Patent Grant
7762029
Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of post tension concrete reinforcing devices and systems. More particularly, the invention relates to structures for anchors used in such concrete reinforcing systems.
2. Background Art
Structural concrete is capable of carrying substantial compressive load, however, concrete is unable to carry significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces on a concrete structure.
The basic principle of concrete reinforcement is simple. In pre-stressing, which is one of two basic types of reinforcement, reinforcing rods of high tensile strength wires are stretched a certain amount and then high-strength concrete is placed around the reinforcing rods. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. The other type of reinforcement, called post-tensioning, follows the same general principle, but the reinforcing rods (called “tendons”) are held loosely in place while the concrete is placed around them. The tendons are then stretched by hydraulic jacks and are securely anchored into place. Prestressing is typically performed within individual concrete members at the place of manufacture. Post-tensioning is generally performed as part of the structure on the construction site.
A typical tendon tensioning anchor system for post-tensioning operations, includes a pair of anchors for anchoring the two ends of the tendons suspended therebetween. In the course of installing the tendon and anchors in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition, thus applying tensile force on the tension to the anchors.
Metallic components, such as tendons, disposed within concrete structures may be come exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If such exposure occurs, and the exposed portions of the anchor and tendon suffer corrosion, then the anchor may become weakened due to this corrosion. The deterioration of the anchor and tendon can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
A typical post-tension assembly, therefore, includes a liquid tight covering or sheathing on its exterior surface. Some anchors are encapsulated in a moisture proof material such as plastic. An example of such an encapsulated post tension reinforcing system is described in U.S. Pat. No. 5,072,558 issued to Sorkin et al. The system disclosed in the '558 patent includes a tendon having an exposed end protruding from a sheath. The exposed end of the tendon is typically fitted through an extension tube. The extension tube has a diameter slightly larger than sheath, such that one end of the extension tube may overlie the sheath. The opposite end of the extension tube fits over, and communicates with, a rear tubular portion of an anchor. The rear tubular member includes an aperture which communicates with a frontal aperture. The frontal aperture defines a cavity or bore in which anchoring wedges are received.
As known in the art, the tendon is disposed through the extension tube and through the anchor wedge receiving bore. The end of the extension tube is sealed to the outer surface of the sheath. After the tendon extends through the frontal aperture, and assuming the far end of the tendon is fixed in place, tension is applied to the tendon, typically by use of a hydraulic jack. While applying this tension, wedges are forced in place on both sides of tendon within the wedge receiving bore. Once in place, teeth on the wedges operate to lock the tendon in a fixed position with respect to the anchor. Thereafter, the tension supplied by the hydraulic device is released and the excess tendon extending outward from the anchor is cut by a torch or other known device. The wedges thereafter prevent the tendon from releasing its tension and retracting inward with respect to the anchor. Moreover, the tension remaining on the tendon provides additional tensile strength across the concrete structure.
It has been determined that the wedge receiving cavity in the anchor body known in the art crated many problems. The wedge receiving bore in the anchor body is typically of a constantly diminishing diameter extending from a forward end of the anchor body to a rearward end of the anchor body. This constantly diminishing diameter is formed during the casting of the anchor body. However, the narrow diameter end of the wedge receiving bore creates problems with the installation of sheathed tendons. When the anchor body is used in the formation of intermediate anchorages, for example, it is often necessary to move the anchor body over a very long length of sheathed tendon. If there is insufficient clearance between the narrow diameter end of the cavity and the outer diameter of the sheathed portion of the tendon, nicks, abrasions, and cuts can occur in the corrosion-resistant sheathing. As such, the integrity of the anchorage system is impaired. Furthermore, there are circumstances where the sheathing diameter may exceed expected tolerances and will prevent the anchor body from easily sliding along the length of the tendon so as to assume its position as an intermediate anchorage. Additionally, in recent years, there has been a tendency to increase the thickness of the sheathing so as to facilitate greater protection of the tendon from corrosive elements. It should be noted that similar problems can occur at a “live end” terminal anchor, the live end being the end of the tendon that is pulled or stretched to apply tension to the tendon.
