primary span that effectively reduces stress amplitudes and deflections in the slab  20  by keeping the slab  20  under heavy post-compression in the directions of primary slab spans around the foundation. The size, distribution, eccentricity and location of post tensioning elements  58  in the ribs  16  and the slab  20  are used, by the engineer, to dictate the natural frequencies of the foundation  100  to be in a safe range relative to operating frequencies of the wind generator according to turbine manufacturer recommendations. 
     The said vertical, radial and circumferential post-tensioning in the foundation keep all the structural components (Pedestal  10 , ribs  16 , slab  20 , thickened slab edges (or integral edge beams)) under multi-axial post compression confinement resulting in lower stress range amplitudes thus yielding higher stiffness, more effective crack control, lower deflections and improved fatigue resistance. Superior fatigue resistance and long life-span are achieved by keeping most of the structural elements of the foundation  100  under multi-axial compression while resisting operating loads or even during normal and abnormal extreme loads from the supported structure (wind power generator). 
     In a preferred embodiment, rib post-tensioning requirement are reduced by engaging fully developed bar dowels from the rib into the pedestal connection as well as extending fully developed radial rebar dowels of the slab  20  into the pedestal  10 , thus allowing passive reinforcing to participate in the said connection especially under extreme loads. Radial slab reinforcing pattern with tapered rib width was found to be very cost effective as the rib to pedestal connection benefits from a large number of top and bottom radial slab reinforcing bars participating in the said connection, as the rib width widens, thus reducing the number of bottom post-tensioning strands required for the said connection. 
     The structural configuration of the foundation  100  reduces the overall cumulative deflections in the structure under tower loads and significantly improves the rotational stiffness of the foundation  100  which is a key factor in determining the size of foundations in wind turbine installations. The rotational stiffness is also improved by the interlocking between surrounding soil (after backfilling) and the multiple surfaces and vertical faces of the foundation structure. The horizontal stiffness is improved by the passive earth pressure on the multiple faces of the structure. Both rotational and horizontal stiffness achieved by this design are much higher than conventional tapered inverted-T gravity spread footings especially for onshore foundations installed below grade in an excavated pit because of the increased interlocking surface area and increased passive earth pressure and increased friction on the multiple faces of the fatigue resistant foundation  100 . 
     The solid-core pedestal  10  comprises a continuous reinforcing cage and a tower receiving component, such as anchor-bolt assembly, with a cylindrical array of bond protected high strength post-tensioning bolts, for connecting to wind tower base flange  301 . In another embodiment and the tower receiving component may comprise an embedded cylindrical metal tower section  56   b  with means  56   c  for connecting to a tower section such as a flange with bolt holes for receiving bolts at its top and with an array of holes to allow the passing of rebar and post tension tendons. The anchor bolt assembly ensures structural continuity between the tower  300  and the pedestal  10 . The post-tensioning forces of the anchor bolts are selected by the engineer to insure that the tower base flange  301  remains in contact with the pedestal  10  under extreme normal and abnormal load conditions. The bolt assembly includes, at its bottom end, a bearing element that may consist of an embedment ring plate that is made of segments that are welded together. 
     Radial post-tension tendons and rebar reinforcing elements extending from the ribs  16  and the slab  20  pass through the pedestal reinforcing cage, or through holes in the embedded metal tower section. 
     As shown on the drawings, post-tensioning elements are flared horizontally, profiled vertically, arranged in matrix groups, spaced and draped in a manner that allows for optimum utilization of post-tensioning and ease of installation while avoiding tendon congestion and stress concentrations as tendons crisscross in the pedestal  10 . The regrouping of tendons to form flat and wide matrix along each axis was found to be effective in avoiding tendon congestion especially in the pedestal  10 . The said flat and wide matrix of tendons is placed as high or as low as possible to maximize their moment arms and optimize their contributed moment capacity. For corrosion protection, bonded (multi-strand and grouted) or un-bonded encapsulated (mono-strand) post-tensioning elements and their associated construction techniques can be used in the foundation  100 . 
     The rib&#39;s thickness can be gradually increased at the connection to the pedestal  10  to increase rib flexural, shear and torsional capacity and enhance pedestal confinement. The post-tensioning requirements can be reduced by engaging dowels at rib-to-pedestal connection and by extending fully developed radial dowels from the rib and the slab  20  deep into the pedestal, thus allowing passive reinforcing to participate in the connection. 
     In another embodiment, ribs  16  top surface can be tapered to a substantial slope extending vertically to an elevation near the top of pedestal allowing the ribs  16  to benefit from diaphragm action at their inner zone and also provide lateral support for the full height of the pedestal  10  and to provide concrete confinement at the highly stressed zone at the top of pedestal under tower base flange  301 . 
