POST-TENSIONING ANCHOR HEAD ASSEMBLY

Abstract: An anchor head assembly (1) comprising a main body member (2) with a plurality of cavities (5) having the shape of a frustum of a cone in an exterior section (3) and a corresponding number of cavities (6) which may also have the shape of a frustum of a cone in an interior section (4). The cavities are orientated such that the narrow ends of those in the exterior section face the narrow ends of those in the interior section. The interior section (4) is dome-shaped so as to facilitate the fabrication of the cavities therein and to facilitate the splaying of strands or tendons as they are passed into and through the cavities in both the interior and exterior sections. The dome-shaped section also facilitates self-centering of the anchor head asembly in a bearing plate. Wedge members (23) are used for capturing the strands and tendons in the cavities in the exterior section of the anchor head assembly. A hexagonally shaped exterior section (9) and the dome-shaped interior section also provides for a lighter weight anchor head than has been heretofore available and the strength of the anchor head is improved using austempered cast iron fabrication techniques.

Description: Post-tensioning anchor head assembly BACKGROUND OF THE INVENTION Field of the Invention The present invention relates, in general, to multistrand post-tensioning cable systems for use in fabricating a wide variety of concrete structures, such as bridges, parking garages, buildings and the like and in particular to a novel multistrand anchor head assembly for use in such systems. Description of the Related Art A typical multistrand post-tensioning cable system used in the fabrication of various concrete structures, such as those described above, comprises a bearing plate and an anchor head. The bearing plate comprises either a rectangular or a cylindrical member having a rear or bearing surface for bearing against a concrete structure. The bearing plate has a bore centrally located therein having an interior end and an exterior end. The interior end of the bore is provided for receiving a plurality of strands or tendons captured in and extending from corresponding strand or tendon anchoring apparatus in a remote concrete structure. The exterior end of πhe bore is provided for receiving an anchor head which is provided with means for capturing the ends of the strands or tendons received in the interior end of the bore. The typical anchor head comprises either a rectangular or a cylindrical member having a rear or bearing surface which is inserted in and/or bears against the bearing plate. It is provided with a plurality of strand or tendon receiving cavities for receiving and capturing, with the help of wedge-shaped members, a corresponding number of strands or tendons. The conventional anchor head typically is made from a relatively massive and heavy casting or forging of ductile iron or mild steel sufficient to withstand the tremendous compressive forces imparted to it via the strands or tendons captured therein. Its front and rear surfaces are generally planar and no attempt has been made heretofore to remove excessive material therefrom or to provide any sort of self-centering capability. A typical strand receiving cavi-ty in the conventional anchor head comprises a first or interior cylindrical cavity and a second or exterior cavity having the shape of a frustum of a cone in which a strand, after passing through the first cavity, is captured using two or more wedge-shape members. In addition to the weight and massiveness of the prior known anchor head and the lack of any self- centering features therein, a further disadvantage of the prior known anchor head is that the bundle of strands in the cable captured therein must be splayed at the anchor head and that the cylindrical shape of the first cavity in the conventional anchor head tends to impart bending moments to the strands which increase in magnitude as a function of the distance a strand is from the center of the bundle. These bending moments are found to impart an undesirably large amount of stress and fatigue to the strands in the bundle, particularly in the outermost strands therein. SUMMARY OF THE INVENTION In view of the foregoing there is provided in accordance with the present invention a post-tensioning multistrand anchor head assembly comprising a polygonal- shaped exterior front surface for reducing the weight of the assembly, a plurality of exterior strand receiving cavities having the shape of a frustum of a cone and wedges for capturing a plurality of strands therein, a plurality of interior strand receiving cavities having the shape of a frustum of a cone for reducing strand stress and fatigue and a dome-shaped interior rear surface for further reducing the weight of the assembly, facilitating the fabrication of the interior strand receiving cavities and the insertion and self-centering of the anchor head assembly in a bearing plate. For greater strength than prior known anchor heads, the anchor head of the present invention is made of austempered cast iron. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of the accompanying drawings, in which: Fig. 1 is a plan view of the exterior surface of an anchor head according to the present invention; Fig. 2 is a plane view of the interior surface of the anchor head of Fig. 1; Fig. 3 is a side elevation view of Fig. 1; Fig. 4 is a cross-sectional view taken in the direction of lines 4-4 of Fig. 1; Fig. 5 is an enlarged view taken within the lines 5- 5 of Fig. 4; and Fig. 6 is an enlarged view taken within the lines 6- 6 of Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1-6, there is provided in accordance with the present invention a multistrand anchor head assembly designated generally as 1. In the assembly 1 there is provided a main body member designated generally as 2 comprising an exterior section 3 and an interior section 4. The exterior section 3 comprises a plurality of cavities 5 having the shape of a frustum of a cone and the interior section 3 comprises an equal number of corresponding cavities 6. Cavities 6 preferably have the shape of a frustum of a cone but may comprise a cylindrical shape or the like. The narrow ends of- the cavities 5 and 6, when the cavities 6 have the shape of a frustum of a cone, are located in the interior of the exterior and interior sections 3 and 4 in communication with and facing each other. The walls 7 and 8 of the cavities 5 and 6 in the exterior and interior sections 3 and 4 of the assembly 1 diverge at a predetermined angle φl relative to a longitudinal axis extending through the center thereof of approximately 7 degrees. As will be described below, this angle facilitates the capturing of a tendon 10 therein and provides for reduced stress and fatigue on the tendon. In addition to the cavities 5, the exterior section 3 comprises a polygonal-shaped member 9 resulting from the elimination of excessive material from the edges thereof so as to produce an anchor head much lighter in weight than prior known anchor heads without any sacrifice in strength. In the embodiment shown, the member 9 is hexagonal-shaped. In addition to the cavities 6, as described above, the interior section 4 comprises an annular planar surface 15 in the center of which is located a dome- shaped surface 16 which slopes inwardly from the annular surface 15 at a predetermined angle φ2 of approximately 30 degrees. The dome-shaped surface 16 further reduces the weight of the anchor head while providing the necessary strength to withstand the large bending moments imparted to the anchor head. In a preferred embodiment of the present invention, the surface 16 has the shape of a frustum of a cone. Extending about the periphery of the planar surface 15 there is provided a shoulder 17. The shoulder 17 comprises a wall 18 which slopes inwardly to the exterior section 3 at a predetermined angle φ3 of approximately 45 degrees, as shown more clearly in Fig. 5. Between the exterior section 3 and the interior section 4, there is provided a center section designated generally as 20, as shown more clearly in Fig. 6. The center section 20 comprises a plurality of cavities 21 having the shape of a frustum of a cone corresponding in number to the cavities 5 and 6 in the exterior section 3 and interior section 4. Each of the cavities 21 provide an interface between the cavities 5 and 6 with the narrow end of the cavity 21 facing the cavity 5 in the exterior section 3 and the wider end of the cavity 21 facing the cavity 6 in the interior section 4. In use, the strands or tendons in a bundle of strands or tendons captured in and extending from a corresponding strand or tendon anchoring apparatus in a remote concrete structure are splayed adjacent to the interior surface 4 of the anchor head assembly 1 in such a manner as to facilitate the passing of individual strands or tendons through the cavities 6 and 5 of the assembly 1, as shown with respect to the tendon 10 in Fig. 4. In practice, the angle φ4 at which the outermost strands or tendons enter the anchor head assembly 1 is approximately 3 degrees relative to a longitudinal axis extending parallel to the centerline of the cavities 5 and 6. This angle greatly reduces the stress and fatigue. heretofore encountered in prior known conventional anchor heads. As the tendon 10 passes through the cavity 6 and thereafter the cavity 5 and tension is placed on the tendon 10 as by means of a hydraulic jack (not shown) , wedge members 23 are placed in the cavity 5 so as to capture the tendon 10 as the tension produced by the hydraulic jack is released. In addition to facilitating the fabrication of the cavities 6 in the interior section 3 of the assembly 1 and providing the strength to withstand the bending moments imparted to the anchor head, the dome-shaped surface 16 serves as a means for self-centering the anchor head in a bearing plate, as described above. To provide greater strength than heretofore available in prior known anchor head assemblies, the anchor head assembly of the present invention is fabricated using austempered cast iron processing techniques. It will be appreciated that the physical size, i.e. diameter, and thickness of the anchor head assembly according to the present invention depends on the number of strands or tendons to be captured therein and the loads expected to be carried thereby. However, in any event, due to the use of austempered cast iron and the dome-shaped structure of the anchor head according to the present invention, greater forces are generally handled with no increase in size or weight of the anchor head assembly. While preferred embodiments of the present invention are described above, it is contemplated that numerous modifications may be made thereto for particular applications without departing from the spirit and scope of the present invention. Accordingly, it is intended that the embodiments described be considered only as illustrative of the present invention and that the scope thereof should not be limited thereto but be determined by reference to the claims hereinafter provided.