An easy solution to the foregoing problems would be to expand the diameter of the wedge receiving bore so as to avoid the aforementioned problems. However, if the overall diameter of the bore is expanded, then conventional (standard size and taper) wedges cannot be used. Other problems may occur if larger or non-standard size wedges or if irregular wedges are used. If the wedge receiving bore were enlarged, then the wedge components would have to be replaced in all such post-tension anchor systems.
It is also known in the art to drill out or ream the narrow diameter end of the wedge receiving bore so as to produce a portion of generally constant diameter. However, drilling and reaming have some limitations. First, drilling or reaming can be very expensive in comparison with the casting of the anchors. Furthermore, drilling or reaming of a constant diameter portion in the anchor body can create burrs and deformations which could potentially cut the sheathing of the tendon and cause adverse corrosion-protection results. Finally, drilling or reaming the narrow portion of the wedge receiving bore can intrude into the wedge-contact area so as to cause uneven and irregular contact between the wedges and the wall of the cavity. Such irregular contact may weaken the anchoring system.
One solution to the foregoing is described in U.S. Pat. No. 6,017,165 issued to Sorkin. An anchor body disclosed in the '165 patent includes an internal wedge-receiving cavity. The cavity has a first portion of constantly diminishing diameter extending inwardly from one end of the anchor body. The first portion has an angle of taper with respect to a center line of the cavity. The cavity has a second portion extending inwardly from an opposite end of the anchor body. The first portion and the second portion are coaxial and communicate with each other. The second portion has an angle of taper which is less than the first portion. The first and second portions are cast with the anchor body. Other patents issued to Sorkin disclose variations of the same general concept, namely that the wedge receiving cavity is divided into a first portion and a second portion, wherein the second portion has a different taper angle than the first portion, such that a minimum internal diameter of the wedge receiving bore is at least large enough to enable free passage of a sheathed tendon therethrough.
One limitation to the anchors disclosed in the various Sorkin patents is the cost of casting the anchor to have more than one taper angle in the wedge receiving bore. It has also been determined that prior art wedges may be more massive, and have more uneven distribution of axial stresses to the anchor base or plate than may be considered optimal. Accordingly, there is a need for an anchor for post tension concrete reinforcing systems which more evenly distributes stress to the anchor base, and which is less expensive to manufacture.
One aspect of the invention is an anchor for a post tension reinforcement system. The anchor includes an anchor base having at least one wedge receiving bore therein. The wedge receiving bore is tapered in diameter at a single selected taper angle. An axial length of the wedge receiving bore is selected so that a minimum internal diameter of the wedge receiving bore is at least as large as an external diameter of a sheath on a reinforcing tendon.
Another aspect of the invention is an anchor for a post tension reinforcement system. An anchor according to this aspect of the invention include an anchor base having at least one wedge receiving bore therein. The wedge receiving bore is positioned in the anchor base such that its longitudinal center is approximately collocated with a basal surface of the anchor base.
Yet another aspect of the invention is an anchor for a post tension reinforcement system. An anchor according to this aspect of the invention includes an anchor base having a specific weight of at most about 0.1 pounds per square inch. Specific weight represents the weight of the anchor base with respect to its load-bearing surface area.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
To better understand post tension anchors according to the invention, it is useful to examine specific differences between various embodiments of an anchor according to the invention and prior art post tension anchors. A typical prior art post tension anchor is shown in side view
Still referring to
The following example dimensions are for industry standard anchors used with 0.500 inch nominal outer diameter (OD) tendons (the OD being defined as without a sheath on the tendon). For anchors used with other size tendons, the dimensions shown in the example of
Returning to
Dimension C also represents the approximate height of the ribs 9. In the example of
As is known in the art, a 0.500 inch tendon that includes a sheath will have a nominal external diameter of about 0.65 inches when using a 0.060 inch (60 mil) thick plastic sheath, and including any lubricant or other protective material between the tendon and the sheath. As a result, the minimum internal diameter of the wedge receiving bore 14 of the prior art anchor 10 is typically too small to allow free passage of a typical sheathed tendon therethrough.