     The foundation may have a circular or polygonal foot print. The thickened slab edge  190   a    190   a  (perimeter beam) may extend above or below the foundation. A shallow perimeter beam profile should be selected for ease of backfilling and improved accessibility for roller compactors during the backfilling of the foundation  100 . A thickened slab ring beam  190   a  may be designed to be at an offset distance away from the slab edge allowing the slab segment, outside the ring, to behave as a cantilever. This configuration reduces slab span and deflections as well as the volume of concrete required in the foundation  100 . 
     As shown on the drawings the size of the slab  20  and its continuous reinforcing including that of the thickened slab is configured to create a rigid composite connection to the ribs  16  with high stiffness which is sufficient to allow adjoining ribs  16  to participate more in resisting the loads and thus reducing local deflections and increasing overall foundation stiffness in addition to reducing the unsupported length of cantilever radial ribs  16 . 
     In a preferred embodiment, as shown on the drawings, the pairing of the ribs  16  on distal ends and the continuous perimeter beam construction yield a cost effective layout of post-tensioning that uses a small number of tendons and corresponding anchors as well as reduces friction losses by avoiding sharp turns in tendon layout. The tendons of the ribs  16  are anchored in a matrix array at the outer end of the rib and extend horizontally and diagonally along the rib to split into at least two groups  58   a  and  58   b  one near the bottom and the other near the top of the rib as it connects to the pedestal  10 . The tendons are more concentrated at the bottom than at the top in a concentric prestressing pattern that is intended to maximize the structural capacity of the foundation and meet the flexure and shear demand of the governing load cases. 
     Ribs  16  may have thickened flanges, at their connection to the pedestal  10 , that may also house post tensioning anchors for tendons  58  extending from ribs  16  on the opposite side of pedestal. The ribs  16  may also have post tensioning anchors along their sides or tops if tendon curtailment methods are applied in the design. The ribs  16  may also have embedded loop anchors if looping of tendons is used in the design. Loop anchors could also be used in the pedestal  10  to support precast concrete towers  300   b.    
     As shown on the drawings the tendons in ribs  16  extend horizontally and diagonally to be split into three distinctive groups as they enter the pedestal. The first group  58   a  with more tendons is placed at the bottom of ribs  16  or in the slab to create camber for reducing deflections and improving foundation soil contact as well as meet the high flexural demand from the governing load cases, and the second group  58   b  slope up diagonally to follow the geometry of the top of the rib as they enter the pedestal  10 . The third group  58   c  is in the middle and it starts horizontal at rib anchor block and diagonally slopes down towards the bottom of the rib to enter the pedestal  10  for optimum use of the tendons. Tendons in the pedestal  10  are fanned and flared into groups to simplify the installation and maximize their utilization by increasing their moment arms measured from the top or the bottom of the structural concrete. Additional post-tensioning groups for shear resistance can be provided by providing tendons that traverse the shear failure plane in the ribs  16 . 
     In another embodiment the post-tensioning in the ribs  16  consist of three distinctive groups:
         1. A bottom group  58   a  that is horizontal at the bottom of the rib  16  and in the slab  20  and may be grouped with slab post tensioning,   2. A top group  58   b  that is diagonally sloped upward to follow the geometry of rib top,   3. An optional middle group  58   c  that starts horizontal at rib outer edge and is diagonally sloped down towards the bottom of the rib to eliminate dead load deflections and keep the ribs  16  and pedestal under post compression during normal operating conditions and also provide the high demand of post-tensioning capacity required at the bottom of the rib for downwind load cases, and traverse the shear failure plane for ribs  16  in the governing downwind load cases and provide additional shear resisting capacity in each rib,
 
such that the number of strands in the bottom of the rib and the pedestal  10  is much higher than that at the top thus causing a multi-axial, heavy, eccentric horizontal post compression in the foundation after the tendons are jacked.
       

     Anchor-blocks for perimeter or circumferential post-tension tendons can be placed at perimeter beams  190 , (ring beams) or thickened slab or at the edge of the foundation or on top of perimeter beams  190  or on sides of ribs  16 . A preferred layout with two anchor blocks on opposite sides of the foundation and with semi-circular (180-degree) tendon arrangement is shown on drawings. Ring tendons with ring anchors  59   b  (such as dog-bone anchors) can be used to avoid having blisters on the foundation  100 . Styrofoam block-outs  59   c  can be placed in the foundation  100  according to anchor manufacturer recommended dimensions. When the concrete reaches the sufficient strength ring tendons are jacked and ring anchors grouted. 
