Patent Application
20080072511
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 are prefabricated and transported to job site, but the pedestal and support slab are poured in situ at the site out of concrete.
A construction site is prepared by excavation and flattening and preparation of soil for the foundation 10. The foundation 10 may be set on pilings, on piers, or have anchors in a conventional manner.
The foundation 10 may be set on a mud slab 14 or on compacted granular fill. The mud slab 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 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 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 10. 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 10 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 100 in the center of the foundation.
Doweling of rebar between ribs and foundation components can be achieved by using rebar couplers, bar extenders or any mechanical rebar splicing system.
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, encased into the web of the rib and extend inwardly into the pedestal cavity at the inner most end of ribs, shall serve as suitable shear force transfer mechanism between rib and pedestal and will also serve as shear reinforcing against pullout shear force of the embedment ring as it crosses the pullout cone of the embedment ring.
In another embodiment the embedment ring, arranged at bottom of bolt assembly, is connected or welded to beams, encased into the web of the array of rib and extend inwardly into the pedestal cavity at the inner most ends of ribs. This configuration will improve the resistance for pullout of the embedment ring by relying on engaging the shear load capacity of the deep ribs.
In one embodiment the ribs are treated with concrete bonding agent along the sides where cast in place concrete is received.
In another embodiment the ribs are provided with water stops or other sealers along the sides where cast in place concrete is received, if corrosion of rebar is a concern.
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 shaped rebar and circumferential rebar is assembled. 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 pedestal cage 50 rebar assembly.
As best seen in
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 and concrete to be poured. The anchor bolts 56 have a top portion which is used to attach the base flange of a tower or column to the pedestal. A grout trough template at the bottom of the bolt template 52 may be used to create a grout trough to ensure a good connection of the tower or column to the pedestal 100. 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 should be spaced to clear anchor bolts and other reinforcement arranged in pedestal cage.
In order to hold the bolt assembly 60 in place for proper alignment of the anchor bolts 56 an alignment apparatus 130 can be utilized. The alignment apparatus 130 can have 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 being of adjustable height relative to the arms 134. The arms 134 may have braces 138 attached to the central post 132 for holding the arms straight. The central post 132 may also have rod supports 135 for holding reinforcement rebars such as reinforcement rebars 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 without central post by relying on the legs 136 supported by ribs, which allows the lower portion of the central post to be removed if desired. Alignment apparatus can be used as at template to ensure proper location, elevation and orientation of ribs.
The ribs 16 can be of any shape or size depending on the specifications of the tower and loads thereon. For example the ribs may be trapezoidal, rectangular, box, tee shaped or I beam shaped. The ribs may have intermediate stiffener plates or diaphragms for improved structural performance. The ribs 16 may have steps 120 or may receive ramps or catwalks thereon for easy access to the forms and pedestal used during construction and maintenance and means for supporting stairs, ramps, ladders and catwalks for use during construction or for maintenance.
Ribs 16 can have means for receiving and supporting forms 18, such as bolts or threaded inserts for receiving and supporting the pedestal forms 102. The ribs 16 may also have attachment means 15 for holding base forms 17. The pedestal forms may be equipped with platform sections for allowing access around the pedestal and the rest of the footing. The ribs may also have steel beams, trusses or girders encased in the concrete along the length of the ribs. The beams or girders can connect to a central steel drum or structure in the pedestal for forming a monolithic structure.
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. The pedestal forms 102 may be attached to the ribs 16 by bolts 18 or by any other means. Similarly the base perimeter forms 17 may be attached to the ribs 16 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 cable or rods of the foundation in place, concrete is ready to be pored into the pedestal and between the ribs. The pouring of the concrete can be accomplished quickly and the area between the ribs can be finished as the pedestal concrete is still being poured. The concrete may all be added at the central portion of the pedestal or at the pedestal and the base simultaneously. Alternatively the base for the entire foot print of the footing can be poured in a first pour then the pedestal can be formed in a second pour.
When the concrete sets, the upper section of the alignment apparatus 130 and the bolt template 52 may be removed by unbolting the connection plate 150 from the top portion of the central post 160, and unbolting the legs 136 from ribs.