Enlargement techniques for the minimum internal diameter of the wedge receiving bore known in the art, as explained in the Background section herein, include reaming or drilling near the first end of the wedge receiving bore 14. Other enlargement techniques include casting the wedge receiving bore to include a second taper angle different from the principal taper angle of the wedge receiving bore so as to provide a minimum internal diameter of the wedge receiving bore large enough to freely admit a sheathed tendon.
Having explained prior art anchor structures, anchors according to the invention will now be explained with reference to
The thickness of the metal structure 7A (forming the basal surface 12A) is shown at dimension AA, and may be 0.21 inches or less. It has been determined that the thickness of the metal structure 7A may be reduced as compared to the prior art structure (7 in
The dimension from the second end 13D of the wedge receiving bore 14A to the metal structure 7A in the present embodiment is reduced to about 0.31 inches (as compared with 0.54 inches in the prior art anchor).
The anchor 10A according to the present embodiment includes one or more reinforcing ribs 9A extending laterally outward from the structure forming the wedge receiving bore 14A. The reduction in the distance between the second end 13D and the upper surface of the metal structure 7A also can provide for a reduction in the rib 9A height. Prior art ribs (9 in
The distance from the basal surface 12A to the first end 13C of the wedge receiving bore 14A in the present embodiment is about 0.53 inches, essentially unchanged from the prior art anchor (see
As a result of having the same maximum internal diameter 18A, the same taper angle and the foregoing shorter overall axial length 16A of the wedge receiving bore 14A as compared to corresponding dimensions in the typical prior art anchor, the minimum internal diameter of the wedge receiving bore 14A in the present embodiment is about 0.68 inches, allowing free passage of a typical sheathed tendon. Another result of the selected axial length of the bore 14A and the resulting longitudinal positioning of the bore 14A with respect to the basal surface 12A is that the wedge receiving bore 14A is located such that its longitudinal (or axial) center is approximately collocated with the basal surface 12A. It is believed that such longitudinal placement of the bore 14A with respect to the basal surface 12A may improve the overall strength of the anchor 10A. In other embodiments, the wedge receiving bore may be formed such as disclosed in U.S. Pat. No. 6,017,165 issued to Sorkin, wherein the bore has a first taper and a second taper such that a minimum internal diameter of the bore is at least enough to enable passage of a sheathed tendon therethrough. The wedge receiving bore in such embodiments can still be located such that its longitudinal center is approximately collocated with the basal surface 12A, thus improving the overall strength of the anchor.
Another feature of an anchor made according to the embodiment shown in
It has also been determined that various configurations of an anchor according to the invention may result in a substantial reduction in the specific weight of the anchor, which is defined as the ratio of the weight of the anchor with respect to the load bearing surface area of the basal surface (12A in
Anchors made according to one aspect of the invention weigh at most about 1.1 pounds, particularly those which are made according to the dimensions explained with reference to
It will be appreciated by those skilled in the art that the foregoing specific weight limitation of about 0.10 pounds per square inch is specifically for industry standard dimension anchors used with 0.500 inch nominal OD tendons. For anchors used with different nominal OD tendons, the specific weight limitation will be a proportional to the ratio of linear dimensions of such anchor to corresponding dimensions on the above example anchor for 0.500 inch nominal OD tendons. Assuming that all anchor dimensions are approximately linearly scaled in relation to the intended OD of the tendon, the specific weight limitation can be calculated by the following expression:
wherein W represents the approximate limit of the specific weight in pounds per square inch of load bearing area, and dt represents the nominal, or load bearing, diameter (in inches) of the tendon for which the particular anchor is sized.
Another possible advantage of an anchor made according to the invention is that having a larger minimum internal diameter of the wedge receiving bore may reduce the incidence of pinching the nose (or small) end of the wedge into the tendon. Pinching at the nose end of the wedge is believed to cause tensile failure of tendons in a number of circumstances. Still another advantage of an anchor made according to the invention is improved quality of casting procedures for the anchor base.