     The foundation is made of a network of prestressed concrete elements that can be structurally analyzed, with the strut and tie method, as to a three-dimensional structure made of an array of vertically and horizontally oriented truss-girders joined at the center, with major tension chords reinforced with prestressing tendons, based on both upwind and downwind load cases wherein tension forces in the structure are resisted largely by prestressing elements and passive reinforcing and compression forces are resisted largely by the concrete elements. The structure can be analyzed as a circumferential array of vertically oriented trusses that are fixed at their inner ends to the central pedestal  10  and are laterally stabilized at their bottom by a horizontal trussed diaphragm formed by perimeter post tensioning  59   a , in the slab or perimeter beam, and radial bottom tendons  58  in the ribs  16  or the slab  20 . 
     In another embodiment the fatigue resistant foundation  100  comprises a circumferential array of vertically oriented eccentrically prestressed cantilevered girders that are fixed at their inner ends to a central pedestal  10  that is laterally supported and confined through most of its height by rib concrete, and the ribs  16  and pedestal  10  are laterally stabilized at their bottom by a horizontal prestressed concrete trussed diaphragm, with a continuous slab  20 , and the prestressing is provided by radial tendons in the ribs  16  (or the slab  20 ) and circumferential post tensioning elements  59 . The radial and circumferential tendons provide eccentric prestressing in the ribs  16  and the pedestal  10 . The pedestal is vertically prestressed and is structurally fixed to a tower base  301  of a pylon. 
     In a preferred embodiment the construction of the foundation  100  may utilize pre-assembled perimeter beam reinforcing cages, built in segments with overlapping spliced bars at their ends, and each having an array of shear resisting vertical ties and flexure resisting horizontal bars as well as local reinforcing at anchor locations as shown on FIG. X. 
     As shown on the drawings, the foundation has specific reinforcing groups. The ribs  16  have flexure reinforcing concentrated at the bottom and the top, vertical stirrups for shear reinforcing that are tightly spaced at high shear zone along rib inner end, rib skin reinforcing on ach face and bursting and splitting reinforcing made of horizontal hairpins extending between the said rib skin reinforcing, as well as straight or U-shaped horizontal dowels for embedment into the pedestal  10  and vertical dowels, at the bottom, for composite action with the slab  20 . As shown on the drawings the vertical stirrups also function as dowels for composite action of the slab  20 . The said dowels are spaced such that they could mesh between slab reinforcing bars without geometric interference, if the rib reinforcing is built in preassembled cages and placed over the slab reinforcing. In order to maximize shear capacity vertical stirrups are placed side-by-side, in pairs, at the inner rib zone where the shear demand is high. 
     Anchor zones are provided with heavy reinforcing with trim bar and ties. The ribs  16  may also have horizontal reinforcing dowels, perpendicular to the ribs  16 , to facilitate the structural continuity of the supported perimeter beams  190  or the thickened slab, across the width of the rib, by means of splicing the said dowels with perimeter reinforcing. 
     The pedestal  10  has a horizontal mesh at the top and skin reinforcing at all faces as well as at least one cage, around the anchor bolt assembly, comprising vertical tightly meshed anti-bursting reinforcing including two cylindrical meshes confining the anchor bolts each comprising horizontal hoops and either C or Z-Shaped bars and a radial array of horizontal hair-pins or stirrups tying both cylindrical meshes or spirals stirrups each housing a number of anchor bolts. The pedestal  10  cage assembly may comprise two concentric tightly meshed cages surrounding the anchor bolts one from the inside and the other from the outside with radial array of anti bursting and splitting resistant hairpins extending between the two cages # and #. Additionally an array vertically oriented pedestal vertical anti bursting and splitting resistant reinforcing group, comprising circumferentially spaced vertical hairpins extending between said top horizontal mesh and a horizontal bottom reinforcing mesh in the pedestal  10  or slab  20 , is included in the pedestal cage. The vertical hairpins # also function as supports to secure tendons in the pedestal  10  during construction. 
     Upper and lower slab reinforcing meshes may have any pattern such as a square grid, a circular array with radial pattern or overlapping pie-shaped segments. Additionally, an array of slab reinforcing locally arranged beneath the ribs  16  and being oriented parallel to the ribs  16  and extending into the pedestal  10  to facilitate composite action. The slab  20  may also be reinforced with post-tensioning elements in any pattern including radial, circumferential, perimeter or a square grid. 
     The foundation system relies on the use of many prefabricated components including rebar meshes and cages, pedestal cage assembly, pre cut post-tensioning strands, preassembled post-tensioning bundles, pre-cut post-tensioning duct sections and prefabricated concrete forms. 