For higher load capacity post tensioning of the foundation is completed by tightening post tensioning cables 110. Circumferential and radial post-tensioning techniques in slab and pedestal can be used if desired.
After the concrete sets, the foundation can be covered with the backfill material to add weigh on top of the foundation base to stabilize the foundation against overturning.
Alternately the bolt assembly can be replaced by a drum with dowels or plates for embedding in concrete and the drum having means for receiving a tower base by means of joining bolts attached to the top of the drum.
Pedestal 100 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 can be in any pattern around the pedestal. An alternative design is shown in
Pre-assembled reinforcement sections (meshes) of the slab components can be lowered down in place to speedup construction. A combination of mechanical bar couplers and splicing techniques can be used provide continuity of reinforcing across the foundation.
Pre-assembled rib forms with all internal components including rebar, dowels and prestressing elements can be used in lieu of precast ribs in the same manner as the described above. Cast in place concrete will be poured into the rib forms as well as the pedestal and the slab. Forms for ribs and pedestal can be removed and reused.
Ribs can also be made in segments and eventually united by means doweling or using structural connectors
Forms for the pedestal and foundation perimeter can be made of precast concrete as separate components or as an integral part of the rib, and can be left as part of the structure.
Ribs can be made with arrangement, mechanisms and connecters for receiving piles or anchors in different configurations.
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 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 ribs, allows the slab to behave structurally as a continuous slab over multiple rigid supports resulting in small flexural and shear stresses in the slab, reducing deflections, improving fatigue conditions and increasing the stiffness of the foundation as well as allowing for the benefits of an economical design.
The prefabricated ribs 16 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, acting as deep stiff girders and similar to counterforts, allow the base of the foundation slabs to have a relatively small thickness using less cast in place concrete and rebar thus saving cost for each foundation. The base rebar will be of smaller size than rebar used on a standard cast in place gravity-style spread footing.
Alternatively ribs 16 can have posts 170, or other means, arranged at the ribs 16 to hold the ribs 16 in place, maintain them plumb during construction and elevate them over sub-base at a predetermined. This style of ribs is intended to be raised above the ground or mud slab 14 so that foundation support slab 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 which extends under the raised rib. When the concrete cures the continuous support slab, extending under the ribs, is united to the prefabricated ribs and the ribs are also united to the pedestal. 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 and support slab. The combination of the high stiffness of the ribs, solid pedestal and continuous slab construction across the pedestal, and under ribs, allows the slab to behave structurally as a continuous slab over multiple rigid supports resulting in small flexural and shear stresses in the slab, 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, and then using heaters or fans as required to regulate temperature, humidity of concrete to allow for proper curing.
Therefore, for wind turbine construction as an example, a known turbine model and tower the base loads and tower base configuration can be matched with site characteristics and geotechnical conditions to select a standard foundation design requirement to build standardized foundations so that engineering time and expense for building wind turbine foundations can be reduced significantly.
Another embodiment of the present invention pertains to a leveling technique that simplifies tower base leveling process and shortens the number of steps required for grouting under 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 into concrete. Such leveling bolts and inserts 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 for leveling bolts. Small cutouts at leveling bolt locations connected can be used. Another benefit of this leveling technique is 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 with base plate connected to foundations at a spaced array. The ribs will be provided column base plate receiving components including embedded anchor bolts and an integrated pier design into the rib. The rib geometry will be widened and enlarged at the integral pier. The array of integrated piers arranged into ribs shall receive the array of columns of the lattice tower. The integrated piers can extend above final grade elevation, while the top of pedestal can stay below final grade elevation. For this foundation style, no bolt assembly or tower receiving components will be required in the depressed pedestal.
This foundation design can also be adapted for offshore wind turbine projects. In this case the foundation may be assembled on a floating platform 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 filling over it with suitable material.
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 appended claims the invention may be practiced otherwise than as specifically described.
| Number | Date | Country | |
|---|---|---|---|
| 60826452 | Sep 2006 | US | |
| 60954502 | Aug 2007 | US |