The foregoing aspect of the invention in which the specific weight of the anchor is at most a particularly defined amount is also intended, in particular embodiments, that the anchor have at least a minimum amount of load bearing area. A minimum load bearing area is preferred such that the anchor can be safely used in post-tension reinforcement. It can be inferred from the description relating to equation (1) that merely reducing the load bearing area of the anchor, such as by reducing the lateral dimensions of the basal structure 12A, would, in fact, result in a reduction of the specific weight. However, such reduced area structures may be unsuitable for post-tension reinforcement of concrete structures. An analysis of why it is necessary to have a certain minimum load bearing area in an anchor, and how to determine minimum useful load bearing area is described in, Post-Tensioning Manual, Post-Tensioning institute, 1717 W. Northern Ave., Phoenix, Ariz. 85201, Fifth Edition, Second Printing (1995). More specifically, because the load bearing area of the anchor is typically smaller than the cross sectional area of the reinforced concrete structure, tensile stress applied to the concrete by the anchor is necessarily unevenly distributed at the ends of the concrete structure. Transferred tensile force from the stretched tendon is concentrated at the load bearing area of the anchor at the axial ends of the concrete, and gradually distributes over the entire cross-section of the concrete at some distance from the axial ends. Such transferred force distribution necessarily means that the force includes some component that is transverse to the axis of the tendon and concrete between the axial ends of the concrete and where the full cross-section distribution occurs. If the load bearing area of the anchor is too small, the transverse forces may cause internal tension in the concrete which in some places may exceed the tensile strength of the concrete (known in the art as “bursting stresses”). Another reason for needing at least a certain amount of load bearing area on the anchor is development of localized tensile stresses at the axial ends of the concrete structure, called “spalling stresses.” If there is insufficient load bearing area in the anchor, the spalling stresses may exceed the tensile stress of the concrete, leading to failure at the axial ends thereof.
In the Post-Tensioning Manual, see pp. 208-236, Section 3.1, Guide Specifications for Post-Tensioning Materials, and more particularly, Section 3.1.7, Bearing Stresses, in which it is stated that the average bearing stresses on the concrete created by the anchorage plates shall not exceed the values allowed by the following equations:
at service load: fcp=0.6fc′√{square root over (Ab′/Ab)} (2)
but not greater than 1.25 f′c
at transfer load: fci=0.6fc′√{square root over (Ab′/Ab−0.2)} (3)
but not greater than 1.25 f′c
where fcp represents the allowable compressive concrete stress, f′c represents the compressive strength of the concrete, f′ci represents the compressive strength of the concrete at the time of initial stressing, A′b represents the maximum area of the concrete structure that is concentric with, and geometrically similar to the geometric area of the anchorage, and Ab represents the bearing area of the anchorage. The dimensions and area of a post-tension anchor are further defined for their intended purpose in, Acceptance Standards for Post-Tensioning Systems, Post-Tensioning Institute, 1717 W. Northern Ave., Phoenix, Ariz. 85201 (1999):
ax=bx+2ex≦2bx
ay=by+2ey≦2by
0.25≦ex/ey≦4 (4)
in which ax, ay, represent the long transverse (to the longitudinal axis) dimension and the short transverse dimension, respectively, of the concrete structure, bx, and by, respectively, represent the long lateral (or transverse) dimension and the short lateral (or transverse) dimension of the anchor, and ex, ey, represent, respectively, the distance from the edge of the anchor to the edge of the concrete structure along the long and short dimensions of the structure. Collectively, the foregoing limitations in load bearing area of the anchor and cross section of the concrete structure are referred to as “post-tension acceptance standards.” In a preferred embodiment of an anchor made according to the present aspect of the invention, the specific weight of the anchor is at most the amount determined by equation (1) and such anchor meets the foregoing post-tension acceptance standards.
It should be clearly understood that any or all of the foregoing aspects of an anchor made according to the invention are applicable to a composite structure in which more than one wedge receiving bore is included, such composite structures being used to anchor a plurality of reinforcing tendons.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3703748 | Kelly | Nov 1972 | A |
| 3912406 | McGrath | Oct 1975 | A |
| 4616458 | Davis et al. | Oct 1986 | A |
| 5072558 | Sorkin et al. | Dec 1991 | A |
| 5141356 | Chaize | Aug 1992 | A |
| 5271199 | Northern | Dec 1993 | A |
| 6631596 | Sorkin | Oct 2003 | B1 |
| 6684585 | Campbell | Feb 2004 | B2 |
| Number | Date | Country | |
|---|---|---|---|
| 20060096196 A1 | May 2006 | US |