     Reusable rib forms  16   b  are utilized to form foundation perimeter, the ribs  16  and the pedestal. Forms can be made to be segmented, universal, expandable and adjustable to work for different foundation sizes. As shown on FIG. X rib forms  16   b  can be made with adjustable supports to elevate the forms above the wet slab concrete during construction if the foundation is built in one pour. Rib forms  16   b  may sit directly on the hardened concrete slab  20  if the foundation is built in two pours. Rib forms  16   b  may be made with two side-panels of stiffened non-stick plates and an array of adjustable horizontal spacers between the panels to maintain proper geometry and resist the lateral pressure of wet concrete. Rib and pedestal forms  102  may be fitted with lifting lugs or means for receiving and supporting ladders, catwalks and work platforms to allow for access around the foundation. The forms may have means for securing post-tension anchors and hardware at specific spacing during construction. The forms may also have means for hanging an supporting rib reinforcing cages. 
     The foundation  100  may be supported on piles, or micro-piles  401  or piers  402  or rammed-aggregate piers  405 . The foundation  100  may receive rock anchors  404  or soil-anchors in a conventional manner. A construction site is prepared by excavation and flattening and preparation of soil for the foundation. The foundation  100  may be set on a mud slab or on compacted granular fill. The mud slab is a thin plain concrete layer intended to provide a clean and level base for foundation installation. 
     In one embodiment, After the foundation site has been prepared, slab reinforcing is placed inside slab forms and the slab is poured in place with dowels extending up from the slab  20  to receive ribs  16  and pedestal in a second pour. Rib and pedestal rebar and cage placement with post-tension tendons (or duct) placement are accomplished and forms are installed in place and a second pour is carried out. Alternatively the foundation  100  can be poured in a single pour with the use of accelerators in the concrete mix and by following a well designed concrete pour sequence. A set of small footings, placed within the mud slab, can be used to support and elevate the rib forms  16   b  and pedestal forms  102  during construction. Slab  20 , pedestal and rib reinforcing elements are assembled in the foundation  100 . Forms are placed in the foundation around the perimeter, the ribs  16  and the pedestal and the concrete is poured into the foundation in a carefully designed pour sequence. One option is to start with slab  20  and the bottom part of the ribs  16  and the pedestal with accelerator in the concrete mix to seal the bottom of rib and pedestal forms  102  by the time the slab  20  concrete is finished, the ribs  16  and the pedestal are poured jointly in small lifts. 
     When the concrete hardens to certain strength, post-tension elements are jacked and grouted as required. The tower base flange  301  is then attached to the pedestal  10  and grouted, and the tower anchor bolts are tensioned after the grout reaches sufficient strength. 
     In a preferred embodiment, the invention relates to a high stiffness, fatigue resistant, wind turbine foundation  100 , supporting a wind generator with a multi-megawatt rating and subjected to extremely high cyclical upset loads that comprise the following components:
         1. a substantially massive and wide central pedestal  10  with substantially solid core concrete construction that is kept, through most of its height, under a combination of lateral structural concrete supports and confinement, high vertical post-compression stress and high eccentric multi-axial lateral horizontal post-compression stress across its width, provided by said lateral supports and post-tensioning elements that traverse the width of the pedestal  10 , through non-segmented concrete construction, along multiple axes in a concentric pattern, and having a set of upright, circumferentially spaced anchor bolts, for providing the said high vertical post-compression stress, extending through said pedestal  10 , and having lower ends anchored relative to an anchor ring and upper ends projecting upwardly from said top end of said pedestal, said anchor bolts being substantially bond protected along their length, said upper ends of said bolts project upwardly from the said pedestal  10  through a base flange of an annular tower structurally fixed atop the said pedestal  10 , and also having an upright heavily reinforced cage of tightly meshed rebar, and concentrically arranged around both sides of the anchor bolt cage with opening to allow the passing of lateral load transfer elements,   2. a support slab-on-grade  20 , cast-in-situ out of concrete against the soil, in an excavation pit, of continuous construction and covering a footprint substantially larger than that of the pedestal  10  and having a thickness that is much smaller than the depth of the pedestal  10  and having thickened edge made of concrete integral with the support slab  20  and having horizontal post-tensioning elements to keep the slab  20  under heavy multi-axial post compression,   3. an array of concentrically arranged ribs  16  made of deep girder construction, integral with the pedestal and support slab  20 , and jointly cast-in-situ with said pedestal  10 , and extending vertically, above the slab  20 , to an elevation near the top of pedestal  10  such that the pedestal  10  is laterally supported and substantially confined below the said tower base flange  301 , and having a width that is substantially smaller than that of the said pedestal, and being arranged such that pairs of ribs  16  outwardly extend from opposite sides of the pedestal with post-tensioning elements inwardly extending from the distal ends of the ribs  16  through the pedestal,   4. reinforcing rebar and prestressed dowels extending from the ribs  16  deep into the core of the pedestal  10  from distal ends, and arrays of dowels, made of rebar, extend between the slab  20  and each of the ribs  16  and the pedestal along their conjunctions,   5. a suitable backfill material  13  placed over the foundation  100 , to stabilize the foundation  100  against overturning, followed by tower base installation and grouting,
 
the foundation  100  is kept under heavy multi-axial post-compression such that tower loads are resisted by pairs of ribs  16 , on distal ends of the pedestal  10 , wherein each pair of ribs  16  form a high stiffness continuous, non-segmented, laterally supported, post-tensioned girder extending between distal ends of the foundation  100  with continuous uninterrupted composite action from the slab-on-grade  20 .
