Fatigue reistant foundation system

Abstract

A fatigue resistant gravity based spread footing under heavy multi-axial cyclical loading of a wind tower having 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 post-tensioning elements that keep the structural elements under heavy multi-axial post compression with a specific eccentricity that is intended to reduces stress amplitudes and deflections and allows the foundation to have a desirable combination of high stiffness and superior fatigue resistance. The foundation design reduces the weight and volume of materials used, reduces cost, and improves 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.

1 . A foundation comprising:
 a concrete support slab having horizontal rebar therein,   a concrete pedestal integral with the support slab having vertical rebar therein,   a plurality of concrete ribs on top of and integral with the support slab and integral with the pedestal, the ribs being radial to pedestal, and having connection elements extending from the ribs into the pedestal and from the ribs into the slab,   post tensioning elements extending from the distal end of the ribs through the pedestal and,   a perimeter beam in the slab having a post tensioning element therein.   
     
     
         2 . A foundation as in  claim 1  wherein,
 the ribs positioned in pairs on opposite sides of the pedestal and having the tension elements extending from the distal ends of the ribs through the pedestal. 
 
     
     
         3 . A foundation as in  claim 2  wherein,
 the post tensioning elements in the ribs are eccentrically arrayed for eccentric loading. 
 
     
     
         4 . A fatigue resistant foundation comprising:
 a horizontally prestressed concrete slab having a structurally continuous horizontal rebar grid for reinforcing the slab and a plurality of horizontal post tensioning elements therein for prestressing the slab, and a perimeter beam proximate the perimeter of the slab,   a vertically extending central concrete pedestal having a plurality of vertical post tensioning elements, the pedestal integrally connected to the slab,   a plurality of reinforced concrete ribs on top of and integrally connected to the slab with structural continuity between the rib and the slab, the ribs integrally connected to the pedestal, with structural continuity between the rib and the pedestal,   at least one post tensioning element running through each rib and the pedestal for providing post tensioning stress in the ribs and pedestal and for connecting the ribs to the pedestal under post compression stress,   such that the pedestal, slab and the ribs are connected to each other to form a monolithic foundation with a three dimensional network of post tensioning elements in the slab, pedestal and ribs keeping the foundation under multi-axial compression while resisting operating loads, and during normal and abnormal extreme loads, thus linking the slab and the ribs to the pedestal under vertical, and horizontally eccentric post compression stress, with reduced multi-axial stress range amplitudes in the concrete and steel reinforcing.   
     
     
         5 . A foundation as in  claim 4  wherein,
 the perimeter beam comprises a thickened perimeter slab portion. 
 
     
     
         6 . A foundation as in  claim 5  wherein,
 the thickened perimeter slab portion comprises a plurality of slab perimeter reinforcing cages, each cage having a plurality of continuous horizontal bars housed in an array of vertical stirrups extending between the horizontal bars for reinforcing the perimeter slab portion and having a perimeter post tensioning element extending through the plurality of perimeter reinforcing cages to further post tension the perimeter slab portion. 
 
     
     
         7 . A foundation as in  claim 4  wherein,
 the pedestal has a vertical inner reinforcing cage and an outer reinforcing cage with the plurality of vertical post tensioning elements therebetween, the pedestal also having a horizontal bottom reinforcing mesh, and a horizontal top reinforcing mesh, with an array of vertical reinforcing hairpin bars extending therebetween for reinforcing the core of the pedestal. 
 
     
     
         8 . A foundation as in  claim 4  wherein,
 the pedestal has pairs of ribs on opposing sides of thereof and post tensioning elements extending from the distal ends of the ribs though the pedestal to hold the ribs to the pedestal under post compression stress. 
 
     
     
         9 . A foundation as in  claim 7  wherein,
 the pedestal has a vertical inner reinforcing cage and an outer reinforcing cage with the plurality of vertical post tensioning elements therebetween, the pedestal also having a horizontal bottom reinforcing mesh, and a horizontal top reinforcing mesh, with an array of vertical reinforcing hairpin bars extending therebetween for reinforcing the core of the pedestal. 
 
     
     
         10 . A foundation as in  claim 9  wherein,
 the pedestal has pairs of ribs on opposing sides of thereof and post tensioning elements extending from the distal ends of the ribs though the pedestal to hold the ribs to the pedestal under post compression stress. 
 
     
     
         11 . A foundation as in  claim 4  wherein,
 the ribs are cantilevered and have a neutral axis, and the post tension elements in the perimeter beam and in the ribs and through the pedestal are spaced relative to the neutral axis of the cantilevered ribs to provide a three dimensional post compression load pattern in the foundation to counter influence the multi-stage and multi-axial fatigue loading of the forces of a wind tower supported thereon when the post tensioning elements are tensioned to the desired stresses. 
 
     
     
         12 . A foundation as in  claim 4  wherein,
 the ribs have a neutral axis and the post tension elements in the perimeter beam, in the ribs and through the pedestal are spaced relative to the central axis of the ribs to provide a camber pattern in the foundation to counter the deformation due to the dead loads on the foundation from self weight, soil weight and the weight of a wind tower supported thereon when the post tensioning elements are tensioned to the desired stresses. 
 
     
     
         13 . A foundation as in  claim 4  wherein,
 the ribs are cantilevered and have a neutral axis such that when perimeter post tensioning elements in the slab are tensioned at an elevation below the neutral axis of the cantilevered ribs it causes an eccentric loading of the cantilevered ribs and the pedestal to satisfy the governing load case of ribs under downwind loading during extreme normal and abnormal load cases when the post tensioning elements are tensioned to the desired stresses. 
 
     
     
         14 . A foundation as in  claim 4  wherein,
 the pedestal is laterally confided by the ribs and the vertical post tensioning elements in the pedestal, and the pedestal further comprises a plurality of bond-protected anchor bolts extending vertically from an embedment ring near the bottom of the pedestal through the top of the pedestal for securing a base flange of a tower in full contact under compression for providing vertical post compression stress in the foundation when the anchor bolts are tensioned. 
 
     
     
         15 . A foundation as in  claim 4  wherein,
 a backfilling material placed over the foundation adds stability and stiffness to the foundation. 
 
     
     
         16 . A foundation as in  claim 4  wherein,
 the size, distribution, eccentricity and location of post tensioning elements in the ribs and the slab are selected to dictate the natural frequencies in the foundation for a desired safety range selected for the operating frequencies of the wind tower and generator supported on the pedestal. 
 
     
     
         17 . A foundation as in  claim 4  wherein,
 the ribs are prefabricated.