Claims: CLAIMS What is claimed is: 1. An anchor head assembly comprising: a main body member having an exterior section with a plurality of cavities having the shape of a frustum of a cone and an interior section with an equal number of corresponding cavities having the shape of a frustum of a cone wherein the narrow ends of corresponding ones of each of said cavities in said exterior and interior sections are located in the interior of said exterior and interior sections in communication with and facing each other; and means in each of said plurality of cavities in said exterior section for capturing a strand or tendon after said strand or tendon has been passed through a corresponding one of said cavities in said interior section. 2. An anchor head assembly according to claim 1 wherein said interior section comprises an annular planar surface in the center of which is located a truncated dome-shaped surface. 3. An anchor head assembly according to claim 2 wherein said truncated dome-shaped surface slopes inwardly from said annular planar surface at a predetermined angle φ2. 4. An anchor head assembly according to claim 3 wherein said predetermined angle φ2 is approximately 30 degrees. 5. An anchor head assembly according to claim 2 wherein the periphery of said planar surface comprises a shoulder having a wall which slopes inwardly toward said interior section at a predetermined angle φ3. 6. An anchor head assembly according to claim 5 wherein said predetermined angle φ3 comprises approximately 45 degrees. 7. An anchor head assembly according to claim 1 wherein the walls of said cavities in said interior and exterior sections diverge at a predetermined angle φ4 relative to a longitudinal axis extending through the center thereof. 8. An anchor hiad assembly according to claim 7 wherein said predetermined angle φ4 is approximately 7 degrees. 9. An anchor head assembly according to claim 1 comprising a center section between said interior and exterior sections in which there is located a cavity having the shape of a frustum of a cone between and in communication with corresponding ones of said cavities in each of said interior and exterior sections. 10. An anchor head assembly according to claim 9 wherein the narrow end and the wide end of each of said cavities in said center section faces a corresponding cavity in said exterior and interior sections, respectively. 11. An anchor head assembly according to claim 1 wherein said exterior section comprises an exterior hexagonal section. 12. An anchor head assembly comprising: a main body member having an exterior section with a plurality of cavities having the shape of a frustum of a cone and an interior section having a dome-shaped surface and an equal number of corresponding cavities, corresponding ones of said cavities in said exterior and interior sections being located in communication with each other; and means in each of said plurality of cavities in said exterior section for capturing a strand or tendon after said strand or tendon has been passed through a corresponding one of said cavities in said interior section. 13. An anchor head assembly according to claim 12 wherein said interior section comprises an annular planar surface and said dome-shaped surface is located in the center of and said annular planar surface. 14. An anchor head assembly according to claim 13 wherein said dome-shaped surface in the center of said planar surface comprises a surface having the shape of a frustum of a cone. 15. An anchor head assembly according to claim 13 wherein said surface having the shape of a frustum of a cone slopes inwardly from said annular planar surface at a predetermined angle φ2. 16. An anchor head assembly according to claim 15 wherein said predetermined angle φ2 is approximately 30 degrees. 17. An anchor head assembly according to claim 12 wherein each of said cavities in said interior section has the shape of a frustum of a cone, the narrow ends of which are in communication with and face corresponding cavities in said exterior section. 18. An anchor head assembly according to claim 13 wherein the periphery of said planar surface comprises a shoulder having a wall which slopes inwardly toward said interior section at a predetermined angle φ3. 19. An anchor head assembly according to claim 18 wherein said predetermined angle φ3 comprises approximately 45 degrees. 20. An anchor head assembly according to claim 17 wherein the walls of said cavities in said interior and exterior sections diverge at a predetermined angle φ4 relative to a longitudinal axis extending through the center thereof. 21. An anchor head assembly according to claim 20 wherein said predetermined angle φ4 is approximately 7 degrees. 22. An anchor head assembly according to claim 12 comprising a center section between said interior and exterior sections in which there is located a cavity having the shape of a frustum of a cone between and in communication with corresponding ones of said cavities in each of said interior and exterior sections. 23. An anchor head assembly according to claim 22 wherein the narrow end and the wide end of each of said cavities in said center section faces a corresponding cavity in said exterior and interior sections, respectively. 24. An anchor head assembly according to claim 12 wherein said exterior section comprises an exterior hexagonal section.

Inventors: CRIGLER JOHN R, CRIGLER, JOHN, R.
Applicants: VSL INT AG, VSL INTERNATIONAL AG
Application year: 1995
Main classification: E04C5
IPC codes: E04C5