       

     In another embodiment, slab post-tensioning can be arranged at any combination of perimeter, radial or diametric, or other patterns. 
     In another embodiment, composite action is further facilitated with radially oriented, reinforcing bars locally arranged in the slab  20 , beneath the ribs  16 , and extended deep into the pedestal  10 , in addition to an array of vertical dowels extending between the rib and the slab  20  that function as shear connectors. 
     In a preferred embodiment, the invention pertains to a foundation  100  for supporting a wind generator with a multi-megawatt rating and subjected to extremely high cyclical upset loads, with increased stiffness and improved fatigue resistant comprising:
         1. a support slab-on-grade  20  of non-segmented continuous construction with a circular integral perimeter beams  190  with circumferential post tensioning elements  59  made of two 180-degree tendon segments forming a 360-degree circle, with anchors at the opposite sides of the foundation,   2. a central cylindrical pedestal  10  integral with the support slab-on-grade of solid non-segmented construction and having vertical post-tensioning elements,   3. ribs  16  integral with the support slab and the central pedestal  10 , on top of the slab  20 , with three or four pairs of ribs  16  radially extending from opposite sides of the pedestal  10  and post tensioning elements extending axially and diagonally from anchors placed at the distal ends of the ribs  16  through the pedestal  10 ,
 
such that the ribs  16  and the perimeter beams  190  function as a prestressed trussed diaphragm structure with infill panels, and pairs of ribs  16  on distal ends of the pedestal  10  function continuous post-tensioned girder, that is free of construction joints, with continuous composite action from the slab  20  and the foundation  100  is kept under eccentric multi-axial horizontal and concentric vertical post-compression, and circumferential post-tensioning in the slab  20  effectively reduces stress amplitudes and deflections in the slab  20  by keeping the slab  20  under heavy post-compression in the direction of primary slab spans which is roughly perpendicular to the ribs  16 .
       

     In a preferred embodiment, the rib extends vertically from the bottom of the foundation to an elevation near the bottom of the tower base flange  301  to enable the ribs  16  to participate in resisting bearing loads under the tower base flange by increasing the area of the cross-section involved in bearing resistance under the tower base flange  301  and increasing the permissible bearing strength under the base flange or the grout bed by increasing the bearing area measured at the surrounding faces of the concrete. The geometric configuration and the improvement in bearing resistance, in this invention, allow the engineer to specify concrete with only one relatively low compressive-strength for the entire foundation structure. In contrast, high bearing stresses under the tower base flange  301  in conventional gravity spread footings, force the engineer to specify concrete with higher compressive strength for the pedestal and a lower compressive strength for the base. 
     The proximity of inner rib ends to the tower base flange  301  allows the inner zones of the ribs  16  to remain under vertical compression stresses caused by vertical post-tensioning forces between embedment ring  54  and tower base flange  301 . The said vertical compression stress zones improves confinement conditions and fatigue resistance in rib inner zones. 
     Bonded and grouted multi-strand system was found to be expensive and lengthy and requires an additional step of grouting and may not be economical for some onshore installations. It is preferable to use un-bonded, encapsulated mono-strands, arranged in bundles and installed in the foundation reinforcing prior to concrete casting, which would reduce construction costs and improve construction schedule. 
     In a preferred embodiment post-tensioning in the foundation  100  is made eccentric, to create cambers in the foundation  100  that could result in reduced deflections and improved foundation-soil contact. As an example, the eccentric prestressing of the ribs  16  creates a convex shaped camber in the foundation  100  that helps reduce the deflections under turbine weight and operating loads. Similarly cambers can be used in perimeter beams  190  and slab sections to reduce slab deflections and improve foundation-soil contact conditions by ensuring a more uniform bearing pressure under the foundation. 
     The vertical profile (elevation) of circumferential tendons in the foundation  100  may be adjusted at mid spans and under supporting ribs  16  to optimize their utilization. 
     In another embodiment gradual transition of geometry at the conjunction of the structural elements is employed to prevent stress concentration and fatigue related problems. As an example the use of fillets and curved transition is desirable at the conjunctions between ribs  16 , pedestal  10  and the slab  20 . 
     In a preferred embodiment, as shown on FIG. X, the inner ends of the ribs  16  are tapered to become wider as the rib connects to the pedestal, in order to satisfy the high flexural, torsional and shear demands at the inner zone of the ribs  16 , and to distribute the multi-axial compression over large surface area to help reduce splitting and bursting reinforcing at the sides of the pedestal  10 . 
     In another embodiment low relaxation post-tensioning strands are used to reduce post tension losses over time. Concrete accelerators and plasticizers and otter admixtures may be utilized in the concrete mix design. The small thickness of the structural elements may allow for on-site steam curing of concrete. 
     A hollow pedestal  10  cross-section may be used, however it can be problematic. Hollow pedestal above the frost depth where there is elevated water table is problematic. Hollow pedestal results in reduced stiffness, stress discontinuity, indirect load transfer and stress concentration with local deformation in the shell resulting in poor performance under heavy flexural and torsional loads from the cantilevered rib, as well as additional problems stemming from stresses caused by frost forming and freezing of ground water in the foundation. 
     In another embodiment the cross-section of the rib may change and dimensions along its length may change. For example, the section may start rectangular and gradually a top flange may be enlarged to reduce stresses in the upper zone of the rib. 
     In another embodiment the pedestal  10  may have an enlarged cross-section at the top followed by a transition into a smaller cross-section below. The upper enlarged cross-section may help with improve bearing strength at the top of the pedestal below tower base flange  301 , or bearing washer plate, and the high strength grout bed according to American Concrete Institute design guidelines. 
     The present invention pertains to a foundation design that overcomes the thermal cracking problem stemming from heat of hydration, in large foundation pours for large multi-megawatt rated turbines, by using a structural configuration coupled with post-tensioning techniques that reduce the thickness of the structural elements, while increasing the surface area of concrete pours, thus improving heat dissipation conditions and causing a the ratio of concrete mass to surface area to be roughly 40% to 50% less than in conventional design for foundations for the same turbine under the same loading and geotechnical conditions. 
     The present invention uses a tower base leveling and grouting method without using tower anchor bolts for leveling, or having to use leveling shims which cause undesirable stress concentration at shim locations which could lead to localized fatigue failure at shim locations. This task is achieved by providing the bolt template at the very top of the bolt assembly with at least three sets of additional bolts and corresponding threaded bolt inserts suitable for embedment into concrete. The said leveling bolts  53  and inserts  53   b  are be located outside or inside the bolt circle of tower base, but directly under tower base flange. This allows for continuity of grout bed construction and provides an easy access for leveling bolts  53 . Small cutouts at leveling bolt locations connected can be used. Another benefit of this leveling technique is having the ability to apply grout a continuous grout bed that is free of construction joints, under tower base  301  in one session and to also have the ability to tension all anchor bolts in one session. 
     The present invention improves safety and accessibility around foundations during construction, and reduces hazardous conditions for construction crew. That goal is achieved by using reusable form sections that are fitted with platform sections for forming a access platforms around the foundation, and connect to at least one access ramp extending beyond the edge of the foundation. The platform and the ramp are fitted with slip-resistant walking surface and elevated ramps all provided with guardrails and designed to applicable industry safety standards. The relatively thin slab thickness minimizes the risk of worker injury during construction. 
     Transformer pad can be supported on precast concrete posts extending vertically from the foundation. 
     Pedestal forms  102  will have openings for running electrical and communication conduits thus preventing problems stemming from randomly placing the conduits in areas that could compromise the structural design. 
     The ribs  16  may have means for receiving and supporting prefabricated trays (or electrical duct banks) for housing power and communication cables. 
     This foundation design can also be adapted for offshore wind turbine projects. In this case the foundation  100  may be assembled on a barge or dry dock then transported or floated to its destination, then lowered into a prepared seabed location. The foundation can be weighed down in place by backfilling it with suitable material. The offshore foundation  100  may be configured to receive any type of offshore piers  404 , suction piers  403 , piles  400 , micro-piles  401 , anchors  404  or any combination of the above. 
     The invention relates to an offshore concrete foundation  100  with high stiffness and improved fatigue resistant comprising:
         1. a support slab-on-grade  20  of non-segmented continuous construction covering the entire footprint of the foundation and having (horizontal) diametric and perimeter post-tensioning elements,   2. a central pedestal  10  integral with the support slab-on-grade  20  of solid non-segmented construction and having vertical post-tensioning elements and also having reinforcing elements of rebar to carry loads diametrically across the pedestal  10 ;   3. a cylindrical or conical stem  11  extending vertically above the pedestal  10  and being fixed to the pedestal  10 , and having a hollow cross section, of equal size or smaller than that of the pedestal  10 , and may be constructed with segmented or non-segmented construction methods and could be made with typical cast in place over the pedestal  10  by using typical construction methods for tall cylindrical concrete structures such as continuous forming, successive pours, segmental construction with precast concrete panels or other known construction methods used conventionally for cylindrical concrete structures such as chimneys, and the stem  11  is kept under heavy concentric vertical post-compression stress by an array of circumferentially arranged vertical post-tensioning elements, and the stem  11  may have an ice cone  11   b  integral with the top of stem  11 , and the stem  11  having means for fixing a tower base  301  of a wind tower  300 , the stem  11  and the ice cone  11   b  are vertically and circumferentially prestressed with vertical and circumferential post tensioning elements,   4. ribs  16  integral with the support slab  20  and the central pedestal  10 , on top of the slab-on-grade, with pairs of ribs  16  radially extending from opposite sides of the pedestal  10  with post-tensioning elements extending radially and diagonally from the distal ends of the ribs  16  through the pedestal  10  and keeping the ribs  16  and the pedestal  10  under heavy eccentric post compression stress and reinforcing dowels extending from the ribs  16  into the pedestal  10  and spliced with pedestal  10  reinforcing,   5. deep perimeter beams  190  extending continuously around the foundation, made of concrete integral with the support slab-on-grade  20  and the ribs  16  and having continuous perimeter or circumferential post tensioning elements,
 
When the concrete sets, the said post-tensioning elements are jacked and the anchor bolts are post-tensioned the foundation is kept under heavy multi-axial post-compression.
       

     The offshore foundation  100  is constructed on a barge or in a dry dock and then floated or transported to an offshore installation site and lowered to be places over a prepared sea bed, a suitable backfill material  13  placed over the foundation  100  to stabilize the foundation against overturning. Scour protection measures  13   b  are provided around the foundation. The foundation is built with marine cement and marine grout and is kept under heavy multi-axial horizontal and vertical pre-stress using bonded and grouted post tensioning systems rated for double corrosion protection and suitable for marine environment. 
     An offshore foundation for wind turbines comprising the following elements:
         1. A vertically extending pedestal that is cast in situ, on a barge, out of concrete, the pedestal has an integral long stem  11  for receiving and supporting a tower structure;   2. A substantially horizontal support slab  20  that is cast in situ, on a barge, out of concrete, the support slab  20  covering an area of ground larger than that covered by the pedestal  10 ;   3. A plurality of radial ribs  16  extending radially outwardly from the pedestal  10  and spaced around the pedestal  10 , each rib being prefabricated and being joined along the base thereof to the support slab  20  when the support slab  20  is cast in situ and being joined along an inner side thereof to the pedestal  10  when the pedestal  10  is cast in situ;   4. A plurality of prefabricated perimeter beams  190  spanning continuously, near the perimeter of the foundation, between ribs  16  and supporting the slab  20 ;   5. Backfill  13  for weighing down the foundation, resisting tower loads and providing scour protection  13   b.  
 
when the concrete sets, the precast components will become integral with a cast-in-place components. Radial post-tensioning tendons extend from rib ends opposite rib ends across the pedestal  10 . Vertical post-tensioning is arranged in the pedestal  10  as well. The stem  11  and the ice cone  11   b  may also benefit from circumferential post-tensioning.
       

     The pedestal  10  has means for receiving and supporting a tower  300  or pylon. The upper portion of the pedestal  10  (the stem  11 ) may be made in multiple consecutive cast in situ pours, depending on its height. Alternatively, the stem  11  may also be made by joining precast segments with circumferential and vertical post-tensioning to form the stem  11 . 
     In another embodiment of the invention, a wind turbine is fabricated on a barge with precast concrete element as following. The barge surface is prepare with a non bonding agent or a thin membrane at the foot print where the foundation to be built. Lower slab reinforcing mesh sections are assembled and placed and the pedestal cage reinforcing is assembled at the center of the foundation. Upper slab reinforcing mesh sections may follow after slab post tension duct is placed. Precast concrete ribs  16  are placed in a radial array around the pedestal cage and precast concrete perimeter beams  190  are arranged around the perimeter of the foundation. Post tensioning ducts in the pedestal space and at perimeter beam-to-rib connections are placed to pair with corresponding duct in the precast members. Forms for pedestal and for closure pours at rib-to-perimeter beam connections are installed. Slab concrete is poured followed by pedestal  10  concrete and closure pours at rib-to-pedestal connections. Stem  11  is fabricated possibly in multiple consecutive pours depending on pedestal height. Stem  11  design may incorporate an ice cone  11   b  at its top. Post tensioning tendons are installed, the jacking and grouting of tendons is carried out. Some pylon sections could be installed earlier prior to transportation. The finished foundation  100  is transported to its offshore installation site using a suitable means of transportation such as towing the barge. 
     In another embodiment of the invention relates to an offshore foundation for wind turbines comprising the following elements:
         1. A vertically extending pedestal  10  that is cast in situ, on a barge or dry dock, out of concrete;   2. A substantially horizontal support slab  20  that is cast in situ, on a barge or dry dock, out of concrete, the support slab  20  covering an area of ground larger than that covered by the pedestal  10 ;   3. A plurality of radial ribs  16  extending radially outwardly from the pedestal  10  and spaced around the pedestal  10 , each rib being prefabricated and being joined along the base thereof to the support slab when the support slab  20  is cast in situ and being joined along an inner side thereof to the pedestal  10  when the pedestal is cast in situ, each rib has an integral pier for receiving a leg of lattice lower;   4. A plurality of perimeter beams  190  spanning continuously, near the perimeter of the foundation, between ribs  16  and supporting the slab  20 , optionally each perimeter beam can be prefabricated;   5. A lattice tower  200  having a plurality of legs structurally connected to the integral piers  180  in the ribs  16 , the lattice tower  200  has, at its top, means for receiving and structurally supporting a pylon or a tower  300 ;   6. Suitable offshore backfill  13  for weighing down the foundation, resisting tower loads and providing scour protection  13   b.          

     When the concrete sets, the pre-cast components will become integral with a cast-in-place components. Radial post-tensioning tendons extend from rib ends opposite rib ends across the pedestal  10 . Vertical post-tensioning is arranged in the pedestal  10  as well. The structural behavior is improved by the added compression in all ribs  16 , edge beams, slab  20  and center pedestal. 
     The lattice tower  200 , preferably incorporating 3-dimentional trusses, transfers the pylon loads down to the concrete foundation  100 . The lattice tower  200  may get connected to the concrete foundation prior to transportation or it can be connected to the foundation at final offshore installation site. 
     In another embodiment of the invention, a wind turbine is fabricated on a barge with precast concrete element as following. The barge surface is coated with a non-bonding agent or covered with a thin membrane at the foot print where the foundation to be built. Lower slab reinforcing mesh sections are assembled and placed and the pedestal cage reinforcing is assembled at the center of the foundation. Upper slab reinforcing mesh sections may follow after slab post tension duct is placed. Precast concrete ribs  16  are placed in a radial array around the pedestal cage and precast concrete perimeter beams  190  are arranged around the perimeter of the foundation. Post tensioning ducts in the pedestal space and at perimeter beam-to-rib connections are placed to pair with corresponding duct in the precast members. Forms for pedestal and for closure pours at rib-to-perimeter beam connections are installed. Slab concrete is poured followed by pedestal concrete and closure pours at rib-to-pedestal connections. A lattice tower  200  structure is prefabricated and mounted atop the concrete foundation  100 . The foundation is transported to installation site using a suitable means of transportation. Seabed is prepared for receiving the foundation by placing a sub-base of suitable material such as crushed stone. The foundation is backfilled and scour protection measures  13   b  are installed. 
     In another embodiment of the invention, the stem  11  is prefabricated separately and provided with means for connecting to the pedestal  10 , preferably an array vertical post tensioning dowels extended between the pedestal and the stem or other segmental post tensioning joining methods. The pedestal may be fitted with means for receiving the prefabricated stem based on segmental post tensioning and grouting construction methods. 
     Piles  400 , Micro-piles  401  or piers  402  or suction piers  403  or anchors  404  can be used with the offshore foundation  100  in a similar manner describe in the application. In this case vertical sleeves will be arranged in the foundation to receive an array of piles  400  or anchors extending through the foundation, and allow for additional loading capacity and improve stability of foundation. Piles  400  are secured to the foundation by filling the sleeves with marine grout. 
     Under some conditions, the use of piles  400 , piers or suction piers or anchors may eliminate the slab  20  and/or the perimeter beams  190  from the design. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the invention.